hesitation at 3/4 to full throttle.....

The engine is cutting out at 3/4 throttle. It just started doing this. At first I thought I was fouling a plug...I put way too much oil on the filter. I assumed this caused the fouling, so I changed the plug. The old plug had black carbon around the edges...but...the the electrode was very light tan. My guess is, it started to foul because of my oil the hell out of the filter technic (not doing that now) and I think it is running too lean...which causes the electrode to be very light tan, right? All settings are stock. So what I can tell from the jetting section I should go to a 170-2 main to 45-6 pilot and the fuel screw 1 1/2 to 2 turns out. I live about 2200 ft. and ride up to 7-8000 ft. does this sound right to start off with. I have NOT removed the airbox lid (too dusty here in Tucson) nor converted to YZ timing...yet. Everything is stock. I do 70% trails 30% MX (MX just for fun and jumping)

If it just started to do this then it may be more than just a main jet problem. Make sure the carb is clean from debris, like the air jets on the intake side might have oil or dirt in them. Just an idea...

speaking of valves, my friend told me to press/kick the kickstarter until it stopped every time after i ride to take tension off of the valve springs at TDC. i assume it will make them last longer.

Kermit: James Dean might very well be on the right track. The carb on my 426 was filthy on the inside...pilot jet was partially clogged...debris in the float bowl....accelerator pump nozzly partially plugged...and a bad air leak at the mating surface of the intake manifold to the head. I carefully cleaned all the passages and jets, then used Yamabond lightly to make a good seal at the manifold/head juncture. What a difference! Your plug should never have anything black on the electrodes or the edges. This would indicate a richness somewhere in the carburetion. Have you checked, or had your valve lash checked and/or adjusted? The valves have a tendency of getting tighter with time due to driving deeper into the seats. Tight valves won't close entirely and will affect engine performance and could do some serious damage if allowed to go uncorrected. Loose valves in this type of head design won't cause damage as serious or as quickly. Loose valve lash will be a noisy clicking sound due to the buckets and shims clicking together. Hope this helps...and sorry if I've told you something you already know.

Bonez: Yes, that is a good habit to get into....especially if you are letting the bike sit for weeks unused. Finding TDC on compression closes the valves tightly and releases as much tension off the valves springs as possible. How much this saves on preventing weaking of the springs is anyone's guess. Here's a reason why it's good practice to close the valves everytime the engine is topped. My son rode his XR100 one afternoon and then washed it. Evidently, water went down the exhaust pipe and got on the exhaust valve stem because he didn't know to move the engine through to TDC. The bike sat up for a couple of weeks and when he went to start it, the exhaust valve had seized in the head and the piston came up and hit it....bent the valve badly. Dad(me) had to replace the valve and seal. Since then, I ALWAYS put the engine at TDC...even between motos and practice.....ALWAYS!

Thanks for the tips guys. My bike doesn't sit too much, I ride to work on it every day. Got the problem fixed, the carb was a little dirty. I also noticed that when I took the cover on the top of the carb off one of the bolts were loose, I am assuming that I was getting an air leak from there. I changed the jets while I was in there; I went to a 170 main and a 45 pilot. I'll see how this works. Thank you Boit I WILL now put it always at TDC.

[This message has been edited by Kermit (edited 10-18-2000).]

[This message has been edited by Kermit (edited 10-18-2000).]

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