I was out on my old Bayou last night and it started working really bad. It hardly had enough power to pull itself in 4th gear and I was down into first gear on even small hills. I decided today to try adjusting the valves and clean the carburetor. The carburetor was full of water and rust so I think that's why it was running poorly. I drained the tank and cleaned it too.
I've adjusted the valves several times before so I'm sure that's not the problem but now it will only idle and dies as soon as I touch the gas. I've pulled the carb and cleaned it a couple times and still the same problem. It seems to be running really rich too, the plug is black and sooty. I'm going to pull the carb again and give it another cleaning but could there be anything else that I should check for?

 

You don't say anything about the level of detail on the carb clean - make sure you remove the main jet, pilot jet, and make sure all the little pinhole size passages are clear by spraying carb cleaner in on opening and it must come out the other.

Also, the float setting may be off. Check the float height measurement from the bowl seam.

Air filter not blocked by mouse nests, etc.
Spark plug cap on solid

Way overfilled oil will cause engine to act very sluggish - seen it happen personally on a MC. Has someone not familiar with it borrowed it because if the removed the oil cap maybe they just topped it off.

 

I found the problem. When I cleaned it I took everything apart I could and blew the passages out so I was fairly sure dirt wasn't still the problem. Unfortunately as soon as I took it off I noticed that I couldn't see the needle that moves up and down. It had came out of place and was lodged in the top chamber of the carburetor where it tore a couple holes in the diaphragm. The needle doesn't seem to be damaged but it looks like a real pain to get that long spring back in there. I fixed the holes with rtv sealant to try it but I'm going to need a new diaphragm.
After looking at a few diagrams of the carburetor I see that someone had put the top part together wrong before I got it. I don't think the needle could have pushed through if I had been put together right in the first place.

 

After looking at it again I think the diaphragm was torn before I even took it apart (at least in one place). RTV sealant was flexible enough but wouldn't stick to the rubber so I'm going to try Seal All. As long as the solvent doesn't damage the rubber it should work fine since it's gas and oil resistant. Hopefully it'll get me going until I get another carburetor. I can't believe a new vacuum valve is over $100.
Here's a great page I found showing the disassembly/ reassembly of these carburetors.
Keihin 32CVK Disassembly

 

I got it back together this morning and had it out for a test drive and it's working great. The only slight downside is that the rpms don't always come back down as quickly as I'd like when I let off the gas (not very often though). I assume it's because the Seal All is a bit stiffer than the diaphragm material. Hopefully it lasts until I can find another carburetor.

 

Are you going to buy an OEM, or a new Chinese unit?

I am rebuilding an 87 KLF300, and my carb is unreparable. I bought
a new Chinese carb, and got it running. Not sure I recommend them,
at $80.

BTW, it seems the 86-04 KLF300 4x2 may be the only quad made with a
rear differential.

 

Honestly, I just planned on getting a used one for now. I'm trying to find just the diaphragm online but I don't want to tear mine apart to measure the old one. Has anyone else replaced only the diaphragm on one of these?

 

There is one on eBay now for $42.55. I was going to list my old carb there as
a parts unit, but may just list the diaphragm. I'd post a photo, but this site
will not let me until I have 20 posts. Tres bizarre!

My carb is not repairable because the idle air screw could not be removed. Attempts
to drill it out ruined it.

I have probably rebuilt hundreds of carbs.

 

Well, the Seal All lasted until a couple days ago until it got hard and brittle and came off. It looks like I'm going to have to find another carburetor. What scares me worse is wondering where the piece that broke off the slider at some point has ended up. It was definitely there when I put it together last time and I don't see it lying in the intake anywhere.
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I just pulled the vacuum valve again and I see another piece broken off of it. Once again, it's not in the intake anywhere so I assume it got chewed up and passed through the motor. Hopefully it hasn't done any damage. I would assume it would have made quite a bit of noise when it got sucked in but my brother swears he didn't hear any odd noises when he was driving it.
I'll know more when I pull the top end to replace the rings and gaskets in the spring.