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

[AceDuece]

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?

 

[racerone]

Think carefully before you spend $$$ on parts.

 

[saltchuckmatt]

Compression numbers?

 

[AceDuece]

Compression numbers?

91psi cold on both cylinders. Reading is from a gauge that consistently reads low as well, so guessing probably 100ish.

 

[saltchuckmatt]

91psi cold on both cylinders. Reading is from a gauge that consistently reads low as well, so guessing probably 100ish.

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.

 

[AceDuece]

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.

I sure hope you're right on the PP. I bought a knockoff one and it was a disaster. Wouldn't plug in right, etc. Found an OEM allegedly new one that should be here tomorrow.

I've also got a new trigger here waiting to be installed. But not sure I've got the time or energy to mess with that before this weekend. Whereas the PP is a piece of cake.

 

[AceDuece]

Think carefully before you spend $$$ on parts.

For sure. Fortunately I'm into the entire boat cheap enough that the outboard was essentially a freebie. Now that I have the transom redone on the boat, I figured let's see if we can get the outboard running solid. If not, no big deal. I'll repower for next year.