MS-440 Carb Tuning Problem

[ErikS]

Interesting problem. New-ish HD-15D (no limiters from the factory) carb on one of my MS-440s. Just a couple weeks ago I tuned it by ear once warm and verified it with a tachometer. 13,480 rpm. Perfect, ready to run.

Fast forward to yesterday. I warm it up, and attempt to retune for current conditions. As i turn the H side screw in, i find the RPMs arent adjusting as I expect, only to run the screw all the way closed and find it's still 4-stroking at WOT. Check it with a tach, 12,720 rpm, super rich, and that's with the H screw completely shut.

Again, this carb can't be but six months old, new OEM Walbro. I can't imagine I'm the first to have this problem, and I'd love to hear from some members with more carburetor experience than myself as to the culprit or suspected culprit in this case.

As a side note, the carb pressure and vacuum tests perfectly. I live at 6,300ft ASL, and had I not known better I'd think my elevation was causing this... However since I've had it tuned to the limit before with no problem, I clearly have another issue at hand here. Thanks in advance guys.

 

[pioneerguy600]

Sounds like the fuel is bypassing the metering needle so I would first check for a loose Welch plug or a defective check valve, either one can allow fuel to get into the engine unregulated.

 

[ErikS]

I'll give it a serious ultrasonic bath. Of course the check valves are non servicable... Welch plug shouldn't be the issue as it passed a pressure test, but i'll go through it in entirety one more time and let you guys know what I find.

 

[pioneerguy600]

What type of pressure test did you run to test a Welch plug?

 

[ErikS]

sorry, it was late last night, and was a few beers in! LOL. visual inspection is the key here. again, i'll let you know what I find.