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The vacuum operated excellerator pump and metering rod were found to be quite troublesome at high elevation. Had to use choke to start while hot, and blubbering above idle. Supposedly you lose 1 inch of vacuum for every 1000 feet of rise in elevation. There were two other M38's there, one with the same set of symptoms and the other had a standard mechanical WO carburator on it. That one ran much better. We also had many Jeeps with vapor lock. They poured cold water on the pump and fuel line or changed to a new fuel pump, and off they went. It's a different world at high elevation. This all took place in the Colorado Rockies. Something to think about for you guys! John

During the old Vic Hickey days re-jetting and Idle mixture adjustments were the standard routine for operating at those elevations.

I was thinking the same thing Wes. You got ready to go to the
mountains and knew a carb adjustment or rejet was going to
be necessary. Tune it back when you hit the flatlands.

Kind of like standard procedure on start up of a cold engine
is pull out the choke handle. Nobody knows that stuff any more.

I was wondering how many folks with vapor lock were using
high vapor pressure gas. Although here in late September we should
be moving to higher VP gas as the season changes, high vapor pressure
gas left over from the spring could be adding to the problems
at altitude and warm temps.

One of the reasons they began blending seasonal blends of gas
was of course to reduce emissions, but a related effect
was to help reduce vapor lock.

Maybe you were near Camp Hale... not much left but it has an interesting history...https://en.wikipedia.org/wiki/Camp_Hale

Base camp was 8400 and we went up from there. Buena Vista Colo. Collegate Peaks. Lots of 14 teeners there. I've heard of high altitude metering rods for the WO carbs, but nothing for the 637S carbs. I think the alcohol high oxygen content gas was screwing with some of the guys, but I ran premium non alcohol. If I lived there year around, fuel injection of some sort would be the ticket. John

Sure wish I knew where to buy non-ethanol gas around here.

Camp Hale had a motor pool so I guess there must of been a way to operate effectively at that altitude... but it is tough that high up.

In the winter there is a Weasel get together at Camp Hale where everyone gets to drive in the snow with their WWll Weasels.

The other 2 M38's there at 12 thousand/ft were mine and Bretto's. Bretto was running the WO with no problems. I was fighting the YS same as Oilleaker. I also had a 24V coil that began to fail at operating temp, thankfully Oilly had a spare in his kit..