Johnson 90HP 1985 - Firing on 3 cylinders

[phlipwright]

Hi, I hope someone can advise me.

My 1985 90HP has new leads and new plugs.
Carb recently serviced by local marine company (June)

Last week the engine developed a fault whereby it would only just manage to idle, and stall as soon as put in gear. Engine sounds very rough as if one cylinder is not firing.

Today I took out the plugs. Alarmingly, one plug's electrode is bent flat against the anode. I'm sure it wasn't like that when I put it in! The deposit was dry, black and sooty.

Also, the top of the piston in this cylinder is bone dry. (no sign of a plug impact though) The other three cylinders are slightly moist.

Does this suggest a problem with a valve? Or is there another explanation?

Thanks in advance

Phil

 

[ezeke]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

Are the spark plugs the recommended ones? Do you have the compression rings on all of the plugs? Is there a helicoil in the cylinder head with the bad plug?

Take the plugs out and use a dowel to feet the piston as you turn the flywheel; the piston should move back and forth through a complete rotation of the flywheel with very little hesitation.

 

[phlipwright]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

The plugs are the correct ones and have compression rings. Helicoil - is that like a valve spring? I can't see any coil or fragments of coil in the cylinder.
The piston moves up and down smoothly.

I re-set the gap on the affected plug and put it in. I ran the engine for a few mins (still wouldn't idle though and was still running rough) I then checked the plug again and the gap was as it should be.

Phil

 

[ezeke]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

A heli-coil is an insert used when the threads on the spark plug hole have been stripped. Something hit the spark plug, maybe it was dropped by your mechanic. If the insulator is cracked, you will not be able to see it, but the plug will never work correctly. I would get a new spark plug.

If you get an aerosol de-carbon treatment and spray it in the spark plug hole, you can get rid of the carbon that built up while that plug was not firing. Deep Creep or Mercury Power Tune are good ones.

BTW, your engine does not have valves.

 

[phlipwright]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

Ah, I see. No, there is no helicoil.
I'll replace that plug tomorrow and do the de-carb and let you know what happens.
No valves? That's a relief
Thank you for your help

 

[phlipwright]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

I replaced all four plugs and used the de-carb but no improvement.
Although the carb was recently serviced, should I check the jets?

 

[ezeke]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

If you are getting spark on that cylinder, then fuel may be the problem. What are the numbers from your compression test?

Did you check spark with a tester? You need to rig a tester that can allow the spark to jump a 7/16 gap. There are lots of ways to do that, some as simple as a board and nails. http://www.iboats.com/Marine_Store/...*********.910499997--submit.x.32--submit.y.10

Here is a link to Bill Kelly's site that may help with your crossflow engine:
http://www.maxrules.com/fixtheory2.html

 

[phlipwright]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

I haven't any compression test info.
I'll check out those sites - Thank you for that.

 

[Dennisanoka]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

I am curious why you replaced the plugs and plug wires to begin with. Was it "just because" or were you suspecting that you had an issue. Did it run well right before you changed them?

 

[phlipwright]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

Hi,
I originally replaced all four leads because one of the old ones was faulty. I replaced the plugs first time because the engine was new to me and I wanted to start with a fresh set.

Still having issues with the engine. The spark seems healthy enough on all four leads.
A friend listened to the engine struggling to run and suspected water in the fuel. So my next step is to drain the fuel lines and try again with new fuel and maybe a new tank & line

 

[phlipwright]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

I have made some progress!
I put in new fuel mix and also used carb cleaner on the carb.
I managed to get the engine running nicely at high revs (before I couldn't get the revs very high at all)
Idling is still a problem though. The engine will stall when I put into gear unless I very quickly move the throttle to a high setting. I took the boat out and ran her at high/full throttle for a while and she ran well.
Coming into the dock the engine stalled on idle again.
Does this indicate a problem with the low-speed jets. If so, can someone please tell me how to access this jet.
Thank you

 

[iwombat]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

I think it's time to get an OEM manual for your engine and do a proper carb rebuild.

When I latch on to a new motor I do a carb rebuild and change the impeller w/o so much as thinking about it. Both are easy jobs and well worth the piece of mind for $$ spent.

 

[phlipwright]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

Hi Wombat,

I think you're right. The annoying thing is the carb was recently serviced and I've only been on the water 3 or 4 times since!

 

[iwombat]

Re: Johnson 90HP 1985 - Firing on 3 cylinders

If it's been sitting awhile it's possible that crud from the fuel lines or fuel pump made its way into the carb and plugged it all up again. You mentioned bad gas also. Do you have a fuel separator installed? If not, that could be the source of clogs. Could also be crud from an old filter.