1967 Mercury 39 3.9hp information needed.

webbrs

Seaman Apprentice
Joined
Jul 28, 2015
Messages
30
Hello, all. New to the forum and to boating although I dive boats for cleaning and repair.
I have a 1967 39 and have rebuilt the carb, fuel pump, ignition and water pump. I have experienced this motor will not run with the tiller grip all the way to the right (in the area marked "shift range") it will idle in the space between "start" and 'shift". Shouldn't it idle throughout the entire range? I feel it should, and if someone could weigh in here I can know whether to mess with it or leave it.
The grip all the way right or closed and the throttle pin is not in contact with the mag cam so I feel the butterfly is fully closed. There is no spec on how much of a gap should be on the butterfly so I am not sure if it should run in this position or not. It runs well after warm up and idles as I described, shifts easily in f-n-r and pulls hard. seems like it is going to be a good little motor as soon as I learn about it. Btw I pull the cowl after starting to adjust and check the internals. Go easy and practice removing it before starting and it is painless. (it does scrape a bit, sort of like playing operation, also I was able to hand roll the motor enough to check the spark fyi.)
I went by the carb schematic and there was the idle jet gasket missing so I replaced it, and the float needle jet is Viton tipped with a solid seat, rather than the solid needle and Viton seat it replaced and I did reset the clips to the proper specs. I don't know if this could cause float level related problems but the old seat was shot. It misses at idle a bit when cold. if this is normal? It idles great after warm-up.
So my question is that shouldn't the motor idle throughout the range and since it doesn't, does anyone have any suggestions or settings that will help?
Thank you.
 
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emckelvy

Commander
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Jan 16, 2004
Messages
2,506
The scale on the throttle is arbitrary and each motor will be just a bit different. What you really want to do is adjust the idle stop screw so that the throttle linkage on the powerhead bottoms out on the screw at the setting where you get the lowest stable idle speed.

You'll have to also adjust idle mixture, with the engine warmed & in gear, for the smoothest idle at low speed. Your "miss" could just be a lean "sneeze", thus the recommendation to adjust idle mixture. It doesn't take much, try adjusting the idle mixture screw in 1/8-turn increments, waiting 10-20 seconds or so for idle to stabilize after each adjustment.

So, a combination of "tweaking" on the idle mixture and idle stop should produce the best, most stable idle possible.

Note, on most models it is possible to realign the shift scale, by loosen an allen-screw on the grip which will allow you to rotate the grip to the proper position. You may have to remove the rubber to reveal the screw underneath. I wouldn't try this without first sorting-out any idle issues.

HTH........ed
 

webbrs

Seaman Apprentice
Joined
Jul 28, 2015
Messages
30
Thanks for that info, I forgot about the throttle stop, I will keep adjusting and see if I cant adjust the grip. Trying to get familiar with this carb, turning the idle set screw cw is richer and ccw is leaner?
 
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webbrs

Seaman Apprentice
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Jul 28, 2015
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After checking the setup I found that the only throttle stop is the position stop for the grip control on the linkage, not on the carb. There is a degree of movement that can be set on the cam pin but it alters the curve.
 

60sboater

Petty Officer 1st Class
Joined
Jun 18, 2015
Messages
307
My 73 9.8 uses a machine screw through with a nut to lock it in position after adjusting. Actually, it has two screws/nuts : one for min and one for max...not sure what your 3.9 has.
 

webbrs

Seaman Apprentice
Joined
Jul 28, 2015
Messages
30
Found out the throttle stop on the linkage along with the pin adjustment to the mag cam give the proper throttle curve. it still sputters and bucks a bit at low idle and when first shifting into gear but I think if I keep at it I may micro adjust it out.
while still in the information topic I would like to know if there is a gasket between the lower and the mid? there wasn't one when I removed the lower to do the water pump, nor is there a reference to it in the breakdown, and I cant seem to find replacement if it did. I get bubbles out of the side between the two parts at low speed. no abnormal water in the case tho. permatex maybe?
 
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