06 yz450f backfiring when reving engine

friend got 06 yz450f today and it backfires bad when reving engine up, air cleaner was real dirty, cleaned, changed plug checked timing marks on cams, ok, unpluged tps no change, cleaned out carb. nothing real bad inside and set air screw to 1 3/4 out, don't know about stator or cdi box any info would help, thanks.

Clean your carb, especially the pilot jet/circuit.

Check exhaust junctions for leaks,also check the hot start,make sure it is closed .

took all jets out and cleaned, even undid cable from hot-start to make sure no tension on cable. exhaust is tight. has aftermarket headpipe and power now in carb., but has been removed at this time.

took all jets out and cleaned, even undid cable from hot-start to make sure no tension on cable. exhaust is tight. has aftermarket headpipe and power now in carb., but has been removed at this time.

What is the condition of the head pipe gasket?,the one that sits inside of the exhaust port.It is back firing when on the throttle?

did not pull off headpipe to check, when you crack the thottle at about 1/3 opening it cracks and backfires pretty bad.

How old is the gas? The float level was not disturbed correct?I know it is very simple,but try a new plug same heat range of course.

Gas was replaced, same plug cr8e, float level was unchanged and looked to be within spec.

Check the pilot jet. '06's were built with 42's, but at your altitude, they should have a 45. That, and a correctly adjusted idle mixture screw will take care of most of it. Some crackling on decel should be considered normal.

Being how warm out it is and being on the gas at 3rd throttle the 42 pilot has no bearing. It is backfiring when on the gas.

Check the things I mentioned,it sounds like it is a used bike from someone else.This is the first day he rode it,correct? I would go over the carb to make sure it has the stock needle,needle pos,main,make sure the needle isn't damaged or worn. Who knows what the previous owner has done. Just make sure everything is stock with the carb.

when you cleaned out carby did you take note which way the plate on the front side of the slide was fitted! it can be put in upside down. the flat side has to be down

did not pull off headpipe to check, when you crack the thottle at about 1/3 opening it cracks and backfires pretty bad.

I am here in Ga (900-1200Ft)and my 06 can not use a 42pj, w/o tons of popping. 1/3 throttle is PJ related. Try a 45pj and a 168mj, needle at 3rd clip. (I prefer a NFLR or 07 needle). Play with Fuel screw - 1.5-2 turns out - make sure you have oring on the fuel screw. Also you mentioned the Hot start - if the plastic nut is semi worn out - air will leak into this/water does anyway - replace with Alum nut. I assume the hotstart is sliding up and down in carb correct?

Do not test on a stand - the bike has to be ridden.

Ditch the Power is not now.....it will only complicate matters.

Sounds lean to me, and that GA450 might have ya taken care of.

One other thing to consider (not sure how succeptable this bike is to it YET) but if you get a tank of fuel that is particularly heavy on the Ethanol, it can cause the bike to run lean. Go to a station out in the boonies that gets regular gas still (alot do, just not in the bigger cities) and try it out if all else fails.

My dads DRZ650 with its stock jetting would run lean as piss with ethanol blended fuel, but ran fine on regualr gas. It was just borderline lean with regular gas. So some jetting had to be done to make it compromise for both fuels.

3rd throttle is the needle it has nothing to at all with the pilot,come on.

thanks alot for quick responces ordered new jets and will move needle to #3 location, even ordered 2 sets to see if I can clean mine up also. ga450 do ya ever get to durhamtown if so let me know, we got a good little group of guys 35+ year old riders.

I plan on going there soon. Just getting back into dirt after about a 7 year hiatus....... im only 28 though.... hope im allowed :thumbsup:

I go there every once in a while - nice place and Mike is great. I prefer mx but I may get me a KTM and become more of a woods rider

3rd throttle is the needle it has nothing to at all with the pilot,come on.

ok IYSS....but his needle is stock and clip needs to be on 3rd position.

I have an '06 and experienced the same backfiring. I tried some different combinations and settled with 45/168 combination and left the needle stock suggested by grayracer. The bike is a whole new bike, very little backfire and pulls strong (especially mid band) all the way thru.:thumbsup:

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