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Steve_I Member
Joined: Nov 29, 2015 Posts: 6
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Posted: Thu Nov 04, 2021 5:33 pm Post subject: Carburetor alterations for High Altitude. |
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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. |
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RonD2 Member
Joined: Oct 02, 2014 Posts: 1916 Location: South Carolina, Dorchester County
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wesk Site Administrator
Joined: Apr 04, 2005 Posts: 16256 Location: Wisconsin
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Steve_I Member
Joined: Nov 29, 2015 Posts: 6
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Posted: Fri Nov 05, 2021 9:57 am Post subject: |
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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! |
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wesk Site Administrator
Joined: Apr 04, 2005 Posts: 16256 Location: Wisconsin
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BB Member
Joined: Feb 14, 2014 Posts: 31 Location: Dunedin, Florida
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Posted: Wed Nov 10, 2021 8:26 pm Post subject: |
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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. |
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Steve_I Member
Joined: Nov 29, 2015 Posts: 6
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Posted: Mon Nov 15, 2021 4:03 pm Post subject: |
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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. |
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wesk Site Administrator
Joined: Apr 04, 2005 Posts: 16256 Location: Wisconsin
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Posted: Mon Nov 15, 2021 10:52 pm Post subject: |
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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:
Quote: | 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. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
Last edited by wesk on Tue Nov 16, 2021 11:14 pm; edited 1 time in total |
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BB Member
Joined: Feb 14, 2014 Posts: 31 Location: Dunedin, Florida
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Posted: Tue Nov 16, 2021 7:09 pm Post subject: |
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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. |
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4x4M38 Member
Joined: May 30, 2014 Posts: 3447 Location: Texas Hill Country
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Steve_I Member
Joined: Nov 29, 2015 Posts: 6
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Posted: Wed Nov 17, 2021 8:39 pm Post subject: |
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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. |
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4x4M38 Member
Joined: May 30, 2014 Posts: 3447 Location: Texas Hill Country
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RonD2 Member
Joined: Oct 02, 2014 Posts: 1916 Location: South Carolina, Dorchester County
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Posted: Thu Nov 18, 2021 4:25 pm Post subject: |
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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! _________________ Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51
“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari
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wesk Site Administrator
Joined: Apr 04, 2005 Posts: 16256 Location: Wisconsin
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Posted: Fri Nov 19, 2021 12:46 am Post subject: |
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Quote: | 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.
Quote: | 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. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php |
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4x4M38 Member
Joined: May 30, 2014 Posts: 3447 Location: Texas Hill Country
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