Momentary stall/power loss symptoms = ?

Greetings! It's been a while but I'm still around...sortof. May I pick your brains? I'm currently having an issue and, at a guess, I'm thinking it may be coil related but it was suggested to me that it may more likely be a valve issue. Pardon the long post but you know me, I don't want to leave anything out that might be important if I can avoid it.


'77 KZ650-C. MAC 4-1, custom baffle. Stock coils. Coil rewire performed. Dyna ignition. B7ES. Non pump Mikuni's. Blue (it's important, right? )


In our last episode, I had just moved to Boston and was in some small amount of culture shock which led me to lock myself in the house and avoid humanity for a few weeks. When I finally went back out, I thought I'd go explore and get acquainted with my surroundings.

I cleaned up Tess, my '77 KZ650-C which was running beautifully in Seattle just a couple of weeks prior and went for a quick half hour ride to get a feel for the freeways. It was summer, about 95 degrees (easily 20+ higher than it's use to in Seattle) It started right up on the first turn, but ran a little rough through the trip. Rough idle, little power, near stall on takeoffs but a quick recovery, although it felt like I was missing a cylinder. I pulled the plugs and found #4 was deep black. not wet and oily but not a dry charcoal-like deposit either. All other plugs were between tan and just slightly fouled but uniformly so.

I went through my fuel and air screws, emptied my bowls, got a fill of gas, put in a spare set of plugs went back out. At that point, the missing seemed to be resolved. Idle was significantly better (not perfect, but nowhere near as rough). Still a slight hesitation at takeoff but reasonable well overall.

About 15 minutes into the ride, however, having come up to full temperature and riding smoothly on the freeway, I started to quickly lose significant power. The more throttle, the slower it got, accompanied by an "empty"/lifeless or muffled exhaust sound. The same sort of sound I'd expect if I were out of fuel or had a massive leak in the cylinder. It wasn't a solid crisp sound. If I backed off, the exhaust sounded normal but that also meant necessarily losing speed...eventually I simply had to pull over to the shoulder.

Approaching a stop, the engine slowed smoothly to a stall. It wasn't an abrupt stall. Rather, it got quieter and slower and by the time I pulled in the clutch and came to a full stop, the engine was already silent. I assume that the engine was only turning at behest of my forward movement.

Trying to restart it, the engine turned but wouldn't fire. So I sat for a few minutes. Said some nice things to it, pet it, looked it over to make sure there wasn't anything obvious. Petcock, plug wires secure, choke in the correct position, no shorts in the coil leads. This was about 5 or 10 minutes. Key, clutch, button, started right up...and ran fine all the way home.

Pulling the plugs, I saw that #4 was again slightly fouled, although nowhere near where it was. The other plugs looked okay. I cleaned and rearranged the plugs and the next day went out again and experienced the same behaviour. About 15 minutes in, the engine loses power, chokes up for a few minutes and then runs fine.

Petcock and screen are clean. Have not yet checked compression or valves as my tools are in some unidentified and yet unpacked box in my storage pile.

Sounds like you are running out of gas. If you have a fuel filter, change it. You can also run out of gas because of lack of venting in either the carburetor assembly or fuel tank.

Yeah, that appeared to be it. I poked around a little more and didn't see anything obvious so the next time it happened, I popped the cap on the tank and shortly afterwards, it recovered. Thank you for your help!

Virii wrote:

Yeah, that appeared to be it. I poked around a little more and didn't see anything obvious so the next time it happened, I popped the cap on the tank and shortly afterwards, it recovered. Thank you for your help!

There's a tinly hole in the washer that seals the gas cap and a passage that has to let air in. You may have to take that assembly apart and clean it.