Doesn’t start after top end rebuild

[tom1104]

I’ve recently rebuilt my top end, bored 1mm oversized, ultrasonic cleaned carbs, all the correct jets and needle, no valve leaks, set gaps and timing. I have bright spark, fresh fuel, new battery. But for some reason the bike won’t start, when I crank it over the left carb backfires fuel every 5 or so turn overs. The bike was running previously before the rebuild but with low compression. I now have 150 on both cylinders. Also the ignition coils are new along with the condenser. I’ve been trying to get it to fire up for a few days now with no luck! Any ideas would be greatly appreciated
Thanks

[redblk63]

This sounds like a classic case of the timing being off between the ignition spark and valve timing. If I recall correctly, the points cam is driven by a shaft ending in a tab that can fit into a slot in the camshaft in two orientations, one of which causes sparks to come at the top of the exhaust stroke. My WAG is that’s your problem.

Take off a valve cover and watch the valve opening vs the spark firing to test.

I guess a simpler case is if the right-side points are connected to the left-side spark plug. Yeah, better check that first.

Good luck,
Larry

[Michael Stoic]

+1 on what Larry said. I've seen similar 180-deg issues with other motors too. It's a great feeling, when after the next 180-deg swap the motor comes to life and settles into idle... Hope to hear tom114 reporting success!

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Mike Stoic