40hp johnson runs rough at low rpm

[kmercury]

I have a 40hp johnson that runs rough at low rpm. It shakes and jerks. At medium and high rmp, no problems.



Thoughts?

Thanks

 

[SparkieBoat]

Re: 40hp johnson runs rough at low rpm

clean your carbs..low speed passages clogged.

 

[kmercury]

Re: 40hp johnson runs rough at low rpm

Thanks... I took the filter screen out at the fuel pump and it had some crud in it. I am ordering a new screen and a complete carb kit... hope that helps.

Thanks

 

[tx1961whaler]

Re: 40hp johnson runs rough at low rpm

What year (model number) are we talking about here? The large twins are not exactly known as a smooth idler compared to a 3hp for instance. It shouldn't shake too much, but it isn't going to idle like a sewing machine.....

 

[1946Zephyr]

Re: 40hp johnson runs rough at low rpm

I would check the point gap and make sure both are set at .020. Points being out of adjustment can easily cause a rough idle. Your low speed should be set at 1 1/4 - 1 1/2 turns out. Running too rich can often make it run rough.

 

[kmercury]

Re: 40hp johnson runs rough at low rpm

It is a 1973 40ESL73D. It is shaking pretty bad.

What year (model number) are we talking about here? The large twins are not exactly known as a smooth idler compared to a 3hp for instance. It shouldn't shake too much, but it isn't going to idle like a sewing machine.....

 

[kmercury]

Re: 40hp johnson runs rough at low rpm

Ill try the low speed setting and make sure it is adjusted. If not, I will check the points. Is that gap still .020 on a 1973 40ESL73D?

Thanks

I would check the point gap and make sure both are set at .020. Points being out of adjustment can easily cause a rough idle. Your low speed should be set at 1 1/4 - 1 1/2 turns out. Running too rich can often make it run rough.

 

[1946Zephyr]

Re: 40hp johnson runs rough at low rpm

Yes, that motor would still have points and coils in it. They didn't convert, till a year or two later on those. Yep, youre motor is based on the old '57 RD design. Just has a few cosmetic differences and a fiberglass hood. Excellent old motors though. Probably about time for a tune up though

 

[kmercury]

Re: 40hp johnson runs rough at low rpm

Thanks for the help. Is that flywheel nut left hand thread and do you know what size bolts I will need to thread into the wheel for my puller?

Thanks again.....

 

[1946Zephyr]

Re: 40hp johnson runs rough at low rpm

Turn it CCW to remove. Thread your puller bolts no more than 7/16" into the flywheel and use grade 8 bolts.

 

[kfa4303]

Re: 40hp johnson runs rough at low rpm

These may be handy. If it's still rough after you clean and dial in the carb, you may only be running on one cylinder, which is not uncommon.

http://www.duckworksmagazine.com/04/s/columns/max/24/index.cfm

http://forums.iboats.com/showthread.php?t=167352