Johnson 15hp two stroke

[shanemax]

Hi. My Johnson will run at full revs with out missing a beat but back it off to half revs and it misses quite badly, its doesnt actually stop but feels like it is about to. Has any one had this themselves. I am bringing it home to work on so any suggestions would be welcome.
Johnson two stroke 15hp.

 

[chris-s]

Missing sounds like an electrical issue. I’d start with checking the plugs, swap them out with some shiny new ones and see what happens from there. Whilst you are doing that, give the caps, leads and other electrical bits a check over to look for anything obvious.

 

[Chiara’s slave]

Missing sounds like an electrical issue. I’d start with checking the plugs, swap them out with some shiny new ones and see what happens from there. Whilst you are doing that, give the caps, leads and other electrical bits a check over to look for anything obvious.

I beg to differ. An electrical issue is likely to affect the whole rev range. Cleaning the carburettor might be more beneficial. A part throttle fuelling issue seems a good explanation.

 

[Refueler]

mmmmm it could be ignition ... at full revs - the ignition may be energised well ... but back off the revs and then it could be failing ignition .... I had a Mercury that did this years ago ... then suddenly one ignition unit failed and engine would not start. Stupid idea linking both coils ... but that was their design.

I know this sounds daft - but are you sure BOTH cylinders are working ? If you are at full throttle - it may seem you have both - but at that revs could be hard to tell. Back of the throttle and that failing cylinder will soon show itself ..

Just thinking out loud.

 

[Refueler]

I beg to differ. An electrical issue is likely to affect the whole rev range. Cleaning the carburettor might be more beneficial. A part throttle fuelling issue seems a good explanation.

Generally I would agree ... but having had a Mercury show similar - then one ignition coil failed ....

 

[Chiara’s slave]

Generally I would agree ... but having had a Mercury show similar - then one ignition coil failed ....

Sure, diagnosing engine problems on a forum is educated guesswork at best. However, were it to be the case that the OP couldn’t tell if his motor was behaving normally at full revs, I’d respectfully suggest he left the job to a dealer.

 

[billskip]

I’d respectfully suggest he left the job to a dealer.

Good advice, but OP may be competent.
I would certainly put new plugs first especially as its 2 stroke....I've had this before on a Mercury ..

 

[Chiara’s slave]

Good advice, but OP may be competent.
I would certainly put new plugs first especially as its 2 stroke....I've had this before on a Mercury ..

Like I said, if he couldn’t tell if it was giving full power. New plugs are always a good idea with 2 strokes, but again, are not otherwise indicated by a mid range misfire. The OP seems pretty clear about his problem, I have experienced the same with several types of engine. Runs fine at WOT, misfires at lower revs and throttle.

 

[VicS]

Hi. My Johnson will run at full revs with out missing a beat but back it off to half revs and it misses quite badly, its doesnt actually stop but feels like it is about to. Has any one had this themselves. I am bringing it home to work on so any suggestions would be welcome.
Johnson two stroke 15hp.

You don't quote the model number ( or the year of manufacture) so nobody knows exactly what we are dealing with!

Have you:
tried new plugs (the correct type will depend upon age),
checked that it is firing on both cylinders,
checked the strength of the sparks with an adjustable air gap spark tester,
measured the compression on both cylinders ?

Does the idle mixture setting have any effect?

Have you:
cleaned the carb paying particular attention to the slow running pathway,
if it is a model with a plastic top to the carb checked that for cracks ?

If an old model ( 1976 or earlier) it will have points ignition so look at points and inspect the coils especially the driver coils under the flywheel.
Information on testing the ignition system can be found in the CDI Troubleshooting Guide but special equipment ( at least a peak reading voltmeter or a dva adapter for an ordinary multimeter ) is required.

You will find just about everything you are likely to need to know about Johnson/Evinrude 15 hp models In LeeRoy's Ramblings at OMC outboard related articles

.

 

[geem]

Mid range misfire almost certainly a fuel issue. Dirty carb. Plugs tend to fail at higher revs. Ditto coils.