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HUUGE missfire YZ426F please help

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Jul 25, 2007
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#1
hi, i recently purchased a yz426f 2001 model from ****, he said it had been recently built etc etc he gave me the old piston, rings, chain etc

got it home, 2 hours use later it develops a huge missfire at any sort of revs, changed the plug and surpressor cap to no avail, checked and clean carb and that made no difference either.

checked valve clearences, all exhaust were fine but the inlets all were 7thou when they should be 4 to 6thou, would this make thta big a difference?

The bike ticks over perfect hot or cold

if not could it be the cdi thats my problem???

any info/help greatly appreciated

thanks, newbie chris
 
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#2
Ive rand around a few bike shops, they seam to think its the CDI stator/magnetos thats the problem

I dont mind spending some hard earned cash on one but ideally i wanna know if its gonna cure it or not!

Somones gotta know something about it

:bang: :bang:
 
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#3
RSTChris said:
checked valve clearences, all exhaust were fine but the inlets all were 7thou when they should be 4 to 6thou, would this make thta big a difference?

No, not a big difference.

Check the hot start button and make sure it is closing all the way. Also check the rubber end of the hot start valve to see if it is sealing correctly and is not damaged.

Check all of your wire connections and grounds. It is unusual for a CDI to go bad like that. Could be a loose connection or a bad ground/earth.

You should also check the flywheel for metal particles. If anything is going wrong in the engine or gearbox, metal bits will collect on the magnets of the flywheel. If there are enough of them they can damage the stator.
 
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#4
Ol'89r said:
Check the hot start button and make sure it is closing all the way. Also check the rubber end of the hot start valve to see if it is sealing correctly and is not damaged.
That was my thought as well. What you describe sounds like a lean condition. Did you use the hot-start to start the bike and then forget to close it? My buddy use to do it all the time on his YZ400F, we would get a great laugh at him in the pits about his "shotgun" bike whenever he did it...
 
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#5
Once started from cold with the choke it will tick over fine, but soon as you rev it even just a little it misses and splutters then returns to tickover and runs fine, this is the same with or without the choke, although pulling the hot start valve out actually makes it worse, like you say its running lean.

Theres no metal particles inside the flywheel casing either

Cheers for all your ideas
 
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#6
You need to do a lead down test,or rip the head off. I suspect the valves are not seating. A few thousandths out of spec is crucial,if it continues going out of adjustment I would not ride it!
 
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#7
Its got LOADS of compression, im 15 stone and struggle to kick it past TDC without the decomp lever.....

Im starting to think its to do with the hot start switch aswell, ive just taken it out and cleaned it, there was loads of muck n sh1t in there.

Soon as it stops raining ill give it another try
 
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#8
Well that hasnt made any difference :(

It still starts, ticks over like it should but as soon as it sees any revs it just bogs down and missfires/pops out of the exhaust

Theres alittle play in the hot start valve, i took it out and it seampt quite "wobbly", is this normal? also on the that goes into the carb, should there be rubber on it? mine was just brass....

The pilot screw is adjusted one and a half turns out from all the way in.

Any other ideas anyone?
 
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Apr 21, 2007
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#9
Just a guess

I'm not familiar with the 426 so this is just a shot in the dark, but have you checked the timing?
 
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#10
Yes, its bang on. It is asthough the timings out, but i garauntee its spot on.

It was running brilliant one minite, the next its gone tits up
 

bxb

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#12
Try cleaning the main jet in the carby. First clue, starts ok and runs at idle, idle jet supplying fuel, Bogs or misfires when throttle cracked, not getting fuel.
Compression sounds fine, we are told that timing is correct and valve clearance is close enough. Most likely culprit left is a bit of crud somewhere in the main jet fuel supply
 

DougRoost

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#13
Assuming you've checked for air leaks external to the carb (I like to spray carb cleaner on everything with engine at idle and listen for change in speed), it sure sounds like a carb problem. The FCR has lots of very small jets, so a jet/passageway could be clogged or it could even be worn slide bushings bushings. A recent Dr. Dirt article in Dirt Rider mag delved into the FCR, including the things you have to especially be careful about. It's a pretty sophisticated carb but this article de-mystified it.
 
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#14
RSTChris said:
Theres alittle play in the hot start valve, i took it out and it seampt quite "wobbly", is this normal? also on the that goes into the carb, should there be rubber on it? mine was just brass....
QUOTE]


Chris.

That sounds like your problem. There should be a rubber pad on the end of the brass valve. The rubber pad should be in good condition with a round indention in it from where it sits against the orifice. If the rubber is damaged or missing or any of the parts that hold the valve against the orifice are missing, the bike will miss as you have described. The valve should not wobble and should snap shut when you close it.

Go online or to your dealer and check the parts scan to make sure you have all of the necessary parts in there and replace the hot start valve. If the rubber on your hot start valve has deteriorated, you will have to clean the little pieces of rubber out of the hot start orifice in the carb.

If your bike still misses, check your main jet circuit and make sure your slide needle is not bent.
 
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#15
Sweet, cheers for that :)

I tried looking at the owners manual for a decent pic of the hot start valve just to see if there was anything missing.

Ill take carb off tomoro, give it a propper blast through and fit a new valve, fingers crossed.....