Spark, Fuel, no start!

Vet261

Vet261

PR Addict
So my '91 RM125 has decided to not start this spring after running completely fine every time I rode/raced it last year.

Has spark.
Getting fuel.
Good ground.
Won't fire, not even a bark.

My only remaining thought is something electrical went south and it's not sparking under compression. Which electrical part would that typically be?
 
Probably a dumb question, but is there anything plugging up the exhaust? Had some mice plug up an ATV exhaust once.
 
Check CDI box according to service manual specs. I had a bike with the same issue once and the CDI box was bad, dont ask me why it still wouldnt fire with spark though.

Check and be sure it doesnt have excess fuel in the crank from getting too flooded from kicking also.


Find a neighbor hood cat and show it to the bike, maybe it will bark then?
 
Similar deal recently with the MX250. Plug wet with fuel, appeared to have decent spark couldn't get it to pop, back fire ar anything until I kicked it about 45 times.

Turned out to be grounding issue.

The older I get, and the better my mechanical and electrical thinking gets, the less tollerant I become to difficult issues. Whether it be one of my bikes, lawn mower or a vehicle.

Small engines are quite simplistic in that it doesn't take much to at least make them pop, even if they don't want to run well. Good spark, timed relatively close, and a good fuel/air mixture should always explode in the cylinder.
 
Thanks everyone. I have the '87 on the bench now, hopefully finish that up tonight and I'll toss the '91 on for full inspection. Hershey I might have you help me with checking the CDI.

The one thing I wanted to avoid was just tossing (expensive) electrical parts at it like our one buddy got into....
 
If you haven't yet, I'd try a new plug. I've had bikes with spark that wouldn't start without a plug change. Just experienced that last week.
 
First thing I told him... especially if you have dogs, the mice stock pile the dog food in little places.

I bet it is air, unless mice ate his CDI wires! Cause there is no way a mouse could find a crumb in that garage or house. Most mice leave there in search of food!

Pit already cleaned the carb and did the obvious to the bike.

Original44 said:
Probably a dumb question, but is there anything plugging up the exhaust? Had some mice plug up an ATV exhaust once.
Click to expand...
 
Already cleaned carb, checked stator, flywheel and timing, exhaust, airbox (I don't have mice in my garage, yet), put in new plug, re-sanded motor grounds, fresh fuel.
It has to be something electrical that's not sparking under compression but it has a heck of a nice spark with the plug laying on the side of the head! I'll tear into it more next week.
 
A second last resort, try an electric impact wrench on the flywheel nut, going the correct direction, this puts some good zing to engine.
 
Well since you've tried all the common stuff...i'll mention something that caused an issue for me a few years ago. Bike suddenly started to run really bad...which is different than your situation I know. I tried all the same kind of stuff you've done and finally called Tim Shepard at the Yamaha shop to see if he had any thoughts. Right off the bat he told me to change the spark plug cap. Yep, that fixed my issue. I would have never thought to change out a plug cap!

Good luck!
 
Especially on older bikes, the spark-plug cap can be pretty lousy…..the screw-in style has given me fits in the past.
 
I third the spark plug cap, even if its a new plug try the cap on or off. when you see spark is it across the gap?
 
After the spark plug our past history with my daughter's 79 CR 125 Honda was no starting when cold was the stator & hot was coil with both
having spark but not a very good one.

Also check all the grounds as rust gets between stuff after a while.

Good luck,
Maico Mark
 
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