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1978 4 hp Mercury Gnat

C

cdbrummond

Contributing Member
1978 4 hp Mercury (gnat model)
I cant get this thing to run right. the carb has been cleaned twice (ultrasonic) with welch plugs removed, reinstalled and selled with nail polish. every part of the carb is clean and clear. new plugs and compression and spark are strong. i still cant get this thing to idle down. it is a 2 cylinder and i suspect the timing might be a little off. i have a timing light but i cant find any info on where the timing should be at in gear wide open. the only info on the motor is where it should be top dead center. it runs well at full throttle but its mid and idle have issues. this motor seems to have a reputation for being a problem child. any suggestions, other than tieing a rope to it and using it as an anchor, would be greatly appreciated...thanks.
 
Max timing on this is 24-26 degrees before top dead center.

And yes, these are a little "odd" when it comes to your idle set-up.

It helps if you have a tiny tach (or the like) to set-up your primary pick-up timing.

You need to start the motor and get it running (warmed up) at a fast idle - 1000-1500 rpms (you can guess and it will work good enough to get it smoothed out a bit).

You should get a timing reading of between 18 and 20 degrees ATDC (if not, adjust to get in this range). If you then quickly back off the throttle (to slow idle - 600'ish rpm range), the timing should advance to 15'ish degrees (ATDC).

To time it you add 2 degrees to whatever it advances to - so if it did make 15 ATDC when you throttled back, then add 2 and time it at 17 ATDC at slow idle.
 
I will work on it tomorrow. I don't have a tack but I can get pretty close to idle rpms just by listening. I will try everything you suggested and hopefully I can get it to run better. Like you said, they are a bit odd and do take a while to warm up and run smooth...thanks so much!
 
The plugs are at 50. I checked and double checked the gap because it seemed really big to me. I will try everything tomorrow. It would be nice to get this little motor running. Its a cool merc but man what a pain in the butt!
 
If you can get her tweaked right you are going to love it.

I have a 2 1/2 horse TWIN (87 Evinrude). The pistons are about the size of a quarter (or so they seem).

Anyhow, a very small twin, when tuned, runs so nice and smooth - just kinda purrs - not like those single cylinder jobs, particularly the 4 strokes, that shake like a wet dog out duck hunting and kinda pop/jerk along.

You are gonna love this motor...
 
Have one of those in my shop, where it sits, dead and unloved. It might run fine if it didn't overheat, and if its Mickey Mouse starting system (what a kluge!) didn't threaten to disintegrate every time it's used.

So it sits.

Jeff
 
Some quality Mercury motors were in production for 25 years +----How many years was this GNAT marketed ?-----Does that say something !!--------Good stuff is produced / sold for many years.
 
i checked the timing and it was dead on. it advanced to 24-26 at full throttle and was at close to 8 when it touched the cam. i suspected a weak spark but i dont have a spark tester. i took out the plugs and shortened the gap to 35 and it ran alot better. it wouldnt come all the way down to around 700rpms in gear at low idle but it ran really well if you had the rpms around 850-900 in gear low idle. i am satisfied with that. considering how fickle this little motor is im just gonna leave it as is. thanks for everything guys! i hope to take it out to the lake on monday and really put it through the motions. i will let you guys know how it goes.
 
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