Right, it's such a small amount of fuel leaking by it takes an hour to foul the plugs up. The last overhauled carb I bought did exactly this and it was the mixture valve.

I have been "taught" MANY things in my life that I later "learned" were incorrect. Just because someone is your teacher doesn't mean everything they teach you is correct. Often not their fault, just bad info passed across generations.

Larry

Your new carb likely has a larger jet than stock or someone otherwise monkeyed with the bleeds, making it a bit richer than normal. Either way, try leaning it and you'll know. If they adjusted the float more than about 1/16" or so high and you get the same over richness.

Well after reading a lot of old posts, it seems this is not an uncommon issue. Though for most the problem is the other way around - mixture too lean, temps too high, and a need to enlarge the main jet. It seems the jet is too large for my setup and the rich mixture ends up fowling the plugs.

It's also interesting that the same carb part number can show quite a range of flows and different performance results. Crude technology indeed.

For me I guess the best thing to do is just be more aggressive with leaning the mixture and fly on. Only other options seem to be exchange the carburetor (again!) or buy and install a smaller jet (probably not worth the trouble and expense).

Thanks to everyone who has posted and helped with this - appreciated!

Wanted to follow-up on this and say thanks to Finn for pointing me in the right direction.

Turns out the problem was with the air filter element. Not collapsed, but otherwise not working as it should. After installing a new one the engine has returned to running normally.

Backstory: For a long time, my engine had a small oil leak. Nothing serious - just a "nuisance leak". But the leak was on the top of the engine and over many hours of operation the oil would slowly work its way around the cylinders, to the bottom of the crankcase, and yes eventually deposit itself right on top of the air box. I now think all that motor oil, over many years, clogged and perhaps damaged the filter element until it would no longer pass enough air. Yes I did service the filter often per the K&N service kit instructions, but I think that motor oil eventually got the better of it.

The K&N filters are good quality (expensive too!) but I don't think they were designed to work in an often oily, dirty, hot place like the bottom of our engines. These are actually "automotive" filters that would normally be found on top of or away from a car engine, exposed to outside air and nothing else.

So, clean them regularity, and be sure not to over-oil them!

I hope my report might help others avoid this problem.

PS The "nuisance leak" has also been fixed.