Bike idles down and dies once it's warmed up

Not exactly sure which technical area this post should fit into, but the Carb section might be the best bet, I suppose LOL


THE PROBLEM:
Bike will start fine, then after warming up (usually 3-5 minutes or so), it starts to drop in idle speed, and will eventually die. Once it dies, it will not restart right away unless I open the throttle quite a bit. Best results are had by just waiting a few minutes, and then trying it again.

DONE, ACCORDING TO PREVIOUS OWNER:
- new battery
- new ignition coil
- new spark plug wire and cap
(these all seem to be legit new)

WHAT I'VE DONE:
- replaced head gasket
- replaced air filter
- rebuilt fuel tap
- installed inline fuel filter (fuel shows to be clear as white wine)
- installed correct spark plug (PO had the wrong one in) (looks like it's running a bit rich)
- verified no air leaks on either end of carb (rubber mounts)
- verified that choke is working properly
- tried adjusting idle speed screw
- tried adjusting pilot screw
- tried with fuel tank cap both sealed and open
- adjusted valve clearance (to nearer the top end of spec)
- fresh gas with Seafoam measured in
- oil level is normal (filter is on order so I can change oil/filter to fresh)

KEEP IN MIND:
- this model (near as I can tell) has no fuel pump

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I've seen DOZENS of threads about this same problem all over the internet, but most of them end up unsolved (or just were never updated with the solution), so I'm not sure WTF to do. It SEEMS to me that the bike is warming up, and that heat is causing something to fail. I can't imagine that the carb would be affected by being at 'warmed up' temperature, so I've been putting off looking at the carb other than to make sure that it seems to be working (by what you can tell from outside it). One thread advised tapping it in the float area to make sure the float isn't stuck - did that, didn't change anything.

The bike is a 1982 KZ250, but from searching online, this is pretty common no matter the year/manufacturer.

I'm seriously stuck on what to do - thoughts??

Two suggestions:
Look for a kink in the fuel line (such a kink will cause the symptoms you describe).
Test to see that the fuel tank vent is operating (just open the tank's cap and see if problem persists).

Would assure fuel level is within specs as shown by the clear tube test.
With otherwise perfect carbs and fuel supply, a too high fuel level results in excessively rich mixture.

Would also double-check valve clearances. As reported symptoms may result from a too tight valve clearance.

Good Fortune!

3 quick things to try:

When it starts to idle down, put on the choke, what happens?

Run the bike with the petcock in the PRIME position. Does it still idle down and quit?

You have a clear filter, watch it to make sure fuel continues to flow while the bike is running.

650ed wrote: Two suggestions:
Look for a kink in the fuel line (such a kink will cause the symptoms you describe).
Test to see that the fuel tank vent is operating (just open the tank's cap and see if problem persists).

Fuel line checked out fine - no kinks.

Problem is the same whether tank cap is open or closed.

Thanks, though!

DiamondSkyBlue1000 wrote: 3 quick things to try:

When it starts to idle down, put on the choke, what happens?

Run the bike with the petcock in the PRIME position. Does it still idle down and quit?

You have a clear filter, watch it to make sure fuel continues to flow while the bike is running.

Once the idle started to dip, I switched the choke on full, and ... nothing happened. LOL

I switched it back off, and then tried setting it on again.. repeated this several times, and no change was made in the idle speed. The bike still kept dropping RPMs until it died.

I verified that the problem is happening whether in the RUN or RES positions.

The angle of the inline fuel filter is such that it's hard to tell if fuel is moving through it, and at what rate. There is definitely fuel IN it, though, and the amount doesn't fluctuate.

Patton wrote: Would assure fuel level is within specs as shown by the clear tube test.
With otherwise perfect carbs and fuel supply, a too high fuel level results in excessively rich mixture.

Would also double-check valve clearances. As reported symptoms may result from a too tight valve clearance.

Good Fortune!

I'm gonna have to read up on that clear tube technique! Thanks for mentioning that.

Well, I started it up and let it run itself to death (urgh) ... then quickly pulled the plug wire from the plug, attached it to the spare plug I have, grounded it against the engine block, pressed the starter and... nothing.

No pop, no spark. Nothing.

This has me thinking that the suggestions of an electrical heat issue might be onto something...

Electronic ignition or points?

I believe it's just electronic (no points).

Has this always been a problem or did it just pop up recently? The first thing I would do is a complete tuneup.

Alright, it seems that the petcock is working so the bike isn't simply just running out of gas.
You checked for spark after the bike died and there was none.
Did the spark return after the bike cooled back down? This will validate that test.
On electronic ignitions, there are no points, obviously, there are pickups instead. These are known to go bad over time due to heat. The symptoms are similar to yours.
Get yourself a can of that electrical contact cleaner. Once you validate the spark test, remove the cover on the side of the engine where the pickups are located. Now start the bike up and repeat your test, that is, let the bike run until it dies. Spray those pickups down with a lot of that contact cleaner, I mean really douse them. If our guess is right, the bike should fire right up after that. What we did was let the pickups get hot and fail and then bring them right back by cooling them way down with the contact cleaner. Make sense? You could even spray them with the bike running to see if the idle recovers.