05 CRF 100 Backfiring

[cjc3fan]

I'm having issues with my daughters 05' CRF 100, it keeps backfiring. I bought this bike used a few months back and when I tested it out then, it did the same thing. The guy I bought it from said it have been sitting in the garage for over a year. I figured bad gas, dirty carb, the fuel mixture screw needed adjusting or needed a new plug. I did all of the above but nothing seems to work. The backfiring is worse when the bike is idling. If i back the idle adjust screw all the way out it seems to run better; better not great. There are no mods to this bike, everything is stock. The previous owner did not rejet anything so i have reason to believe that the factory jets are still in there. Any assistance would be greatly appreciated.

 

[Drifz1]

seems like jettings off imo what colour is the plug? and did you check valve clearences?

 

[xKroniK13x]

Jetting, valves adjusted?

 

[darederek]

compression? drifz1 just said in another thread his yfz backfired when the compression went bad

 

[Awk08]

pull and completely disassemble and clean the carb, and also check the valve adjustment schedule, possibly due for a valve adjustment
but i'd clean that carb first, if it's all stock, then stock jets should be fine

 

[cjc3fan]

She starts up fine and rides great (other than the backfiring), but I'll do a compression check just to be safe. As far as the valve adjustments go; is that any easy thing to do? I'll take the plug out to see what the color is as well.

 

[Drifz1]

compression? drifz1 just said in another thread his yfz backfired when the compression went bad

yea has to be real bad though

 

[Drifz1]

She starts up fine and rides great (other than the backfiring), but I'll do a compression check just to be safe. As far as the valve adjustments go; is that any easy thing to do? I'll take the plug out to see what the color is as well.

yes its easy enough if its just the shims but you will need to buy new shims. each shim would probly be about $2 ,what its does is moves the cap which is directly above the valves closer to the cam lobe... you will need a set of feeler gauges aswell

 

[00blaster200]

I would say it could possibly be valves.