Tips for Carbs in Cold Elevation - Don’t want EFI

[m_m70]

When I lived in the sierras, around 6500ft, I had an Edelbrock 1405 manual choke and to start at cold temps, at least under 20* I used half choke and one pump at the accelerator. Worked well and my rig was always outside (no garage). I know yours is more setup for Phoenix elevation so that may not work for you......

I know on the Edelbrock I had I could swap out the metering rods and springs in a couple minutes. Used to keep a set in a sandwich bag in the glove box and swap them out in Auburn while I gassed up/grab a bite, to get over the summit to Tahoe or the other way to Sacramento. I'm not familiar with your Carter and if it's as easy to tune on the fly as the Edelbrock was.....

 

[nvrstuk]

remember full choke at altitude on a carb tuned for low altitude means lots of fuel, that's not vaporizing.

I was going to buy an identical offroad carb tuned for altitude , and swap it at the first stop, seemed like easiest way to make it work. I did not want to be playing with removing top of carb to swap needles out. Then I found the carb guy in Glendale who built me up a Qjet, that had an automatic altitude compensator, and turned it into a manual adjustment, 1/2 turn either way of a screw gets me set 2,000 or 6-7,000 ft up here in Payson.

Yours sounds more like a cold issue than altitude issue, or maybe both piling on. The fuel is not vaporizing. Simplest trick, try some starting fluid spray? Pull a plug when at altitude see how bad it looks. Might try hotter plugs buy one step hotter.

Exactly right as air at altitude is at a premium. You NEED to rejet and change your starting procedure at altitude.

Also, 20-50wt is too heavy. I have read so many tests on the load that 20-50 wt creates. I ran 20-50wt for 40 years in my Bronco. Not any more. Yes it needs the slippery stuff in it but not 20wt for those cold starts.

I'd like to hear what Bruce (I'm assuming) built you for an "automatice altitude compensartor". How does that change metering rods and jet sizes with 1/2 a turn of a screw? I have built more than a few Q-jets, I'd like to hear how this works.

 

[gnpenning]

A manual choke is a big help once you learn how to use it. You reduce the number of people that can drive it effectively.

I'm interested in hearing about the automatic altitude compensator as well. I do know how you can adjust the travel on the primary side metering rods on a qjet from the outside of the carb. Is that what you are talking about?

 

[FordFarmer]

I’m curious about this automatic altitude adjustment as well. The way I read it is that it was removed and replaced with a manual adjustment. One of our other family transports has auto altitude compensated carbs but I wasn’t aware they were available for our trucks.

 

[mpboxer]

When I lived in the sierras, around 6500ft, I had an Edelbrock 1405 manual choke and to start at cold temps, at least under 20* I used half choke and one pump at the accelerator. Worked well and my rig was always outside (no garage). I know yours is more setup for Phoenix elevation so that may not work for you......

I know on the Edelbrock I had I could swap out the metering rods and springs in a couple minutes. Used to keep a set in a sandwich bag in the glove box and swap them out in Auburn while I gassed up/grab a bite, to get over the summit to Tahoe or the other way to Sacramento. I'm not familiar with your Carter and if it's as easy to tune on the fly as the Edelbrock was.....

Thank you, Carter is the same and I actually have metering rods and springs with me, might have to pop those in.

 

[nvrstuk]

I learned and continue to state that 4bbl carbs have essentially 7 "circuits" (some say 5 but...).
Float, choke, idle, main metering, power enrichment (example- Holley Power Valve), accelerator pump and of course those HUGE Q-jet secondaries.

If all you are doing is changing the idle mixture screws (unless all you are wanting is a better idle at 10,000ft) your engine is running super fat the rest of the time. I am really curious how a 1/2 turn of some screw can adjust several different "circuits" at once and make this a 2k-8K ft elevation carb.

Just curious- love to learn new stuff.


When cold starting at elevations above 5K ft I would never use the choke. Crank for a couple seconds and then hit the accelerator 2-3x and it would always fire up. When your air density isn't high enough to run at the AFR that you need then why restrict more air flow with the choke plate being closed to restrict more air?

LIke Broncobowsher said too- are you 100% POSITIVE ??? that you have fuel flow into your engine during those first 5 seconds of cranking? Remember the decades of fuel "boiling off" posts so your fuel bowls are dry in the am and you need to crank for 5 seconds just to get gas into your carb. Very common

 

[cldonley]

Aircraft pressure carbs (some, at least) have a bellows that compensates fuel flow for air density, leaning the mixture in relationship to lower air pressure. These carbs are really more like throttle body fuel injection though, no float bowl, hence the "pressure carburetor" designation. They were designed for high altitude aircraft ops during WWII. I think I have read about the same type of set up for some Qjets, but I don't remember where. Adds a pretty significant layer of complexity to the carb for sure. This picture gives you an idea how it works. Reduced pressure on the back side of the diaphragm allows the spring to reduce the size of the jet, leaning the mixture. http://2.bp.blogspot.com/-dkOl76hvA...600/Schematic+of+the+PS+series+carburetor.jpg

A quick search says the M4MEA Qjet on an Eldorado was an altitude compensating carb.

 

[FordFarmer]

Aircraft pressure carbs (some, at least) have a bellows that compensates fuel flow for air density, leaning the mixture in relationship to lower air pressure. These carbs are really more like throttle body fuel injection though, no float bowl, hence the "pressure carburetor" designation. They were designed for high altitude aircraft ops during WWII. I think I have read about the same type of set up for some Qjets, but I don't remember where. Adds a pretty significant layer of complexity to the carb for sure. This picture gives you an idea how it works. Reduced pressure on the back side of the diaphragm allows the spring to reduce the size of the jet, leaning the mixture. http://2.bp.blogspot.com/-dkOl76hvA...600/Schematic+of+the+PS+series+carburetor.jpg

A quick search says the M4MEA Qjet on an Eldorado was an altitude compensating carb.

Yup, we have ‘em on our Twin Bonanza (Bendix PS-5BD) and they work awesome. I make a few small adjustments once a year and they hold their “tune” very well in between. Wouldn’t mind having something similar for the Bronco. It’s all mechanical so I wouldn’t have to concern myself with the electrical/programming issues some of the TBI users seem to have.

 

[nvrstuk]

Looks pretty slick, knew there had to be a high altitude carb for planes but how come OE did nothing for autos/trucks that live at 6kft? Probably "good enough" & "it costs more"... answered my own question I think. lol

Never ran across one on an Eldo- did more than a few of those since they were the 850 cfm Qjets but never ran across one of the special altitude compensating ones.


Cool stuff

 

[cldonley]

Yup, we have ‘em on our Twin Bonanza (Bendix PS-5BD) and they work awesome. I make a few small adjustments once a year and they hold their “tune” very well in between. Wouldn’t mind having something similar for the Bronco. It’s all mechanical so I wouldn’t have to concern myself with the electrical/programming issues some of the TBI users seem to have.

Twin Bonanza is such a sweet airplane to fly. Hate working on them though!

 

[cldonley]

Looks pretty slick, knew there had to be a high altitude carb for planes but how come OE did nothing for autos/trucks that live at 6kft? Probably "good enough" & "it costs more"... answered my own question I think. lol

Never ran across one on an Eldo- did more than a few of those since they were the 850 cfm Qjets but never ran across one of the special altitude compensating ones.


Cool stuff

Most airplanes have manual mixture adjustment so you have a little formula you go by to manage it. The altitude compensating one take a little work load off the pilot, but add a lot of work on the back end maintaining them. Gets harder and harder to find shops to maintain them. They are not completely automatic, they still have a mixture control in the cockpit. I imagine the added maintenance and cost is why the OEM didn't make them for vehicles.

 

[FordFarmer]

Twin Bonanza is such a sweet airplane to fly. Hate working on them though!

I feel the opposite. It’s so big and roomy. Designed to be field serviceable by GI’s…..I just deleted about two paragraphs of off topic drivel about pressure carbs. Sorry OP! My personal experience with them is very positive and would lead me to want to try one on the Bronco. I have a new rabbit hole now.

 

[cldonley]

I feel the opposite. It’s so big and roomy. Designed to be field serviceable by GI’s…..I just deleted about two paragraphs of off topic drivel about pressure carbs. Sorry OP! My personal experience with them is very positive and would lead me to want to try one on the Bronco. I have a new rabbit hole now.

I'm planning on going Qjet, but hadn't planned on the altitude compensating model. Maybe I should re-think that.

 

[FordFarmer]

I'm planning on going Qjet, but hadn't planned on the altitude compensating model. Maybe I should re-think that.

If you find one please start a thread on it…..I’ll do the same. If nothing else it’ll be fun to tinker with.

 

[gnpenning]

Off idle circuit. The vertical slots above the idle mixture holes in the Venturis.

Used by those with light loads along with light throttle.

No, I don't call it a 8th circuit.

 

[73azbronco]

Exactly right as air at altitude is at a premium. You NEED to rejet and change your starting procedure at altitude.

Also, 20-50wt is too heavy. I have read so many tests on the load that 20-50 wt creates. I ran 20-50wt for 40 years in my Bronco. Not any more. Yes it needs the slippery stuff in it but not 20wt for those cold starts.

I'd like to hear what Bruce (I'm assuming) built you for an "automatice altitude compensartor". How does that change metering rods and jet sizes with 1/2 a turn of a screw? I have built more than a few Q-jets, I'd like to hear how this works.

So, my understanding is the carb is out of early 80s eldorados or some such, the altitude compensation was an option on those cars in Colorado, and worked automatically, but it didn’t work well, efi followed soon after. However, taking off the automatic contraption part, and making it a simple screw under a plug at the front of the qjet, acts like a variable jet. Works great. 1/2 turn from PHX to Rim at 7000 feet is all I need, another 1/2 turn for 10,000 feet in Colorado.

 

[cldonley]

So, my understanding is the carb is out of early 80s eldorados or some such, the altitude compensation was an option on those cars in Colorado, and worked automatically, but it didn’t work well, efi followed soon after. However, taking off the automatic contraption part, and making it a simple screw under a plug at the front of the qjet, acts like a variable jet. Works great. 1/2 turn from PHX to Rim at 7000 feet is all I need, another 1/2 turn for 10,000 feet in Colorado.

Found the altitude compensating Qjet for a 75-77 Eldorado. https://www.caddydaddy.com/shop-par...ester-quadrajet-m4mea-carburetor-rebuilt.html
I can see how making it a manual adjustment vs. automatic would be a good thing. Read some articles on it that indicated it was not very reliable in the day.

 

[nvrstuk]

Thx for posting up the pic.

Bet those old 501cu in Caddy's got great mileage at 5k ft! lol

 

[73azbronco]

Found the altitude compensating Qjet for a 75-77 Eldorado. https://www.caddydaddy.com/shop-par...ester-quadrajet-m4mea-carburetor-rebuilt.html
I can see how making it a manual adjustment vs. automatic would be a good thing. Read some articles on it that indicated it was not very reliable in the day.

Third picture top view of carb, left side of carb, the round hole with brass plug above the oval fitting is the port. Bruce punches out the plug, threads it, and inside that is the screw, easy prays. I’d just buy one from him though buying from that site and experimenting, that and they want over $700 with core charge, that’s about what I paid Bruce, but that was about 10 years ago, it seems.

 

[gnpenning]

I would like to see the inside of that carb for a reminder. I believe it is doing what I posted about in #23 ???