Combustion at key on

[barber1303]

Looking for some advice/experience on a problem I am having with my '85 model year. I finished a ground up rebuild over the winter of '23, repainting and replacing parts as needed with OEM. It rode and performed well over the spring/summer/fall of 2024, but had an intermittent issue where cylinder 4 (I think) would combust to varying degrees of intensity at key on. I would estimate this happens roughly every fourth key on and varied from a slightly audible POP, to completely blowing the carbs off the manifold (rare). The frequency seems to suggest that occurrence depends on which cylinder is left with the intake valves open during shut down. I don't remember it doing this prior to the rebuild, and for the life of me, I cannot think of what would be occurring that would (1) yield a combustible mixture in the cylinder right at key on, and (2) cause a spark within the cylinder to trigger combustion at key on. Time between events only seems to matter in that it won't do it twice in a row...I believe the engine needs to rotate before it will do it again. BUT, it can sit for days (maybe weeks) and still do it right at key on.

For clarity...this happens right KEY ON...the engine is not running. You can start to hear the v-boost cycle and then BOOM (or whoosh...depending upon which intensity is showing up that time)

Anyone experience this or a similar issue? Any thoughts at where to start trouble shooting?

As always, thanks in advance for your excellent advice...

Chris

 

[gentsvmax]

Looking for some advice/experience on a problem I am having with my '85 model year. I finished a ground up rebuild over the winter of '23, repainting and replacing parts as needed with OEM. It rode and performed well over the spring/summer/fall of 2024, but had an intermittent issue where cylinder 4 (I think) would combust to varying degrees of intensity at key on. I would estimate this happens roughly every fourth key on and varied from a slightly audible POP, to completely blowing the carbs off the manifold (rare). The frequency seems to suggest that occurrence depends on which cylinder is left with the intake valves open during shut down. I don't remember it doing this prior to the rebuild, and for the life of me, I cannot think of what would be occurring that would (1) yield a combustible mixture in the cylinder right at key on, and (2) cause a spark within the cylinder to trigger combustion at key on. Time between events only seems to matter in that it won't do it twice in a row...I believe the engine needs to rotate before it will do it again. BUT, it can sit for days (maybe weeks) and still do it right at key on.

For clarity...this happens right KEY ON...the engine is not running. You can start to hear the v-boost cycle and then BOOM (or whoosh...depending upon which intensity is showing up that time)

Anyone experience this or a similar issue? Any thoughts at where to start trouble shooting?

As always, thanks in advance for your excellent advice...

Chris

Man! I thought I'd heard it all LOL. Yea, we have amazing machines that can throw a hell of a range of problems.

 

[gentsvmax]

Maybe check the condition of the connector blocks at the ignitor box.
Swap out that # 4 coil for another by plug only without the brain damage of actually removing that coil to see if the mystery follows.