Thread: motorcycle backfiring

Standard 34mm Mikuni carby. Sounds like the needle setting for the midrange slide opening has been altered.

Two type if backfire:

- mild popping = too lean
- harder banging = too rich

Originally Posted by jakeslouw

Standard 34mm Mikuni carby. Sounds like the needle setting for the midrange slide opening has been altered.

Two type if backfire:

- mild popping = too lean
- harder banging = too rich

It is the mild popping.

For the midrange is it an external screw you adjust or an internal adjustment?

- tilt the carby
- remove the slide
- you'll see a needle going thru the slide
- remove the screw holding the needle and cable in the slide
- shift the circlip on the needle DOWN one slot (which will RICHEN the mid-range)
- reinstall by reversing the process
- test
- repeat the adjustment if required, or reverse if it gets worse.

It might help just asking the local Yamaha dealer/bike shop to check the carb for the correct needle-jet size and needle profile.

I have let an auto elect mate have a look at it. It is looking like it is a faulty winding that is giving the problem. This particular winding is suppose to be putting out over 200volts, mine is just making 75volts. It took him awhile to track things down and did alot of testing including double checking things on his own bike which is the same.