Hey guys a quick question I have a 140cc thumpstar that's ildes fine and starts fine but when you rev it it backfires and flames come out the muffler I have pulled the carb off and set the mixture screw by winding it all the way in then wound it back 2.5 turns this didn't fix the problem I have also removed and cleaned the spark plug just want to ask what to try next. Thanks in advance
140cc thumpstar
- Thread starter King905au
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my67xr
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First thing, drop in here and introduce yourself
With the mixture screw being set at 2.5 turn's out is quite lean, you want it somewhere between 1.5 turn's out and 2 turn's out.
Do you have a Mikuni china 22/26mm copy carby on it ?
If so they are jetted too lean from the factory
Get yourself a new pilot jet, a #20 Pilot jet will change the carby into something a lot better
Do you have a decent, oiled, foam air filter on it ?
To tune the carby the easiest way is to make yourself a tool.
You need a small flat headed screwdriver bit, and a short piece of 5mm pvc tubing
Heat up the end of the tubing and push the driver bit into it, let it cool and it's ready to go
Warm the bike up, and let it idle, turn the idle speed screw in so it raises the revs about 500 rpm.
Now turn the mixture screw in till it start's to run rough.
Then turn it out till it reach's the highest/smoothest rpm
Turn it in about 1/8th of a turn.
Now Turn the idle speed screw back out till it idle's at a normal speed.
Take it for a test ride and see how it goes, it may need a touch more of a turn on the mixture screw to find it's sweet spot
First thing, drop in here and introduce yourself
With the mixture screw being set at 2.5 turn's out is quite lean, you want it somewhere between 1.5 turn's out and 2 turn's out.
Do you have a Mikuni china 22/26mm copy carby on it ?
If so they are jetted too lean from the factory
Get yourself a new pilot jet, a #20 Pilot jet will change the carby into something a lot better
Do you have a decent, oiled, foam air filter on it ?
To tune the carby the easiest way is to make yourself a tool.
You need a small flat headed screwdriver bit, and a short piece of 5mm pvc tubing
Heat up the end of the tubing and push the driver bit into it, let it cool and it's ready to go
Warm the bike up, and let it idle, turn the idle speed screw in so it raises the revs about 500 rpm.
Now turn the mixture screw in till it start's to run rough.
Then turn it out till it reach's the highest/smoothest rpm
Turn it in about 1/8th of a turn.
Now Turn the idle speed screw back out till it idle's at a normal speed.
Take it for a test ride and see how it goes, it may need a touch more of a turn on the mixture screw to find it's sweet spot
