Lawnboy powered minibike question

Walter Keller

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tampa bay area
Lack of power/RPMs is the main issue - starts easy, idles fine - accelerates to a point then goes flat. Theres no governor (completely removed) current carb is a 26mm PWK. If you lift the rear wheel and crack the throttle wide open it acts like it has a governor. My first try at carbs was a 19mm - about the same size as the factory carb that was on the mower. The engine acted the same as the 26mm carb I installed today...accelerates than goes flat.
Looking for some ideas why the engine is so gutless - thinking it could be the coil? Also running a 30 series TC.


Staff member
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Ruhrpott [Germany]
original lawnboys are two stroke engines (we talk D400 iron horses no?)

two stroke engines always have some "sort of" governor.. or rather "rev limiter"

and that is dictated by port timing and exhaust geometry.
at a certain point the engine cannot inhale properly anymore and fresh intake is pushed out the exhaust port more or less directly (lack of back pressure)

not an issue on a lawn mower (hence they don't have expansion chambers at all)
very much an issue on minibikes and karts ...

And true.. westbends and old McCullochs don't have an expansion chamber either and still rev much higher..
BUT that's because they have different port timing than your lawnboy.

Now that this is out of the way..
chances are you just have a bit of a weak spark,
or lost a good bit of compression;

keep in mind that you only have 3.5horses to work with so don't expect too much from it.
but from what I've seen I'd say solid 6k should be doable w/o load

you can up the compression a notch and ggain a tiny bit extra.

but essentially you'd need to fab a proper exhaust and adjust timing a bit (where possible)
to gain a few more horses and a much nicer overall performance ;)



New member
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Tampa Bay Area
5hp C21ZMR

Had to re register
No its the 5hp C21ZMR - have a expansion pipe in the works plus I have toyed with the idea of adding a reed box.....Dont really know if anything will be gained from this effort. Could throw a 4 cycle on and be biking in short order.