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Begging for help, 88 Merc 20 won't idle

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Bowhunter1661

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As my title states I am in desperate need of help. I cannot get my Mercury 20 to run at idle, it runs great at mid to high range. Compression is 120 on both cylinders. I have rebuilt the carb in its entirety, thoroughly cleaned with wire and cleaner. New impeller assembly, Fuel pump, primer pump rebuilt, new fuel, tank, lines from tank to motor-motor inlet to filter-filter to carb. Coils, plugs and wires checked ok.

After running at the lake the bottom plug had a slight amount of milky oil on it. Pulled the exhaust covers, replaced both gaskets. No more milky oil on plugs. Still runs like crap at idle.

Pulled the carb and reed valve cage, found a slight hole in one valve. Ordered a set of Boysens because OEM are discontinued, also ordered new reed valve seals and upper/lower crank seals (just in case). I don't know if this pin hole would cause rough idle/stalling or not. Please help!
 
A crappy idle (which includes stalling unless you give it gas) on these portables is usually a sign that they are sucking air somewhere.

The hole in the reed could account for the extra air but is unlikely to cause the milky oil in the lower cylinder - that's usually water.

The milky oil could be indicative of a bad power head base gasket - they do go (not uncommon in a motor of that age) - since these motors are headless this is one source (along with the exhaust cover gaskets) that will let water into the heads and suck in some extra air.

If this is not the issue my second guess on the hit list would be the gaskets on the transfer cover (intake side of things) - there is a rectangular gasket (o-ring) under the cover (cover is identified by it's rectangular in shape with two smaller inset rectangles on the right side of the (head) from the perspective of facing the prop).

These also "blow out" due to age - again, not uncommon on a motor of this age.


If the transfer gasket is gone (crappy idle/stalling on idle symptoms) it usually leaves an oily/goopy mess in the lower cowl directly below the cover - like maybe someone sprayed some WD-40 or Fogging Oil on the side of the power head and you get a black slimy mess right down in the pan.

The black mess in the pan in "other than" directly below the cover could be indicative of a bad base gasket as well (at least towards the rear of the engine) - if the base gasket is toasted closer to the front (carb end of things) you sometimes also get a "sneeze" before she quits.

The o-ring and the base gasket are both available from Merc for under 25 bucks total - I would be inclined to replace both.

It may sound like a lot of work to pull the power head, but it's actually not a great deal of work - after removing the fuel line, the tell tale, then unplugging the stop circuit and unhooking the shift shaft the entire power head can be pulled "intact".
 
Thank you so much for the reply. The reed valve will be replaced once they are delivered. As far as the exhaust gaskets, I have already replaced them. after this, the milky oil on the plug completely stopped occurring. Though the motor continued to run poorly.

When I removed the reed valve cover there was an odd thing I noticed. A portion of one of the gaskets was missing. I figured it was stuck to the reed valve cover plate but I couldn't find the torn piece anywhere. It did not appear to be a fully ruptured gasket, so until I get the replacement I cannot say for sure if this was also part of the issue.

when the engine was running poorly I tried spraying water on all of the gaskets to see if idle speed would change. No change.

Also, there is a puddle of oil/fuel under the carb. This motor hasn't been cleaned in lord knows how long. So not sure if that is from sitting so long or if it is indeed indicative of an issue occurring at this moment.
 
So in that case I would get your reeds sorted out and take it from there.

If the gasket on the reed plate was also shot that could cause some oil/gas/air to "blow out" which would also result in poor idle/stall at very low rpms AND create a "goopy mess" up at the carb end of things.

The new reeds and gasket may cure your issues.
 
So, not sure if anyone is familiar with this model, but, there is a "C" shaped rubber seal that seals the two sides of the reeds apart from another. This deal appeared to be in ok shape. However, on the bottom of the seal where fuel may collect, the seal was beginning to degrade from ethanol. It did not appear to be fully compromised but non the less damaged. The gasket type seals were also saturated in fuel all the way through at the bottom on the reed cover plate. The screws that hole the bottom of this reed plate were only loose. With only a very slight twist of the screw they came loose. Wondering if that was also part of the issue. I could get the motor to run "ok" on the muffs by turning the idle screw about 3 turns out, which I know is likely way rich on a normally running engine. At that adjustment the motor was still running poorly and puffing smoke. But, if the reed valves, "C" shaped seal, and gaskets were all leaking in a air I would think that would be the reason I could get it to run so rich. Because the extra amount of fuel entering matched somewhat the extra amount of air entering.We shall see!
 
Should also add, there is no thermostat in this motor. I have heard that some of these donor have them from the factory. could it be the motor is running too cool?
 
Saw another individual that had nearly the same issue, his outcome was the primer ball bearing was pushed into the spring. Just in case I pulled mine apart after rebuilding it. I may have been drinking a bit when I did this rebuild :eek:. I pulled it back apart today due to this other individuals issue. I found the brass plug is actually a scosh longer. When I tightened it down the split ring was forced around the brass plug, oblonging it and damaging the end. Not sure if this is my issue or not..

There was a lot of fuel in the reed valve cage, also, there has been excessive smoke issues.

i
 
I think I may have found my problem... And I feel like an idiot. I bought this boat and motor from my ill grandfather. Because of his current state he couldn't tell me how to run the motor. Well, I FINALLY found some info online for an operators manual.

When starting the motor I would prime the engine, but, I would not adjust the idle speed knob (ignorance). In fact it was always set to full CCW, which is the slowest setting. So I believe that my issue is my idle was just too low and it would die.

Not it totally sure until I get the motor back together Friday when my parts come in. But it sure seems likely. Here's part of the instructions. Let me know what you think.

 
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Well... Just got done at the river. You all are reading a post written by the happiest man in the world right now! First off, the motor fired right up without hesitation. Idled smoother that any merc I have seen. When I would take off and run full throttle then come back down to idle, it would run as smooth as can be! Not one hiccup, missed beat... Nothing. Idle screw is set to 2 turns out from lightly seated. Running slightly rich, but, Boyesen recommends this due to the possibility of lean pops destroying the carbon reeds.

I am not sure wether it was the reed replacement, seal replacement or what. All I know is the motor is running great. I do not even have to mess with the idle knob to get it to run. RPMs are around 1100 at idle and 900 in gear.

MAN AM I RELIEVED!!!!!
 
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