I recently purchased an '04 yfz. It had stock exhaust and GYTR air filter w/out lid. The carb was jetted for aftermarket exhaust(the plug was black). I changed the plug, installed full Abla exhaust, K&N, outterwares, and no lid. Now when at idle I get a backfiring noise coming through the carb. I have a 48 pilot, 175 main, and an NCV-Q needle(my forth jetting set up). I don't think the jetting is the problem, because I have had the same noise with several different jet combos. The mixture screw is out 2 1/4, and may be too lean. At idle when you punch the throttle it cuts out. Does anyone have an idea where my problem might be? Since I bought this machine used I have no other information on what has been done to the motor (exhaust timing changes??). Thanks in advance if anyone out there has a solution.

 

Is the backfire or popping caused by going on the throttle...or does it happen by just idleing? If the choke is pulled partially or fully on...does the backfire/popping stop or get better?

 

The popping stops when on the throttle. Any ideas?

 

The popping gets alot louder when it is choked.

 

Have you checked the intake valve clearance yet? Any vacumn leaks in the intake area?

 

No I haven't checked intake valve clearance. I have mainly focused my attention on the carb. Do you believe my problem is in the motor? Can you recommend a site that might give me instruction on checking valve clearance? As far as vac leaks, I could use a pointer on checking that as well. I've checked every hose that is present. And everything is secure, and all the hoses are free of debris. That doesn't mean there isn't something missing. My manual should be here soon, which will help. Thank you very much for your help. Hope i can return the favor some day.

 

Usually if the idle circuit is lean, then the popping or backfire happens going on the throttle. And if it gets worse with the choke on, then its not too lean either. I was thinking if the intake valve tightened up, it doesnt close properly, and the gases spit back in the intake. Also when you check the clearance, check to see the punch marks on the cam sprokets line up correctly...its posible the last guy has them out of time. Be aware the cams need to come out if the clearance needs adjustment, they are shim under bucket. You might want someone else doing it, if youre not very mechanicly inclined...but checking isnt a big deal, and if they are in tolerance,youre money ahead. So at least do the checking, and then decide if you want someone else to do the adjustment. Heres a link to a generic checking of a cam with shim under bucket
how to

That carb has a throttle position sensor..is it still hooked up...and working? As long as you have the service manual coming, there is a troubleshooting guide in there, and itll explain the valve clearance checking, and adjustment if necesary, and the throttle position sensor adjustment.

I'd also check the accelerator pump timing, which is also in the service manual.

heres another how to on valve clearance checking

 

Thanks again hondabuster. I'm sure this will help.

 

Sounds like a it's lean on the pilot jet/circuit. You can try turning your pilot screw out. If you go past 3 1/2 turns then you'll have to change pilot jet or air jet. It could also be an intake leak, you can spray some carb cleaner around the intake manifold to see if you engine changes. If it does then you have a crack or leaky gasket. Lastly you could have a timing issue, maybe a chain tensioner not taking the slack up or intake valves that have tightend up... Honda did a great job of explaining all of that.

 

Did you happen to check on the air cut off valve? If its installed wrong, or removed without being properly plugged, it can cause weird vacuum leaks.