Topic: HELP cb750 Cylinder 1 firing intermitent

[vinny9744]

I am having a very strange problem. I have been bring a 73 CB750 back to life for around a year. This is my latest issue. Cylinder 1 is not firing all of the time. I noticed this when I hooked up a timing light to the wire and got no flash. This is the only cylinder this does this with. The weird thing is if the motor is running and i pull the wire off with the timing light attached it flashes. However when I put it back on the plug it stops. Pipe is cold / colder. Plug is sooty.
Here's is what I have done. (Not all for this reason though)

Tappets checked.
Timing checked, (Dyna S) set to full advance.
Carbs rebuilt with new parts. correct float height and synced with gauges
New NGK plugs and wires.
K&N airfilter clean
New carb boots, manifold boots and bands (no leaks)
New Petcock new fuel line
New Battery
The bike has Dyna 5 ohm coils installed from the PO which I thought may be the problem. They look  ancient.
I cleaned and changed the plugs. Switched the plug wires even switched the wiring to the coils.

Then something really strange happened, all of the cylinders except  #4 cut off, no spark. As I stood there shaking my head I waited a minute and tried again. Now 2 3 and 4 are firing but one only fires when the wire is pulled and held  near the plug. It seems to me there is a short somewhere but I have no clue when it comes to electrical stuff.  I checked the connections and didn't see anything too unusual. Please help.

[Duke McDukiedook]

Possible short in the coil, did you test the questionable coil with an ohmeter?

Is that spark plug OK?

[vinny9744]

No not real sure how to do that, I did just buy an multimeter though.
This is what I did as far as trying to isolate it. First I exchanged plug, then plug wire, then position on coil. All attempts met with no change . I changed the wiring to the coil and moved all the wires to the corresponding cylinders. No difference. The wierd thing is that with the motor running and a timing light attached to the #1 wire, when I pull the #1 wire and just sltightly re-attach it to the plug in the cylinder, I get light. I dont notice any difference in how the motor runs. It actually runs pretty good. cant wait to see how it goes on all cylinder.

Any tips on how to test the coil or where else to look? I am really lost.

[Duke McDukiedook]

Service manual located in the faq section for download tells you how to test it and what the readings should be.

Download it and read a little before you ask any more questions.