Topic: Mystery Problem - 78 CB750K

[maconnoise]

So I have a mystery problem that is stumping me. I went for a ride a few days ago and ended up having to limp my bike home running on what sounded like 2 cylinders. Here is what's going on:

- When cold, it was a bit rough on takeoff with a bit of missing, then cleared up over about 2500 rpm.
- When it finally warmed up, started to miss below 3000 RPM (especially when going up hill)
- 2 miles later, would miss badly under 4k RPM (sounds like losing 2 cylinders) and power way down
- would stall if let idle almost immediately
- fuel level good, petcock at correct position
- fuel flowing through clear filter & lines, no visible water in tank, no additives
- pulled plugs (week later) - all had orange/rusty coloration on tops
- #4 piston head looked like it had some moisture or oil on it, but no heavy gas smell
- all the electronics seem good, but I haven't touched the ignition system since I bought the bike
- Carbs need to be cleaned, but have been running well lately (possible clog?)

What am I missing? Seems like a blown head gasket would only effect one cylinder and there is no sign of exterior leaks.

[TomsK8resto]

Maconnoise,

When and where did you last put gas in the tank?  It sounds like you have water mixed with your fuel due to how rough it ran and the color on the plugs.  If it were me, I'd drain the tank and carb bowls, clean the plugs and put fresh fuel in it.  If that is it, I might also change the oil and filter.

Is it possible that somebody messed with your bike?

Good luck.

TomsK8resto

[Wobbly]

all the electronics seem good, but I haven't touched the ignition system since I bought the bike

The points would be the first thing I would check.
It wasn't raining by any chance?   

[BobbyR]

When you say "Electronics" do you have a Dyna or other aftermarket ignition system. Dynas are known to drop two cylinders when the engine is warmed up.

[BobbyR]

When y9u say "Electronics" do you have a Dyna or other aftermarket ignition system. Dynas are known to drop two cylinders when the engine is warmed up.

As are points and condenser systems. There's more threads about condensers failing than the ever popular assertion that someone who's bike is not firing on all 4 cylinders must have a failing Dyna...

I know all about condensers failing and rubbing blocks breaking off. It is also true that some Dynas fail when warmed up. In making a diagnosis you need as many facts as you can get.

[Wobbly]

My electronic Dyna S ignition never gave me any trouble, by the way.