Merc 75 HP running rough at idle

[richve]

Greetings Guy's. I have a 1994 Merc 75 ELPTO which I bought last year. I took the plugs out and the piston heads appeared to be covered with a lot of carbon. I sprayed Power Tune on each and let sit for 3 days. Took the boat out over the weekend. Motor starts up okay,runs great at high speed, but in neutral it idles rough and will sometimes stall. At low RPM's motor will bounce back and forth, increase RPM's a little and it runs smooth. I pulled the plugs and all three are wet. The low speed mixture screws are all set differently. Only changes I have done to the the motor are ,water pump,thermostat and poppet valve , fuel filter,and spark plugs.Using fresh fuel with Stabil for ethanol and a Racor. Need your thoughts on this approach. I would prefer going the Seafoam and Power Tune route and adjusting carbs before I attempt on rebuilding carbs. Don't know if I'm up to that. Have Merc Service Manual. Did compression check 130,130,135.
Thanks for any opinions.

 

[Tommyuk1]

Re: Merc 75 HP running rough at idle

I am having a very similar problem with my 1991 merc 100 hp 4 cyl and I have been researching what steps to take to solve it and i am hoping to this weekend. If you have the manual follow the instructions for setting up your idle in the carb section. Also its not to hard to take the carbs apart you cant really go wrong if you have the manual, and if you were able to change those other parts you listed in your thread your more than capable of doing it.

Are the tops of the pistons now clean/cleaner?

Have you checked your fuel lines are intact and aren't drawing in air?

Good luck with it

 

[richve]

Re: Merc 75 HP running rough at idle

Tommyk1, Thank you for the reply. The tops of pistons are a little cleaner so I intend on repeating the procedure. Fuel lines ok. I rebuilt carbs on a 500 Merc with 2 carbs, however the linkage on this motor simply scares me.