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After much searching here on the site for quite some time, I've not comg across an answer to my specific inquiry, so I thought I'd post it directly. As a moderator on another forum, I do try to make sure I "search" for answers before asking a question that has been asked 1000x before, but I cannot locate any direct info on my question, so here goes ...


I have a Tiger Cub (shown in sig line) that I'm happy with; it's been reliable and done a great job. However, I'd like a bit more power from it. There are times when it loads up a bit in heavy grass, and could use a touch more "oomph". It has the Kaw FH601V engine. After looking over countless parts manuals and shop manuals, I've come to the conclusion that I'm unable to tell how they differentiate the HP models in how they achieve that power differential. As I recall that TC chassis and series of engine was available in 19hp, 21hp, 23hp, 25hp, all from the same identical long-block per the shop manual. When visiting the Kawpower site, I can see there are slight variations in carbs from revision to revision, but I cannot tell if these are the SOLE means of upping the power or not. And since there are so many variations, I'm not sure which one I'd need to select.

In fact, according to the manual, page 3-14 for the fuel system, here is the info I see:
FH601V, FH641V, FH661V, FH680V, FH721V
Carburetor Specifications:
Nikki 621266
Throttle Bore Diameter 26 mm (1.02 in.)
Venturi Diameter 21mm (.83 in.)
Main Jet (MJ) L: #122, R: #128
Pilot jet (PJ) L: #46, R: #44
Main air jet (MAJ) 1.7
Pilot air jet (PAJ) 1.1,
etc, etc ...
The above info shows me that even the carbs are the same for all those five engines.

Is there a throttle restriction plate behind the carb that is different?
Is there a different setting in the max throttle travel when the govenor opens up, allowing more WOT flow?
In short, just what makes a 25hp engine out of a 19hp engine, when it appears that the entire long block and carb are essentially the same?????


Does anyone have direct, specific info as to how I can "up-power" my Kaw? Got a site link with specific parts numbers or tuning procedures, etc? Perhaps an older script on sandstone that was done back in the good ol' days? It would be presumably far cheaper for me to simiply upgrade the induction system than to "repower" the chassis with a new engine.

I believe the numbers in the engine ID 641-721 etc., is the cubic centimeters of displacement. That means, as the number gets larger, the bore gets larger or the stroke gets longer or both. The larger the CC displacement the more you need to feed it. That means different caburation. gregjo1948

At least for my engine series, the 19-25HP engines all have identical long blocks (block, heads, crank, bore, stroke, etc) as well as identical induction sytsems (same carb, intake manifold, carb jets, etc). This is all from info in the Kaw FH series engine manual.

Hence my quesiton about how the differnt HP ratings are achived.

I suspect it's in how far the gov allows the throttle plate to open up under load, but I cannot get any confirmation of this.


If you follow the link to the manual in this thread, you'll see what I refer to ...
http://www.lawnmowerforum.com/kawas...v-engine-service-manual.html?highlight=parish
page 1-7 and page 3-14 show that the displacement and induction systems for the 601 - 721 engines are all identical.


Being an old traditional hot-rodder type guy, I am used to the displacement and induction systems being the foremost ways to grab more power. Presuming these are all the same (and according to the manual, they are for the 601 through 721 engines) then the ONLY way I can think of is perhaps a restriction on throttle opening under load???


Any ideas are welcome, but I don't think the "typical" answers are going to satisfy the question. It's not displacement or carb sizing as far as I can see.

I believe I may have stumbled onto the dirty little secret on how these engines are made to offer different HP ratings ...

It is, as simple as this sounds, a matter of which throttle shaft is present in your carb. Each engine gets a different throttle shaft part number. When I looked at crankshafts, connecting rods, pistons, etc, I realized that the engines are the same. What differs is the carb assembly. But the carbs themselves all have the same pilot jets, idle jets, main jets. They have the same isolator plates. They have the same floats and needles. Everything is essentially the same EXCEPT for the throttle shafts.

Now, I made sure to compare each series to the same series in a larger HP engine. When I looked up the parts info at the Kawpower site, I used my engine as the base model, and then looked up the same series in larger HP models.

My engine: FH601VBS29
Others: FH641VBS29, FH680VBS29, FH721VBS29

Looking at the throttle-shaft for each engine, the base part # is 16041A-xxxx. (Do not confuse this with the choke shaft, which has number 16041 but does not have an "A" suffix").
Looking at each engine, there are four butterfly shafts; note the last digit changes:
engine......throttle shft ........... jets ...
FH601:.... 16041A-7022...... 104 & 108
FH641:.... 16041A-7023...... 104 & 108
FH680:.... 16041A-7024...... 104 & 108
FH721:.... 16041A-7025...... 104 & 108


Each shaft has a flat section on the forward upper edge. That "flat" is made to contact a stop which limits the shaft travel. In essence, the HP is manipulated in how far they allow the throttle butterflies to open!

Much of this has to do with how a "governed" engine works; this is different from a vehicle engine where WOT is available at any time under your foot. A governed engine is similar to a cruise control in a car, except that the cruise holds a vehicle speed, where a governor holds the engine speed. In a goverened engine, the rpm is controlled by the governor assembly. Under high rpm, but no load, the butterfly shaft may only be open 1/4 to achieve that 3600rpm typical of small engine equipment. As the load increases, the govenor opens up the buttefly shaft to hold that desired rpm setting (typically maxed at 3600 rpm, but could be less if you set the throttle for mid-throttle or idle ...)
So, with a restricted throttle shaft, the engine will attempt to achieve 3600rpm AS LONG AS THE LOAD IS NOT GREATER THAN THE POWER OUTPUT FOR THAT RESTRICTED SHAFT TRAVEL! However, if the shaft is artifically restricted in it's travel, the HP generated will only be as much as it can make under heavy load with partial throttle opening. Hence, a 19hp engine has a shorter travel than a 21hp, a 23hp, a 25hp engine. That is why there are four different throttle shafts for engines that have essentially the "same" engine (same displacement and same carb jets).

In short, at light loads, any engine can use partial throttle to achieve 3600rpm. But under heavy loads, the engine needs more air/fuel to maintain that 3600rpm; it needs more energy to produce power in response to the demand. Kaw limits their power by limiting the throttle shaft travel!

Want more power from your 19hp engine? Swap to a different shaft! Viola - more power!

Now- a word of caution here ...
The 25hp engines do have some extra items to deal with the greater power production. They include an oil cooler that helps deal with the extra heat generated. Etc, etc ... I have already added the oil cooler previously; easy to do and relatively inexpensive. Also, "more power" sounds great, but it can add stress to other components in the driveline and equipment, so be forewarned ...

If anyone has other thoughts, or sees this differently, then I'd love to hear your ideas. But to me, this seems the most likely way they "up-power" the FH engines in the same displacement engine.

MadMackie - would love to hear your thoughts here. Seems like I've discovered the root difference, but always open to other input for discussion!

Hi dnewton3 and group,
Your research is interesting. I have done some part number research on this engine series and only came up with different main jet part numbers and sizes. I'm really not very up on Kawasaki engines in general, I did find that the FH series has two displacement groups, the higher HP models having the larger displacement. Kawasaki parts manuals are more difficult and time consuming to search thru and I never did find actual part numbers for carburetor assemblies which is usually the first indicator of internal differences. Knowing that Kawasaki parts are pricey to say the least, I would consider repowering with a higher HP later model engine to get more power. Of course, new engines are also pricey, but do have nice warranties.

Mad Mackie said:

Hi dnewton3 and group,
Your research is interesting. I have done some part number research on this engine series and only came up with different main jet part numbers and sizes. I'm really not very up on Kawasaki engines in general, I did find that the FH series has two displacement groups, the higher HP models having the larger displacement. Kawasaki parts manuals are more difficult and time consuming to search thru and I never did find actual part numbers for carburetor assemblies which is usually the first indicator of internal differences. Knowing that Kawasaki parts are pricey to say the least, I would consider repowering with a higher HP later model engine to get more power. Of course, new engines are also pricey, but do have nice warranties.

Click to expand...

I find dnewton3's reseach very interesting also. I was under the assumption that the FH series gained power with more displacement. I did a quick check of part numbers, finding anything to do with displacement, the same numbers from 601V to721V. If you found 2 displacement groups, how can the part numbers be the same?I didn't look into carburation but, if all I need to do is install a different throttle shaft, I'm all for it. gregjo1948

gregjo1948 said:

I find dnewton3's reseach very interesting also. I was under the assumption that the FH series gained power with more displacement. I did a quick check of part numbers, finding anything to do with displacement, the same numbers from 601V to721V. If you found 2 displacement groups, how can the part numbers be the same?I didn't look into carburation but, if all I need to do is install a different throttle shaft, I'm all for it. gregjo1948

Click to expand...

A Kawasaki FH series engine on an older Scag Tiger Cub is a now discontinued line of engines although some models are still available in the aftermarket. The series starts with a FH 451V, 500, 531, 541, 580, 601, 641, 661, 680 and 721 all of which are vertical shaft engines. There are two displacements in this series. The new Kawasaki FH line is a horizontal shaft engine, I haven't seen any on a machine so far.
I think that the FR, FS and the FX lines are the only vertical shaft twins now available from Kawasaki. The former FH line was discontinued due to non compliance with EPA regs.

Correct - we need to be specific here, because my FH series is an older one.
I typically try to be very clear in my questions and answers; hence why I listed the specific models of the engine data I looked up on the Kawpower website.

So I also found this same disucssion on another forum over at www.lawnsite.com and the same topic ended up with the same conversation, although it took them about 10 pages to come to the conclusions that it only took me an hour to discover. You can see the conversation here, but just focus on the photos on page 9; there are a few photos:
a) photo of the part used including the oil cooler set and throttle shaft
b) installed oil cooler
c) another pic of oil cooler
d) view of the 19hp shaft sitting next to the 25hp shaft (this is the most credible evidence and makes it so very simple to understand)
You can see the info and photos here:
http://www.lawnsite.com/showthread.php?t=317558&page=9


Now I am constraining my comments to my older series of FH engines; those that are all at the top of the displacement list from 601-721 series.
The ONLY thing (and I truly mean only thing) that separeates those in HP ratings is the little throttle butterfly shaft, which limits the travel of the shaft in its rotation.
Kawasaki has made a bazillion different variants of many engines, but when comparing and contrasting info within the same bloodline, this is a true statement.
You can "up-power" your FH engine (FH601V - FH721V) by simply removing that restriction. You have two choices:
1) buy the shaft for the HP you want (the 25hp shaft is 16041A-7025 is is currently about $40 give or take a bit) and install it after removing the carb, govenor, etc and put it all back together
2) get out a dremmel and cut off the offending metal on the shaft tab (I did this; took me about 60 seconds ....)

I cut off the matieral using the photo link from above as a guide. I printed off the photo (copy/paste caputred and put into Word) and then used sciscors to overlay the "new" part on the "old" part, so I could scribe a line that would represent the material to remove. If you refer to that photor link above, what I did was essentially cut off the portion of metal that has the "A" stamped upside down in the picture, and made the cut at an angle that replicates the 25hp shaft on the right.

Viola! 25hp out of my 19hp Kaw engine. The engine will now open up the butterfly shaft MUCH further, allowing for a much larger air/fuel mix under heavy loads.
Under light lights, it works just the same as any other engine would. The govenor controls the throttle shaft.
Because I didn't take anything apart, and my mower ran fine previous to this little experiment, I had no issues in having to "return" the idle speeds or govenor setting. I highly recommend that anyone doing this actually take the short cut (pun intended) and just cut off the metal material. Doing this is quicker, and there is no need to mess around with your carb and govenor settings.

I would remind those who try this to seriously consider adding the oil cooler as well. If you expect that high-loads would be rare for long periods, you can probably get away without it. But it you intend to use the new-found power for sustained periods, I really suggest you add the oil cooler; it is very inexpensive - only cost me about $25 when I added it a few years ago. (You'll need the heat sink cooler, the o-ring and the threaded extension). You can find those part numbers on the Kawpower website under the FH721 (25hp) engine parts listing. You can see my thread about this upgrade here: http://www.lawnmowerforum.com/scag-forum/8933-kawasaki-oil-cooler-upgrade.html


Yes - it truly is that simple to achive a 30% power gain.

How is the oil cooler fastened to the engine? Disreguard that question. I went to your "Kawasaki oil cooler upgrade" I have an old Z that I think I'll boost the FH680V up a bit. Have you noticed a fuel consumption gain since you've tweeked your engine? gregjo1948

This is great stuff guys!!!
Never have gotten very deep into Kawasaki engines, but being an old carb guy from the 50s, I have often wondered about Kawis and how they did this. In 1970 I was working for a performance boat dealer and they sent me to carburetor school in Boston, MA. The school was 4 1/2 days. who would have thought a carb school could be that long!!! Later the same year I was attending a Mercruiser marine engine service update school and nearby there was a Holley carb school going on and I was able to get in on some of it. The Mercruisers all were GM engines along with Daytona Marine, but Holman Moody was primarily Ford and then Chrysler Marine 426 hemis along with their 440s.
Anyway, the Kawi stuff is very interesting and if I get the chance, I will get a look at the carb on the Hustler X-ONE/FX730V that I service, but I know that it is at the top of its HP range, but still interesting!!!
Mad Mackie in CT:laughing::biggrin::smile:

Regarding the fuel economy issue, there really is not a big shift as you may first expect.

Because the gov responds to the load, the only time the throttle plate would open up and consume "more" fuel than the lower-power units is if you loaded the engine up hard enough to cause the shaft to rotate past the "normal" position. If the load does not surpass the typical position of any "normal" use, then the extra available power is not actually in use.

Lots of folks mistakenly think that when they run the throttle lever all the way up, that the carb butterflies are wide open, but this is not true at all. Under no load, at 3600rpm, I estimate that the throttle plates are perhaps only 20% open. Under high load, the gov will try to open them as much as possible until the rpm setting is met. The artificial limit of the various throttle shafts limits that rotational travel; hence the different HP limits. But as long as the gov can be satisfied for any given load, then the throttle will normalize to some opening at the butterfly plates. Under mowing loads, it may be perhaps 35% or 45%, but certainly anywhere near 100%. The only time the plates will "open up" is when you get into a heavy load such as wet grass or heading up a steep slope.

Generally when I'm mowing, I'm probably not running under a high load because most of my area is flat and I keep my grass fairly well groomed, so the difference between my "old" 19hp setting and the "new" version since I've modified the shaft is pretty much non-existent. However, I do have a few sections that the extra HP is warranted; just a couple of uphill pulls while mowing. Since this represents a very small portion of the overall mowing job, the fuel economy really is not altered any perceptible amount for me. But if you have a LOT of HEAVY load situations, it will consume more fuel for sure. But it would only be that percentage of fuel PAST where the plates would have been limited anyway.

Now, to get even "more" power, you could pull the carb and install larger main jets; that would give more power across the board, but also consume a lot more fuel across the board. But I'm not interested in that. I'm happy with my shaft modification; gives me plenty of reserve power (30% more) but really costs me nothing in normal operation and it was free to do!

Thanks dnewton3 for all this info. Do you know if the other engine manufacturers use this technique for HP? gregjo1948

Generally it is a displacement and or carb change. Years back some manufacturers made their carbs with replaceable venturis and thereby having the ability to change airflow without having to cast another carb body.
As for Kawasaki carbs, having done the throttle shaft modification and keeping the same high speed jet may cause the air/fuel mixture to be lean on fuel. Monitoring the sparkplugs for color changes may indicate a leaner fuel condition. I think that I would consider increasing the jet size. Depending on the type of jet, sometimes they can be increased in size by running a particular size number drill thru them. I've done this many times on two stroke outboard engines to get them to run smoother and idle better. I haven't done this to very many four stroke engines. I use a micrometer to confirm drill bit size and hold the drill bit in a pin vice and run the bit thru the jet with it removed from the carb. I have an assortment of jet screwdrivers as the jet can get damaged by using the wrong tool. The Briggs engine on my wood chipper had a service bulletin on the high speed jet and recommended it be replaced by another part number. I did this and found the new jet to be .0015" larger than the original jet. This engine appears to have more power since the jet change, but difficult to determine however. The sparkplug did change slightly in color, but I changed from 87 to 89 or 91 octane fuel so the fuel change may had caused the color difference on the sparkplug.

I would disagree here, but I will be specific as to why ...

The amount of throttle opening has only a minor effect, if any, on how "lean" (or rich) an engine runs, especially at part throttle settings. The main jets will certainly effect the lean or rich condition, but the throttle blades at mid-open will not. The task of the throttle plates is to meter airflow into the engine. The task of the jets is to provide a proportional amount of fuel for any given airflow. This is the venturi effect that all carbs are based upon. The throttle shaft butterflies control the air by restricting full volumetric airflow as a variable to generate a desired power level, and the jets supply fuel in proportion to that flow stream.

Some carbs are so simple that they ONLY have a main jet (some folks refer to these as the "high speed" jet because they are the feature that controls at or near operational rpm settings). I recently worked on a small Briggs engine for a push mower that has no throttle lever or cable at all. You simply push the primer bulb and yank the starter rope. It only runs at 3600rpm, and there is no idle setting. Hence, no need for anything but a main jet and needle. These are easy to work on and tune because there is only one intended setting for rpm steady state.

Other carbs are expected to perform at a wide variety of throttle settings, and at varying loads at those ever-changing settings, so they are much more complex. Hence; idle jets, idle valves, booster valves, tip-in air bleed slots, air bleed jets, main jets, adjustable needle positions, etc etc ... There are multiple ways to achieve these functions, but the overall concept is the same; the air-to-fuel ratio needs to be as close to 14.7:1 as pratical for good running across a broad rpm range.

Running a tiny bit lean is not a bad thing; it can be more fuel efficient and also a bit cleaner for the environment. But I'm talking about a small amount of lean condition at steady state, not grossly lean as to risk overheating or burnt plugs.
Conversely, max power is often found at WOT at slightly rich conditions, but there are obvious downsides to that as well; wasted fuel, fouled plugs, etc ...
Overall, it's a target of compromise.

In a perfect world, the stoichiometric goal of 14.7:1 would be ideal all across the rpm range. In reality it's hard to achieve this at all throttle settings. That is why more complex carbs do have idle air ports, idle and main jets, booster valves, etc. But a reasonable compromise can be found, and then jetted for that condition. As we use these engines in lawn service, they run a very consistent 3600"ish" rpm during all normal operation. Therefore, it's easy to find a jet that works well 99% of the time. This is where electronic fuel injection has an advantage, because it is easily "tuned" throughout the rev range. With a carb, the only way to alter a lean or rich condition is to change the jets; altering the throttle setting will not have effect because the mains are downstream of the butterflies. The throttle plates alter the amount of air; the jets alter the amount of fuel PER airflow.

In general, our engines run steady-state most all of their life, and it's easy to find an appropriate jet size. So ....
As long as the jets were sized correctly at the factory, the amount of throttle opening via this shaft modification topic should have no bearing on a lean or rich condition. And that would also be borne by the proof that all four engines (19-25hp; models 601 - 721) use the same main jets from the factory per the Kawpower website for sea-level condition up to 3500' elevation. Kaw does not change the jet size for the HP range offered in this engine series; they ONLY modify the allowable throttle opening limit. By the way, even the highest HP version (25hp) does not allow full WOT condition; the modification I've done still does not allow the plates to go totally parallel to the airflow. They still provide some restriction at max condition, so to refer to this as WOT (wide open throttle) is a bit mis-leading. They are not "wide open", but they are "maxed out" as far as Kaw allows for any given HP rating.

I don't disagree that it's a good idea to keep track of fuel condition (lean or rich), but this modification I speak of should have no bearing on that condition as it ONLY controls the max allowed airflow into the engine, and does not alter the proportional metering of the main jets.

Your are correct about the high speed jet sizes, just did some looking thru the Kawasaki parts manuals, very time consuming however!!! The only time that I have seen the throttle plates come close to WOT was on a generator that I purposely overloaded. Engine manufacturers call the faster RPM setting "high idle".
Possibly Kawi changes the governor sensitivity between HP ratings. On some newer small engine carbs I have seen that air bleed jets have been installed and the size of the air bleed jets have a direct affect on fuel flow thru both the low and high speed jets. I didn't see anything that would indicate air bleed jets in the Kawi manuals, but I have seen air bleed holes that have been bored into carb bodies, the size of which can be changed during manufacturing. They can also be placed in areas in the carb where airflow varies depending on throttle plate position and thereby vary fuel flow thru metering jets. You do realize that this particular Kawasaki FH engine line has been discontinued due to failure to comply with EPA reg changes, not that I am a fan of the EPA!!!!! There are a few FH721V engines available from engine suppliers, but the lower HP models are pretty much gone. This type of fuel management in the carbs may have contributed to the high emissions of this engine line that couldn't easily be corrected by Kawasaki, thus the discontinuation of the line.

Most certainly you are correct; this FH series is long gone.
Not really sure how the current FH (horizontal shaft only if I see the site info correctly) compares and contrasts.

Yes - after more than an hour of pouring through info, the ONLY thing I could find that varied was the throttle shaft. Even the carb bodies are the same part number, which would indicate there are no differences in bleed ports, if they exist in this model.

As for the gov assy, there's no difference there either. I have the full shop manual on PDF, and the procedure is the same setting for all. The gov parts are all the same across the four models. For these models, it is literally as simple as how much they artificially choke down the power with the throttle stop arm on the butterfly shaft.

Like I said previously, I like to be specific. This topic ONLY applies to my older FH series engines 601-721. Whether this applies to any other series of Kaw is something I cannot speak to.

But you've got to admit it's a cheap and easy bump in power! Always helps to know the tricks inside the trade. It's an easy mod that took longer to access (had to pull off the high-end fancy dual element filter) than it did to perform less than a minute's work on the shaft stop with a Dremel. Now I have more reserve power for longer uphill pulls or heavier dewy grass.

Anyway - thanks to all who chimed in; been a good learning lesson and hope it helps the next guy!

If all this is fact, and the only dif is an oil cooler on the 721, how can Kawasaki justify charging so much more for a higher powered engine? gregjo1948

It's this simple (to coin a phrase from the movies) ...
If you build it, they will buy it.

There are some applications where more power is desired, and easily managed by the whole powertrain system (be it a mower, a chipper, a pump, generator, etc).
There are some applications where more power cannot be tolerated by the entire powertrain system (same examples).
They offer these various power limits so that the OEM of the equipment can match up power offerings for needed or desired outcomes.

As we know with the Scags, most of the series in each model can handle the "top" power producer for any given model range. My Tiger Cub was available from 19hp up to 25hp in two differnt deck widths. Why offer those four power levels? Because Scag can charge more money for what amounts to very little (if any) added content; it's a money maker. It's not so much that Kaw charges a lot more, but the equipment OEM charges more, for the increase in available power, and perhaps more robust drivetrain. Whereas Kaw may charge a bit more money for a tiny little part change, Scag charges a LOT more money for the "privilege" of mower power for your mower. (This is not unique to Scag; they all do it. It's a concept done in many industires across all manufacturing.)

Hence, my desire (and now newfound joy) wanting more power from the same package. I paid nothing to get 30% more power!

There is a variety of oil coolers out there. Perhaps you need not buy a pricey "kawasaki" cooler to achieve the same results. . . Is the power "transmission the same, so you can safely transmit additional power? Interesting thread.

I repowered my 2008 Tiger Cub with a 30 HP Briggs Pro Turf engine, I like that it came with a engine oil cooler. The only thing that was noticeable was that there was less of a decrease in RPM with the collection blower running compared to the original 26 HP Briggs ELS engine, and I think that fuel consumption is less with the new engine, but difficult to calculate however.

I believe the throttle shaft in the fx651 is the only difference limiting this motor to 20.5 hp when compared to the fx730 which is rated at 23.5 hp, just like in the above discussed FH motors. Can anyone confirm this? Does anyone have a good link I could check out regarding any adjustments to the governor which would be needed when swapping shafts or is it just as simple as swapping the shafts with no adjustment to the governor necessary? Thanks.

Also, is anyone aware of oil cooler equipping on these different motors within the fx line? i.e. does the fx 730 have an oil cooler while the fx 651 does not?
Edit: looks like the FH motors have the oil cooler added to the higher hp model. The fx line do not seam to have this. Debating on whether or not to simply cut the tab/restriction or replace it. Replacing it would be less susceptible to any potential warranty fowl ups but cutting the tab/restriction would likely be easier given I wouldn't have to remove as many parts. Any thoughts...???

No oil cooler added to the FX line until the next model larger than the FX730V.
I'm going to say that if you had identical machines next to each other with the only difference being that one machine had an FX651 and the other an FX730, it would be very difficult to put them thru the same load and speed to become aware of the difference in rated horsepower. If you replaced the throttle shaft or removed some of the throttle stop material, I doubt that you would see any difference in power level produced. On a governed engine of this type it is almost impossible to load the engine to where the throttle stop would limit throttle opening when the governor is trying to push for more throttle. With an engine in a test cell that has the capability of measuring and varying torque load and monitoring horsepower, then you may see a slight difference. On most governed engines at 3,600 RPM and high loads, the throttle plate will not be open very much. Several years I had a 10,000 watt continuous rated generator in my shop for service. This machine had a 50 AMP plug on it and I plugged my AC/DC welder into it. I shorted the leads together and with low amps I turned the welder on. As I was cranking up the amps on the welder, I was observing what the engine governor was doing with the throttle plate. The throttle plate would quickly cycle to a fairly wide setting and then drop back as the engine RPM would increase to 3,600 and stabilize. I slowly increased the amps on the welder until the throttle plate opened more and to the point where the governor quickly pushed the throttle plate to full open at which point the engine quit. I did this after a complete service of the engine and a replacement of one of the large capacitors in the generator field excitation. The owner wanted me to test demonstrate it for him and this was the only way that I could accomplish a test. I still don't really know just how many amps load I put it, just guessing based on the amp calibration on the welder. This generator is still used as power outage backup in a pizza shop to keep the large coolers going and has been doing a fine job.

Regarding the FX motors, it's entirely possible that the power ratings are limited in the same or similar fassion as the older FH motors. It is, after all, a very easy and simple method of restricting max power.

The best way to tell if this is true is to go to their website and research the parts numbers as I did. Yes, it's tedious and boring. But as often is true, the answers lay in the details. That is how I confirmed my suspisions. And the icing on the cake was the photos from the other website, where folks posted the two parts side-by-side.

Find your specific model and series of FX motor, then look up the parts for the larger models in that same series. Drill down to the carb, and the throttle shaft of the carbs. If the jets stay the same and the throttle shaft changes based upon HP, then you have your answer.

It is far easier (at least on my engine series) to cut the offending part tab off the exiting shaft than to have to remove/replace the carb. It took me less than 5 minutes total, and that included removing the cyclonic air filter to get to the carb. If you do a nice job of carefully cutting the tab with a Dremel, and gently file it after the cut, it will look so stock that I doubt anyone could ever tell the difference anyway.

The jets, carb, etc. are all the same between the 651 to 730 motors. I took material off the shaft until the throttle blades open nearly flat. The factory 651 shaft seems to limit the throttle blades to roughly around 3/4 of opening. I used the machine briefly in a neighbors vacant lot with thick grass and thus far the power increase/resistance to bogging seems very noticeable.

As I am the type that would do the same and have done similar mods in the past there is no doubt that if I had a Kawi that was in this category I would do the same mod on the throttle shaft stop. I used to have a Bridgeport mill and medium size South Bend engine lathe in my shop and did lots of mods over the years. both of these machines have been replaced with much smaller machines. My Scag Tiger Cub originally had a 26 Briggs ELS engine and although it had low compression in one cylinder, the only difference in power/torque that I noticed with a new 30 Briggs was less of an RPM drop with the collection blower installed. The RPM difference was so small that I could barely notice it listening and I had to confirm it with a laser tachometer. I had previously done and recorded RPM checks with the original engine. Although the new engine has more HP, it appears that the fuel consumption is slightly less that the original engine. Having replaced the intermittent hourmeter, I may be able to closely determine the per hour fuel consumption.
It will be interesting to see if you notice any power/torque changes/improvements as you operate this machine.

NickTF said:

The jets, carb, etc. are all the same between the 651 to 730 motors. I took material off the shaft until the throttle blades open nearly flat. The factory 651 shaft seems to limit the throttle blades to roughly around 3/4 of opening. I used the machine briefly in a neighbors vacant lot with thick grass and thus far the power increase/resistance to bogging seems very noticeable.

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It's great that you noticed a power increase, I had mixed feelings as to if it would be noticeable. A lot also has to do with the governor response and I think that FX Kawis and possibly other Kawi models have a quick governor response which stays there until the engine RPM increases accordingly. My 18 Briggs Vanguard horizontal engine has quick governor response, but it seems to quickly drop as there are several RPM surges before it settles in. My 30 Briggs Pro Turf engine responds and stays there, but more HP in this engine.

NickTF said:

The jets, carb, etc. are all the same between the 651 to 730 motors. I took material off the shaft until the throttle blades open nearly flat. The factory 651 shaft seems to limit the throttle blades to roughly around 3/4 of opening. I used the machine briefly in a neighbors vacant lot with thick grass and thus far the power increase/resistance to bogging seems very noticeable.

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After extended use of the machine after this modification I would definitely recommend it as it has made a very noticeable difference in the resistance to bogging out in high load situations.

dnewton3 said:

Correct - we need to be specific here, because my FH series is an older one.
I typically try to be very clear in my questions and answers; hence why I listed the specific models of the engine data I looked up on the Kawpower website.

So I also found this same disucssion on another forum over at www.lawnsite.com and the same topic ended up with the same conversation, although it took them about 10 pages to come to the conclusions that it only took me an hour to discover. You can see the conversation here, but just focus on the photos on page 9; there are a few photos:
a) photo of the part used including the oil cooler set and throttle shaft
b) installed oil cooler
c) another pic of oil cooler
d) view of the 19hp shaft sitting next to the 25hp shaft (this is the most credible evidence and makes it so very simple to understand)
You can see the info and photos here:
http://www.lawnsite.com/showthread.php?t=317558&page=9


Now I am constraining my comments to my older series of FH engines; those that are all at the top of the displacement list from 601-721 series.
The ONLY thing (and I truly mean only thing) that separeates those in HP ratings is the little throttle butterfly shaft, which limits the travel of the shaft in its rotation.
Kawasaki has made a bazillion different variants of many engines, but when comparing and contrasting info within the same bloodline, this is a true statement.
You can "up-power" your FH engine (FH601V - FH721V) by simply removing that restriction. You have two choices:
1) buy the shaft for the HP you want (the 25hp shaft is 16041A-7025 is is currently about $40 give or take a bit) and install it after removing the carb, govenor, etc and put it all back together
2) get out a dremmel and cut off the offending metal on the shaft tab (I did this; took me about 60 seconds ....)

I cut off the matieral using the photo link from above as a guide. I printed off the photo (copy/paste caputred and put into Word) and then used sciscors to overlay the "new" part on the "old" part, so I could scribe a line that would represent the material to remove. If you refer to that photor link above, what I did was essentially cut off the portion of metal that has the "A" stamped upside down in the picture, and made the cut at an angle that replicates the 25hp shaft on the right.

Viola! 25hp out of my 19hp Kaw engine. The engine will now open up the butterfly shaft MUCH further, allowing for a much larger air/fuel mix under heavy loads.
Under light lights, it works just the same as any other engine would. The govenor controls the throttle shaft.
Because I didn't take anything apart, and my mower ran fine previous to this little experiment, I had no issues in having to "return" the idle speeds or govenor setting. I highly recommend that anyone doing this actually take the short cut (pun intended) and just cut off the metal material. Doing this is quicker, and there is no need to mess around with your carb and govenor settings.

I would remind those who try this to seriously consider adding the oil cooler as well. If you expect that high-loads would be rare for long periods, you can probably get away without it. But it you intend to use the new-found power for sustained periods, I really suggest you add the oil cooler; it is very inexpensive - only cost me about $25 when I added it a few years ago. (You'll need the heat sink cooler, the o-ring and the threaded extension). You can find those part numbers on the Kawpower website under the FH721 (25hp) engine parts listing. You can see my thread about this upgrade here: http://www.lawnmowerforum.com/scag-forum/8933-kawasaki-oil-cooler-upgrade.html


Yes - it truly is that simple to achive a 30% power gain.

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I know this is an 8 year old post, but does anybody have a good link the info that Dnewton posted a link to? I tried it but it no longer works.
thanks,