High Altitude Tuning

a place to discuss anything of interest to owners of M151 jeeps

Moderators: rickf, raymond, Mr. Recovery

Post Reply
jim4164
Private First Class
Private First Class
Posts: 23
Joined: March 22nd, 2015, 1:40 pm
Location: Huntington Beach, CA

High Altitude Tuning

Unread post by jim4164 » March 4th, 2019, 9:49 pm

My M151A1 lives at or above 6,500 feet altitude. While driving my precious Mutt, if I press on the gas pedal in an attempt to quickly accelerate as when starting up a rise, the engine instead seems to lose its mojo and simply stall or at least react by bogging down and nearly or actually stalling. When I restart the engine, which is possible only if when cranking I hold the throttle fully open thereby "clearing out its throat", it smokes briefly until it clears out and returns to smoothly idling. Then I can drive again as long as I don't try to suddenly gas it. Is there a carburetor adjustment, other than the idle mixture, which I have already fiddled with, that can fix my problem? It almost seems like there should be an feature like an accelerator pump that would adjust the mixture when more power is suddenly called for. But why then would the exhaust be smokey when recovering from a stall or near stall resulting from pressing the gas pedal?
M151A1
BRP Can-Am Outlander 650
Jensen Healey 2.2
MB ML430
MB GL450
Grumman 15 & 17
Giant Dirt Bikes
Gone: 356A, TR4, Saab Turbo, ++
Missed: 901, Shelby Mustang, 300SL, Ford GT

User avatar
rickf
General
General
Posts: 19740
Joined: November 26th, 2007, 1:28 pm
Location: Pemberton, NJ.

Re: High Altitude Tuning

Unread post by rickf » March 5th, 2019, 11:23 am

It sounds to me like the accelerator pump is bad. Have you ever had the carb apart? High altitude will cause it to run just a hair rich down the road but you are not really high enough for that since these run lean to begin with. The bog is from lack of fuel at acceleration.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

jim4164
Private First Class
Private First Class
Posts: 23
Joined: March 22nd, 2015, 1:40 pm
Location: Huntington Beach, CA

Re: High Altitude Tuning

Unread post by jim4164 » March 7th, 2019, 5:27 am

Aha! I'll have to check for a bad accelerator pump. That makes sense. I didn't know they had one. Probably a bad accelerator pump diaphragm would cause the problem?
M151A1
BRP Can-Am Outlander 650
Jensen Healey 2.2
MB ML430
MB GL450
Grumman 15 & 17
Giant Dirt Bikes
Gone: 356A, TR4, Saab Turbo, ++
Missed: 901, Shelby Mustang, 300SL, Ford GT

User avatar
rickf
General
General
Posts: 19740
Joined: November 26th, 2007, 1:28 pm
Location: Pemberton, NJ.

Re: High Altitude Tuning

Unread post by rickf » March 7th, 2019, 11:05 am

VERY common problem on these. Be aware that the accelerator pump is vacuum operated so there will be no pump if you push the pedal while it is not running. Also, if you shut it off and the air cleaner hose is off and the pump is working then you will get a shot of gas and some gas leaking out of the mouth of the carb, this is normal. IF, the pump is working.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

jim4164
Private First Class
Private First Class
Posts: 23
Joined: March 22nd, 2015, 1:40 pm
Location: Huntington Beach, CA

Re: High Altitude Tuning

Unread post by jim4164 » March 7th, 2019, 3:58 pm

More very useful info. When I get it out in the Spring (late this year due to heavy Sierra snow pack) Il dig into the acc. pump issue. Thanks for your insights.
M151A1
BRP Can-Am Outlander 650
Jensen Healey 2.2
MB ML430
MB GL450
Grumman 15 & 17
Giant Dirt Bikes
Gone: 356A, TR4, Saab Turbo, ++
Missed: 901, Shelby Mustang, 300SL, Ford GT

Post Reply