Carb Trouble YZ426

So I go to take my YZ for a rip around the block and it starts fine. Soon as I turn the cold start off the damn thing dies, and wont re-fire. Kick,Kick,Kick nothing! So I pull the plug out and its got spark but its dry. Shouldn't my accel pump have gotten some fuel in there?? Because I did twist my throttle a bunch of times. Any you guys have any ideas what to check? Im going out to the garage now to pull the carb off.........Thanks

Well the carb was spotless clean and the accel pump was working I put a new spark plug in and set the fuel screw back to 1 1/2 turns out and it starts right up. I thougt I had this spark plug eating prob fixed. Its been starting great for the last 2 months. I guess I will just ride it and hope it doesnt do this when Im 20mi from home.

I guess I spoke too soon..........when I took it for a spin it ran like shit. Sputtering and popping. Anyone have any ideas on whats going on here?? it ran perfect when I parked it last night.

I just went through this . Turns out I had some BAD GAS. Water was in my carb. It only takes a drop to foul things up.

Seems like a simple thing to try........I will change the gas, with any luck that will fix it. I did check the valve timing just in case it jumped a tooth but it was spot on.

Just curious, was that the prob?

The bike chewed up another plug..... it still had spark but not enough to start the bike. I ordered a 158 main jet to bring some color back into the spark plug. I think its just a tad on the rich side. I had the bike out for a rip tonight and it seemed like it was running good. I think that the box of plugs Ive been buying from might have been defective aswell since the last 2 I used only lasted 10min. I got my new plug from somewhere else and its working for now. Thanks for the help :) ..........James

When you start the bike, do not turn it off until it has reached full temp. Once it's warm all will be OK, but if you start it cold and shut it off cold you'll foul a plug (at least mine will).

When you start the bike, do not turn it off until it has reached full temp. Once it's warm all will be OK, but if you start it cold and shut it off cold you'll foul a plug (at least mine will).

Ya.....these bikes are bad for that. Im not sure exactly why my bike would have chewed those 2 plugs in a row but I hope by leaning it a tad that it will be easier on the plugs. The spark plug was a little black instead of tan colored so maybe thats not good for the plug? Have you ever tried one of those Iradium or platinum plugs?? maybe they will have longer life.

OK... So I changed my main jet from a 162 to a 158 in hope to stop my bike from eating these plugs. The plug on the left is with the 162 and the one on the right is with the 158 main.


What do I have to do to get a nice tan color in the plug?? The bike is still eating 1 plug per ride &%$#@!?? Is there something Im missing here? Any help would be greatly appreciated!! Thanks guys..............James

Any Ideas??

What is your altitude and average temp? Is the float at 8mm? Float shut off nice and round, not worn in a ring? What pilot do you run? Do you shut off the fuel at the end of the day? I would shut the fuel off about 5 minutes from the truck and run the bowl dry (or drain her with the screw) next ride. Are the coil leads top and bottom clean with no rust? Wicking the throttle before a start should not be needed from what I hear. This will really load the intake with raw gas. I run the NGK iridium and never foul, but I have only fouled any plug once and this was a jetting experiment. My '02 YZ426 runs 165 main, 40 pilot, stock needle 3rd clip, at 1300ft, boysen cover, bk mod, #58 leak jet. I am terrified to go leaner on the main. What muffler do you have? Do you oil the air filter thinly and evenly? What air jet do you have in there? I bought mine used and the fella before me had a different jetting mindset. Can you experiment and start her without the choke? Are you sure the choke doesn't have some crud that won't let her pull out? Did you pull out he fuel screw(carefully, to save the o-ring, washer and spring). If the O-ring is damaged or gone, she may bleed fuel into the pilot-ask me how I know. You might try calling Boysen and JD for help. The guys at Boysen talked with me a long time to clean up a few issues and they know what commonly breaks first. Good luck to you.

Well first of all...... the trouble I have is always when I start the bike, it either starts and runs fine, or doesnt run at all. When I take the plug out its dry and has no spark... there is nothing wrong with the ignition because I can take another spark plug and it will work. I always turn the fuel off after every ride, always use fresh premium fuel, air filter is clean and oiled proper, carb is spotless, float level is good etc. Right now it has a 162 main, 48 pilot. The spark plug at first didnt look like it was burning right and when I showed it to the dealer he said it looked rich on the main jet. So I dropped the main from 162-158. That didnt really make any difference in the plug color so I went back to the 162. So for the lack of selection on pilot jets, I put in the 48 just to see what happens. Right away the bike seemed to run a little stronger, and the plug had a little more color. The air screw is 1 1/2 out. So I dont really know what to try next. The bike is running pretty good and has been staring up good now for the last 2 days, but the plug still isnt getting a nice tan and stutters slightly at 1/4 throttle at steady pace.

I'm nobody's expert, but I thought you said you wicked the throttle before starting. I would never do this. The AP pump will load up the intake with raw gas. It's possible I misunderstood. With all the detergent and different blends in modern pump gas, I don't trust plug color as an indicator any more. Fuel screw is tuned for RPM? Boysen told me a lot of guys ruin the o-ring (on the fuel screw). IT breaks and little pieces get down in the pilot. That's a big 'ol pilot for my area with Summer temps-but all bikes are a little different. When you wick the throttle, do you get black smoke for a second? You should get a little when it hits the rev limiter. Do you always have to use hot start when she's hot even if you don't dump her (rich)? I just got some AP issues figured out-what a pain. Jetting a 4 stroke is tough. I run iridiums NGK dr8eix I think. I never foul, but I jet my pilot leaner than you do. I did fight a bog when it got hot (ambient air temps were 90+ degrees F) though. It sure feels great when you get her jetted right. Good luck.

Ya, I did twist the throttle a few times before starting it, but only because after it didnt fire up and I checked my plug and it was dry, so I tried twisting the throttle a couple times to get some fuel in there and it still didnt fire. Then I noticed that the plug didnt have spark, so I put a new one in and it fired right up, then next ride the same bullshit all over again. I think my jetting is so far out that its shortening the life of my plug. Trouble is that Im not sure if its rich or lean. Did I mention that I have a WB exhaust?? I think that might have something to do with this too. But the bike has always been hard on plugs......usually 1 plug every 2nd ride, and Im startin to get sick of it. Its always been like this since new and has gotten worse recently.

Do you ever start the bike and then shut it off before it's up to full temp? For example, "Hey bob listen to this.. kick...VVRROOMMM... <shut off>"? Doing this causes my 426 to foul a plug, but if I let it warm up all the way then no more fouled plugs.

In regards to the plug color: in short, don't use this as your primary tool for jetting. Jet by the seat of your pants and you'll be happier I think. If you insist on using the plug color, then don't forget that you have to run it at the opening that you're adjusting for. For example, if you're checking the main jet then you've got to hold it wide open for some time, then hit the kill switch BEFORE you let off the gas. If you're examining the pilot jet then run it for some time at 1/4 throttle or less, then hit the kill switch before changing the throttle, etc..


Change the gas. Just drain it and put some good fuel in there. Try a tank of race gas and see how it does; or some fresh 94-93 octane.

Just try it before completely modifying the carb.


Change the gas. Just drain it and put some good fuel in there. Try a tank of race gas and see how it does; or some fresh 94-93 octane.

Just try it before completely modifying the carb.

Thanks for your concern......but in case you didnt notice, I ride this bike on the street and I cant have it killing spark plugs and leaving me stranded on the highway. Unfortunatly I cant get race gas at the pumps and 94 octane is just as hard to find. I usually run fresh 92-93 octane.

Well this is where Im at tonight:

Main #162

Pilot #42

Needle groove #3

Air screw 1 3/4 turns out

I went back to stock jetting tonight and went to groove #3 and the bike seems to be more responsive, no more stuttering, fires up real strong, but the plug is still white looking in the center (plug is still very new) but has a little black around the outer ring. So far this is the best its been.

In the past ive tried:

Main #158

Pilot #42 and #48

Needle #4

Air screw 1-2 turns out

My engine was completly rebuilt this winter and maybe has 20hrs on it now. Ive recently fitted the bike with WB exhaust, Hot cam decomp, Electrex stator. So now I will be extremely happy if the plug starts to tan brown with more hours. Its running excellent now so I think Im getting real close, but time will long as the plug doesnt go for a shit then Im laughin. Thanks alot guys for all your support.......If any of ya ever need a favor, just PM me. :)


I'd stick with either the 162 or 165 main and go up one or two in your pilot jet. You might be close to spot on with your main, but your pilot may be choking your bike, causing hard starting. Also, that could be why your bike runs better with the choke on- needs more fuel. . . I had a perfect tan plug, but my bike ran like crap until I went to a bigger pilot.

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