Got a 1987 YFM100 champ that my 10 year old rides. Recently it got very hard to start. In fact, it won't really start. I replaced the fuel filter and it didn't help. I pulled the plug and sprayed carb and choke cleaner on the end and replaced it. It fired right up. It doesn't run great but runs well enough to ride around the house and as long as you are riding it, it keeps running. As soon as you stop and let it sit for a few minutes, it won't start back. If I spray the spark plugin again it will start.

Any idea what might be wrong with it?

 

Install a new plug...then go from there.

 

Thanks for the reply. Always a good first step when dealing with fire problems. The only thing that bugs me is that it's a new plug. I put a brand new plug in it just a couple of months ago.

 

Hi rykr, I have the exact same atv. That sounds to me like a fuel problem. Probably the air/fuel mixture isn't right. Since it runs after it gets started, it might just need a set screw adjustment. Another possibility is bad or contaminated gas. When I had a similar issue it ended up being that I had an extra o-ring in one of my set screws that wouldn't let it close in all the way. They're super small and easy to miss. Maybe start with checking your gas condition, clean the carb, then play around with your set screws to see what works (always remember where you started for reference).

Hope this helps.

 

Also check your oil level. Too much or too little can make it act weird as well.

 

I see. Your spraying the plug trying to "prime" the starting, not cleaning the plug. Sounds like a fuel delivery issue ( as another had mentioned ) and probably time for a good carb cleaning/rebuilding.

 

did you happen to figure out the issue? Mine did the same thing and I'v got no fire. Ive replaced pulser, coil, spark plug, and CDI and checked most wires for connectivity. Still exact same issue as yours. It has not fire when I try to restart it.

 

spray it and let it cool off and it'll run a little while

 

Earlier posts were almost certain to be no compression due to valve clearances needing setting, common problem with little Yams, or an air leak due to cracked inlet manifold, if it can't suck through carb you don't get fuel into cylinder. However if you are not getting a spark when engine is hot, most likely a stator coil gone bad, either power or pulser coil, and as you have replaced pulser that leaves power coil.

 

The issue with having to prime the plug with carb cleaner was resolved, I replaced every ignition part on mine or so I thought until I actually found a service manual. Turns out it after spending 400$ on CDI, COIL, and Pulser it was actually what is called a source/charging coil. which is located behind the magneto. you will need a special puller to remove the magneto. The coil and new seal was 61$. you will need to solder it back on and be sure to cut the wires very close to the old one! Ive included the service manual on testing the ignition system and the part number/name below.

3GB-15451-00-00 GASKET,CRCS COVER (Yamaha) $15.02 Available Thursday, October 29, 2020 5G3-85520-M1-00 COIL,CHARGE (Yamaha) $33.22 In Stock
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$15.02
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$33.22 Subtotal: $48.24 Shipping: $9.95 Tax: $3.49 Order Total: $61.68

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