Running on two cylinders

[bellison80]

Hi guys,

So the bike has been running well for some time now. I have been mostly commuting back and forth to work from one side of town to the other. Yesterday I went for a ride after work and headed out towards the country for some fun. I took it up through the gears on a long straight stretch, was well up over a 100mph then backed down, about a quarter mile later I felt hesitation in the throttle and pulled in the clutch to downshift, I held it to watch and see if the rpms would rest at idle like it always does and the bike died. I pulled over also realized when I went to start it again that the headlight was out. The headlight has gone out before from the fuse coming out of the block located under the handle bars. It was the same thing again, not sure if this is at all relevant but thought id include it.

Anyways I started the bike again and it was clearly running on two cylinders, choke on struggling to stay running, choke off it would die. Turned and headed for home and within a quarter mile was running fine again. Few more miles at a four way stop same thing. Pulled in the clutch and it died. started again on two cylinders.

I have rebuilt carbs, tuned and synched, screws are 2 1/2 turns out, plugs looking as they should, new rubber from airbox to carbs, inline fuel filter, new iridium NGK plugs and NGK wires.

My question is this.......if it were a coil would it be intermittent like that? I was under the assumption that if a coil is bad it just goes bad. Not sure what direction entirely to go in. I was thinking of checking and looking for corrosion in the electrical connections.

I can also say that every time I have tried to push this bike it acts up like this. I was also wondering if this could be a vacuum issue. The intake boots and clamps look on the rougher side, dont leak at idle but that could be a different story at higher speeds and once the engine is nice and warm?

This is a 79 cb750f super sport, stock everything accept for cosmetic mods.

[TOOLS1]

I have been told that coils can work at low RPM, and then be bad at high RPM. I am dealing with this same issue on a bike right now.
TOOLS

[rrgunslinger]

Sounds like the plugs were fouling to me. When you back off at high speed like that you suck alot of gas into the cyl and possibility oil. Mine did that to me the other day and I know the plugs were firing cause I have those fancy spark plug caps that you can see through.  They glow red when firing. After a block it cleared up but I had to wind it up to get it to clear.   I still have back firing on one or two of my cyl but I contribuite that to the carbs and maybe the timing (1973 with points).

I would put a set of new (cheap) plugs in it just to see.

https://www.denniskirk.com/ngk/spark-plug.pd8ea.prd/D8EA.sku

[bellison80]

Thanks for the info fellas. I just got done inspecting the wiring a bit and cleaned up the connections to the coils for the heck of it. I still have some stock NGK plugs that I could switch around to see if it may be a pluf fouling.