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Stock 1976 R90 /6. Here's the problem: when the motor is not yet warmed up, only the right hand cylinder fires. The left one stays cold but fires at high rpm. If I pull the spark plug cable off of the left cylinder and hold it so that the spark can jump a half inch or so from the cable to the plug, then it fires and runs great. In fact, even if its throwing a spark over to a cylinder fin, it still runs well. If I press the cable back on, the cylinder no longer fires. Once its fully up to operating temperature, the motor purs like a kitten.

Points gap and timing are spot on (had the same problem when the timing was retarded). Carbs are well balanced. I know the choke is set well because it ocassionally runs great even when cold. Spark plug cable appears to be in great shape.

I sure would appreciate any ideas you have!

Thanks,
Peter

Have you swapped sides...ht lead/cap/plug? Test and eliminate each component?

Oh, and welcome.

Steve's suggestion is the key.

Trade components between cylinders until you find which component the problem stays with. The first thing I'd do is just pull the caps off the plugs and put them on the opposite ones. If the problem switches sides, you will immediately know it is the coil, plug wire, or cap. If it stays on the same side you can still switch plugs, or even whole carbs.

If switching none of the easily switched components switches the problem, then it could be as simple as a valve adjustment or as serious as poor compression on the bad cylinder from bad rings or valves.

That was one of our frequent tests. You have a bad spark plug. Replace and that symptom should go away.

You have come across a very useful test accidentially.

What you are doing is raising the output of the coil,the extra gap increases the Kv required.Usually the symptom of a fouled plug.Replace the sparkplugs.

Thank you all very much. That all sounds like good advice. Just for kicks, I'll try swapping components from the other side before I change out the plug. I hope to get a chance to do it today, then I'll report back here.

Thanks again!

--Peter

Swap the parts and plugs one at a time and test between.

Garnet wrote:Swap the parts and plugs one at a time and test between.

Yeah, I thought that was clear, but maybe not. Very important.

Spark Plug. Don't waste time with the other stuff.

Jean wrote:Spark Plug. Don't waste time with the other stuff.

My money is on Jean.