1975 Mercury 7.5 slight cough at idle

[1962]

Please Help I have a 1975 Mercury 7.5 that has been sitting for about 4 years. I went over the engine, ie new carb kit, water pump, plugs and fuel pump. Engine starts and runs up ok, but at idle it has a random miss, almost like a cough thing. Tried everything to get it out but still no luck. I'm hoping someone here can offer some advise on something I may have missed with the tune up. It also seems to come down off high idle slower that usual, I bring the throttle back to idle but it stays at high speed for a few seconds before coming back to idle. Any help will be surely appreciated. I'm going to give this engine to my Grandson if I can get it running properly.

 

[tpenfield]

If it is a dry cough, it could be COVID . . .

Actually, it may be what is called a 'lean sneeze' . . . which is when the fuel/air mixture is too lean and the fuel is igniting as it enters the cylinder. I believe there should be an idle mixture screw on the carburetor which you can adjust.

 

[1962]

Thanks Ted, I've tried the adjustment from closed to many turns open with no change. It's almost like a random miss but seems to run up on high idle just fine. I can keep it running high idle in a tank and it appears to run as it should. When it does it at idle it will sometimes shut off. Carb has been disassembled, soaked in carb cleaner, jets cleaned high pressure air and kit installed. I've been around engines all my life but this little devil has me stumped. 2 cycles are not exactly my cup of tea LOL !!!!!

 

[flyingscott]

If you turned the screw and there is no change you need to clean the carb better,

 

[1962]

While turning the adjusting screw through it's entire range the engine will either slow down or speed up while making adjustments, it's working properly. It does not remove the random miss at low speeds. Have rebuilt many many carbs in my lifetime, I'm very watchful right down to the tiny little holes in the jets, cleaning them with a tip cleaner. The carb has been cleaned properly, I just can't get that low idle miss out. I hoping someone here may have experienced the same thing and was able to determine the cause.

 

[1962]

My "no change" comment above referred to no change with the problem after making adjustments to the adjusting screw on the carb. The adjusting screw on the carb will run the engine from lean to rich as it should, so I know it's working. Low idle cough, sneeze miss or whatever you want to call it remains ???????

 

[3boydad]

Sure it’s not ignition?

 

[racerone]

Check upper and lower crankshaft seals.-----If original they are likely hard / brittle and not sealing properly.