Carburetors at high altitude

[WHATROAD]

When we were talking about this, in Ouray, I forgot to ask if your fuel tank was baffled. We considered several "unique" ways to keep fuel at the ready. Perhaps a baffled fuel tank and the in-tank pump in the lowest camber would maintain the fuel need and avoid the vapor problem. Or not

 

[mriplaybass]

According to my calculations, here in Wisconsin where I live, the atmospheric pressure is around 13.07psi. @70°F. At the same ambient temperature, the pressure at 10,000ft drops to 3.96psi, and at 13,000ft, it drops to 2.50.

I don't know what I did wrong, but even though everything else should be correct, I will correct this error.
Here in Wisconsin, the ambient atmospheric pressure at 70°F is 14.19psi. At 10,000ft, the pressure drops to 10.19psi. At 13,000ft it drops to 9.08psi. The boiling point of gasoline, and everything else is still pretty close to the same. I guess I need to check the math a little closer!

 

[Posi]

Posi-May I ask your source for info on the no timing advance of the jumper wire.
My Limited GOOGLE search'n found a couple of references that it did. But no hard evidence.
I will look into my FSM and see what/if it has any info.
LG

The link that Torx posted doesn't mention anything about timing, only that the black/wtr wire is for high altitude operations so I'm not sure where folks are connecting it with timing capabilities.

The timing has to be set manually by turning the distributor. It has a mechanical and vacuum advance. This system doesn't magically switch over to computerized ignition system simply by attaching one ground wire.

The CJ stock computer doesn't have a timing map with spark advance values for all the combinations of engine speed and engine loads. It's just not that advanced.

The black ground wire runs back to the computer, somewhere along the line it has a splice. My guess is that the splice goes to vacuum switches to control the amount of air and fuel.

You could always put it to test. You have the ground wire on your Jeep. Check your timing, then check your advance timing before hand. Now attach your ground wire and check your timing at idle and advance. Reset your idle if need be before checking timing. Post your results.

 

[Lumpy Grits]

Posi-What you say makes sense, and what I'm think'n in the back of my pea brained head.
Then I start think'n about how the computer can control ign timing to a point, and does so via the 'knock-sensor' that the late model jeeps like mine has.
http://home.sprynet.com/~dale02/knock.htm
Gonna dig into the FSM.

LG

 

[mtnwhlr]

I thought the reason for the tricked ing upgrade when you wire the dissy directly to the control mod and take the computer out of it so it wouldn't adjust the timing or air/fuel.

 

[Lumpy Grits]

OK-here's what I found dig'n into my FSM, MR-252. About the Knock Sensor and Altitude Jumper wire.

Page B-273, in Fuel Feedback Systems section sez....
The Knock Sensor can control the ign timing "selectively" to the individual cyl or combination of cylinders.

Page B-275, again in the Fuel feedback Systems section....
Altitude Jumper Wire does 'tell' the MCU to adjust the air/fuel mixture, ign timing and idle speed.

Wish I knew how to take a picture of those pages, and post them-
That's W A Y above my skill level.:dunno:



LG

 

[Belizean]

LG looking at my FSM, MR-252 that info is verified

 

[Torxhead]

I would like to think that a manual timing advance of 5 degrees, premium gas, carb adjustment, and an oem fuel return line. just might do the trick. The electric fuel pump is a pita, as they use a lot power, you have to carry a spare, be able to change it on the trail, also require a safety shutoff, and you can't let the gas tank fuel level get to low.

 

[Lumpy Grits]

you can't let the gas tank fuel level get to low.

I do believe that is one of best remarks, in this entire thread.
The only thing I would add is a fresh fuel filter so the gas doesn't have to 'fight' to get to the carb.
LG

 

[Posi]

Good find LG.

Can you post a wiring schematic from your manual so I can see just how it works? Even a picture of it would help. It would still be a good test for you to ground the altitude wire and report the results.

The directions I posted previously still applies according to the FSM. You have to manually set the timing and reset your idle, the computer will not do it for you.

http://oljeep.com/gw/mr253_IS/MR253-IS-1E.pdf

 

[tarry99]

I don't know what I did wrong, but even though everything else should be correct, I will correct this error.
Here in Wisconsin, the ambient atmospheric pressure at 70°F is 14.19psi. At 10,000ft, the pressure drops to 10.19psi. At 13,000ft it drops to 9.08psi. The boiling point of gasoline, and everything else is still pretty close to the same. I guess I need to check the math a little closer!

All good conversation..........but to truly get the motor adjusted correctly for altitude.......you must first establish a "Base Line" somewhere at a given "Adjusted " or "Density Altitude" that your motor performs its best at. With that Jetting & timing information you can now Jet your carburetor correctly at any given altitude with a simple weather station.

Modern fuel injection systems have the ability to tailor fuel and timing to get the best air / fuel ratio for conditions.

Anywhere in the country daily there is a huge difference between " Mean" or established actual ground Altitude and "Adjusted Altitude"..........

Your local airports all have established ground level altitudes.......as will as the mountain passes in Colorado........change the barometric pressure , temperature & water grains for the day and that "Adjusted altitude" your Jeep motor see's could be several thousand feet either higher or lower.

Find the base line and adjust accordingly when at altitude.........normal basic adjustments for a normally aspirated motors at altitude are: Jetting, Timing , compression ( if you had a way of changing it) that could also be cam timing and lower gearing........

And during High temperatures as stated above get your fuel lines away from heat , there are plenty of heat wraps out there for that purpose or in extremes do a "Cool Can" which is just a gallon +/- can with several coils of fuel line inside.........drop a bag of ice in it and no more vapor lock for the day.

 

[Posi]

OK-here's what I found dig'n into my FSM, MR-252. About the Knock Sensor and Altitude Jumper wire.

Page B-273, in Fuel Feedback Systems section sez....
The Knock Sensor can control the ign timing "selectively" to the individual cyl or combination of cylinders.

Page B-275, again in the Fuel feedback Systems section....
Altitude Jumper Wire does 'tell' the MCU to adjust the air/fuel mixture, ign timing and idle speed.


LG

http://oljeep.com/gw/mr253_IS/MR253-IS-1E.pdf

Correct me if I'm wrong, I don't see anywhere in the FSM that says the altitude wire or the computer sets or advances the timing.

States you have to set the timing beforehand.
If you follow the proper steps the the computer will adjust the timing, idle, and air/fuel mixture.

The knock sensor creates a a signal based on vibration caused by engine knock. The computer uses the signal to "retard" the timing. See low.

Knock Sensor

The knock senor is designed to retard the timing not advance it.

 

[Torxhead]

Looking at pages B-269 and B-274, in the M.R 252, it does state that the mcu does change ignition timing. I also think this subject is off topic since Bass does not have the mcu.

 

[Posi]

By changing the timing they mean retarding the timing not advancing the timing. This topic is about running carburetors at high altitude, discussing a stock carb with altitude option is on topic IMO.

It's important to get the info correct.

 

[PistolDave]

All of these issues are why my jeep now has fuel injection. Gas has a much harder time boiling at 15psi. Constant recirculation through the regulator on the throttle body does not hurt either.

Vapor lock seems to be a lot worse now than it was back in the days before you had an ear of corn in your gas tank, coincidence?



Side note: In cab timing control, if you already have an MSD ignition.

https://www.msdperformance.com/products/rpm_and_timing_controls/timing_controls/parts/8680

Just noticed something about this product, if you are from California you cant use it, because it will give you CANCER. You have been warned.

 

[Lumpy Grits]

Only the knock sensor states it retards timing to 1 or more cylinders as needed.
The Altitude Jumper section clearly states it 'adjusts timing', did not indicate more.
Posi-if I knew how to post the schematic, I would gladly do so for you.
Now-For those without the MCU. What do you folks do for high altitude. I can understand resetting idle mix, idle rpm and timing. Do many of you change out the carb's jets also?
Pistol Dave-That MSD timing control box looks to be a help for altitude issues. First I've heard of it.
LG

 

[Lumpy Grits]

Good find LG.

Can you post a wiring schematic from your manual so I can see just how it works? Even a picture of it would help. It would still be a good test for you to ground the altitude wire and report the results.

The directions I posted previously still applies according to the FSM. You have to manually set the timing and reset your idle, the computer will not do it for you.

http://oljeep.com/gw/mr253_IS/MR253-IS-1E.pdf

Your PDF is for MR-253 and the adjustments are for a High Altitude setup that is in place and operational from the factory and THEN, bringing it down below 4K' ASL.
There is no mention of having to do those procedures in the MR-252. All that is stated in the MR-252, is to GND the HA jumper wire.
LG

 

[mriplaybass]

Looking at pages B-269 and B-274, in the M.R 252, it does state that the mcu does change ignition timing. I also think this subject is off topic since Bass does not have the mcu.

A bit off topic for my particular application, but I threw this out there for anyone and everyone to discuss ideas.
I am learning a lot from this post. I did not realize that there was a device on any CJ that could change timing or fuel mixture. However, as Posi's research indicates, the device appears to retard the timing in order to eliminate spark knock. The exact opposite of what we want at high altitude. I usually advance my timing 5° when going above 9,000ft, and so far that has worked pretty well.
As to premium vs regular gasoline, (pure, not 10% ethanol) what I have found so far is that there is no difference in the boiling temperature of the 2 fuel grades. Premium is more resistant to pre-detonation in high compression engines. There is no performance gain if the engine does not have high compression. But, will it help spark knock when we advance our timing to operate at high altitude? I have never experienced spark knock at altitude after advancing the timing using regular gas, but it is an interesting point.
What I have found however, is that the premium has a much longer "shelf life" than regular gas. Regular can start to deteriorate inside a properly closed container in as little as 3 months. Premium under the same conditions does not begin to deteriorate for nearly 9 months.

 

[Lumpy Grits]

Bass, out here in PRK, we call 87 octane 'regular', is that the same for you?
Our premium is 91 octane.
Note: I have noticed my Jeep starts easier/quicker at altitude(7K+' ASL)with premium in the tank. This is for cold(sits overnight)and hot engine starts...
I belive premium gas, carries far less ethanol.
I have stored premium gas for longer than 12+ months(with Sea-Foam added)with no issues.
LG

 

[mriplaybass]

Yes, same ratings here. I was referencing non treated gas. I have actually used 2yr old gas in outboard motors treated with Stabil.