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Discussion Starter · #1 ·
Hi, I looked at the forum but it seems that nobody got this problem by now. I have a Indy 500 '90 the engine is EC50PL. I've just rebuilt it because the crank bearings went too hot. I think the problem is the carbs. At idle, the engine is running good but when I put full throttle, the engine spitback (reject) the air and gas by the carbs. After about 10 or 15 seconds (after the rpm rise) at full throttle, the engine run well without any spitting.
I tried many different jettings, the carbs are sync. but don't have a strong vaccuum (but I still get one).
I stopped the engine when it was spitting and the sparkplugs were dry.
Ignition timing is good and I tried 3 different CDI.
The engine is .040 oversized and carbs are VM38.
 

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So, the problem is that you are having fuel and oil spit back out of the carburetors when the throttle is opened up right? Does the engine still seem to run okay, or is it having running issues? Sleds always spit some fuel and air back out of the carbs, it shouldn't be a garden hose or anything, but there will almost always be some coming out. Are you running it with the airbox off all the time? If you are, did you rejet it? Removing the airbox and riding the sled will lead to a lean condition that can melt down a motor. You need to put some larger jets in there if you are keeping the airbox off. What color are your sparkplug electrodes? Black is rich, brown is good, white is lean.
 

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Just a thought but check all you vacuum lines
and double check the bolts that hold the jugs to the crankcase.
Just a little vacuum leak causes problems.
 

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Discussion Starter · #4 ·
Thank you for the replies!
I run my sled with the airbox. The sparkplugs are brown and very clean since I use synthetic oil. I sealed the jugs with special gas and heat resistant silicon.

Here are other infos if any other ideas.
I canceled oil pump and now I mix the gas.
Recommended jettings are for a stock engine (270 main, 50 pilot). Since it is oversized to 509cc, and it is really cold here (-25 degrees celsius), I've put 310 for main jet.

Here is a better description of what the engine is doing.
Idling perfectly at 1500 rpm (sparkplugs color check OK)
Putting full throttle (rpm rising slowly to 2000...3000) and spitting for about 10 seconds. When rpm reach 3500-4000, engine runs well until 8000.

When I put jets less than 310, engine seems knocking at high rpm.

Thank you
 

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Stock jets, dial indicator timing marks, then time. Silicone??? if you spread it on outside of "jugs" it won't help and it won't hurt. Why pre-mix when oil pump can be used? Make sure your carb boots are not cracked.
 

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Discussion Starter · #7 ·
Thank you all for your help, I finally found the problem. My engine needed gas, the fuel pump was pumping but not enough. It seems so obvious now after all I have tried. I pre-mix because I am not sure enough about the oil pump.

Thank you again
 

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if you can put up with it...mixing oil is fail safe (as long as you do it right) I have had oil injection systems fail, and it sucks.
 
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