cracking/ back firering 250cc

[HSV_Hunny04]

i have a 250cc just had a new top end and now when ever u idle it or rev it a bit its cracks and back fires, put a bigger jet in it and it still dose it so i put the original one back in also bought a racing CDI n i replaced the old one back and still doing it so i was wondering if any one has an idear of what it could be?? thanks

 

[sean01]

weren't you having a similar issue a while back when swapping carbs?

sounds like an air leak or serious carb issue.. running lean

 

[HSV_Hunny04]

yea but it wasnt running right until now i had a top end rebuild now its running right just every now and then it backfires and cracks

 

[sean01]

have you tried changing the slide needle height?
check for manifold leaks
loose exhaust can sometimes give similar trouble

maybe a video of it might help us

 

[HSV_Hunny04]

i moved the needle today it was on the second bottom one then i put it back to the middle but then its so much drama to put the carby back on every time and then start it but i might have another attempt 2morrow and its hard to adjust the air screw cause its right underneath the carby but i might try all these things 2morrow and see how i go n take a video and show you wat the go is thanks for ya help

 

[clunky64]

make sure your hot start valve is sitting in correctly and not sucking air had same problem on yz250f and this is what it was, you have not said what bike it is

 

[sean01]

pretty sure its just a vertical 250cc china

racing cdi sorta gives it away too lol