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35 hp Merc outboard running high idle-Please Help!!

S

Steelshp

New member
Hello All,

I've been messing with this motor for a month and can't get it figured out. Looking for some guidance.

Let me give you some back story.

I've had this motor (84 Mercury 35 hp) for about 9 years on my 21' pontoon. I've never had any real issues with it. Had the carb cleaned the first year I bought it, and the water pump done 3 years ago. The first time out this season it started and ran fine. Had it on the lake for about an hour. On the way back to my dock, running wide open, it suddenly bogged down and acted like it was going to die. I backed it off to half throttle and it ran back to the dock but still didn't seem right.

I went back down a few days later and when I started it, it began coughing and sputtering. I called a guy who is very knowledgeable about marine engines, and he came to look at it and advised I needed a carb rebuild. I took the carb to him and he did the rebuild but when I put it on the motor it still coughed and sputtered and stalled out after a few seconds. Upon further investigation, I found water in my fuel tank. So I pull my tank, drain the fuel, pull the carb again, clean and dry the tank, reclean and blow out the carb, and install a new fuel line/primer bulb.

So now, when I start it, it will start and run fine except it is running high rpm's at idle speed and won't idle down. I've scoured the internet and have tinkered around with it but I am stumped. I tried calling the guy who rebuilt the carb but he failed to return my call. I got deparate and called the local Mercury dealer but they refused to work on it because it's "too old". It seems like it's up to me to try and figure this out, so if anyone can give me any ideas to point me in the right direction it would be greatly appreciated.

Also, FYI- did a compressioin test=good--did a spark test also=good

Thanks
 
Check that your carb throttle plate is full closed at idle - adjust linkage if needed. . Otherwise I Think you may have a vacuum leak. Since the carb was removed/replaced check all the vac hoses on the carbs/bock, etc. Try spraying starting fluid at one spot at a time around the carb base, hoses, fuel pump and see if RPM changes to find the leak. You could pull the carb and reinstall with a new gasket.
 
Canufixit said:
Check that your carb throttle plate is full closed at idle - adjust linkage if needed. . Otherwise I Think you may have a vacuum leak. Since the carb was removed/replaced check all the vac hoses on the carbs/bock, etc. Try spraying starting fluid at one spot at a time around the carb base, hoses, fuel pump and see if RPM changes to find the leak. You could pull the carb and reinstall with a new gasket.
Click to expand...

I did check carb throttle plate = closed.

I recently spoke with a Mercury mechanic and he also advised I try adjusting the linkage. After thinking about it though, should I really have to do this since it idled fine before?

I do have a carb gasket, so I might go ahead and throw that on to see if it helps, it would be easy enough to do. I'll also try the starting fluid trick to narrow down any leaks. If it's not the linkage, it does seem to point to a vacuum leak. Thanks much for the reply.
 
Sometime the Throttle linkage / wire/ adjustment can change to pull the throttle to higher RPMs. Often the instructions for throttle/idle adjustment is as follows.

- Disconnect throttle remove cable from engine.
- At Idle - run engine in neutral and ensure that idle adjustment screw cannot be adjusted to lower RPM when engine is warm. (Adjustment screw is off stop.)
- ensure that Throttle plates(s) are FULLY closed on all carbs.

IF the above does not work - check for a vacuum leak next. Any and all gaskets and hoses that connect to the Carbs or intake manifolds. Use Spray as stated.

Once the leak is found / fixed start over and adjust idle to about 800 rpms (1000 rpms approx when in neutral and in water) when in gear. Then re attach throttle cable - if cable will not go on without changing rPm adjust cable so it will.

Let us know how you make out . it's not that complex.

Good Luck
 
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UPDATE:

Went down this afternoon to see what I could accomplish. Disconnected the throttle cable, spun the barrel around 5-6 time to lengthen the cable and bring the throttle down a bit. Re-attached cable and fired it up. It coughed a bit so I backed off the idle mixture screw just a tiny bit. Adjusted the idle adjustment screw just a little and now the thing is purring. Took me all of 15 minutes to fix this thing. I've had the motor 9 years and after the carb re-build and idle adjustment this may be the best it's ever ran since i've owned it.
HUGE thank you to Canufixit on this forum and Rick at Knox Marine in Fredricktown, Ohio for guiding me in the right direction on this after a couple weeks of frustration.
 
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