Yet another coil thread

I have spent a little time reading up on, and testing my coils today. Bike would run great until it was warm, then the longer it ran, the worse it would misfire.

Both primaries measured about 3 ohms

Coil for cylinders 1 and 4 measures 20 on the 200k range
Coil for cylinders 2 and 3 measures 60-63k on the 200k range.

Plugs 1-4 in order



I had cleaned up the 1st plug, and swapped it with the second one. This is after about 20 min of riding. I am fairly certain this is a coil issue, but just wanted a bit of feedback since I can't afford a $130 guess.

 

Which bike? Idk what all you've done in the resto process but imo it's out of synch and the cylinders are working against each other versus together. Are the valves in spec?Are you testing to (in-put) and through (out-put) on the coils? Have you swapped the coils and re-tested? How old are those plugs? Are those plugs gapped correctly? Are the plug wires making good contact to the caps? Are the caps correct? Are the resistors in the caps screwed in tight? Hope this helps.

Gary H.

 

Then that coil would appear to be bad. My 750 had a hell of a time starting for a while, then I discovered that one of the secondaries measured out at 80K or thereabouts.

Did you measure the secondaries with the plug caps installed on the wires? That would explain the 20K on the other coil. Stock coils should measure out at 11K on the secondary without the caps.

You might want to consider getting ahold of Chacal about swapping in a set of Dynas. That's what I did with mine, and they work great.

As for how that's done... Installing Dyna Coils

 

Yeah I measured them all with the caps on. I will likely pickup a used coil from someone. This engine might not be long for this world anyway.

 

wouldn't it seem that if 1 & 2 weren't firing they would come out wet ? 3 & 4 are so lean, i'd suspect a vacuum leak. but those plugs are so old i wouldn't trust reading them at all
buy some plugs

 

The #1plug was initially the 2nd plug. I cleaned it up and swapped them. The other three were all the same at the time.

 

are you sure this isn't carb 2 running rich from a stuck float? a quick float level check will tell the true story. oh, and how's that oil level?

CN

 

Because of the waste spark system, the plugs are wired with pairs in series. 1+4 are wired together and 2+3 are wired together. Because of that, electrical caused misfires (like a coil problem) can run in pairs, but those pairs wouldn't be 1+2, they would be 1+4.

In theory, if cylinder 2 is sparking, then cylinder 3 must be sparking as well. There are some failure modes by which that's not the case (like jumping from the plug boot around the plug to ground), but I would consider them to be less likely.

The converse is also true... In theory, if cylinder 2 is misfiring due to an electrical issue, then cylinder 3 must be misfiring as well.

The point is, if you've got a misfire on cylinder #2 only and NOT on #3, then I'd focus on fuel, not spark. It could be a corner case electrical issue, but I think it would be unlikely.