Box Stock

why is the motor/carb blowing gas mist out of carb and missing at ~5500+

A SIMPLE PLAN

A SIMPLE PLAN

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valve lash is good. HEMI, CM .265" cam, CAST ALUM 9999 RPM flywheel with 4* advance key, so total of 32* of advance?. BILLET ROD, stock carb with .033" jet AT 3600 FEET. header, with 3/4" BRIGGS Hot dog muffler with center divider drilled out.

tried stock jet .028"
then tried .032" jet
now at .033 (85) jet. all the same thing. missing at 5500+

less O2 at 3600 ft so shd need LESS GAS, not more, as LESS O2 to burn it. plug is kinda the right color light tan. each jet will get its own spark plug. but why is it missing and blowing gas mist out of the carb, (no air filter on test stand) when it misses. seems to do ok... til... ~5500+ when it starts to miss and blow gas mist out of carb... ??? simplest motor, after the Briggs flathead... or Cox .049" engine.

12.8HD red stripe springs ... shd be able to go to more than 5500'ish without valve bouncing off the seat? if i can get to 6500 without it missing i wud be very content. for Coleman 200x

Im trying to get it dialed in with STOCK CARB before changing carb to 22mm and trying all over again.
 
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A SIMPLE PLAN said:
valve lash is good. HEMI, CM .265" cam, CAST ALUM 9999 RPM flywheel with 4* advance key, so total of 32* of advance?. BILLET ROD, stock carb with .033" jet AT 3600 FEET. header, with 3/4" BRIGGS Hot dog muffler with center divider drilled out.

tried stock jet .028"
then tried .032" jet
now at .033 (85) jet. all the same thing. missing at 5500+

less O2 at 3600 ft so shd need LESS GAS, not more, as LESS O2 to burn it. plug is kinda the right color light tan. each jet will get its own spark plug. but why is it missing and blowing gas mist out of the carb, (no air filter on test stand) when it misses. seems to do ok... til... ~5500+ when it starts to miss and blow gas mist out of carb... ??? simplest motor, after the Briggs flathead... or Cox .049" engine.

12.8HD red stripe springs ... shd be able to go to more than 5500'ish without valve bouncing off the seat? if i can get to 6500 without it missing i wud be very content. for Coleman 200x

Im trying to get it dialed in with STOCK CARB before changing carb to 22mm and trying all over again.
Click to expand...
Valve float? Dyno recommends 20lb springs for that cam
 
STOCK SPRINGS CAN DO 5400? i wud hope that 12.8# HD red stripe springs cld do better than 5500 before giving up??? someone wrote on here they're running 12.8 HD Red springs to 7400???? seems a little much but??? i wud think that 12.8HD RED STRIPE springs cld make it past 5500???? maybe not? blowing gas mist out intake at 5500... and it is pretty much done at 5500... it just blows MORE GAS vapor out the carb after that????
 
I am no professional, but I would think the cam aggressiveness will affect how much spring weight is needed for a set RPM. So if the cam that was used recommends a heavier weight it may be required because of the steepness of the profile.

Not as sure, but if you are progressive in timing heavier springs may be required to close quick enough.

This is no dyno numbers or experience so forget it if it seems wrong to you. Just my thoughts from reading.
 
A SIMPLE PLAN said:
STOCK SPRINGS CAN DO 5400? i wud hope that 12.8# HD red stripe springs cld do better than 5500 before giving up??? someone wrote on here they're running 12.8 HD Red springs to 7400???? seems a little much but??? i wud think that 12.8HD RED STRIPE springs cld make it past 5500???? maybe not? blowing gas mist out intake at 5500... and it is pretty much done at 5500... it just blows MORE GAS vapor out the carb after that????
Click to expand...
Why wouldn’t you run at least as stiff a spring as the cam manufacturer suggests?
 
i thought i wud want the springs to keep the revs in check and give up at 6500 ish. i figured that the 12.8#HD springs wud to that. maybe i was wrong. i have seen posts where ppl are running 12.8HD red stripe springs on karts to 7200'ish? so...??? but 22# springs are supposed to be good for 6500. ??? that doesnt make sense, if 12.8# springs have been run to 7k? 19.5# over the nose of the cam shd be enuf, maybe not?
 
You see 10.8s on clones running 7000rpms but if you will pull a box stock predator cam you will see why you can only get 5000ish before valve float. The lobes almost resemble a perfect triangle
 
Valve float may not be the only reason you are seeing spray out the carb.

Cam overlap combined with the signal from the exhaust can cause this at certain rpm, when the two align.

May, or may not indicate a problem.
 
Are you doing this on a stand or running it down the street? We don't advise to free rev them and trying to tune them without a load like that. get it off the stand and make a run down the street.

The clones you read about turning 7000 with 10.8 springs have lighter valves and rockers and they have been working on the valve geometry to get them to turn 7k with those springs for a few years.
 
racinnut15xm said:
Are you doing this on a stand or running it down the street? We don't advise to free rev them and trying to tune them without a load like that. get it off the stand and make a run down the street.

The clones you read about turning 7000 with 10.8 springs have lighter valves and rockers and they have been working on the valve geometry to get them to turn 7k with those springs for a few years.
Click to expand...
on stand. cuz of the altitude here at 3600' i want to get the motor tuned RIGHT before i put it in the Coleman 200x as sadly i doubt if i cld go around the neighborhood before KAREN calls 5-0 on me so that is ~<5-15 mins? sadly. cant have it dying on me halfway around the hood ... and it costs more in impound fees in a week than ...
 
A SIMPLE PLAN said:
on stand. cuz of the altitude here at 3600' i want to get the motor tuned RIGHT before i put it in the Coleman 200x as sadly i doubt if i cld go around the neighborhood before KAREN calls 5-0 on me so that is ~<5-15 mins? sadly. cant have it dying on me halfway around the hood ... and it costs more in impound fees in a week than ...
Click to expand...
you can't tune them at high rpm free wheeling on a stand. It's not the same
 
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