So my Father-In-Law has a few 4 wheelers and one of them is a 1990 Yamaha YFM250B. When I first married my wife 10+ years ago we rode it with her family on occasions but it hasn't been ridden in 5+ years and has been sitting under his back porch. It has seen a decent amount of wear over the years. Anyways, since it has just been sitting (and he knows I love all things outdoors including 4-wheeling and also fixing things) he told me if I wanted to fix it up and get it running then it was mine. So I brought it back home to Colorado and have been working on it. So far I have done quite a bit (forgive some of my crude methods as I learn as I go and do what I can). I fixed up the body (gave a new paint job, conditioned the leather and sanded/primed some of the rusted body), new spark plug, new battery, emptied the old oil and replaced, brand new air filter, drained small amount of gasoline that was in it (checked gas tank and clean/clear of debris), pulled the carb (soaked it in 90% isopropyl alcohol as this was recommended somewhere else) and cleaned out all the junk that was in it (not too bad but yet a moderate amount of debris/buildup). I verified the best I could that all the jet holes were blown out but did this just manually by blowing through them myself but I could tell they all were patent as air passed through freely by creating a seal with my lips around the holes (wish I had a an air compressor)! I reassembled it and had a bit of a clutch issue as the clutch slide in the top of the carb wasn't sliding correctly but I figured out and now the clutch is working fine. So after that long drawn out story here is the problem...

I believe the electrical system is all running well. I get power to the lights and the neutral indicator light. When pushing the start switch (with the engine set to 'run' on the left handle and the fuel intake to the carb set to 'on') it keeps trying to 'turn over' or start but it never does even with choking it and trying to give it some gas from the throttle. Thinking that maybe it was just 'cold' and needed a push start that is what I tried. I actually hooked it up to the back of my truck and had my wife pull me on it. I was pleasantly surprised that I was able to get the motor running when I shifted it into 1st gear. However, when my wife stopped I wasn't able to keep the motor running (w/ or w/o the throttle). So I had her pull me again but this time I thought that maybe I could 'warm it up' if she pulled me further after the motor started to keep it running. So she pulled me around the block a few times. During that time the motor kept on running and I shifted up to 2nd, 3rd and 4th while she pulled me and it kept up just fine. However, while she pulled me I tried pushing on the throttle and I was able to hear SOME engine reving but was never able to gain any speed beyond that of the speed that my wife was towing me at which must have been between 5-15 mph. After the few laps around the block she stopped towing me and once again I wasn't able to keep the engine going even by throttling it. Thinking that maybe now being a warm engine it might start on its own I tried starting it again withe the electronic push button and that kept firing but wasn't able to fully start the quad. So I am trying to figure out what is going on. My best guess is that it has something to do with the fuel intake/injection but am not sure where or how to fix. Any thoughts at all are helpful!!!!!!!

 

I'm no expert but it takes 3 things to make a motor run. Air, fuel and fire.
are you sure your getting good fire. when you use the electric start is the fuel getting to the plug?

 

maybe a compression test is needed

 

how would I verify if the fuel is getting to the plug? I verified that I have good spark with the new plug. I also held my finger over the spark plug hole when trying to start and verified there is good air pressure or whatever it is called. Would fuel getting to the plug be a concern if it won't keep running after I've already got the engine running from towing it?

 

How would I do a compression test and what does that verify/test for?

 

Sounds to me like a carb issue. I had a similar problem on my small atv where it would only start while pulling it behind my car and popping it into gear. Using the electric start it would never run on its own. I had to replace the o-rings on the air and idle screws and get the adjustment right. It may need new o-rings and gaskets in other places too. Before doing any of that, I would just make sure everything is adjusted properly and that your fuel line itself isn't clogged and that your carb float is working properly (no leaks in it if it's plastic and the height is set properly). Also check your fuel shut off valve.

 

Thanks, I'll give those a try and see what happens. I'll let you know!