Topic: Mystery (suspected) ignition problem, K6 CB750

[spider-man]

OK, this one has me frustrated. Maybe someone has some ideas for things to check.

Motorcycle is a 1976 CB750K with a bit under 18,000 on the odometer. Problem is that cylinders No. 2 and No. 3 are not firing. I highly suspect an ignition problem, but I've been unable to trace it and it doesn't add up.

A bit of background: It was a neglected machine when I got it, owner supposedly "just had it tuned," but it ran like crap. I sorted out a number of things, installing the correct spark plugs, replacing points and condensers, setting points using timing light and dwell meter, tensioning cam chain, setting valves, etc., etc. I didn't sync the carbs, but it idled evenly and ran great. New spark plugs were a beautiful light tan.

I took the machine down a few weeks ago to replace a leaking tach cable seal. When I fired it back up, everything was kosher at first, but then something wasn't right later when I went to take a ride and I aborted the mission. (Was on my way to work.)

Should have ran the bike the next day to see what the problem was, but, well, too late for that now. Instead, I decided to undertake a project I had in the works: install a set of GL1000 coils (eBay purchase). I dove in and did the swap. Same problem.

Now the catch: I'm getting fantastic spark from all four plugs. I'm also getting gas to the non-firing No. 2. and No. 3 -- plugs are wet. (It also doesn't make sense to me that if it were a gas problem it would affect No. 2 and No. 3 simultaneously, especially since everything was cool not long ago.)

Any ideas? I've wracked my brain trying to figure this one out.

[MrFry]

well if you are getting spark, i'd say that isn't your problem (assuming you have the correct plugs and correct gap)
could be too much fuel or too little

or perhaps compression?

i know - i'm not much help

[spider-man]

I haven't done a compression test, but it was running great just a month ago, with no smoke, no hard starts, consuming virtually no oil, etc.

[spider-man]

Mystery solved, we have liftoff.

I thought about what I wrote and what I'd done. When I tested for spark, I was doing it with a shop plug, not the plugs that were in the bike. That didn't make sense; I needed to see if I was getting spark from the plugs that were in the machine.

So I pulled them and tested them for spark. Sure enough, nothing. I installed some Champions I had lying around, and voila, it's an inline four again.

MrFry and others who pondered my problem, thanks. I appreciate the time.

I love it when a plan comes together.