1976 johnson outboad 40hp only goes 12 mph

teelaw64 · May 9, 2011

cant get over 12 mph had carb cleaned and soaked starts and idles good

Joe Reeves · May 9, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

Check compression. What are the psi readings?

With s/plugs removed, check the spark which should jump a 3/8" gap with a strong blue lightning like flame... a real SNAP! Does it?

If you're not using Champion UL81J s/plugs gapped at .030, try them. If memory serves me correctly, that's what the engine calls for as per a OMC bulletin.

With engine NOT running but in gear, at full throttle, check to see that the throttle butterfly is opened fully (horizontal).

If you didn't manually clean the high speed jet that is located in the bottom center portion of the float chamber, do so with a piece of single strand steel wire.

Let us know what you find.

F_R · May 9, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

It is running on one cylinder. Two most likely causes are no spark on one and/or no compression on one.

crablegs · May 9, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

What size boat?

teelaw64 · May 10, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

hi the compression readings are 115 on both cyl. i had spark friday i had it running in lake now i cant get it started looks like no spark i used spark plug tester motor calls for l4j thats whats on the inside cover but there useing b5hs this on 16' deep-v alum. the carb butterfly stays open horizonal when at full throttle

Joe Reeves · May 10, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

The L4J plug was superseded up to the Champion UL81J that I mentioned previously.

F_R · May 10, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

No matter what plug, it won't run if it has no spark. You gotta get into the ignition system and see what's wrong. That motor has the Low-Tension magneto. Probably dirty/worn/contaminated breaker points

teelaw64 · May 10, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

yes it has spark i had the off swich off so good compression and spark i will change sparkplugs i put seafoam in tank seems like it wants to take of then backs down could a weak impeller cause motor not to run full speed

Tim Frank · May 10, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

Don't think so.
If you have any significant cooling problem, it unfortunately doesn't usually show itself until there is some major damage.

Not likely here, but if you aren't sure when the impeller was changed last, it never hurts to R&R.

Are you getting a good spark now...jump a 3/8" gap?
That plug (B5HS) is really hot....are you sure that is a reasonable x-ref?
Better to use the Champion.

metriccrescentwrench · May 10, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

seems like it wants to take of then backs down

Could be a fuel delivery problem. Check the tank vent. Check to see if primer bulb is collapsing. See if pumping the primer bulb makes it run better.

could a weak impeller cause motor not to run full speed

No, but a bad impeller could make the engine not run at all

(after it's been toasted to a crisp).

emoney · May 10, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

Coils?

Joe Reeves · May 10, 2011

Re: 1976 johnson outboad 40hp only goes 12 mph

As per my original reply "If you didn't manually clean the high speed jet that is located in the bottom center portion of the float chamber, do so with a piece of single strand steel wire."

If you're not absolutely sure that this jet is clean, do the above procedure. It may not be the cause of your problem BUT it is one of the usual causes.

1976 johnson outboad 40hp only goes 12 mph

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