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1988 20 hp Mercury off-idle stutter

M

mcycleboy

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OK.....its become a matter of pride now as there is no reason why I can't just live with this minor but (for me) frustrating problem.

I have a 1988 low hour 20 hp Mercury 2 cycle that has always had a stutter between 1100 and 1600 rpm that I just cannot eliminate.

By stutter I mean it seem to run rich and "4-cycles" in the small off-idle range. At about 1600 rpm it smooths out all the way and including WOT

It starts very well. It idles well. At WOT is runs very well. I have changed out all the ignition components from a donor motor (stator - trigger - coils - switchbox - wires). Carb has been swapped with a known good one. The typical carb primer check ball has been checked and is not the problem (I wish it was). Motor crank seals changed both ends. I changed the reeds to Boyesen reeds with a new reed cage seal and gaskets....which improved my idle but did not eliminate the off-idle stutter.

Compression is 125 each.

Exhaust manifold gaskets were updated as per a recall notice.

The 2 bleed check valves are working and the hoses to them are good.

Timing has been checked and is correct at WOT and at idle pickup.

New Plugs and Plug wires did not help.

Plugs are the recommended NGK BP8H-N-10 gapped at .040 I don't dare run hotter ones for this motor.

I am pretty sure my problem is with Carburetion not with ignition.

The stutter that I hear indicates the motor is running rich in that range.

no amount of LS adjustments helps the off-idle stutter ...but the LS adjustments do properly affect the idle is all the typical ways.

The carbs for these small mercs do not have (that I can see) any way to adjust the mid range. I have tried a smaller HS jet with no change

Main Question--------------

Is there any way that anyone knows to affect the off-idle or mid range of a WMC carburetor ? The carbs for 18 20 and 25 hp Mercury / Mariner from 1983 to I think 2004 of are designed about the same for that 2 cycle motor.

Is there a carburetor diagram or schematic available than anyone knows of ?

I am not completely sure this problem is all that unusual for my motor but this bugs me while trolling.

I am tempted to try--------------

There is a small non-replaceable brass air-jet that leads from the top of the carb to the main jet emulsion tube that I believe is responsible for the mid range. I am getting to the point where I might slightly open that up to see what happens. I sure would like someone else's opinion however before I do it. Its not designed to be screwed with.

Any opinions are welcomed.

signed

Frustrated in East Winthrop Maine
 
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I assume you problem occurs as you give it gas off idle. If so, try drilling the hole(s) in the throttle butterfly a few drill sizes larger.

That mod made a world of difference in my Merc triples.

Jeff
 
Thanks. I very much appreciate your response. I have a spare butterfly plate that I might experiment with. By drilling out the butterfly "idle" holes...wouldn't that be almost the same as retarding the timing? I would think that if you make the holes larger... you would have to drop the idle speed back to compensate... which is (on my motor) done by retarding the timing ? Hey..if it works it works ! Where did you find the information as to that fix ? Thanks much for the info.

Tom Johnson
 
Don't mess with THOSE holes! I believe they work opposite of what you think--drilling lets in more air to LEAN it out.

Trust me on the butterfly mods. I've done two motors that way now with great improvement.

Jeff
 
thanks...I misspoke...I should not have said "idle holes"..as I was not referring to the tiny LS jet holes in the carb body but instead to the preexisting holes in the butterfly itself... my mistake. I used the word "idle" holes because the butterfly on this motor is 100% closed at idle (no stop screw) and the holes drilled in it I assume are needed to allow the motor to idle with the butterfly fully closed. Either way I have a spare butterfly which I will test on. I think I understand what you are saying. Thanks much. If I don't understand - you can PM me...I am rather thick at times !

Tom Johnson
 
Jeff

I increased the 2 small holes in the spare butterfly plate I had by 2 machine drill sizes larger and ran a quick test on it. It seems to be better...still has a slight burble to it but I think it is better...I don't want to over correct so I will run it this way for a week or so before I try anything else. Is there any way to describe how much larger you had to make the holes in your butterflies on your merc ? We are talking about machine drill sizes, not the typical graduations of a typical drill set at home.

I will let you know within a couple of weeks what transpired.

Tom Johnson
 
My three holes were about 5/64 " each. I went to 8/64 (1/8) inches on all three. I suggest you go another drill size, maybe two.

Jeff

PS: A later version of my carb has the center of the 3 holes drilled out to 9/64 inch (with the other 2 holes as before). I guess Merc figured the same thing out!
 
Jeff

Thanks much for the info...I took the boat out for a long ride yesterday and there is no doubt it runs much better. The hard "4 cycling" is now more of a hit or miss at that rpm. Thanks so much. I think I am going to open them one drill size more. I want it to run rich but not skip so I know enough not to try to get it too perfect for fear of running too lean. I very much appreciate sharing what you did with me...you have saved me a lot of frustration. If you look at the number of things I have done (see first post) you would know how many hours I have been working on this.

thanks much

Tom Johnson
 
You're wlcome, Tom. Why this works remains a mystery to me. I think it speeds up the arflow going through the carb so--when the throttle is whacked open--the lean stumble that happens before the fuel delivery "catchs up" with the air flow is lessened.

Jeff
 
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