1976 MERCURY 1150 115hp

Hello all! There are a lot of knowledgeable folks here! I hope you can all help me please!

I have enjoyed this oldie but goodie for the last five years. However, since the fall of 2010, I have been struggling with getting it to run smoothly. Sometimes it is a rough start, sometimes it won't idle at all, sometimes it kills when I try to put it in gear... The one thing it won't do is kill once in gear and running. HOWEVER, it will lose power periodically where the RPMs will drop and hold at a steady level, then, whenever it decides, pops back up to where it should be and back down again whenever it feels like it. Other times, it runs perfectly... often enough to not give-up yet! When it is not acting up, it runs well enough and idles down smoothly enough to troll with!

A "local" who used to work for Mercury had looked at it several times. First time or two, it ran perfectly and so little he could do but enjoy a ride around the lake. When it did start acting up with him on board, he checked it out and could not quite put his finger on the problem. He mentioned next season borrowing some special tool from the Mercury dealer/shop that could test for something or another on the motor, but is otherwise currently at a loss.

So, until then... I took it back home for the winter and had another shop (reputable one, not the bad one!) look at it with the explained the problem. They came back with a report that there is NOTHING mechanically wrong with the motor. While at the shop, the motor ran perfectly. Spark is good, gas is good, compression good, etc... Their prognosis is an electrical part is going bad, but won't know which one (switch box-$609, Starter-$326, distributor-$545) until it completely breaks down. According to them, I can either go ahead switch all those parts now, paying double what I bought the entire boat for, or wait until it completely won't start!?! =(

What am I to do? I can take the boat out and bobber fish. I'll take the kids out when the weather is calm as I otherwise do not trust this engine. It is a big lake (Lake Osakis, MN) and I would be a fool to go out in rough weather, even a little wind, with a bad motor on that lake.

Any advice or suggestions would be much appreciated! I am barely a novice as far as the mechanics go, so bear with me in advance! Thanks so much!!!

Re: 1976 MERCURY 1150 115hp

Well it sounds like you have an ignition component breaking down but then again it sounds like it could be fuel to. Buy a spark tester first. Check your spark. This ignition should be able to jump a 1/2 inch gap. Should be bright blue on all cylinders.


Three other things you can do, if you have good spark skip number 1.

1. buy a manual a dva if you have a spark problem, then run through the ignition tests.
2. Take it out on the lake and when it is acting up,start pulling the spark plug wires to try and isolate the problem to a single cylinder. Which would be trouble in the -distributor cap, plug wire, or plug. If all the plugs seem to be firing when you pull them off its probably fuel. The Motor will lose rpm if the plug is firing right when you pull it off. They should all take away the same rpm when you pull them.

Problems like this can be a pain.
3.This could be a fuel problem and not be ignition related at all. Try running it with a known good tank and gas line also. Not the one your currently using.

Re: 1976 MERCURY 1150 115hp

I have a lot of experience with that motor, but your symptoms do not fit any single problem. The loss of RPM could be a weak fuel pump, or partially clogged antisiphon valve on the built-in fuel tank. The fuel pump on that motor was marginal, so a rebuild is likely in order and cheap to do. The pulse hoses may have springs in them to keep them from collapsing. Make sure that are in good shape and well clamped. Check the fuel lines and connections for air leaks.

The bad idle is usually the result of a bad fuel mix or more likely slightly dirty carbs.

Check the spark plug wires for arcing to ground. Check the spark plug wire caps for cracking. Check the motor wiring harness for bad insulation. Don't replace any ign components until you know they are faulty. The CDIElectronics web site has a diagnostic test for that motor's ign.

Re: 1976 MERCURY 1150 115hp

Thanks to both of you!

Naturally, it got cold here in MN. Was hoping to do check these suggestions out on the motor as the weather pushed 50degrees recently!?! Not so this week! =(

I will follow-up on these suggestions and report back when done. Sincerely, thanks again!!!

*All- Keep the advice and input coming! I am all ears!!! =)

Re: 1976 MERCURY 1150 115hp

I too have expieriance w/this motor.I have one right now. When i first got it. It did th same thing. So i cleaned the carbs out. It ran better but not right, so then i put an inline clear fuel filter on it. I was getting trash in the carbs. So i cleaned them again. It runs perfect now. As far as an electrical prob. is concerned the stator charges the battery only. This is not going to cause this problem unless the wire's have bare spots and are grounding out. You can actually take the stator of and it will still run. THE BATTERY WILL GO DEAD. The coil on this motor might have a crack in it . If it does, that could cause the problem, all though i would think it would do it all the time if this is the case.Check the fireing pin in the distributor-(IT IS LOCATED IN THE DIST. CAP IN THE CENTER)-It is spring loaded. Some times when they get worn down the material from the pin winds up in the hole it is mounted in and causes build up. This will cause the pin to become stuck, when this happens when the pin wears down the spring cant push the fireing pin down. USUALLY THE SWITCH BOX QUITS COMPLETELY.