1986 Johnson 90 hp backfiring mystery

[Anyox]

I recently removed the powerhead on my 1986 90 hp to have a crack on the block welded shut, but now that I have put it all back together the motor is backfiring and I don't know why.

During removal of the powerhead, I did not disassemble the front end of the motor so the carbs were effectively not touched in the whole process. I did pull off the starboard side cylinder head because the crack was through the lower part of the water jacket to the outside and have since reinstalled the head with a new gasket. Otherwise I disconnected wires and hoses then hooked them back up in the original position (as far as I can tell). I have gapped the plugs for some reason the motor runs a little better when gapped to .032 instead of .030 like the manual says.

The motor will run, but it needs to be started with fuel for ~2000 - 2500 rpm where it will backfire every 3-5 seconds but when throttled up to higher rpm it runs smoother. When I lower the throttle the backfires kill the engine before I can get to true neutral to shift the engine into forward or reverse.

Any thoughts or ideas would be greatly appreciated. Thanks!

 

[petryshyn]

Re: 1986 Johnson 90 hp backfiring mystery

did you swap plug wires on that head by accident?

 

[Anyox]

Re: 1986 Johnson 90 hp backfiring mystery

I'm pretty sure I didn't, but how would I double check for sure? One is longer than the other so it seems that there is only one way it will fit. Those wires did get a little charred by a torch though....but that is a story of a seriously stubborn 5.25" bolt that I sheared off then didn't want to come free for way too many hours. Could a crispy wire be causing the problem?

 

[petryshyn]

Re: 1986 Johnson 90 hp backfiring mystery

also check the primary wires on your coils for being swapped as well.

with a little persuasion, plug wires or primary wires can be switched!




cheers.....

 

[fire7882]

Re: 1986 Johnson 90 hp backfiring mystery

I agree, sounds like some wires got crossed or timing is way off. Timing is most likely fine becaause it dosn't sound like you messed with it.

 

[Anyox]

Re: 1986 Johnson 90 hp backfiring mystery

I never intentionally adjusted the timing during the process. From the service manual it seems that the only real way to not have proper timing is to have shifted or obliterated the cam under the flywheel. I never removed the flywheel, but I did put a fair amount of lift on it when trying to remove the powerhead that inconveniently had a seriously stuck sheared bolt. Could just lifting from the flywheel cause a shift in the cam? If not...then it sounds like I need to go recheck the wires. How do I determine for sure which wire leads from the powerpack go to which cylinder?
Thanks!

 

[petryshyn]

Re: 1986 Johnson 90 hp backfiring mystery

fastest and easiest way is to connect a timing lite to #1 and observe where the flash occurs when attempting to start. If it flashes near the TDC mark, that bank is correct. If it flashes 180 deg from flywheel marks, you are swapped on that bank. If it is correct, simply swap the other banks wires and restart listening for backfires.

This saves looking up schematics and colors.


Cheers.....

 

[fire7882]

Re: 1986 Johnson 90 hp backfiring mystery

Could just lifting from the flywheel cause a shift in the cam? If not...then it sounds like I need to go recheck the wires. How do I determine for sure which wire leads from the powerpack go to which cylinder?
Thanks!

Its highly unlikely and I would even say impossible to change timing by lifting from the flywheel.

You can also start it and pull the plug wires one at a time to see if the motor runs different. If there is no change, that cylinder is not firing. Be careful with this method, you could get shocked so wear some thick gloves and pull the plug wire fast. You could also pull the wire from the power pack to coil if you motor is equipped with that type ignition.