Johnson 90HP - Wont rev to potential - WOT

[sixdeep]

J90TLETS
Will idle ok and seems to run well but when you go full throttle it doesnt seem to rev out all the way. Took it out when it was choppy yesterday and it was near impossible to get on the plane. would only rev to 3200rpm full throttle and pull about 12mph into the chop, riding the wave it would rev better and pull decent speed.

I pulled the plugs and 3 of the 4 were pretty oiled up, black but not caked on, the other was pretty clean. Pulled the intake cover off and it was pretty oiled in there - bluey normal clean colour. One of the carb butterflies wasnt fully seated closed and when full throttle seems a bit more tilted than the other. I have pulled this off and replaced it with a spare that looks better. (I had previously overhauled both the carbs and this one i had to get threads fixed and new bolts so may have messed up) It seems to be a bit rich but the manual i've been reading only really states how to alter the idle mixture.

Any thoughts on what else i can check and that should be done? I'm going to take it back out to test in the next day or so but any other ideas are greatly appreciated.

 

[emdsapmgr]

Re: Johnson 90HP - Wont rev to potential - WOT

If the carbs were recently removed, the linkage adjustment between the upper and lower carbs may be slightly off. The carbs have to both be fully closed when at idle-matched to each other. That's adjustable in the linkage. When at WOT, both carb butterflys should be parallel to the body of the carb, or flat. If the idle is off, they may also not be level with each other when at WOT. You'll have to pull the airbox cover off and visually check the carb linkage when you make the adjustment. The factory manual describes a "sync and link" procedure which will sync the carb butterfly opening with the advance of the spark. It's rather detailed and a factory manual does the best job of explaining this. One other thing. That engine will seem to run just fine on the trailer with 2 or 3 of the 4 spark plugs firing. You need all 4 firing once you get it in the lake and in gear, so check for spark on all 4 plugs next time out boating. Weak ignition components tend to fail once the engine heats up to normal operating temps, so make sure you fully warm it up when testing for spark.

 

[sixdeep]

Re: Johnson 90HP - Wont rev to potential - WOT

Thanks for the info, i forgot to add that i did the syncing part when i changed the carb. The link part i haven't done, i have read up on the timing but not attempted yet.

What is the best way to check for spark? The manual doesnt like simply removing the plug and then earthing it. I did regap all the plugs as they were slightly too close

 

[emdsapmgr]

Re: Johnson 90HP - Wont rev to potential - WOT

Use an inductive timing light to check for spark when it's cranking. The flashes from the gun will tell you a lot about the quality of spark.

 

[boobie]

Re: Johnson 90HP - Wont rev to potential - WOT

Your best bet for checking the spark is an open air gap spark tester set at 7/16 " open air gap. If it doesn't pass that, you've a problem.

 

[sixdeep]

Re: Johnson 90HP - Wont rev to potential - WOT

thanks, i dont think its spark given how wet it was inside the carb throats but im happy to test anything at this stage. looks like there's no adjustment of mixture on the carbs so im not sure how to resolve that if it is too rich? Hoping its just the carb syncing that ive fixed

 

[sixdeep]

Re: Johnson 90HP - Wont rev to potential - WOT

Took it out this afternoon after swapping the carb. It was much less choppy than last time. full throttle would pull a little over 3100rpm then it got up on the plane and away it went. Still seemed a bit less power than i would have expected but it got up and went to over 4200 and around 27mph. At 3600 odd rpm it was doing 22mph. Does that sound about right ?
Got it home and flushed the motor then pulled the plugs. number 1 still very clean compared to the other 3. Is it possible it's running on 3 and still performing as described above?
I got hold of a timing light so i'll check the spark when we take it out fishing on the w.e
Also checked the timing point yesterday and its already set correctly.

Appreciate peoples thoughts. I'm pretty happy with how it went with the carb replacement and a mate of mine that owned his own boat for a few years thought it went pretty well.

 

[sixdeep]

Re: Johnson 90HP - Wont rev to potential - WOT

Hey guys still after some info.
Should add the boat is a 16' fibreglass Sea Nymph Ranger. I think it weighs in the region of 1400 on the trailer so maybe 1200 alone ?

Ran a timing light on all the leads whilst running slow forward and it was ok but the clean plug cylinder (1) was hard to get a feed from. Swapped to brand new plugs and got some new leads. Took it out on a pretty flat day, full trim down and full throttle 3100rpm, once up on the plane trimmed back and to stay on the plane it has to run about 3600-3700rpm and thats about 21-23mph. Full throttle i was getting around 4000-4100 and 28mph. It all seemed ok but i pulled the plugs afterwards and number 1 cylinder plug is still clean as a whistle whilst the other 3 are black.

I'm pretty stumped as to why. Is it possible for the motor to run as good as it does on 3 cylinders? It doesnt sound like its missing or running with 1 down but i dont get why the plug looks so different if it is running the same as the others.

 

[emdsapmgr]

Re: Johnson 90HP - Wont rev to potential - WOT

#1 cyl sounds suspect. Why not do a cyl drop test? Run the engine on the trailer, in gear. Pull one spark plug wire of a plug with a pair of insulated pliers. Replace it, pull another plug wire off, etc. When each plugwire is disabled, the engine rpm's should go down-noticeably. If you pull one wire off and there is no rpm change, that cyl is not contributing to firing the engine. That engine will seem to run fine on 2 or 3 cyls on the trailer, till you get it under load in the lake. Then you need all 4 cyls contributing. Possible your spark to #1 is intermittent.