KZ440 bogs when hot has no power

I have a 1980 kz440. I've had it out for a few weeks this spring. Changed the oil, and put in new plugs for the season.
It starts great and runs smoothly until it heats up.
When it gets hot, it bogs down and feels like it's running under water. If I let it sit and cool down, It picks up pretty well again. I didn't have this problem last week, but coming in to work yesterday, I lost power and had to limp in.
I did some searches and the suggestions I saw were to check the coil and the valve clearance.
Are there any other suggestions? Or is anyone dealing successfully with a similar issue? Did I do something that broke something?

Valve clearances are checked and set when engine is cold to allow for the decrease in gap as the engine warms. So valves barely closing on cool engine may begin leaking more and more as the engine warms and the gaps disappear. And this causes loss of compression and loss of power and other bad stuff.

OOOOOOOooo yeah. Ditto to that!

Sounds exactly like what was happening to my KZ400 until I took it to the shop. Started great, ran great until hot, then would bog down with no power.

My mechanic found one of my carb diaphrams was partly torn. If I understand the mechanics correctly, the diaphram causes the needle to raise, mainly between 1/2 to 3/4 throttle, allowing more fuel to flow. I'm guessing that when the engine got hot, the diaphram got more plaible and the hole stretched out. This eliminated the vacuum and the needle wouldn't rise up, causing the engine to bog down. Fortunately Ray had a used diaphram and slide in his boneyard so it only cost an arm to fix. Runs pretty good now.

The other problem he discovered was a split in my vacuum petcock diaphram causing fuel to dump down my vacuum line. I've gone from 20 mpg to 42 mpg since that was fixed, but I'm not sure what kind of performance problems that was causing other than fouling the plugs.

Good Luck!

:pinch: duped again

Post edited by: Link14, at: 2007/05/15 14:54

These samll displacement engines are expecially sensetive to air/fuel variations. My bike did not idle for crap when I got it, I assumed the carbs where pluged but they were not. The problem was air leaks at the carburator holders and the wrong type of air filter. When I got the air filter issue fixed it ran like it was new. So the smallest thing can make these motors problamatic. Getting these motors out of sync ( what happened with one torn carb diaphram) makes them run like crap under load. Also causes strange noises, vibration, and wierd feelings when riding them.

SO, Friday I ran fine, Saturday I changed the oil - and adjusted the idle and it ran great, but I didn't run it hot till Monday.
Could my idle adjustment have been the culprit once it got hot?

I don't think the idle adjustment has anything to do with the problem. The idle is just that, idle speed.
I would go through all the tune up steps in the manual first, including the valves. See what happens, if the problem is still there then write back.

Idle adjust is not your problem.. could be a few other things.

Tight valves, that is really bad...

Your ignition advance may need to be lubed, when it gets hot it is sticking mabey.


If it sputters or pops I would suspect the coil, plugwires, ignition, etc.

This one may sound dumb, but verify your brakes don't hang up...

Chack you plugs and see if they are white, brown, black, or oily.

Post edited by: Tyler, at: 2007/05/15 22:16