One cylinder not firing properly

[azr]

My 78 1000. So it seemed like one of my dyna coils failed after only 2 yrs. Primary tests fine but the secondary is high at 25K when it's supposed to be around 15K. I threw in an old set of coils that seemed to test fine (primary around 4 and secondary around 15K..I know everyone says it's supposed to be around 25K but I have 6 OEM coils and I tested them all and they all show around 15K so I figure what are the odds of all of them being low?, correct me on this if I'm wrong please). I tested all the caps (which are new) and they are all on spec. What is happening is my #2 cylinder is not firing unless I pull off the cap slightly to allow it to arc across onto the plug, if I push the cap on all the way the cylinder stops firing??? for the life of me I can't figure out what is wrong. Any thoughts. #3 is firing fine as are the others.

[Redman]

...
...... What is happening is my #2 cylinder is not firing unless I pull off the cap slightly to allow it to arc across onto the plug, if I push the cap on all the way the cylinder stops firing??? ....
...

Interesting that you discovered this. I woulnd never have thought to try pulling the cap part way off.
But I am no ignition expert at all.

I can not explain it either, but seems like the solution is either a new plug or a new cap. Maybe do some swap-sta-tution to determine if is the plug or the cap that causes this to happen.

<< LAT#ER NOTE: I suspect the plug. Maybe there is a crack in the porcellin, and when cap is all the way on the spark can jump to the crack and then to ground somewhere. And with the cap pulled back away fropm the tip the spark jumps to the plug tip and then the plug gap beacause maybe the crack is too far away. But anyway, just try swapping one plug for the other, or one cap for the other.

[Counter]

Is there a step by step diagnostic anywhere I can use to diagnose a dead cylinder? I just rebuilt my carbs and the #4 cylinder is dead as a doornail... I am going to open the drain to make sure that gas is getting to the bowl but the motor is too hot to do it now.

I think it may have happened late last year or even during the winter while it was down (not being ridden).

I'd really like not to have to take the carbs off again and open up #4.

[BassCliff]

Hi,

There are troubleshooting procedures in the Electrical Odds and Ends section of my little website as well as in your manual too.


Thank you for your indulgence,

BassCliff

[azr]

Well I swapped out for another coil with no improvement, changed the caps no improvement, replaced the spark plug with spares I had, no improvement, then replace the spark plug with a new one and bingo I have combustion. I haven't examined the old plug with a magnifying glass yet but your theory 'Redman' sounds plausible. Always bothers me when I can't figure out the 'ya but why?'. Thanks for the help.

[BigD_83]

Well I swapped out for another coil with no improvement, changed the caps no improvement, replaced the spark plug with spares I had, no improvement, then replace the spark plug with a new one and bingo I have combustion. I haven't examined the old plug with a magnifying glass yet but your theory 'Redman' sounds plausible. Always bothers me when I can't figure out the 'ya but why?'. Thanks for the help.

...and I'm now going through a similar problem. Last night I had the engine running, but had no spark on #1 cylinder. I could not get the GS650G to fire up this evening so I put the battery back on the charger.

After a full charge, I tried pulling the caps off partially, and my #s 1 and 4 cylinders started firing again. They had been deader than dead since last night, and this evening I managed to get the rpms up again and the engine idling better just by pulling the caps up slightly so they weren't seating on the plug.

I'll try switching plugs when I replace the new plug socket I bought today for that purpose. Turns out that the plugs (NGK DR8EA) currently installed fit neither a 5/8 or 13/16th hex, but are 18mm, which was counter to the catalog information at NAPA

What would be the consequences of the plug caps being on the wrong plugs? The two sides didn't match up in that the plug caps were in opposite positions on the left vs the right (i.e. the longer/straighter of the two was on cylinder #1 on the left)