'81 KZ650 Surging / cutting off when hot

"...but I was just replacing the piston rings at the time and I wanted to get everything back together. It's not hard to take this motor apart, so I may go ahead and order new valve seals and install them just to be certain."

I'm fairly confident in my mechanical abilities. It only took about 2 hours to replace the piston rings and have her running again. I'm pretty sure I'm capable of measuring the valve bucket clearances

I just found the numbers I think, it says .08-.18mm. Does that sound right?

always start with the simple things first,run a compression test before tearing down a motor to do a ring job.if compression goes up by squirting some oil down the cylinder theres a good chance its rings,if not look at valve adjustment

not sure if this has been mentioned or checked before, but gas tank vent, also the screen filter on the petcock.
it sounds kind of like a fuel delivery issue rather than mech.

TriniTT wrote:

"...but I was just replacing the piston rings at the time and I wanted to get everything back together. It's not hard to take this motor apart, so I may go ahead and order new valve seals and install them just to be certain."

I'm fairly confident in my mechanical abilities. It only took about 2 hours to replace the piston rings and have her running again. I'm pretty sure I'm capable of measuring the valve bucket clearances

I just found the numbers I think, it says .08-.18mm. Does that sound right?

Yes that is the measurement.

Oops, sorry, forgot about the whole rebuilding-the-top-end-of-the-motor thing. Valve shims should be easy comparatively.

"So I posted a while back about some difficulties w/ my kz650, and I ended up replacing the engine, and having to replace the piston rings in the new engine. That's basically all I did, put it back together, and I'm now making decent compression again."

As I mentioned at the beginning, I replaced the rings, "and I'm now making decent compression again." Suffice it to say, if I have the common sense to tear apart an engine, it's likely I also had the common sense to make sure I had a reason first

My compression is decent now since I replaced the pistons and used a different cylinder head. I was making 30-60-90-70 before, and it jumped up a bit when I poured oil down the cylinder. Since I changed the piston rings and have driven it a bit, it's up to 120psi across them all. This is just to clarify on the background as to what I've tackled thus far, which is what led me to the valve stem seals.

The smoke I'm seeing right now is bluish-white, and looks and smells like oil smoke. Thus, I'm replacing the valve seals.

Now another issue I just noticed after a brief drive today is the float bowl overflow lines are periodically dumping fuel out, and quite a bit at that. I was checking out my battery issue (yeah, can't seem to keep it charged either, despite good readings from the alternator and a solid connection to the battery and grounds), and I noticed a puddle forming under the bike. I looked up and it was just dumping fuel out of those lines, but only for a second or two, then it stopped and started again a few seconds later.

I would go back to basics. The fuel overflow suggests you may have a supply problem. Make sure you have your petcock lever set to 'run'. If it is on prime you may get overflowing. Prime is only used when bowls have been run dry, say after stripping the carbs. I would check the vacuum and supply lines and the petcock diaphragm. With the tank disconnected from the carbs if you suck on the vacuum line you should get a good flow of fuel thro the supply line.

Ok, in essence of saving time and posts, let's just assume I've checked everything common sense would suggest to check. The fuel petcock is switched to "on" at all times.

To clarify the issue with the fuel, it appears to only be pouring from the #4 bowl. I know the cylinder is firing, however, because if I block the intake pod for that carb, the engine dies.

Wouldn't fuel overflowing from the float bowls mean there was too much fuel? If it's not getting enough fuel, how would the bowls be overflowing? Clearly the bowl is getting too much fuel, and thus is overflowing. Now, I've seen a few pics of the vacuum lines, but just to clarify:



In the pic, the brown tubing is venting to the back/atmosphere, as pictured. The fuel line labeled goes to the fuel part of the petcock, and the vacuum goes to the vacuum of the petcock. The only thing I don't see pictured are 2 lines on the back of the carbs (I believe #1 and #4 each have one) that have been blocked off with a bolt in the vacuum line. Is this all correct?

TriniTT wrote:

The fuel petcock is switched to "on" at all times.

Wouldn't fuel overflow from the float bowls mean there was too much fuel? If it's not getting enough fuel, how would the bowls be overflowing? Clearly the bowls are getting too much fuel, and thus are overflowing....

...The only thing I don't see pictured are 2 lines on the back of the carbs (I believe #1 and #4 each have one) that have been blocked off with a bolt in the vacuum line. Is this all correct?

As for the fuel level, do the Servce Fuel Level Check by removing the carbs from the bike, rigging up an auxilliary supply of fuel, then check the fuel level in each carb. Do so by connecting clear tubing to the overflow nipple at the bottom of the carb, hold it up high & against the carb body, then opening the drain screw. Fuel should flow into the tubing. Lower the tubing & hold it at the lower level & note the fuel level again. It should be 3mm +/- 1mm below the carb body (or 1/8"). If you raise the tubing, start over.

As for those other ports, do you mean the engine side of the carbs? And are they actually on the carbs? Or are they the nipples on the carb holders - if so, two of those normaly have the air induction system tubing, while the other two are capped. Some folks remove the air induction system (smog device) & cap the ports.

The nipples on 1 and 4 are actually on the carb holders, and there's no smog system currently installed. I'll likely install caps there so it's sealed better.

Yes, install good vacuum caps from the auto parts store.
And it wouldn't hurt to replace the other two on #2 & #3.
Z1Enterprises.com sells them as well. By the way, I sent you a private message.

Received and replied.