Kaw Power Up

[dnewton3]

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.

 

[gregjo1948]

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

 

[dnewton3]

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.

 

[dnewton3]

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!

 

[Mad Mackie]

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.

 

[gregjo1948]

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.

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

 

[Mad Mackie]

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

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.

 

[dnewton3]

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.

 

[gregjo1948]

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

 

[Mad Mackie]

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: