1979 70hp Merc - stalls going into fwd/rev

My 1979 70 hp Merc OB frequently stalls when shifted into forward or reverse. It doesn't seem to matter if it is warm or cold. I just got it back from the local repair shop and they could find nothing wrong. They changed the plugs. Last year I added some Seafoam to the gas and this improved the situation a little but not consistently. I bought the boat and motor about 3 or 4 years ago and this is the only real problem with it. Once the boat is moving the motor runs fine.

Re: 1979 70hp Merc - stalls going into fwd/rev

It is likely teh carbs idle mixture is mis adjusted or the idle is too low. The motor should idle in neutral at about 900-1000RPM. This will drop to 800-850 in gear. if these RPMs are correct, you may richen the idle mixture 1/8 turn on the top carb. If this doesn't get it, richen the bottom carb 1/8 turn.

Re: 1979 70hp Merc - stalls going into fwd/rev

The rpm's look about right but I will double check them. I will try the idle mixture adjustment this weekend. I was checking the manual last night for just that adjustment. Many thanks and I will post my results.

Re: 1979 70hp Merc - stalls going into fwd/rev

My idle rpm was low so I increased it to 800. The idle speed screw doesn't seem very sensitive. Still stalling in gear. I adjusted the top and then bottom mixtures in 1/8 turn increments about 1/4 each way, rich and lean. Still stalling, mostly. I sprayed the carbs with Deep Creep and put some Seaform in the tank. I got a sunburn and some advice from my neighbours in the marina to hire a professional. One of them mentioned something about a solenoid that prevents my problem, but I couldn't see anything that looks like a solenoid on the engine and my manual makes no mention. I was wondering if I should try doing a full idle mixture adjustment as per the manual. I was also wondering if I should remove the mixture valves completely and check for wear. Lots of question, very few answers.

Re: 1979 70hp Merc - stalls going into fwd/rev

The locals were talking about the idle stabilizer module. I do not know if you have one. Look for a electrical device that is not the coils or switchbox.

it sounds like you need to clean the carbs and rebuild them.

Re: 1979 70hp Merc - stalls going into fwd/rev

I would check for even and strong compression across all cylinders... stalling under load at idle can also be attributed to poor or uneven compression.

Re: 1979 70hp Merc - stalls going into fwd/rev

Thanks for the feedback. I will start looking for a carb kit and will check the compression.