Pismo10

This is the dist setup I have.

Thanks for the ideas so far.

BadDog

I had a 502 with a circa 1990 ignition setup. Odd miss at 3500 to 4k, came and went. Changed or rotated out with known good ignition components and eventually I disconnected the tach wire. Problem solved. Apparently, something in the tack was pulling the signal down at certain points in the sweep. At the time I re-connected and tested again to verify the cause. It was repeatable. From asking my buddy who was a very good marine mechanic he did say it was an uncommon problem but he had seen it before. Not certain if related to specific tach brand. Simple to try.

BadDog

Did some google searching seems that is a more common problem now with that era of boat. Tachs are getting old. Simply pull the gray wire at coil and test.

Is problem dead repeatable at X exact RPM each time? From my symptoms some years back it seemed to come and go but a tach shorting to ground would likely not be totally repeatable and likely temperature, vibration humidity, etc related or affected. Let us know what happens...

Pismo10

Will pull tach wire and report back. Coil on order. Dist pickup next.
Thanks, will get there.

SB

Had a customers boat whom’s coil issue would cause engine to die after same amt of time, every time.

105Fountain

Let us know what it was?

Griff

I would start with replacing the coil and if no change, then the pickup module Mercury-Mercruiser 87-892150Q02 SENSOR ASSEMBLY Genuine factory part (mercruiserparts.com)

mike tkach

tgnition coil,s don,t last forever,also the coil wire carys rhe load 8 times more than each plug wire,i have seen a coil wire cause strange issues.

F-2 Speedy

Hey Mike good to see you're still around

SammyJ

If your running a QuadraJet carb check to see if the stone fuel filter in the carb inlet isnt clogged....... happend to me years ago