1981 90 HP Johnson - H-E-L-P !!!

[metriccrescentwrench]

My recently purchased boat has a 1981 90 HP Johnson. I know nothing of this motor's history, (and of course the previous owner had zero problems with it). The first time we took the boat out it ran great, no problems at all. Later that same day it seemed to be a bit rough at partial throttle, and woudlnt idle when putting the boat on the trailer. Checked the plugs (NGK) and they were fouled so I replaced them with the recommended Champions.

Next weekend the engine wouldn't start. It would occasionally run for a few seconds and die, and backfired a couple of times while cranking.

Checked compression (cold), 125, 125, 122, 121.

Spark looked good on all 4 cylinders (checked by observing a spark plug resting against bare metal on the engine - I don't have a spark tester).

Removed carbs and pulled float bowls. Everything looked clean as a whistle, no trace of gum or deposits. Sprayed carb cleaner through all jets etc., everything seemed normal.

Replaced fuel line on fuel pump inlet, outlet, and both lines to carbs. Old fuel hose had a few minor surface cracks.

Rebuilt fuel pump. Old diaphragm looked somewhate stretched but had no holes or tears. Fuel flow looks somewhat stronger now.

Drained tank, pulled strainer. Everything was perfectly clean with no sign of sediment or dirt. Put fresh fuel in tank.

Now (with muffs and water hose) it'll start and run for maybe 15 seconds before quitting.

I'm quickly running out of ideas. Any suggestions are appreciated.

 

[HighTrim]

Re: 1981 90 HP Johnson - H-E-L-P !!!

I would reccomend getting yourself an inline spark tester, they are so cheap nowadays, and very important in outboard testing. Your spark should jump a good 7/16 of an inch with a strong blue ZAP!! Have you tried taking the fuel line off and running it into a can while turning it over to ensure you are getting fuel? Will pumping the primer bulb keep it running?

 

[tashasdaddy]

Re: 1981 90 HP Johnson - H-E-L-P !!!

back fire is a sign that the woodruff key in the flywheel is sheared, and the flywheel has moved on the crank shaft, just the slightest. throwing the timing off. if the previous owner had, had the flywheel of and not torqued properly this is highly likely.

 

[HighTrim]

Re: 1981 90 HP Johnson - H-E-L-P !!!

I agree with TD, also make sure when you installed the new plugs that you didnt cross the leads in any way, and the plugs are torqued to spec, 18.5 to 20.5 ft/lbs.

 

[commonarcwelder]

Re: 1981 90 HP Johnson - H-E-L-P !!!

I agree. Seen it many times. Gap plugs at .30 .

 

[metriccrescentwrench]

Re: 1981 90 HP Johnson - H-E-L-P !!!

back fire is a sign that the woodruff key in the flywheel is sheared.....

Ahhh, didn't think of checking that. Many years ago I raced go-carts and had that happen, I forgot all about it.

I don't have the manual in front of me right now, will I need a puller to get the flywheel off or should it come off fairly readily?

(And I'm absolutely positive beyond a shadow of a doubt that the plug wires are where they belong.)

 

[HighTrim]

Re: 1981 90 HP Johnson - H-E-L-P !!!

quality puller with grade 8 bolts.