80cc chinese motor problems

#1
Ive got this little motor from a minibike. it sat for a while and got a small amount of pitting in the cylinder. other than a small loss in power that is not the problem.
the coil fires great and the carb is in decent condition. The oil is new and the valves work correctly from what i can see.

The problem is every few days it will start and run great for a few minutes, about 30 if im lucky. then it will refuse to run more than a few seconds. i though maybe it was getting flooded but i am not sure becuase it will not just fry out in a few minutes like normal it simply wont start and run for a few days. this thing drives me crazy evey time i get it running again it just gives up after me making big plans trying to use it again. i have never had this problem.

My theory is the coil is messed up and when it heats up it strops working. but idk
any ideas?
 

Addicted 2 Minis

Well-Known Member
#2
In order to help you, it's best we know everything that you have done to this engine. You claim that there's pitting in the cylinder, the only way you would know this is if you removed the cylinder head. Unless you pulled the spark plug and shined a light in the cylinder and managed to look into the cylinder while holding the light which is a real pain to see anything, I think you're not giving us the whole story. If you did remove the head, chances are it's not sealing after you put it back together. Some head gaskets are re-useable, most aren't. A poor sealing head gasket will perform as you are describing ( doesn't want to run after warming up ). Another thing that would cause the problem you're describing is lack of fuel. If the fuel filter is plugged causing very little fuel to get to the carburetor, it'll run until the carb runs out of fuel and may take a day to get enough fuel in the float bowl to run again. Some of these engines have screens in the gas tank, others have a pre-filter on the carburetor inlet. Post some pictures of your engine so we know what to look for, 80cc Chinese gives us a clue but there are several different ones, all having their own quirks.
 
#4
Ive got this little motor from a minibike. it sat for a while and got a small amount of pitting in the cylinder. other than a small loss in power that is not the problem.
the coil fires great and the carb is in decent condition. The oil is new and the valves work correctly from what i can see.

The problem is every few days it will start and run great for a few minutes, about 30 if im lucky. then it will refuse to run more than a few seconds. i though maybe it was getting flooded but i am not sure becuase it will not just fry out in a few minutes like normal it simply wont start and run for a few days. this thing drives me crazy evey time i get it running again it just gives up after me making big plans trying to use it again. i have never had this problem.

My theory is the coil is messed up and when it heats up it strops working. but idk
any ideas?
Do you mean it won't run at ALL? Or just at idle? Are you sure its jetted right?
 
#5
ok so i took this motor apart after sitting for a while. took the head cap off and cleaned everything. did not replace the gasket but that is a good piont. i will get some tomorrow. Ocationally it will start and run for about half an hour hot or cold. it runs good but then it just wont. i have taken the carb completely apart and cleaned thoroughly. (there is not fuel or air filter i know but i dont care)
i have tested the coil numerous times with the same result (it works well.) i know it gets fuel but even spayed in the carb it will not fire. this is strange becuase if it decides to run that will help it start. i will get some gasket sealler and try that first. i will also get pictures tomorrow. but the piston is vertical and the tank is on top with the carb around back. the jets are good i have cleaned them numerouse times and made sure thay flow well but did not really enlarge it any. i am pretty experienced with these that is why it boggles me.
 
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#6
Everett, there is a chance you could have an intermittent coil or intermittent condenser (if it has points). A problem of this nature is VERY rare and it can be he!! to find because the conditions don't happen at any specific time, temperature or rpm. If the engine has a history of being turned off by pulling off the spark plug wire, that causes voltage surges that can arc inside the coil windings and will damage the delicate varnish insulation on the wire. With time and use the coil windings suffer enough damage to make the coil operate erratic or not at all. This winding damage won't always show with a test meter because it needs high voltage to arc over if the wires don't actually touch where the varnish has burnt away.
 

SAS289

Well-Known Member
#7
Everett, there is a chance you could have an intermittent coil or intermittent condenser (if it has points). A problem of this nature is VERY rare and it can be he!! to find because the conditions don't happen at any specific time, temperature or rpm. If the engine has a history of being turned off by pulling off the spark plug wire, that causes voltage surges that can arc inside the coil windings and will damage the delicate varnish insulation on the wire. With time and use the coil windings suffer enough damage to make the coil operate erratic or not at all. This winding damage won't always show with a test meter because it needs high voltage to arc over if the wires don't actually touch where the varnish has burnt away.
That's interesting but you would think that once resistance gets higher than that of the plug gap it would be all over for the coil. Are you saying the coil can find ground and arc to it without the plug wire connected?
 
#8
Everett, there is a chance you could have an intermittent coil or intermittent condenser (if it has points). A problem of this nature is VERY rare and it can be he!! to find because the conditions don't happen at any specific time, temperature or rpm. If the engine has a history of being turned off by pulling off the spark plug wire, that causes voltage surges that can arc inside the coil windings and will damage the delicate varnish insulation on the wire. With time and use the coil windings suffer enough damage to make the coil operate erratic or not at all. This winding damage won't always show with a test meter because it needs high voltage to arc over if the wires don't actually touch where the varnish has burnt away.

That is very interesting! i have never heard of that. i will look up that problem and see what i can find. Would you happen know how to identify such a coil? from a quick search it seams they are mostly found in large engines.
 
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#9
That's interesting but you would think that once resistance gets higher than that of the plug gap it would be all over for the coil. Are you saying the coil can find ground and arc to it without the plug wire connected?
it seems to me the coil would arc with in itself. much the same as over volting a tesla coil or other high voltage coils.
 
#12
That's interesting but you would think that once resistance gets higher than that of the plug gap it would be all over for the coil. Are you saying the coil can find ground and arc to it without the plug wire connected?
When the wire is separated from the spark plug that gap plus the plug gap equals a space that can require several times the normal firing voltage. Ignition coils usually can't tolerate that blast of over voltage repeatedly, it will cause shorts in the secondary winding where the high voltage is flowing.
 
#13
I went and bought gasket sealer and applied that. It worked well and it ran for a little while but I think I need to clean the carb again
 
#14
i reinsalled it on the tinybike and and made a quick strait pipe. i also jetting the carb slightly with and exacto knife and like i said it doesnt have an air filter. it has some pitting but it is very minor and my mods seem to have overiddeen that becuase it puts out more power than every before. i did remove the governor from the crank case but i am not worried about the flywheel becuase it never gets to those rpms. over all this thing takes a while but goes about 30 mph and spits fire. this sat for while and the cables are seized so i removed them and took and old brake lever bolted on next to the foot pedal serves as a gas pedal. when i made that strait pipe i made the mistake of turning it up. that makes it fun at night becuase you can see the fire shooting strait up but maybe not great i the long run. the pedal makes for a "unique" riding experience for sure.
 
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