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1962 Johnson 10HP - help with rough idle - tried everything else

T

Tom Gillispie

New member
10HP Johnson outboard I rebuilt after 15 years of non-use has a rough idle that I cannot figure out.

Starts easily, runs well at WOT, but at idle, it runs OK for 2-3 sec, then RPM drops down and its really rough, then RPM will pick back up and smooth out for 2-3 sec, then drop down, etc. If I don't keep the throttle open a bit, it will die at the point when the RPMs drop down. I've spent a while adjusting the high and low speed needle valves per the service manual I have, but nothing seems to smooth out the idle.

Video of my dilemma at http://youtu.be/fd395KC8bMw

During the rebuild I repaired/replaced:
- compression a little low but even
- new coils, points, condensers, fuel filter, some fuel lines, water pump, lube, etc
- carb rebuild including thorough soak/cleaning; replaced high-speed needle valve; low-speed valve looked OK - maybe try replacing?
- new gas can and new fuel hose

Suggestions on where to look next and/or how to diagnose this would be really helpful - thanks.
 
Throttle is opening too soon.-------Throttle plate should be closed at idle and roller should NOT be in contact with throttle cam.
 
Racerone's point is valid except that I didn't see you get it down to a low idle to confirm if it runs when the carb roller comes off the throttle plate. To set this correctly, if you haven't already, you just ensure that the little line on the throttle cam (the part that pushes on the carb's roller to move the butterfly) is in the center of the roller on the carb just as the carb butterfly starts to open. Pretty much once the roller hits the throttle plate but some carbs have some slack between when the plate hits the roller and when the roller moves the throttle butterfly. I set it to be in the middle of the roller just as the butterfly on the carb just starts to open. This ensures your timing advance is synchronized with your carburetor. When the roller moves off the throttle cam, the only thing changing as you turn the throttle lower is the spark timing is being adjusted to happen later, reducing your RPMs. This is important for a smooth and low idle and very important to maximize top end performance, as well.

The other thing to look at is if both cylinders are working. It's possible that when the RPMs drop down that it is a function of you dropping a cylinder. You could simply try to start it on just one cylinder at a time and see if it dies, instead of dropping RPM. If it dies when only running on one cylinder, you know the one you are on is the culprit. You could also try and pull each boot at a time when it is running rough to see if it smoothes out or dies. These motors can run pretty good on only one cylinder.
 
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That sneezing when you throttle down is caused by running too lean. Now you have to figure out why it is running too lean. Some possible causes are throttle opening too soon (as already suggested), slow speed needle not open far enough, trash in idle speed circuit of the carburetor, slow speed needle damaged or tip broken off, air leak anywhere letting air into crankcase (besides through the carburetor). I think I hear the last one (air leak) when it sneezes. See if you can see it blowing out somewhere as it sneezes.
 
Thanks everyone - really helpful. You all are the best!
I had checked the throttle/timing sync but sort of quickly - I'll go through that carefully.
Other suggestions I will try:
- get a spray bottle of oil pre-mix and spray it into carb - can help seal that air leak temp and smooth it out
- listen intently for an air leak someplace; try to figure out if the cough is coming back through intake or through exhaust
- run on just one cylinder to see if its just one cylinder or both
- Carb cleaner on a few spots to help identify the air leak - the carb cleaner is sucked in at the leak and smooths out the cough
- Double check the low speed needle valve and packing. I cannot recall if the engine dies when the low speed needle is backed way out (really rich); if it doesn't, that points to a clog in the low-speed passages in the carb - perhaps
- I'm thinking I'll hook up a timing light and watch the flash to see if I can "see" any skipping. Probably unlikely but that can help rule out any basic ignition issues
- Confirm the ignition path can throw a "big" spark, not just a small one

I will do these things this weekend and report back - again - thanks for the suggestions on what to try next!
 
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The symptoms are " classic lean sneeze "--------Check to make sure that throttle is not opening too early and check that the low speed needle circuit is clear.-Check that the low speed needle is indeed moving in and out when you turn it.
 
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