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Old 12-01-2017, 05:07 PM
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Join Date: Apr 2017
Location: Tasmania Aus.
Posts: 4
Default Rough Idle.

My 1972 50HP Johnson runs well when underway but idles really rough, have tried to adjust mixtures on both carbies to no avail. Any one have a suggestion?
Thanks, Bill.
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Old 12-01-2017, 05:20 PM
Senior Member
Join Date: May 2016
Posts: 491

Check the compression first.... then check the spark.

With all spark plugs removed.........
Let us know what the compression PSI readings are on all cylinders.

Use a spark tester whereas you can set a air gap of 1/4" for the spark to jump/ The spark should be a strong blue lightning like flame... a real SNAP!... Is it?

(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

Note 1: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
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Old 12-03-2017, 07:41 PM
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Join Date: Apr 2017
Location: Tasmania Aus.
Posts: 4
Default Thank You

Thank you, will do as suggested.
Cheers, Bill.
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Old 12-03-2017, 07:45 PM
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Join Date: May 2016
Posts: 491
Default Rough Idle Suggestions

Originally Posted by Bounty Bill View Post
Thank you, will do as suggested.
Cheers, Bill.
Very good Bill. Keep me updated with the results. (Joe)
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