clutch engagment rpm vs gearing purple plate

Thanks for that, I was speaking about a gas animal, I guess they don't run those anymore. I'm unsure what you meant by when you said
"The percentage change in area will be even greater."
I'm interested in the reasoning for increasing the the big jet by 100% but not the low-speed jet. I don't race 4 cycles, no experience, looking to learn.

No one runs gas animals that I am aware of, there may be a few Baker Pro-Gas engines still competing.
The Briggs LO206 runs on gas (animal platform engine with a sealed bottom end and spec jetting - ie cannot be changed legally.)

I don't understand your comment on "increasing the big jet by 100%."
If this fellow's engine is hitting a "barrier" at peak rpm, that could be an indication that it is overly rich on the main. It will wet foul the spark plug and, if bad enough, shut the engine completely off for 1 second (only at peak rpm.)
Changing the low speed jetting will do nothing to fix this problem.
My suggestion is that he go down .001 or .002" on the main jet to see if this alleviates the "barrier" problem.
We have no idea what size main jet the OP is using. My suggestion of a .052" is a good starting point for his alcohol purple restrictor plate animal engine.
 
A cut and paste from my spreadsheet files showing percentage differences in area between 2 jets. I've always gone with the idea that you need twice as much Alky as gasoline. The low-speed jet, I assume, can contribute to this, it always did with the 2 cycles. But I could be wrong.
jet change 1.jpg
 
Al,The man said they are running a purple plate animal.
They must (to be legal) compete using alcohol for fuel.
The stock gas jet is a 95 (mm) for these engines - so the equivalent in English would be .0375."
Then consider that the carb is blueprinted, uses a different fuel nozzle and needle, different cam, ignition timing, and rpm range.

I think you'll find that my suggestion on starting jet will be pretty spot on.

Then again, this might be some local track option class that is totally different from the norm and we're all throwing darts in the dark.
 
Here's the same spreadsheet with a #95 starting jet. It makes some difference in the mixture percentage. Going to a .053" makes it 100.7% bigger. I'm not saying the .052" is not right, I just have the numbers.
jet change 2.jpg
 
Here's the same spreadsheet with a #95 starting jet. It makes some difference in the mixture percentage. Going to a .053" makes it 100.7% bigger. I'm not saying the .052" is not right, I just have the numbers.
View attachment 6054
Ok, but how much did you open up the nozzle and did you figure in the different taper of the needle (if different was used.)
Typically .110" nozzle with BHA, or .106" with BGB needle for purple. Stock gas needle is .100." Adjust the clip position to tune (similar to how you would change main jets.)
Also the air side can be manipulated on a WKA blueprinted carb. Typically the emulsion tube holes are resized and reworked too.
There's a lot more to changing one of these carbs over from gas to alcohol to make them work best. "Can" you just change the jets? Sure, Briggs sells the kit for that (and I bet that it's a very slow moving part number currently.)
 
I'm totally with Brian on this... you should be turning way more RPM with a purple plate animal than 4800rpm. We turn our purple plate clones 5800-6100 all night, every night. Usually when running a track that tight, you even turn them harder to make up for the RPM loss in the corners. You need to get that engine back to the builder and find out what is going on, you are leaving a TON on the table if that really is your peak RPM!
 
Somewhat of an update.
Rain outs, missed a few events while my son was away at camp.
Kart sat a bit and I forgot to drain the methanol..........wouldn't start. Pulled the carb and found the gel penalty for being lazy. Cleaned it up.
Not sure what I did besides clean it and re-seat the carb. Seems to hit 5800+ rpm now consistently.
 
Somewhat of an update.
Rain outs, missed a few events while my son was away at camp.
Kart sat a bit and I forgot to drain the methanol..........wouldn't start. Pulled the carb and found the gel penalty for being lazy. Cleaned it up.
Not sure what I did besides clean it and re-seat the carb. Seems to hit 5800+ rpm now consistently.

I hope your not spinning it up and hitting 5800+ rpm while it's on the stand. bad bad idea
 
I hope your not spinning it up and hitting 5800+ rpm while it's on the stand. bad bad idea
I didn't.......but honestly wouldn't have known better to not to do that until you posted.
Figured load was the only way to know if the thing was healthy after cleaning it up. So we tested it locally and I made an oval to simulate the normal track, and got the higher RPMS to my surprise.
 
Your clutch engagement isnt going to change your RPM. Once its engaged, its engaged.

Setup, gearing, speed, etc will determine your top RPM, but clutch engagement will not
They say clutch engagment for a box stock predator is 2800. Why wouldn't it be a little higher closer to peak torque like other motors?
 
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