1979 Johnson 4hp one cylinder rich

[Dabbler_E]

Hi iBrains,

I have a 1979 4hp Johnson that runs pretty well, and the bottom spark plug looks fine (light tan) but the top plug consistently shows a rich condition. Both cylinders share a single carb, both have good compression (within 2 psi), both have strong and consistent spark (based on gap tester and inductive timing light). I replaced with new plugs (gapped), and the situation didn't change.

Part of me says "if it runs OK, don't mess with it" but I have a feeling I could be getting more out of this little motor if I can fix the rich condition.

What am I missing that could cause this issue?

Thanks!

 

[kbait]

Pin hole in fuel pump diaphragm could still function, but dump fuel to the cylinder it draws it's pulse from (top cylinder?). When idling, it will likely flood that cylinder when you squeeze the primer bulb, forcing fuel though that hole..
Good luck!

 

[Dabbler_E]

kbait -- sorry, I forgot to mention that this motor has gravity feed from an on-board tank. No fuel pump.

 

[kbait]

Ah. Well, since compression is even, I'd pull the carb, and then reed plate, and check that leaf valves are seating properly for each cylinder..

 

[Dabbler_E]

Bingo-- reed valves show a gap on that side. thanks kbait!

 

[boobie]

I've seen a lot of reed valves that had a gap in them and still worked fine.

 

[AlTn]

check that the top cylinder is firing consistently

 

[Dabbler_E]

AlTn -- the inductive timing light shows very consistent spark to both cylinders.

 

[Dabbler_E]

OK, more clues: I pulled the exhaust cover and the LOWER piston shows scoring and carbon deposits.

 

[Dabbler_E]

 

[Dabbler_E]

Solved: The fuel mixture problem ultimately was caused by the wrong orifice in the high-speed jet. Once I replaced that, everything sorted itself. I don't know why the problem manifested so much more in the top cylinder, but it's running great now.