Big Backfire!

[GDW57]

Last summer my '74 Bultaco Alpina was running fine, then, BANG!!, huge backfire! Could not restart, had to get towed out of the woods.

Since then, all I can get is some sluggish running for a few seconds that never really takes off or more backfires.

I have checked the timing and points, and cleaned the carburator.

I have spark, barely, but it seems to look no worse than before when it ran fine.

Any ideas ???????

 

[JE223]

Huge backfires are usually caused by burned, bent or broken valves. Could also be a slipped timing chain. I am assuming this is a 4 stroke. If it is a 2 stroke maybe a stripped key on the mag. Anyway, the spark is happening when a valve is open.

 

[VB Racing]

If it is a 2 stroke, you may have lost a reed valve also(if it has reeds). just a thought :think:

 

[SULLY]

Big Backfire, no backfire since and sluggish running....Hmmm, is it difficult to start?
You also say the spark looks weak. I would check the charge ingnition coil winding (the one on the stator plate in the side casing) I have has mine fail on a Gas Gas trials bike, and the winding had gone high resistance. I later rewound the coil myself after getting hold of the correct gauge winding wire.
One word of caution though, if the bike has a CDI unit, disconnect all this from the loom before checking the winding, a circuit tester can damage the electronic components if testing is haphazard.

I can't help with what readings you should get when you test the coil, you would need to look in the w/s manual.

 

[GDW57]

Thanks for the input

I should of given more info; the bike is a 2 stroke, no reeds, and has never completely started (only sputtered) since the big bang!!

With no valves or reeds to contend with, we are down to the timing of the spark or maybe the quality of the spark!

The timing is spot on, the woodruff key is in good shape, so I sent the ignition off to Hughes Bultaco for testing.

I'll let you know how it turns out!

 

[GDW57]

Problem Solved: Condenser wire shorting out !

Tim at Hughes found that my condenser wire was shorting out. What I figure is that since the spark was hit or miss, fuel would build up and bang !!!

Anyway, the Bul is back to its old self, BRAAAAAAP !!! :yeehaw: