Bizarre: starts to miss after a few miles..?

[Alizard]

Well, just to remind me that I'm not nearly as smart as I thought I was:

I have an elderly 1979 KZ-750 (twin) and it actually runs great..... mostly. lately it has developed a bizarre problem:

Around town on city streets it runs perfectly. But: if I get on the expressway and drive a few miles, it starts to feel a bit "weak" at the throttle and I will feel an occasional misfire, but keeps running and I can still drive it.

After I pull down off the expressway, I can still feel it miss a few times but overall it runs decent. Within maybe 1/2 mile of low speed riding, it feels fine again.

This morning, after I had run a few miles and I felt it miss, I pulled off and checked the clear fuel filters for gas and they were full. I have cleared the air vent so it's not a "vapor lock" at the gas cap.

A few months back, I flushed the tank and cleaned the carbs, replaced the fuel lines and filters. I also adjusted the float levels at the time. It was running great until I noticed this. It also has relatively new plugs and points.

It feels more like a fuel issue, but I don't really know where to start. Since it only happens after cruising at 55 mph for a few miles and goes away quickly afterwards.

[StarGate]

Just fought that gremlin on my bike. The only time it would fail would be when I had run at 70+ for several miles. If I ran at 85+ it would get even worse and may not clear up until I replaced the fouled plugs but normally it would clear up soon after slowing down to normal city speeds.

Turned out that I had one float adjusted so close that it would flood that cyl when the throttle was opened to 75% or more and the more you open it, the sooner it would foul the plug. Once I discovered which one it was, I adjusted the float on that carb for a 2mm lower fuel lvl and the problem went away. Not sure if your problem is exactly the same but it sure sounds like it might be since you just adjusted your floats.

[Alizard]

I was thinking along those lines myself..... but, the float levels were dead on and it hasn't been very long since I adjusted them.

BUT: I have an aftermarket electronic CDI ignotion which I have run for MANY years. It has blown up a couple of times, but I fixed it (electircal engineer).

On a hunch, I restored the bike's original ignition at lunchtime and rode it back to work.... and did not feel the funky miss after a few miles. I will run it a while on the stock ignition and verify that it is the case.

This is wierd, because the CDI should either run or not.... never had it get marginally flakey before.

[elr658]

Had that deal on a 99 Sporty I'm working on. I was going to say heat related coil breakdown or module.

[Alizard]

Originally Posted by elr658

Had that deal on a 99 Sporty I'm working on. I was going to say heat related coil breakdown or module.

Looks like it is the CDI ignition. I rode it to work again today using the stock ignition only and there was no miss at all. I guess it could be the coil breaking down under high voltage because the CDI puts a lot more voltage on the coil than the standard 12V points configuration.

Wierd.

[Alizard]

Originally Posted by elr658

Had that deal on a 99 Sporty I'm working on. I was going to say heat related coil breakdown or module.

It turns out you were right, the coil was breaking down.... or, it was flaky somehow. It fooled me because with the CD ignition on, it would start very easily and run well until after a few miles then have a miss..... when I bypassed the CDI and ran on the standard ignition, the miss was gone and the bike ran good....

EXCEPT: on the standard ignition, it was hard to start cold.

Here is the diagnosis: flaky coil, so using the standard ignition there was low spark output under cold crank (when battery voltage is low due to starter load). But, coil put out enough spark when the bike was running and the system voltage was up to 14V, so it ran fine then.

With the CD ignition on (which boosts the spark voltage) it started up easily when cold, but after a while, the higher spark voltage was apparently arcing over someplace or breaking down somewhere inside the coil causing a miss.

I found a used coil for my bike on e-bay and now the problem is all gone: cold starts easily with or without CDI, runs very well on CDI and there is no misfire.