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84' Mercury 2.4L 200 H.P. Rough Idle

R

rotax21

New member
I have a 1987 21 Enchanter with a 84' Mercury 2.4L 200H.P. Engine on it. I recently had a mechanic rebuilt the carbs and fuel pump as it was running rough and smoking a lot at idle and would sometimes stumple on acceleration. It ran great once I was moving. He found a tear in the fuel pump diaphram and the carbs were not to bad, but he went thru them anyways. I took the boat out for the first time today and it runs better, doesn't smoke as much, but it shakes a lot at idle and still stumples sometimes when I jump on it out of the hole. Still ok once I am moving.
The mechanic said that he couldn't get it to idle smoother as maybe there is a cylinder or 2 that are oversized causing an imbalance. I am not believing that. What kind of adjustments can be made to smooth out the idle? Could the reeds need replacing? This is my first outboard and it ran rough when I bought it, so I am not sure how smooth it should or could be if it was tuned correctly. Any ideas? The serial number on the engine is 6392841
Thanks
 
Not that this helps any but when I asked a few boat guys about my 89 merc 200 about it's rough idle they all said "typical mercury of that era". Not that it is a bad thing as they say they are good motors but they all, or many of them run rough down low. These motors don't have all the technology that helps todays engine run smooth at idle. They often load up at an idle and need to be cleared out once and a while. So if you idle out and then go to get into all of a sudden it can stumble until it clears out.

I accept it as normal and when people ask why it runs so rough I just say it's angry ;)
 
It could be a number of things from as easy as screwed up carbs or leaky reeds or it could be that you have a couple of weak cylinders. If you have an oversize cyl it will not make it idle poorly. What kinda mixture do you run? it could be loading up as said before. And older mercs idle fairly descent when right.
 
Well I am changing my stance a bit after yesterday. I took my front cover off and saw that the carb butterflies were very uneven at idle. The top and bottom ones were slightly open and the middle carb was closed completely. I set them all as close as I could get them by eye and when I took it out later on it ran sooo much better. It was much smoother at idle and the transition off idle was much cleaner.

So.. just may be something else to check.
 
Surely the mechanic would have sync'ed the carbs...but you never know. As stated, Merc's idle poorly...even the EFI's. That's why you should run a Yamaha. Smooth as butter even at 500 RPM.
Guys say Boyesen reeds will smooth out the idle. In an older motor chances are the reeds are a little out of shape. I hate the way my 200 EFI Merc idles...and everything else is up to snuff. I'm putting in Boyesen's this winter...see if that makes a difference.
 
my 86 2.4 idles rough as well. i want to add solid mounts but dont want to get rattled right out of the boat.
 
has your thermostat ever been changed?

sometimes theyll stick open not allowing the engine to reach operating temp at low rpms,
and that will cause a number of prroblems
smoking, fouling plugs so its only running on 5 cylinders (that would msake it rough) , hesitation or stumbling,

it seems to me that this could be the problem
hope that helped..
 
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