1985 Johnson 25hp J25TELCOS firing half as often on bottom cylinder at idle

A

AceDuece

Cadet
Joined
Apr 30, 2014
Messages
28
Hey y'all!

Latest boat project has a 1985 Johnson 25hp that I'm trying to get running better.

The bottom cylinder is firing half as often as the top one at lower RPMS. I have a knockoff tiny tach. Wrap it around the top cylinder, reads 1,000 RPM. Wrap it around the bottom cylinder it reads 500 RPM. Obviously the engine is actually cranking the same RPM and not oscilating 500 rpms.

The bottom spark plug wire is just half the pulses. I've tried switching the coils and the plug wires. Same problem.

I'm thinking power pack or the trigger sensor? The power pack is only pulsing the bottom whole half as frequently. But is it actually the power pack or something upstream in the system (trigger sensor)?

Is there a way to diagnose and tell?
 
S

saltchuckmatt

Captain
Joined
Jul 19, 2019
Messages
3,338
AceDuece said:
91psi cold on both cylinders. Reading is from a gauge that consistently reads low as well, so guessing probably 100ish.
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I run a lot of those, 100 they will run decently, 90, not so much. Might come up and like you said those compression testers all come out different. I rely mostly on one I got in the 80's but it reads the lowest of three that I have.

120 or better is where you want to be.

Best guess, power pack. Unless there's a problem with your flywheel or woodruff then that has the highest probability. Pulling the flywheel, examining and ohming out the trigger doesn't cost anything.

If you get a pp, make sure it's OEM. Used is fine to.
 
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