Double Spark (?) at Idle

[JoeAyers]

I have a 1977 Johnson 140hp Model# 140TL77S

Compression = 132, 135, 135, 138

THe motor is on a 16' Silverline Grenada Closed Bow Runabout. From about 2K RPMs through full throttle it runs like a scalded dog. (44-50 MPH) Sounds great, runs great, stays cool (since replacing the thermostats). My problem is if I set idle where it is supposed to be ~850RPMs the motor will run for a little bit then stalls. It sounds like there is a small cough like the plug is double firing once at TDC and once at bottom. If I turn up the idle to about 1300-1500 RPMs the cough is still there but it will stay running. Where should I start? What should I look @ first?

I checked the carb and it is clean and operating as expected.

Thanks for any help you could provide.

Thanks,
Joe

 

[metriccrescentwrench]

Re: Double Spark (?) at Idle

You didn't say if you've checked the timing or not. As a first step you really need to get the timing and carb linkages set properly, the factory manual will have a step by step procedure for doing that. If that doesn't help then invest in carb rebuild kits.

 

[jonesg]

Re: Double Spark (?) at Idle

I would do a compression test, then a spark check , then rebuild carbs with new kits and link / sync.
Somewhere in all that you should find the problem.
If its new to you then do everything else, change gear oil etc.

 

[watergecko]

Re: Double Spark (?) at Idle

Can't say I kno what a scalded dog looks like, don't think I wanna either. Lol.

 

[blimp]

Re: Double Spark (?) at Idle

sounds like a lean sneeze. try covering the carb throats with a (clean) shop rag one by one. This will effectively choke, or richen, the mixture. If the idle evens out, you have found your offending cylinder. I would guess there is still some blockage of the idle circuit, or maybe an airleak.

 

[daselbee]

Re: Double Spark (?) at Idle

What blimp said. Lean sneeze. Locate which cylinder by using a suitable method to choke it down ONE CYL AT A TIME. You can't use the choke butterflies, cause they do two cyls at a time. You must use a rag or two fingers down the carb throat to block some air flow, thus enriching the mix. Take your time, and you will eventually find one cyl that smooths out when richened. Concentrate on the carb for that cyl, looking for varnish, dirt, debris in the idle circuit passages. Do this testing in the water, or in a large tank to simulate backpressure. I would not do this on muffs.

 

[JoeAyers]

Re: Double Spark (?) at Idle

Thanks for all of the replies.

I checked the idle timing. If I set it within spec it coughs and dies. If I set it high 6-8* ATDC then it coughs but stays running. I've done the carb linkage according to the manual and it seems okay.

Looks like I'll do a rebuild on the carb and check for air leaks as soon as the temp gets above freezing. I water tested the new water pump/thermostats on Saturday in a snow storm, but it was about 35*. It's been highs of 20 the last week.

Thanks again for all the help.