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First, I apologize for bringing up such a frequent topic but, I have searched endless threads and have yet to find one with a definite solution.

Background: 2001 YZ125 that I overhauled top (B OEM piston and replate) and bottom (crank rebuilt, crank bearings, seals) last year about this time. SST pipe with 450MJ and 25PJ. The bike had no problems with plugs before rebuild but, I had only road it a handful of times before rebuild. Since overhaul it has been fouling plugs relentlessly.

With a new plug installed it will start and run fine thru-out the band with very little smoke and minor spooge. The plug shows a nice chocolate brown. After it sets more than a day it will wet foul the plug within 30 seconds of start-up. I can pull the plug, dry/clean it and re-install again, run for about 30 seconds then bog and die. If I put in a new plug the process (runs great until next start-up) starts all over again.

I have recently re-checked float height and re-packed silencer. It ran well for about 1 hour, shut it off then next day wet fouled on start-up as noted above.

So, it appears to only wet foul plugs that are already a little dirty from a previous running. While recently trouble shooting I checked the stator and coil resistance per the manual specs. All seemed OK however, the coil resistance appears to be a little higher than spec (sorry, I don't have the numbers with me right now.) I also noticed that while doing a spark check that the spark intensity seems to be greater the harder I kick it.

So the question now is, should the spark intensity be the same regardless of how fast the flywheel is turning? If the problem is a weak spark what is the probable culprit, coil or stator?

Thanks for any assistance and bearing with the long description.

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Did you check your air filter? and choke circuit? for me is just a carb problem, may be your needle doesn´t cut or what i said before.

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I am using new plugs. If I put in a new plug it will start and run well then re-start and run well as long as the motor is still warm. It is only after the bike has cooled that it will wet foul the plug about 30 seconds after starting. If I dry/clean that wet plug it will start again but only run for a few seconds before wet fouling again.

Basically, I have to put in a new plug every time I ride it. That plug will last all day as long as the bike has not cooled off much. The next time I start the bike when cold on the same plug, it will foul right away. Put in a new plug and the process starts all over.

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Ok that made it clear, now this: what oil ratio do you use, and how is your jetting?

It sounds weird, but maybe oil making it to the plug somehow? Have you done a plug chop? Just to see what happens to that fouled plug. And what kind of plugs we are talking about?

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What are you doing with the choke during these 30 seconds of warm up?

Needle clip position?

Air screw setting?

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Depending on outside temp it will start after 1-2 kicks without choke. With choke on it revs quite high so I usually leave it up for only a few seconds then close it and twist the throttle slightly, either way it will foul soon after cold start without a new plug installed. I have tried clip in 3rd and 4th position with no noticeable difference. I have tried the air screw any where between 2 and 2 and a half turns out. I recently switched from Techniplate at 32:1 to Bel-Ray synthetic at 36:1. One other thing that has me concerned is I had the frame powder coated when I had the motor out for rebuild. I cleaned the coating out of the threads where the coil attaches with a tap but am still wondering if maybe there is a grounding problem causing a weak spark. I have used BR8ES, BR8EG, BR9ES and BR9EG with the same results on all plugs.

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Try turning off your fuel petcock after shutting the bike off. Then turn it on when kicking the bike over. If this solves your cold fouling problem then it could be the needle valve rubber seat has degraded and needs replacing.

Another thing to mention. Did you rejet when switching oil ratios? Going to a leaner oil/gas ratio makes your air/fuel ratio richer.

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Try turning off your fuel petcock after shutting the bike off. Then turn it on when kicking the bike over. If this solves your cold fouling problem then it could be the needle valve rubber seat has degraded and needs replacing.

Another thing to mention. Did you rejet when switching oil ratios? Going to a leaner oil/gas ratio makes your air/fuel ratio richer.

Could definitely be a leaky seat, as it will allow the crankcase to build up a lot of extra fuel/oil, flooding the bike when restarting.

I wouldn't sweat rejetting going from 32:1 to 36:1. The difference in the amount of fuel passing thru the jets is miniscule. People really oversell the jetting/mix ratio thing.

In a 32:1 mix 96.97% of the mix passing thru the jets is gasoline, 3.03% is oil. In a 36:1 mix it's 97.30% gas, 2.70% oil. What's this mean? It means that you need a jet 0.34% smaller by flow area on the 36:1 bike to get the exact same jetting (% gas in new mix ratio, divided by % gas in original mix ratio). Means your 450 main would need to be changed to a 448.5, and your 25 pilot would need to go to a 24.9 (I'm assuming the pilot is sized based on fuel flow, like the main jet is). Like I said, neglegible.

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I always turn the fuel off after riding. With the tank of the bike I have checked the petcock, it does not leak in the off position. I only changed the fuel recently just because I had some 36:1 mixed up for a newly acquired KDX220 and wanted to try a different type oil to see if it would make a difference in the YZ, it did not.

I am really leaning toward a weak spark or maybe a leaky right side seal. I replaced the seal when i rebuilt it but, it may not have sealed properly. If the seal is leaking would it be blowing excessive smoke while running? As I noted before, when it is running well after installing a new plug there is little to no smoke from the pipe and it pulls hard thru the mid to top, better than I would have expected from a 125 anyway!

Thanks to all for the help.

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If starts with no choke is because doesn´t idle properly, so close screw and try to start up again, if starts with out throtle and idles is because your float level is high.

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Making some progress. I made a small float height adjustment and cleaned the coil contacts with some emery cloth. I then took the plug out of my KDX and put it in the YZ to see if an already dirty but not fouled plug would still fire in the YZ. I noticed the gap on the KDX plug was at .030, I had been using .020 on the YZ. Doing a spark check with the plug removed I noticed it was throwing a better spark than before. Anyway, the YZ fired right up and ran well and has started and ran well 3 more times since with no problems. That's the first time I have been able to ride more than once on the same plug.

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