1992 Johnson 48 SPL high rev dieseling

I picked up a skiff with a 92' Johnson 48 spl 2-stroke. Motor hasn't been started in 5 years. Serviced the lower end and plugs. Was able to start it and it ran fine. Ran the engine for about two hours at at different speeds not exceeding 2500 RPM. Check thermostat operation runs below 140 deg.

Now here is when it gets interesting, when I exceeded 3k rpms the engine went into free running at a very high rpm. Throttling down to idle had no impact and neither shutting the key switch or pulling the safety lanyard. I ran back to the outboard and yanked both spark plugs which did not do anything. The motor just kept running at a very high rpm.....I thought the motor was gonna blow. I then yanked the fuel line out and loosened the cover for the fuel pump to bleed the remaining fuel....it just kept running. I then went to carbs and tried to block the air intakes no luck. I then moved the lever that controls the choke plates and that killed the engine.

Fortunately no catastrophic damage, but definitely unnerving.... What if I was out on the water and this happens....scary!

Any thoughts on where to look.....appreciate any feedback!

Re: 1992 Johnson 48 SPL high rev dieseling

Those runaways are freaky. I normally pull the fuel line and dive behind the other side of the house, i dont hang around loosening covers.

Were you running it on muffs at the time?

Re: 1992 Johnson 48 SPL high rev dieseling

Was running engine in barrel of water....if I was on the muffs good chance I may toast the impeller and possible heat damage the motor....after this episode motor seems to run good at low rpms....I am getting a little nervous to test it at higher rpms...lol.


I think tomorrow I will remove the carbs and clean them and then check the timing. I am baffled on this.

Re: 1992 Johnson 48 SPL high rev dieseling

Were you supplying new water to your barrel while it was running?

If not, the motor will dump heat into the cooling water and will eventually overheat the motor.

Obviously the horn didn't go off, so that needs to be checked.

Regardless, check compression and do a good decarb.

Re: 1992 Johnson 48 SPL high rev dieseling

I agree with Dazk, also double check that the water level when in the barrel is at least four inches or so above where the LU separates from the leg.
Any lower and it will starve for water as there is no forward movement of the engine to throw water up the inlet

Re: 1992 Johnson 48 SPL high rev dieseling

No problem on the water level and temp....even though I ran it for a total of two hours....it was probably 20 minutes at a time. The hottest it got was lukewarm and coolant temp never exceeded 140 deg.

Looking at some other threads on two strokes on free running...a piece of carbon in the cylinder maybe igniting and providing an ignition source. I still don't get how the carbs are feeding it fuel for WOT when they are set for idle position. It's either the carbon or a air leak in the carb gasket or some other leak in the system. I checked the spark plug color and it looks normal (brown color).

I think I will start over with the basics and do a compression check and then run a can of sea-foam for the decarbing.

Re: 1992 Johnson 48 SPL high rev dieseling

Update...found the carb gaskets were leaking replaced and it seems to working now. Throttle response seems to be a lot better and varying idle speed I was having before went away.