Johnson 90hp stalls at high speeds

[kermit606]

Recently purchased an 85 Checkmate Trimate I with a 4 cyl Johnson 90hp. Engine starts fine and runs at lower speeds fine but anytime I increase the speed too high (either slowly or by quickly jerking it forward) the engine stalls completely unless I immediately return it to idle speed. Was thinking it is a carb problem but I am much more proficient with cars than boat mechanics. Plug wires have a good bit of carbon on them but since it runs at low speeds ok didnt think this was the issue. Trying to avoid rebuilding the carb if I dont have to. Anyone heard of this problem before? Any ideas?

 

[jtexas]

Re: Johnson 90hp stalls at high speeds

so, you can't get it to run at full speed at all?

That's a classic symptom of clogged hi-speed jets. I highly recommend a compression test and spark test first. Spark should jump a 7/16" gap on an inline tester.

If you're familiar with automotive carbs, you're going to be surprised at how simple these are.

Welcome to the forum.

 

[kermit606]

Re: Johnson 90hp stalls at high speeds

Took your advice: Got spark on all 4 cylinders. 3 cylinders compressed at 120 psi however the 4th cylinder failed to compress at all and also the plug on that cylinder had the bridge completely pressed into the tip of the plug. The piston appears to be moving, Im assuming that there is a worn seal? Any ideas? Also do you think this would account for the lack of high speeds possible or might this be another problem on top of a clogged carburator? Is it ok to run it out on just the 3 cylinders in the mean time while I work or repairing the non-firing one?
Thanks for the help.

 

[jtexas]

Re: Johnson 90hp stalls at high speeds

sorry to hear it. the piston is contacting the spark plug. tear it down now, maybe just hone the cyl, or maybe one sleeve. Keep running it with everthing out of balance, it'll be a parts motor before you know it.

You do need to find the cause of the failure. Pulling the head is the first step.

Start a new thread with the words "rebuilding" and "engine" in the title, to get some mo betta minds than mine involved.

You can do it.