Backfire/Engine fail

Cylon

Cylon

Member
Hey all I was riding my bike to work yesterday and the engine was backfiring a ton almost like it was four stroking but in fact it was backfiring. So I changed the plug almost made it to work then the engine shut down and failed to restart.

So far I've pulled the whole top end off nothing wrong with it, in fact it looks amazing. I did some more porting and filed down the intake piston skirt.

Next I checked the magneto magnet and it seemed Okay. I pulled it off anyways just too look at the woodruff key and that was fine maybe a little loose.

Tonight I'm going to put the engine back together with new rings and try it out if that fails I'll just buy a new cdl because my magneto is fine.
 
when my bike was backfiring and eventually died it was the electrical system. Make sure you are not running a resistor plug boot, that your magnet is correctly placed, your magneto has right ohms, wiring isnt grounding out. etc.

The other thing you might try is a clogged carb jet. It could be backfiring due to a clogged carb jet, then once the engine got cold you couldnt get it started at all.
 
Check your magneto. Could had came out of time somehow by snapping the piece that keeps it in place. This would keep you running probably with it already warmed up, but wouldn't allow you to restart. Internals look good so it has to be the timing of things.
 
You might want to check the spark-wire. In fact, I advise you to change it with automotive-grade spark-plug equipment. I had a similar experience a few years ago, and it turned out my spark-wire was corroded and was only firing intermittently. Uncombusted fuel-vapor built up in the exhaust which would backfire.
 
So magneto is still looking good, I rebuilt the motor with new piston rings and I went to start it and still nothing.

I took the cdi off and tried to take the spark wire out of it and was glued so I ruined it trying to get it out, next I'll order a new cdi and see if that works.

I also took the carb apart and it isn't blocked up at all.

Thinking about just replacing the entire electrical system.
 
CDI timing going bad and head gasket leaks are the two most common cause of backfiring (though they sound different)
 
Might want to order a couple cdi/coils. Replace your defective one, keep a spare with you while riding, along with spare zip ties and a phillips scewdriver . Zip tie the cdi on, don't use the screws and straps. The ears break easily.

Unless the magnet is spinning free or the head nuts are real loose, backfires can be 99% sure to be from the cdi.
 
Had a day off today and I went ahead and dicked around with the bike. I have loads and loads of spare parts because I own 4 bikes and sure enough the CDI was junk.

Put a new CDI on there and fixed all the air leaks from tearing apart the motor yesterday and greased the flywheel, runs great now.

Next I'm going to figure out how to make bigger manifolds. My ports are twice as big as the intake and exhaust manifold! Talk about bottle necking.

Thank you everyone for your input. Helped me solve this a bit quicker.
 
before you do too much, realize that the carb is same size as intake & muffler flange is same as the rest of the pipe
 
Cylon said:
Had a day off today and I went ahead and dicked around with the bike. I have loads and loads of spare parts because I own 4 bikes and sure enough the CDI was junk.

Put a new CDI on there and fixed all the air leaks from tearing apart the motor yesterday and greased the flywheel, runs great now.

Next I'm going to figure out how to make bigger manifolds. My ports are twice as big as the intake and exhaust manifold! Talk about bottle necking.

Thank you everyone for your input. Helped me solve this a bit quicker.
Click to expand...

Time for a straight pipe!
7sHLw1T.jpg


Just kidding. Bigger exhaust pipe helps, but not much as long as you can cause a sort of funnel for the exhaust via port matching you are pretty much set to go until you get a big expansion chamber. Also the intake has to be matched with a carb and with that tuning it. NT carbs are simple which is why I am sticking with it. Best of luck with your other adventures!
 
Two strokes need an expansion chamber to control the power band. It's 4 strokes that can be straight piped without power loss.
 
Oh sorry misunderstood at first l read it to quit I'm in a back of a car atm head back home from Boston lol.
 
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