Carburetor alterations for High Altitude.

[Steve_I]

Finished restoration on an M38A1 last year. All stock parts. Runs fine at sea level where I live. Took it up jeeping in Idaho last month at around 7,800 ft. Ran ok but lacked power and flooded out constantly. Back home now and runs fine again.
I grew up at altitude running 50’s era civilian jeeps at 9,000 ft with no issues. I remember my granddad (who was a mechanic) telling me once about re jetting carbs for high altitude. Soooo…. Can any one out there give me any insight, or provide any resources as to carburetor alterations to run this M38A1 at altitude.

[RonD2]

Hi Steve,
Not sure it helps or not, but searching on "altitude" turned up this old post: http://willysmjeeps.com/v2/modules.php? ... t=altitude

Good luck!

[wesk]

If you don't mind changing carb jets on the road here's where the YS637S & YS950S main jets fit in the Carter Jet List.



If you google Carter Jets you'll find suppliers that still stock them. Just pick a jet with the same thread size.

[Steve_I]

Rod,

Thanks for the info. I must have done something wrong when I searched as I could not turn up anything. That surprised me a bit as I surly was not the first to have a issue with Altitude.

Wes's response in the link is to not worry about it unless the jeep will remain at altitude. I am thinking of parking this one at my family's cabin in Colorado so I would like to fine tune it for about 9,000 ft. If anyone has done this or could direct me to any resources that would be great.

Steve Ingram

PS I love the expertise available on this site. good job to all!

[wesk]

Not sure who Rod is but the altitude correction is simple. Remove your main jet and accurately determine it's size and thread pitch then select one from the chart I provided that is leaner running and matches the dimensions of your current jet.

[BB]

When I lived in Colorado Springs in the 70s I did a lot of carb work and what I did was to use number drills to measure the jet hole diameter if it wasn’t marked on the jet. Then solder the hole closed and re drill the hole three to five thousands smaller. That way you didn’t have to locate another jet and you didn’t ruin the original. Another thing you will need to do is advance the timing a couple of degrees.

[Steve_I]

Been away from my computer for a few days and just catching up. Thanks to all for the insight and advice. Just to make sure I am understanding all correct.

Wes: the "Carb Jet" you are referring to is the Jet that the metering rod goes through, correct??

BB: when you say advance the timing do you mean going further from TDC (so like 7 deg BTDC to 9 deg BTDC).

With the Gas I am using, both my jeeps seem to like the timing set at about 8 or 9 deg BTDC for here in Oregon.

[wesk]

Steve, First be sure to download all the free manuals in PDF here that you do not already have. Second try to use them. Your best illustration and parts list is in the M38A1 ORD 9 SNL G-758. Use it in conjunction with TM 9-1826A dtd 1952 using the general Carter YS chapter and the most current spec sheet for the YS950S.




Item AD is the main jet.







You said:

BB: when you say advance the timing do you mean going further from TDC (so like 7 deg BTDC to 9 deg BTDC).

You must be very careful when selecting your terminology. In your above statement: going further from TDC is possible in both directions, before/advance & after/retard! BTDC makes it very clear which direction from TDC you are going.

Rather then experiment by trying different settings use a vacuum gauge and set timing to a setting that gives the best vacuum readings at open throttle and you may have to compromise a bit if starter labors too hard to turn her over when starting the engine at the new setting.

[BB]

Advance the timing to about 10 degrees before top dead center.You will have to experiment with the timing depending on the temperature and the fuel you are using. If I remember the regular fuel in that area was only 85 octane and might give you ping or detonation. If you are using ethanol free as you should because ethanol is corrosive to metal fuel tanks etc. the octane will be higher and run cooler with less chance of detonation. Remember 60 hp at sea level but about a 20 percent drop if I remember at 8000 feet so it’s worth experimenting.

[4x4M38]

Unless I missed something the R+M octane of gasoline as noted at the pump is tested with or without octane depending on what you buy.

You do lose mpg and other issues related to ethanol blended gasoline but 85 octane clear and 85 octane ethanol blend is 85 octane gasoline.

[Steve_I]

BB, thanks for all of the good advice and guidance. I am running Ethanol Free fuel in my jeeps here in Oregon, but can only get 87 Octane. Have not tried to source ethanol free in Colorado but I always try to find the lowest Octane available, with in reason.

[4x4M38]

Hi Steve
Some guys buy av gas and cut it. It’s a little hot for a 60 hp jeep!

My previous contained a typo:

“
Unless I missed something the R+M octane of gasoline as noted at the pump is tested with or without octane depending on what you buy. ”

It should read with or without ethanol.

[RonD2]

Maybe the good thing about aviation gasoline (commonly 100LL, which is 100 octane, low lead) might be the lead that an old L134 motor probably still likes for the lubrication. All gas being leaded back then. Does anybody remember what leaded gas smelled like --- compared to the corn-cut stuff we use today? Real man gasoline.

I don't know if adding a gallon or two of avgas to your tank could help or hurt high altitude jeeping, but there's plenty of forum discussions about the pro's and con's of using high octane gas in an L134 motor that was made to run carb jets and timing using 68 octane back in the day. Stuff about higher octane burning slower (but more evenly) than lower octane gas. How would using high octane affect L134 carb jets and timing at high altitude? After all, airplanes fly at altitude, don't they?

Interesting thread topic to follow.....keep us posted please!

Last time I looked, I recall that 100LL is dyed an unmistakable blue. Without wings, don't let The Man find you with it in your tank.

Good luck!

[wesk]

Does anybody remember what leaded gas smelled like --- compared to the corn-cut stuff we use today? Real man gasoline. Laughing

The smell of leaded gas has always been a ton better then the STINK of unleaded gas! I get to smell leaded gas every day since I spend all of my duty time around 100LL aircraft. It's rare for me to take on a job on a customer's light piston aircraft that has approval for operating on unleaded auto gas. If the job includes smelling that stinky gas I just turn my nose up at the job.

It doesn't hurt anything to use 20 to 25% 100LL with regular unleaded gas in a 60 to 72 HP jeep 4 banger. It does add the lead which these engine really like and the octane ends up down around 85-87 anyway. Seldom above 90.

I recall that 100LL is dyed an unmistakable blue. Without wings, don't let The Man find you with it in your tank.

Yes the 100LL uses a light blue dye as the old 80 Octane LL did which used a light red dye. The stuff I like absconding with was the old 115/145 Oct LL but it had a purple dye. Yes eventually these dyes tend to stain the fuel system including the throttle bore of the carb. So yes the dye could give your use of the 100LL up but then you always have the defense that you only serviced with it when you were using the jeep off road!

One word of caution about using dyed fuels though! The dye sticks round long after the gas evaporates. So if you park the jeep for long terms then best to make sure there's no dyed gas in the tank during the storage. The dye that is left behind when the gas evaporates gets very sticky and can plug things up.

[4x4M38]

The same is true for ethanol blended gasoline. If your vehicle sits for extended periods get that junk out of your tank and drain everything. Using the stabilizer only works where there is liquid.