Cub backfire thru varn

bobgrimes

Member
I have a 64 cub. It is backfiring through carb. Mostly at medium to high speed. At idle it is ok. Any thoughts will be appreciated.
 
One possibility is that a valve spring broke or the keepers have broke or come off the valve allowing the spring to not tension the valve. To those that question my reply saying the valve would fall in the cylinder, the Cub engines are flat heads.
 
there are a few possibilities for your problem. timing off a bit,spring or keepers as already stated broke or worn to a point they don't hold the valve tight enough, a bad valve or seat that leaks a bit of a loose hole for the valve so it sets different part of the time. To be through the carb would need to be an intake valve. Loose shaft in distributor could also be affecting the point timing so they let the spark be a bit early or late when the valve is cracked open.
 
the first thing to do is a compression check to verify the vales are sealing. right down the readings in order. as this indicates a possible intake valve. exhaust valves usually burn before an intake valve. do you know how to check compression, no sense me explaining something you know how to do. it can also be a tight valve meaning there is no lash left or its running at zero lash. valves are set to a specific setting the same as points. and that is the next thing to check, make sure your points are set to factory spec. timing is ALWAYS set after you set the points. the reason... is because different point setting will change the timing setting. then LAST you adjust the carb load screw , then the mixture screw for the most rpm, and last the idle setting all in that order. you said at idle it is ok . does that mean it is not backfiring which it wont be. is it running smoothly and not missing? pulling the throttle wide open from idle you will get a backfire through the carb with a bad valve. there you go , you have some work to do now and report back with your findings.
 
If it just started doing that, it could be a cracked distributor cap or carbon track. Higher speeds need more volts. Jim
 
Will do. Will spend some time tomorrow and go through all the suggestions. Another question. Could a weak coil cause this?
 
a weak coil will cut the spark out under load. i cant remember ever hearing one causing a backfire. a backfire is the spark happening at the wrong time basically.
 

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