top cylinder not firing 1976 75 johnson

[75-JOHNSON]

all three cylinders have spark, timing is correct,110psi on all 3, can run on middle and bottom cylinders fine. switched the #1 and #3 carbs and the top cylinder still wont fire, leads me to beleive its not a carb issue. just backfires and sputters. service manual is on its way but does anyone have any advice as to what might be wrong? something with the reed valves??

 

[wilde1j]

Re: top cylinder not firing 1976 75 johnson

Backfire suggests timing is wrong due to plug wires mixed up, sheared flywheel key, someone messed with timing, etc.

Coil may be bad as well for no fire hole.

 

[75-JOHNSON]

Re: top cylinder not firing 1976 75 johnson

alright. i checked the timing of the top cylinder with the engine idling and the light flashes just before tdc. is this what is suppossed to happen?

 

[wilde1j]

Re: top cylinder not firing 1976 75 johnson

How could timing light fire if hole is dead?

Check all cylinders with a needle gap spark tester ... should get a strong blue spark over 7/16" gap if OK.

Are you sure it's backfiring and not just coughing?

 

[75-JOHNSON]

Re: top cylinder not firing 1976 75 johnson

o you could be right about it coughing and not backfiring. it just makes terrible noises. when i said the top cylinder wont fire, i ment that it would not burn fuel and run correct, but it DOES have spark.

 

[wilde1j]

Re: top cylinder not firing 1976 75 johnson

But maybe weak ... Test that coil with a needle gap tester to eliminate that possibility.

 

[75-JOHNSON]

Re: top cylinder not firing 1976 75 johnson

alright im still looking around town for a needle gap tester but to test the coil could i switch the coil on the top cylinder with one on either the middle or bottom, both of which seem to be working fine? or is that not a good idea

 

[wilde1j]

Re: top cylinder not firing 1976 75 johnson

Like this:

http://www.sjdiscounttools.com/the404.html

Switching coils should be fine. If problem moves, you have the fix nailed.

 

[75-JOHNSON]

Re: top cylinder not firing 1976 75 johnson

So I used a needle gap spark tester andf I can get a spark over 7/16. I also switched the coils between a working cylinder and the top one and the problem still remains at the top cylinder....my book should be here any day but could this be a power pack or some other electrical problem down the line?

 

[wilde1j]

Re: top cylinder not firing 1976 75 johnson

CDI/Rapair website has a good ignition trouble shooting guide which is a free download.