Kaw Power Up

[dnewton3]

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!

 

[gregjo1948]

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

 

[Mad Mackie]

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.

 

[dnewton3]

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.

 

[Mad Mackie]

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.

 

[dnewton3]

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!

 

[gregjo1948]

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

 

[dnewton3]

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!

 

[monkfarm]

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.

 

[Mad Mackie]

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.