Johnson Ultra 4 question

[slowpoke47]

I have a Johnson Ultra 4, model #J4RLCCS, year 1988, owned since 1989. This is the 2-cylinder 4hp engine with the throttle operated by a slide lever on the top of the housing. It was barely used when I got it, and I stored it until 2000, when I began using it on rare occasions for auxiliary power on a sailboat. My best guess is that it has less than 100 hours total run time (I did say sporadic use). It has received regular maintenance, disregarding the minimal run time, on a yearly basis. When using it, we normally run it at about ? throttle or less, at a constant speed. Here's the oddity- on occasion, the engine speed increases by itself dramatically, at least several hundred rpm's, and holds this speed as though we had moved the throttle. When this happens, the throttle slide has not moved, but we can slow it back down with the slide. Typically, if this occurs, the engine has been running long enough to be fully warmed up. It always runs smoothly, at the set speed as well as the higher speed.

I have chased this for years. The OMC tech is at a loss. I can't duplicate it at home in a barrel, as I can't run it much above idle in the barrel. Due to the mounting setup on the boat, there is no way to diagnose it when it acts up. The carb and fuel pump have been serviced, and there have been two different fuel tanks and hoses used. The plugs are new. The throttle linkage is a cable from the slide control to a quadrant under the flywheel, and from the factory there is provision for some "free play" where the cable attaches to a bellcrank on the quadrant. A couple of years ago, I carefully cut and fitted a piece of aluminum plate to fill the triangular gap to eliminate any possible independent movement of the cable and the quadrant, which opens the throttle valve on the carb. Nothing has succeeded in correcting this intermittent problem.

So, I'm appealing to the collective wisdom on this forum- any suggestions?

 

[Rick.]

Re: Johnson Ultra 4 question

When it's running normal try pulling one plug at a time to see if it stalls out. I'm just guessing that maybe your running on one cylinder and every once and a while the second one kicks in and speeds up your RPM? Best of luck. Rick.

 

[Haffiman]

Re: Johnson Ultra 4 question

A couple of possibilities.
1: The engine starts running hot and leanes out.
2: Partly as above, but it might create a water mist leak in either exhaust or head gasket that works at 'water injection'.
3: You may be in a speed/load area where the engine may pick up rpm just by a slight increase of power due to some of the above.
4: You may have a 'siphon effect' occurring from your fuel tank if tank is above engine. A slightly leaking float valve might be the reason.
Change impeller/pump/
Pull down head and exhaust cover gaskets, clean all passages and install new gaskets.

 

[slowpoke47]

Re: Johnson Ultra 4 question

When it's running normal try pulling one plug at a time to see if it stalls out. I'm just guessing that maybe your running on one cylinder and every once and a while the second one kicks in and speeds up your RPM? Best of luck. Rick.

Interesting possibility- not sure how the engine sound would differ one cyl. vs. two (other than rpm, of course), as my wrenching, although extensive, has been auto/truck, 4-cycle multiple cyls. If this is in fact the issue, then 1 cyl does an impressive job of moving almost 2 tons of boat at a respectable clip! Can't troubleshoot on the boat because the outboard is basically out of reach while in use, and removing the engine cover is somewhat of a production. I may be able to beg the use of a test tank.

A couple of possibilities.
1: The engine starts running hot and leanes out.
2: Partly as above, but it might create a water mist leak in either exhaust or head gasket that works at 'water injection'.
3: You may be in a speed/load area where the engine may pick up rpm just by a slight increase of power due to some of the above.
4: You may have a 'siphon effect' occurring from your fuel tank if tank is above engine. A slightly leaking float valve might be the reason.
Change impeller/pump/
Pull down head and exhaust cover gaskets, clean all passages and install new gaskets.

Thanks for the feedback. I'll keep your suggestions in mind when I am able to work on the engine.