1978 or 79 700 Merc problem

waterinthefuel · Mar 31, 2006

Ok, why is it that every engine I own has a problem regarding idle? Anyway... We took our boat out fishing today and it ran like a champ, at anything above idle. We put new plugs and fresh gas in it. Occasionally it would let you shift into gear without killing, but they were few and far between. It has a tach, but it hasn't worked since long before we got it. How can we tell if it's a low idle or something actually wrong? Any suggestions? We will take it in Monday and I'll fill you guys in on the results, but I'd love a heads up on what the experts think it might be.

Silvertip · Mar 31, 2006

Re: 1978 or 79 700 Merc problem

We are at a greater disadvantage than you. You can at least "see" and "hear" the engine. We can do neither. No tach and nothing to go by, the best you can hope for is a lucky guess. Lots of things can cause a low RPM stall. Bad fuel, water in fuel, gunked up idle circuit in carbs, misadjusted carbs, synchronization needed, low idle speed, weak spark, low compression, to name a few.

waterinthefuel · Apr 1, 2006

Re: 1978 or 79 700 Merc problem

It occasionally sounds like it's hitting on 2 cylinders. When it idles smoothly it shifts fine. The fuel is 2 years old, with stabilizer. It's not low compression or weak spark. Other than that, anything else could be it. I tried my best to describe the problem.

WillyBWright · Apr 1, 2006

Re: 1978 or 79 700 Merc problem

Dump the gas and get fresh! Geez! You spend hundreds of dollars maybe over a grand, and you're running old gas??? Stabil extends gas life my months, not years. Any adjsutments you do with that junk will be totally meaningless once you use that up and start burning real gas. That !very! low octane paintbrush cleaner can detonate a piston or two in that model in a very short time. You might not even last the first tank. Don't just top it off with Premium. Get rid of it!

waterinthefuel · Apr 2, 2006

Re: 1978 or 79 700 Merc problem

It was drained prior to our last outing. It has fresh new gas now, and is still misbehaving. It does have a little bit of the old still in it, just residual of what we couldn't siphon out. We are taking it in tomorrow for a test on a dyno.

timmathis · Apr 2, 2006

Re: 1978 or 79 700 Merc problem

If idling up a little don't help. Try turning your air screws out a 1/4 turn at a time and see if that helps.

tashasdaddy · Apr 2, 2006

Re: 1978 or 79 700 Merc problem

how long did the motor sit (no use) before it started acting up.

waterinthefuel · Apr 3, 2006

Re: 1978 or 79 700 Merc problem

First of all, what is an airscrew? Second, it sat for over 2 years.

timmathis · Apr 4, 2006

Re: 1978 or 79 700 Merc problem

Do you have a manual? Airscrew- is the carb adjustment screws. The air mixture screw. Sat for 2 years: you are lucky it will start. You probably need to pull carbs and clean them also you should change the impeller every 2 years or less.

waterinthefuel · Apr 4, 2006

Re: 1978 or 79 700 Merc problem

We brought it to get it worked on yesterday. In addition to fixing the problem addressed in this post, we are replacing the water pump. We do not have a manual for it. We'd love one, but don't know where to look.

Motor Boater Bill · Apr 5, 2006

Re: 1978 or 79 700 Merc problem

You can get a manual at www.oldmercs.com. The factory manual will set you back around $50 which you will make back on the first hour of shop time you don't have to pay for!

waterinthefuel · Apr 6, 2006

Re: 1978 or 79 700 Merc problem

Ok, this is an update. The guy said something was wrong with one of the carbs. Something was moving wrong and allowing the float to come up too far, making it 4 stroke at slow speed. I have no idea if we were "had" or if this was honest. We haven't tested it yet. We are going to drop it in the river this weekend. He also said change the water pump every 3 years, NO LESS OFTEN than that.

Motor Boater Bill · Apr 8, 2006

Re: 1978 or 79 700 Merc problem

Water pump impellers cost less than rebuilds! The first impeller you install yourself will also pay for the manual in saved labor cost.

waterinthefuel · Apr 8, 2006

Re: 1978 or 79 700 Merc problem

Another sad update. Winds 20-25mph and temps in the 50's this morning. Same forecast for tomorrow. The boat is covered and back in it's spot until we are done with these cool windy days. We did start it to run the gas out of it, and it surely seemed to idle alot faster than it had before. Maybe that was his "fix." Who knows.

timmathis · Apr 8, 2006

Re: 1978 or 79 700 Merc problem

I hope it works well for you . I don't think it will be long before the weather turns In our favor for the lake. Keep us posted, Tim

waterinthefuel · Apr 9, 2006

Re: 1978 or 79 700 Merc problem

Will do.

Clams Canino · Apr 9, 2006

Re: 1978 or 79 700 Merc problem

It NEEDS to idle under 1000 RPM in neutral no matter what. If it's too high, he did it wrong. A good trick is to put it away for the winter with 1/2 tank of gas and Stabil, then in the spring add 1/2 tank of fresh gas. A compromise but better than a tankfull of old gas. -W

waterinthefuel · Apr 22, 2006

Re: 1978 or 79 700 Merc problem

Well we took it out fishing today, and it runs great. Seems to idle a little fast in neautral, but once in gear putters like a kitten. It only died on us shifting twice in the whole day. Whatever he did worked great. It never missed a lick other than those two moments.

1978 or 79 700 Merc problem

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