Weak & intermittent spark on 91 Moto 4 250

[kenikas]

My grandson and I are trying to resurrect a 91 Moto 4 250 that we got as a basket case a while back, and are having trouble getting a consistent spark out of the ignition system. We've been able to get it to cough and pop a few times, but can't get it to start up and run. When we pull the spark plug out and hold it against the cylinder while cranking it seems to get a few bright sparks, and then they just get weaker and weaker. We've gone through the troubleshooting steps in the manual, and the ignition coil resistances checked ok ( it had a new coil on it when we got it, appears to be an aftermarket one ). The main switch, and the engine stop switch checked ok, so we moved on to the magneto and the source coil, and pickup coil resistances were both within limits. So we ordered a new CDI box off Amazon ( unfortunately the only things we can find are cheap chinese knock offs ), but that didn't seem to make any difference either.

But just to give a little more background on this machine, it seems to have been at least partially submerged and not cleaned at some point in it's life. And the guy we got it from had  left the spark plug out so the piston was rusted into the cylinder wall. It now has a new cylinder, piston & rings, and we looked everything over and cleaned it up.

Back to the magneto, I decided to pull the stator and check it and the magneto rotor for any damage or crud, and that's when I found something that struck me as a bit odd. They seem to be inside the oil bath of the crankcase with the pickup coil and the lower part of the stator sitting in the oil, when I separated the spacer that holds the stator from the crankcase, it drained the oil from the engine. Does anyone know, is that normal?  I've been an aircraft mechanic for 40 years and anytime there was oil getting in to the magneto's on an aircraft engine it was time to overhaul them.

I've worked on CDI motorcycle ignitions ( albeit many moons ago! ) but they were all 2 strokers, and this one's got me pulling out what little hair I have left!  My grandson has a Timberwolf that's fairly close in year to this one, we think, but of course none of the plugs on the CDI (or anything else) match so it's had to swap parts for troubleshooting.  Any ideas anyone have would be appreciated

[kenikas]

Wanted to post a follow up to this since we finally found the culprit and got the old beast running.  It turns out everything was good with the magneto, so we put it all back together and went through all the wiring and switches again just to make sure and everything checked good per the wiring diagrams and the manual. Still no spark! Tried several different spark plugs, nothing. The guy we got it from had replaced the ignition coil with an aftermarket "racing" coil and the resistances checked "close enough for government work", but luckily the original coil was in one of the misc. parts boxes that we got with it. The spark plug connector on it was broken but we were able to take the one off the "new" coil and use it on the original one, and after cleaning off all the rust and crud on the mounting contacts, it tested perfectly within specs so we bolted it on and hooked it up and nice big fat sparks!

So I guess the moral of the story is never throw anything away!

[Mech]

My motto is "perseverance always wins, giving up never does.".

Well done you..

[spock58]

Just to add to the story, it is normal on 4 stroke quad engines to have the ignition running in oil. Of course when you're used to working on 2 stroke dirt bikes usually, like I was - this may come as a surprise alright!