Carburetion and jet sizes for restrictor plate racing

[Madlogger]

I am trying to make a single cylinder Briggs & Stratton Animal racing engine compete in a restrictor plate class for my 10 year old son. The engine is running rich and sounds flat when he is on the track. The main jet in the Walbro PZ22 carb measures 0.96 mm in diameter. The resrictor plate has three 0.225 inch diameter holes. I have been unable to locate any information regarding what the recommended jet size should be to make the engine run well with this restrictor.

 

[Tmoose]

if the symptoms described are at full throttle, with some serious revolutions
I'd confirm the float height is correct and the air cleaner is in good condition and not over oiled and then install a slightly smaller main jet. I'd go by jet number, not measured size, altho it is a good idea to confirm approximate physical jet sizes just in case someone has been in there with numbr drills or jet reamers.
As a quick check restiricting the carb bowl vent makes the carb go lean. If the engine perks right up it suggests leaner is the way to go.

http://wem.walbro.com/distributors/servicemanuals/HIGHPERFORMANCEPZCAEBURETOR.pdf

 

[Madlogger]

Tmoose, Yes, this is at full throttle, the revolutions are not where I would like them to be but are in the 4000 range. The float is at the specified level, the jet is the factory jet. According to the spec sheet for the carb it is a #95. I am guessing that that stands for 0.95mm as I measured the hole with a tip drill that measured 0.961mm by my uncalibrated caliper. A 0.035 inch guage pin passed through the hole easily. I never thought of restricting the bowl vent, that is a great idea, thanks. I will let you know what I find out.

 

[Tmoose]

I forgot to ask if these parts are all meeting for the first time, or you bought them from somebody caliming they were a good running combination. Or some other situation altogether.

 

[Madlogger]

I bought the engine from Briggs and Stratton, brand new in the crate, the restrictor plate is handed out by the sanctioning body.

 

[Surestick]

Is the restrictor plate before or after the carburetor?

 

[Madlogger]

The restrictor plate is after the carb.

 

[LionelHutz]

Reading the plug should confirm that it's rich. Put a smaller jet in. If it goes faster then go another size smaller and try again. If it slows down then you've gone to far or need to go the opposite way in jet size.

 

[bigTomHanks]

Buy an innovate mtx-l and get exact about it.