This has the boat mechanics stumped

[macca123]

I have a 1982 40 hp mariner, the problem with it is it idles too fast, now some details about the motor, compression 120 on both cylinders, timing spot on, mixture correct, carby's clean and idle screw backed of so it is not touching and the butterflies are compleatly closed. The only way the mechanic can get it to idle slow enough to put it into gear without a big crunch is to lean it up, or rich it up but then it runs rough, has any one come across this problem in their time working on motors.

thankyou in advance for any replies

macca123

BTW it's a jap mariner serial no 40e 6e9l 405360

 

[Faztbullet]

Re: This has the boat mechanics stumped

Odd problem but several possible causes:

1) air leak in manifold from missing screw/bolt
2) leaking/improper carb mounting gaskets
3) flywheel key partially sheared

 

[macca123]

Re: This has the boat mechanics stumped

Thanks fastbullet for the reply, I think I may have solved the problem, while take the carby's of for the millionth time I found stuck in the reeds the 5 inch long tube that goes in the end of a wd40 can, so it is obvious that someone has sprayed wd40 into the motor and it's sucked in the tube, that would cause my problem ???? will not know till I put back together, what's your expert opionion.
How the meechanic at the boat shop did not pick this up amazes me.

 

[ufm82]

Re: This has the boat mechanics stumped

Holding a reed open will result in a lean condition and thus the high idle. Ever pull the fuel line off an outboard to "run the gas out of the bowls" before storing it? It always revs up right before it runs out of fuel. Lean condition.
That's why the mixture was causing issues- rich mix was dumping one cylinder. Good luck with the fix- that seems pretty obvious to me. No damage to the reed? Don't want a steel reed breaking and going in the engine- that gets ugly really quick.