1989 Merc 100hp 4-cyl - Cyl#2 Lean

[MartyTn]

Hi All, <br /><br />This motor has a lean pop on cyl#2 when cold and in neutral. The pop goes away when engine is warm or in gear. Engine has good power. Last fall a merc mechanic did a "full" check, replaced gas lines, rebuilt all 4 carbs (carb per cyl) rebuilt fuel pump, replaced all reeds, and made sure no air was getting in fuel. He said compression was good at 110 even across all 4 cyls. After he had done all he could he admitted he was stumped and the cyl#2 lean pop still exists. He went back thru the #2 carb twice more looking for blockage but couldn't find any. He said that when he would put his hand partially over carb #2 the engine would smooth out and run correctly. The carb rebuild kits were the "full" kits. Could a carb jet be clogged and not visible ? With only #2 acting up could this be crankcase seals or some other non-fuel root cause ? <br /><br />Many thanks for your help and ideas,<br />Marty

 

[DJ]

Re: 1989 Merc 100hp 4-cyl - Cyl#2 Lean

Marty,<br /><br />I would suspect a crankcase seal or seal between cylinders.

 

[MartyTn]

Re: 1989 Merc 100hp 4-cyl - Cyl#2 Lean

Thanks Djohns,<br /><br />Is there a way to verify a seal problem other than teardown ?<br /><br />I'm trying to decide if the motor is still usable as-is for this season or not. I keep thinking "lean eats pistons", but since the lean pop is only on the low-speed jets, does this necessarily mean the hi-speed jets would be lean also? The motor runs fine when warm or when 1800 rpm or more. Thanks, Marty.

 

[DJ]

Re: 1989 Merc 100hp 4-cyl - Cyl#2 Lean

Marty,<br /><br />I believe the technician did diagnose it correctly. By blocking air flow to the carb. throat the engine returned to normal. In other words, the cylinder in question is finding an air supply somewhere else.<br /><br />Realize that the cylinder is more likely to find an alternate air source at idle with the carb. throat throttle plates closed. At higher speeds, it probabaly works fine. It can find all the air it wants past the open throttle plate.

 

[MartyTn]

Re: 1989 Merc 100hp 4-cyl - Cyl#2 Lean

My main worry about this is I'm wondering if I could possibly be running too lean at higher rpms and inviting a piston melt-down since additional air can be had from an alternate source? The temp guage shows only around 120 degrees at higher rpm, but I don't know how close that would be for a cyl#2 temp.<br />Thanks, Marty.