engine problem -need some input !

[richirich1113]

i have got a wird one here. i just reacently completed building my bike. everything seemed ok for about 6-7 10 min runs while i was breaking it in. then i put an expansion chamber exahust on. the it could be a coincidance but not to long after that i can not get the bike to idle right. it starts doing wird things like it would be idleling the i give it a alittle gas and let off and it revs out and wont stop till i turn the idel screw down the it idels down and wants to die so i turn it in alittle and it will just rev out again without touching the throttle. another thing then ill get it to idel again and i try choking the card and istead of killing the motor it will rev out like im opening up the throttle. This has got me perplexed i feel ive tried everthing. ive changed the carb (i have another bike with a nt that i know works right) and it still does it .ive put a new intake gasket- nothing. a new throttle cable thinking it might be stickin -nothing Im wondering if maby the carb is to small for the motor or maby a problem with the ignition. to get a feel for the bike it is a 58cc phantom eddition moter, expasion chamber exhaust , jaguar ignition system. any ideas would be appreciated if you need any other info to make a diagnosis let me know . frustrated and out of ideas

 

[motorpsycho]

It's possible that the expansion chamber is making it run too lean now.
If an engine is too lean the idle will increase. If you increase the air in / air out of the engine, your air-fuel ratio will become leaner
If the float is stuck (which is not uncommon for the nt carbs) the float bowl could be running out of fuel too soon which would also cause it to rev up like that (again, it would be running too lean). It could be running out of fuel, revving up, and then once more fuel gets into the carb bowl, it will settle down.
I would throw the stock exhaust back on and see if it still does it.
I know that would be working backwards, but it will eliminate the exp. chamber as the problem if it still does it with the stock exhaust on it.

 

[richirich1113]

thank you for responding! I think i might know what the problem is. after scratching my head all day i noticed that when i prime the carb till its full there is gas leaking all around the bowl. i tightend the screws tight but it still leaks. so now i have to wait to get a gasket, but i bet thats the problem. brand new carb go figure two thumbs up for chinese quality control LOL LOL LOL
again thanks for the info ill probably need it at some point it was informative.
richirich

 

[motorpsycho]

actually when you prime the carb gas will come out (if you pump the primer too much).
when you pump the primer, you are making the float drop and the needle and seat open up to over-fill the float bowl. the extra gas from the overfull float bowl will either go into the intake manifold, or out the carb overflow. every time you pump the primer more gas is getting let into the float bowl. pump it too much and the gas has to go somewhere.
gas will even come out of the primer button itself, and sometimes it makes it look like the float bowl gasket is leaking.

 

[richirich1113]

well anyway yesterday i ordered the performance carb from JNM motors. the Nt carb is still a crappy carb in my opinion. also it is about the only thing that isnt modified.
Usally it only takes a couple of days to get stuff from them since they are not to far away. Thanks for the info ill let you know if it works.
i also want to post pics of it cause it is really cool for my first build but itsgot to be perfect before i can brag about it!

 

[richirich1113]

Well i just installed the performance carb from JNM motors and im impressed. i had to jet it down to a .70 jet . it was a piece of cake installing the carb. the motor fired right up. my bike now runs like a small motorcycle. A+ to JNM motors.