01 YZ 426 troubleshooting


9 replies to this topic
  • G-BRO

Posted December 18, 2013 - 06:44 PM

#1

Just recently did a top end on my once tired 01 YZ 426. It was my first top end and I am a still a bit of a novice mechanic. I did a lot of research and followed the manual as much as I possibly could. Overall, I changed the piston/rings, flex honed the cylinder, cleaned/removed/inspected the valves, replaced the valve springs, changed the stock exhaust cam to a hotcams auto decompression, changed the timing chain, cleaned the carburetor and some other smaller things.

 

That being said the bike starts up cold or hot very easily, but when I rode the bike today it didn't run smoothly between about 1/8-1/4 throttle. The bike runs awesome between 1/4-full throttle, but I noticed that the more I opened the throttle the worse the bike seemed to get "loaded up" when I backed off the throttle and would stumble if I tried to blip the throttle. Also, after the bike was warm if you were to abruptly open the throttle from idle it would stumble and die.

 

I have checked and rechecked the valve clearances and the timing. My other thought was the idle mixture adjustment, but I couldn't seem to find a sweet spot to make it much better. I've had the carb off this bike many times and have never had the bike run like this. I'm stumped....any ideas??



  • grayracer513

Posted December 19, 2013 - 08:24 AM

#2

That being said the bike starts up cold or hot very easily, but when I rode the bike today it didn't run smoothly between about 1/8-1/4 throttle. The bike runs awesome between 1/4-full throttle, but I noticed that the more I opened the throttle the worse the bike seemed to get "loaded up" when I backed off the throttle and would stumble if I tried to blip the throttle. Also, after the bike was warm if you were to abruptly open the throttle from idle it would stumble and die.

 

If I understand this, you say that it doesn't run well at smaller throttle openings, and when you run it hard, it runs even worse when you let it idle down?  Stumbles and gasps more?

 

If so, it sounds like a fuel supply problem.  Check the float level, check that the float drops freely on its hinge pin, and check the flow when the float is lowered.  There is a screen on the top of the needle seat that's serviceable by removing the seat from the carb body, and two on the fuel petcock. 

 

It's also possible that you might have a varnished pilot jet contributing to the problem.  Clean the jet with a fine wire, or the appropriate number drill:

 

http://www.thumperta...o/#entry6879695



  • G-BRO

Posted December 19, 2013 - 10:37 AM

#3

Yes that is correct. Ok I will try that today. Is there anything else aside the carb that could play a factor? Something I forgot to mention was that there is more valve noise since installing the hotcams exhaust cam. A ticking noise. It's not obnoxious but it is a new sound. I was told some extra noise is normal for an aftermarket cam. Does this sound right?

  • grayracer513

Posted December 19, 2013 - 10:45 AM

#4

It can be true, yes.  The "quieting ramp" of the cam is a section between the heel and the lobe that is ground to very gently begin and end the lift of the cam so as to reduce noise and reduce the impact on the valve train components.  A lot of the aftermarket stuff has a little less of this feature, especially when the grind profile is more aggressive than stock.

 

As to the carb, there could be other problems, but it sounds like your complaint is centered around the fuel supply.  With that in mind, also see if loosening the gas cap improves the flow of fuel to the carb at all (issues with the vent).  Check the whole thing carefully for any sign of dried fuel deposits in any orifice.



  • G-BRO

Posted December 19, 2013 - 10:52 AM

#5

Ok I will do that. The bike did sit for a while during the rebuild. Probably doesn't help the carb.

Thanks!

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  • alfie

Posted December 20, 2013 - 02:55 AM

#6

this was the same problem i had with my wr 426 when i had the hot cam installed, the bike would just not run. right.

For me it turned out be a bad cam, but im not saying that is your problem its just something to bear in mind.



  • G-BRO

Posted December 20, 2013 - 03:11 PM

#7

Are you refering to the sound of the cam or the performance? How did you discover it was the cam? Did you end up getting another one or did you go back to stock?

  • G-BRO

Posted December 30, 2013 - 07:30 PM

#8

Hey Gray, 

 

I'm still trying to track down the source of my fuel problem. I ran the bike today and as I was applying some chain lube with the bike leaned over on the kickstand I noticed the bike sputtered and died. I kept playing with it and discovered that if I leaned the bike over to the point where fuel would come out of the overflow tube the bike would either die or become extremely unresponsive to throttle input until it cleared out. Once the bike cleared out it seemed to run normally. 

 

If the float is functionally correctly shouldn't the bike run the same even if fuel is coming out of the overflow tube? Could this be an indication of incorrect float height? 



  • grayracer513

Posted December 31, 2013 - 07:56 AM

#9


If the float is functionally correctly shouldn't the bike run the same even if fuel is coming out of the overflow tube? Could this be an indication of incorrect float height? 

 

No, of course not.  With the bike on its side, the float can't function, and all the fuel runs to one side, and is at the same height as the carb throat. 

 

If leaning the bike over no farther than the side stand does causes flooding, you haven't fixed your float level problem, though.



  • Jake_mx426

Posted September 07, 2015 - 03:28 PM

#10

My 426 did the same thing ended up being a warped head took it in to get shaved down and then it was fine





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