Sluggish 40 hp Johnson

[Echoman]

I got a 1970 40 HP Johnson from my neighbour. I rebuilt the carb using a kit. The idle and low speed works GREAT ! but the high speed jet is a fixed one and the high speed is sluggish. If the carb had an adjustable High speed jet, I think it would run better if I could lean it out. I have washed it out with lacquer thinner and blown it out with compressed air. It looks really clean.

The compression is OK.

Does anyone know if you can replace a carb having a fixed high speed jet with a carb that has an adjustable one?

 

[DJ]

Re: Sluggish 40 hp Johnson

You may also be losing spark on one cylinder. Did you look at the coils?

 

[tashasdaddy]

Re: Sluggish 40 hp Johnson

did you remove the high speed jet and clean it? http://forums.iboats.com/showthread.php?t=244013

 

[F_R]

Re: Sluggish 40 hp Johnson

If it truly is running rich, somebody may have been trying to soup it up by drilling out the fixed high speed jet orfice. If that is the case, you will have to replace the jet, not the whole carburetor.

How sluggish is sluggish? Like half speed? If so, it probably is running on one cylinder as DJ has already suggested. Dinking around with the carburetor won't fix that. Have you check compression? If that is ok and equal, move on to the ignition system (don't ignore the spark plugs).

EDIT: One last thought; that motor has a hot-air choke that is heated by air drawn through a heat exchanger in the exhaust. If that heat exchanger burns out --and they do--, it will draw exhaust gasses to the carburetor, making it run lousy. Another thing, the air intake for the heat exchanger is a threaded hole a few inches below the heat pipe. Some people seem to think that hole should be plugged up. Not so. It needs to be open or it will run rich due to not getting the air that normally flows through that circuit.

One last-last thought: Does it run better with the hood off or door open? That is due to an exhaust leak because of the o-ring missing off the top of the water pump housing.

Now that you know all the odd quirks of a Johnson 40, go find out what is really wrong with it.

 

[Echoman]

Re: Sluggish 40 hp Johnson

Thanks for the replies. The coils have been replaced and the compression is OK and approximately equal. I wasn't able to remove the high speed jet from the carb, but I might try to find a rebuilt one.

I'll also the hot air choke as mentioned, it's not something I'd even considered.