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1987 Johnson 90hp misfire

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Joycejh11

New member
I've got 120+ compression on all 4 cyl, just rebuilt the carbs, I've got spark on all four cylinders but I seem to be having a misfire on both port side cylinders. Top left plug looks like it's running normal but the bottom left looks brand new after an hour of being run. Drop test showed the bottom left plug misfiring and the top left was definitely firing but seems as if it stopped firing after my drop test. Not sure if it caused something to short...

I'm open to any suggestions, I'd love to have this boat out on the water this season.
 
Joycejh11 said:
I've got 120+ compression on all 4 cyl, just rebuilt the carbs, I've got spark on all four cylinders but I seem to be having a misfire on both port side cylinders. Top left plug looks like it's running normal but the bottom left looks brand new after an hour of being run. Drop test showed the bottom left plug misfiring and the top left was definitely firing but seems as if it stopped firing after my drop test. Not sure if it caused something to short...

I'm open to any suggestions, I'd love to have this boat out on the water this season.
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I've got it running and its idling. The drop test confirms all cylinders are firing but it doesn't appear to be on time. Going to hookup a timing light later today to confirm. Also the starboard side head appears to run 20 degrees cooler than the port. I imagine this is a thermostat sticking open. Might as well replace all of them. Any other diagnosis suggestions are welcome.
 
Make sure you use something to hold the timing plate so it does not advance by itself. You can get the timing procedure from the factory manual (a must have in my opinion) but I suggest you do a google search on the "Joe Reeves timing method" which allows you to do it without putting a load on the motor.
 
PhilSch said:
Make sure you use something to hold the timing plate so it does not advance by itself. You can get the timing procedure from the factory manual (a must have in my opinion) but I suggest you do a google search on the "Joe Reeves timing method" which allows you to do it without putting a load on the motor.
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why would the timing plate advance itself at idle? I am pretty much going to verify #1 spark plug is firing at tdc. Not sure if that’s the proper route but a timing light will allow me to make sure all the plugs are firing consistently at the least. I have a factory service manual and have read through it. I don’t have the test prop nor the tdc tool so I won’t be setting timing anytime soon. Still haven’t out ruled any other problems yet though.
 
It sounds fine to me....your flywheel is not out of time.

AGAIN...for the future, you do not need to hook up a timing light just to check flywheel indexing...a pencil will do that nicely and that procedure is described in the FACTORY manual.

Below is info on Joe Reeves timing method. The factory procedure requires putting a load on the motor at high RPM but Joe's method allow you to accurately time your motor when cranking. Key difference is you need to use 4 degrees less than the specified amount for your motor:

[FONT=&quot]NOTE-1: If your engine has the "Fast Start" feature", you must disconnect/eliminate that feature in order to use the following method. The "Fast Start" automatically advances the spark electronically when the engine first starts, dropping it to normal when the engine reaches a certain temperture.[/FONT]

[FONT=&quot]NOTE-2: A fellow member (Klink) from one of the various marine forums suggested having water supplied to the water pump (flushette or barrel) simply to provide lubrication to impeller. A worthwhile suggestion I thought, and entered here as an option.[/FONT]

[FONT=&quot]The full spark advance can be adjusted at cranking speed,"without" have the engine running as follows.[/FONT]

[FONT=&quot]To set the timing on that engine, have the s/plugs out, and have the throttle at full, set that timer base under the flywheel tight against the rubber stop on the end of the full spark timer advance stop screw (wire it against that stop if necessary).[/FONT]

[FONT=&quot]Rig up a spark tester on the #1 cylinder plug wire. Hook up the timing light to the #1 plug wire. Crank the engine over and set the spark advance to 4? less than what the engine calls for.[/FONT]

[FONT=&quot]It's a good idea to ground the other plug wires to avoid sparks that could ignite fuel that may shoot out of the plug holes. I've personally never grounded them out and have never encountered a problem but it could happen.[/FONT]

[FONT=&quot]I don't know the full spark advance setting your engine calls for, but to pick a figure, say your engine calls for 28?, set the timing at 24?. The reasoning for the 4? difference is that when the engine is actually running, due to the nature of the solid state ignition componets, the engine gains the extra 4?.[/FONT]

[FONT=&quot]If you set the engine to its true setting at cranking speed, when running it will advance beyond its limit by 4? which will set up pre-ignition causing guaranteed piston damage! You don't want that to take place.[/FONT]

[FONT=&quot]No need to be concerned about the idle timing as that will take care of itself. The main concern is the full advance setting.[/FONT]
 
Brand new plug after 1 hr of running means ----No spark.----Or no fuel.----Or too much fuel.----Or no crankcase compression.----Bad plug.-----Does spark jump a gap of 7/16" on all 4 leads ?
 
racerone said:
Brand new plug after 1 hr of running means ----No spark.----Or no fuel.----Or too much fuel.----Or no crankcase compression.----Bad plug.-----Does spark jump a gap of 7/16" on all 4 leads ?
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I haven’t hooked up a spark tester to it. I have grounded the plugs to the block and they seems to work fine. I’ve got it on an 18ft boat and should be pushing 35+mph but I’m only reaching 28-29. I’m sure pretty positive it’s due to that cylinder with the clean plug. Going to bench test my coils in a bit, I believe they’re fine though. I’ll order a spark tester.
 
Joycejh11 said:
I haven’t hooked up a spark tester to it. I have grounded the plugs to the block and they seems to work fine. I’ve got it on an 18ft boat and should be pushing 35+mph but I’m only reaching 28-29. I’m sure pretty positive it’s due to that cylinder with the clean plug. Going to bench test my coils in a bit, I believe they’re fine though. I’ll order a spark tester.
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Grounding a plug and seeing a spark is not an adequate test. Get a spark tester.
 
flyingscott said:
Nothing wrong with your motor you cannot diagnose anything on Muffs!!!!! Get it in te water.
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I had it out in the water last week. Wouldn’t break 29mph on my 18ft boat. Should be reaching around 40mph. Felt very low on torque. Ran it around the lake for about an hour then pulled it up on the trailer. 3 spark plugs looked normal and 1 (bottom port side) looked very clean, as if it was brand new. (I had just replaced the plugs and gapped at 0.040).
 
Not 100% positive but sounds like an intermittent misfire. Coils, spark plugs and spark plug wires are all capable of causing an intermittent misfires. Consider swapping coils/wire and plug (as a unit) between two cylinders and see if the problem moves. It likely will. If so then you can narrow it down to one of those 3 components.
 
Are the carbs opening all the way? Is the timer base moving until it hits te stop? What happens if you pump the primer ball while that is happening?
 
PhilSch said:
Not 100% positive but sounds like an intermittent misfire. Coils, spark plugs and spark plug wires are all capable of causing an intermittent misfires. Consider swapping coils/wire and plug (as a unit) between two cylinders and see if the problem moves. It likely will. If so then you can narrow it down to one of those 3 components.
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I’ll start by doing this. Thanks for the suggestion.
 
Took it out again today. Same thing, one plug super clean. I hit the choke at WOT and it made the rooms jump way up. Taking the carbs apart again. received_427131201924622.jpeg
 
Stop running this motor and look for the " smoking gun "-----If a carburetor is blocked ( high speed jet ) then you starve one cylinder of oil as well.-----Motor will go ---" boom " on you.
 
Took the carbs back apart and found a bit of paper towel from cleanup plugging a jet. Got it all back together and in the water. I'm making about 33mph on my 18ft boat. Sounds very good and gets up and goes good. I looked at the plugs and the port bottom side is still very clean as if it hasn't been run... It looked a bit wet, cylinder has 115+ compression. I couldn't imagine it could be a head gasket leak with those compression numbers. Any thoughts?received_427131201924622.jpeg
 
Glad to hear it's running good now. I would run it some more before making any judgements about the spark plug. Might just take a little more time for the plug to show signs of combustion.
 
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