Walbro WYL Mystery

[Conquistador3]

I haven't got much time to play with chainsaws right now due to weather and the few dry moments inbetween being occupied by spade, chipper and manure, but I have a little something to keep me busy.

My uncle has a 20 years old (1998 manufacture date) Honda UMK-431 brushcutter. Last year it started to become hard to start when cold and this year became nearly impossible to start so it ended up in my hands to be (hopefully) fixed.

To cut a long story short while rebuilding the carburetor I found the metering arm very tight and "below" level. I didn't think much of it and I just set it using the Walbro metering gauge during reassembly.
Started well after rebuild, so I set idle, ran it for ten minutes until satisfied, then I drained the fuel, let it run until it died and put it outside overnight to simulate a cold start. I've learned it's better to err on the safe side. And I was right.

Next morning there was no way to start that damn thing. Having learned my lesson over the years, I took the carburetor off and, much to my surprise, I found the metering arm had become very loose. I set it back with the gauge, reassembled and the thing started... albeit not not as promptly as it should. But ran extremely well and smooth.

To be honest I've seen several poorly set carburetors (metering gauges are very expensive here and seem to hurt big egos) but I've never seen what this old Walbro is doing here.
If it can help it's a WYL-74A and according to both Honda and Walbro it's no longer available. There may be a replacement model but I haven't had the time to visit the nearest Honda dealership yet (very nice people... but 20+ miles roundtrip, so it will happen when I am in the area) so I don't know that and it may be more expensive than my uncle can stomach.

I am just curious to understand what in the heck is happening here.

 

[alexcagle]

Replace the fulcrum, needle and pin again.
Although hard to see, either the needle can rust or the aluminum fulcrum can corrode, causing a burr or catch, that during operation bends it out of shape.

If the two forks holding the needle are not at the same equal height, this can cause problems as well.

 

[Conquistador3]

Thank you.
I should have another fulcrum/needle/pin combo from a previous rebuild that only required gaskets and membranes. If I find that I'll give it a shot and see how it works.

 

[Conquistador3]

Yesterday evening I tried to replace the whole needle assembly (fulcrum, pin and the needle itself) with a brand new one I had in the spare drawer. The needle stuck and flooded the engine after a few seconds of operation.
Then I put in a spring (same length) salvaged from a junked Chinese carburetor to rule that out. Again the needle stuck and flooded the engine, this time so badly it poured into the fuel filter housing.

I tried looking at the needle and pin seat with a magnifying glass under a strong light but there doesn't seem anything that "catches" the needle or the pin, but these things are so small it means little. Tonight I am giving the cast body another good clean. If even that doesn't help I have an aftermarket carburetor on its way. If even that isn't enough I am out of ideas.