New to me 426f...need help

[rickwenham]

I justed picked up a 2002 yz426f appears to be in great condition...

the guy hadn't ridden it in like 8 months and it reeked of stale gas so i drained the tank, put fresh gas in, and cleaned the jets (1 was plugged)

I fired it up and it is still backfiring outta the exhaust like crazy... The mixture screw was out 2.5 turns, so i turned it in a 1/2 turn at a time until it was only 1/4 turn out.....STILL BACKFIRING!

what do I try next?

I'm new to these hi-perf 4 strokes so any help is appreciated...thanks

 

[biglou]

I'm in a hurry at the moment, but "air leak" is what to look for next. Also, upside down slide (easy to do) in carb. More later if needed.

 

[rickwenham]

the rubber manifolds look good...i don't know where else an air leak would be...

Isn't backfiring from the exhaust caused by a rich condition?

thanks for any tips

 

[blanc]

I have the exact same bike which had the exact same problem you just mentioned. Get hight temp silicone sealent and seal the exhaust everywher, make sure that the bolts connecting the header to the cylinder are extra tight! Thats what my problem was after I overlooked it on numerous occasions.

 

[SFO]

its popping so you go leaner on the fuel screw?
Perhaps we have larger issues?

 

[rickwenham]

i fixed part of the problem...

i found half an o-ring in the carb where their should have been a whole one...it was on a screw that also had a spring on it.

so i replaced the o-ring and put on a boyesen accelerator pump (that the sales guy said is worth it) and the bike sounds a lot better

the mixture screw is now 2.5--3 turns out and the bike sounds much better now... the only problem is a slight bog if you give the throttle a quick twist off idle.

whick direction is leaner? Srew in or screw out?

and could changing the screw's position help with the bog?

thanks

 

[Matt90GT]

2.5-3 turns means you are to rich on the pilot jet. You need to go down.

The 426 has a bad bog. Yamabog is the affectionate name for it. You need to do what is called the BK carb mod for the bike which will time the AP properly. Next thing would be to get a JD Jet kit and install it per instructions. That will cure your carb issues.

 

[rickwenham]

Isn't my Boyesen accelerator pump supposed to basicly do the same thing as a BK carb mod?

Also..I was told by a guy recently that backfiring from the exhaust is caused by a rich (too much fuel) condition.
Is this true?

Is 1 turn out on the mixture screw more lean or more rich than 2 turns out?

thanks for the clarification...step by step I am getting closer to riding the bike :ride:

 

[biglou]

Out is richer, in is leaner. If you're 3 out, you need to go richer on the pilot jet and turn the screw in to 1.0-2.0 range. You might try just going to 1.0 out as is now, and see how she does. FWIW, mine always performed just fine when riding and under load even when they had a slight hesitation on the stand. Only once or twice did I have any bog issues while riding, and that was after extreme temp changes that I didn't adjust for.

 

[Masterphil]

No, the AC pump cover that you bought is a band-aid fix, the BK mod allows adjustability of squirt volume.

 

[rickwenham]

i think i'm going to leave the bike as is for a couple weeks till I take it to the desert...then I'll do a post-ride assessment.

the bog seems to be just when you snap the throttle with no load on it like biglou said (although i can't really get on the throttle too much on my short street) :yikes:

thanks for the help guys

 

[wayneo426]

the bog seems to be just when you snap the throttle with no load on it like biglou said (although i can't really get on the throttle too much on my short street) :yikes:

BK mod will clear that right up. :laugh:

 

[Ol'89r]

the bog seems to be just when you snap the throttle with no load on it

Rick.

Make sure that your slide opens a little bit BEFORE the accelerator pump squirts fuel in the carb. If it squirts fuel too soon it will cause a bogging condition. It can be adjusted by bending the little tab on the cable bellcrank. Also, you may want to check your valve adjustment. Tight valves will cause a backfire condition. Another thing to check is the hot-start valve. Make sure it is closing and sealing properly. If it is leaking air it will cause a backfire or popping.

Just my $ .02