YZF426 not to healthy

[Quiksilver]

My brother has had a 426 for about a year now. When he first got it it was really fast and all, kind of made you do this- :yikes: but after a few months it started to run not like it used to. It has not been used too much however so at first we thought it would not be the piston/rings. It is mostly the top end that has been effected. It revs up but then spluters and backfires on the way back down. It was second hand, done one season. So, is it the piston rings, carb or something completely different?

 

[ironworker]

My first thought would be Valves........ If the valve clearances tighten up it will lose power, my next guess would be piston and rings, either way If it were my bike bike I would check into it or have someone do the work for you.
try HERE and take a look at the FAQ page.

Good Luck

Rich

 

[YZ165]

Good call IW. Check those valves and then do a compresion check. YZ165

 

[biglou]

I wonder if the carb could use a good cleaning also? Clogged main jet might cause these symptoms too. A good once-over the whole bike wouldn't hurt!

 

[Ol'89r]

I wonder if the carb could use a good cleaning also? QUOTE]

Check inside the little plate on the bottom of the float bowl, held on with two screws. Remove the plate and make sure there is no water or dirt in there. That is part of the accelerator pump system.

Also, make sure the hot start button is sealing and the rubber boot around the hot start button is seated. If the rubber boot is not seated all the way down, it can hold the hot start button open a little bit. Will cause the condition that you describe.

An air leak at the head pipe and muffler joint or at the head or most anywhere in the exhaust system will cause a popping on decel.

And, like others have stated, check the valves.


Just my $ .02

Ol'89r

 

[Matt90GT]

what kind of maintenance do you do on the bike?

clean the airfilter. you have not mentioned doing that.
tear down the carb and clean it.
install a new plug
check the valve clearance.

if you do all that, it should run like new.

 

[Jay Moffatt]

I am the brother that owns the problamatic YZF426. The bike is very well maintained, and the carb is spotless, i have stipped it down a few times thinking that a bit of rubbish may have snuck in somewhere and made sure the air filter is seating properly. The power has gradually been going since i got the bike, that is why i suspected the piston and rings. It also started eating spark plugs.
However it is a good shout on the valves keep the suggestions coming, Jay.

 

[Ol'89r]

I am the brother that owns the problamatic YZF426
However it is a good shout on the valves keep the suggestions coming, Jay.

Jay.

Probably the most important part of maintaining the new style four strokes is checking the valve clearance. If the valves are allowed to close up, it can cause many problems and can do great damage to your engine. Not only will the valves be damaged, but you can destroy the camshafts and followers. The cams rely on the clearance to provide a film of oil between the cam lobe and the followers. If there is no clearance, the oil will be squeezed out from in between the cam and follower resulting in metal to metal contact.

Also, if there is not enough clearance the valves will not close all of the way and eventually will burn the valve faces and seats. Lack of compression, hard starting, rough running are the first signs of this. If you keep running the engine it will only get worse.

 

[Jay Moffatt]

Thanks for the advice, i will not ride or run it again untill i have sorted the valves and hopefully i haven't done to much damage. All the problems that you have mentioned fit my bike. I still have a way to go till i compatantly maintain 4-stoke engins, but i better get used to it because i'm not going back to 2-strokes.
Jay.

 

[Jaybird]

The gradual loss of performance sounds very much like a valve issue, as stated above.
But be certain to address all of the above mentioned items as well.
The carb and the valves control how the engine runs more than anything. The accelerator pumps are known for acting up.

Before you jump into the valve work, (which is easy once you've done it once) you are going to for sure need some shims. You could wait till you have it torn apart and measured so you will have an idea of what size shims you will need, then see if your local dealer has what you want.
Another option is to have shims ready to go. You can purchase a shim kit and replacements from HotCams inc. http://www.hotcamsinc.com/ (you need the 9.48mm size)

Yamaha service manual explains this procedure very well and provides a chart to help.
If it isn't clear to you, do a search in here for Nikki's threads on valve clearance. She gives a very easy to understand explanation of the procedure.