Reply To: Wisconsin M RBM poppet valve carb

[PM T2]

I think you are making more work for yourself than is necessary. My personal experience is that even a worn poppet valve stem wouldn’t necessarily be causing the issue you having with the motor running continually. I’m inclined to agree with Harry that fuel starvation is more likely.

WRT to the poppet valve stem – I’ve been working on a Blakely gearless motor (aka Gray Gearless) that has a very worn poppet valve stem. I had issues with getting it to run at all, which actually turned out to be compression ring related. before I went after the rings, I machined a new poppet valve with the stem machined back to original diameter (3/16″ in this case). After I got the compression issue solved, I have run it with both the new poppet valve and the old worn one. It makes no difference whatsoever which one is in the carb, it starts and runs exactly the same. The old poppet valve stem is worn down to 0.155″ so there is more than 0.030″ clearance between the stem and the section of carb that supports it. If you really want to address the fit of the poppet valve stem, then machine a new one and leave the casting alone. You can only cut the casting once, and if you are the least bit misaligned, you’re going to cause yourself more problems than you’ll solve. Transversely, you can make as many poppet valves as it takes to get it right.

Those are my thoughts, not yours. Hope it helps.

Best,
PMT2

He's livin' in his own private Idaho..... I hope to go out quietly in my sleep, like my grand-dad did..... and not screaming, like the passengers in his car...