Carburetors at high altitude

[mriplaybass]

Getting back to the fuel issues. A little more information from personal experience. Although there has been a lot of discussion on the carburetor, fuel filter orientation, OEM pump, etc., the problem that several of us have had is not occurring under the hood. As I stated before. The fuel is vaporizing in the fuel line before the pump, whether it is electric or mechanical.
I specifically wanted to test my theory, so we installed a digital fuel pressure gauge on the dash board. The sending unit is between the pressure regulator and the carburetor. That way, I know that if there is fuel pressure showing, barring a stuck float, gas will be getting to the carburetor. Every time we had problems, the fuel pressure suddenly dropped to 0. Once the gas in the float bowl was depleted, the engine quit. I could hear the fuel pump cavitating. After the Jeep sat for a while and cooled off, I would turn the key on, and could hear the pump catch prime. Once I could hear the pump slow down, I knew gas was flowing because the pressure gauge would go back to where the regulator was set. Once this occurred, the engine started immediately. I have seen and heard this happen to others who used a similar set up.

 

[Posi]

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

LG please re-read the bulletin!
1, 2, and 3 in BOLD numbers and letters are for vehicles that are originally set up for under 4000ft being operated above 4000ft.

Also, the reason you can't find it in your manual is because it's a bulletin and came after the manual was printed.

I'm starting to think you ready believe all you have to do to your low altitude CJ is attach a ground wire and your engine will run great at high altitudes.

 

[Lumpy Grits]

Posi-Your MR-253 refers only to Grand Wagoneer and truck. Nowhere is it mentioned about the CJ. It also states the MR-253 is for '84 models, no mention of later models. Could things have changed for the 85's and 86's as in updated MCU?
Mine is an Ca. mod '85, AMC 258 i6 / 4.2l . Build date was Feb '85 and I bought it in March of '85. The FSM MR-252 clearly states the HA jumper wire can control the timing. I have no reason not to believe it.
FWIW: My MR-252 shows a publish date of January 1984.
I have never ck'g the ign timing with the HA jumper connected to GND. Never saw the need. But, I can tell you that changes are going on from the increase in idle rpm and throttle response at altitude.

"I'm starting to think you ready believe all you have to do to your low altitude CJ is attach a ground wire and your engine will run great at high altitudes."

YES-I do believe this, have no reason not to. I have run the HA connected many, many times over the last 32+ yrs at altitudes to just over 10K' ASL, with no other adjustments made. Ran very well.

LG

 

[Posi]

The high altitude GND wire on my '7' sure helps above 6K' ASL.
I have had my OEM, carb'b '7' above 10K' ASL with little issue on a few cool days. Power loss was noted.
I usually do have to up the idle some.
Here's my question-What do folks do with their ign timing at the altitudes you folks saw on the trip?
LG

Power loss
Need to increase the idle
All the symptoms of an engine that needs to be advanced.

 

[Lumpy Grits]

Some power loss is to be expected from the thinner air. Don't you think?
I run my idle just a bit higher than factory specs. Takes at most, 1/4 turn of the screw.
At the 12K+' ASL seen in the Colorado run. A timing tweak may well be in order, as the MCU will only change so much.
May well look into the MSD timing adjuster........
LG

 

[Posi]

Some power loss is to be expected from the thinner air. Don't you think?
LG

I'm not sure what to think after reading these conflicting posts.

I have run the HA connected many, many times over the last 32+ yrs at altitudes to just over 10K' ASL, with no other adjustments made. Ran very well.
LG

I have had my OEM, carb'b '7' above 10K' ASL with little issue on a few cool days. Power loss was noted.
I usually do have to up the idle some.
LG

I can tell you that changes are going on from the increase in idle rpm and throttle response at altitude.
LG

What I can tell you, 2 years ago I set my my timing and adjusted my carb at 7000ft and it ran fine every day on the trail at 7-13k ft. If there was a power loss I didn't notice it. Anyone on the trail ride in 2015 can attest to that being the case. Zero fuel problems.

The difference in traveling 10k to 12k is minimal. Basically hitting the peak and coming down and it's all steep so your not at 12k long.

MSD timing adjuster........
LG

It's my opinion you don't need it. You already have the altitude adjustment wire. If it were me I would manually advance the timing 5°-7°, attach the wire and see how she runs. I would also take some extra vacuum line if further adjustments are needed. Again, just what I'd do if I had the jumper wire.

 

[mriplaybass]

My altitude fuel problems did not really start until I was over 11,500ft. But even then, ambient temperature, wind, steepness of trail, etc. all seemed to play a part.

 

[Lumpy Grits]

Posi- No conflict that I can see. My CJ did run well, like I said. I run the idle a little higher than what the MCU sets it to, when the HA is activated. The CJ has always run fine on the road and trail, when the HA jumper is in use.
Don't forget-I still run ALL of the OEM, Ca model SMOG junk on mine.
After thinking it over-I agree about the no-need for the MSD timing controller. Rather put that $$$ towards gas.
What did surprise me, was the fact I never had to adjust the idle air/fuel mix after activating the HA jumper...

Bass- What were the general air temps folks saw at the higher altitudes?
How full did you keep your gas tank?
Could any of the fuel starvation/v'lock issues you had, been from the gas tank not venting/equalizing?

LG

 

[mriplaybass]

Air temps starting out at around 9,000ft could be as much as 75°F, but once we got around 11,000ft, temps could be anywhere from around 65°F down to I am guessing around 32°F since it was snowing on several occasions. Rarely had any problems when ambient temps were below 50°F unless I sat at idle and on an incline for a long time.
Tank would be full at the beginning of a trail ride, and usually around 1/2 at the end of a ride. Although once, it did get down around 1/4 tank, although the amount of fuel in the tank did not affect the vapor problem.
Tank vents properly.
As stated earlier, even when removing the gas cap and blowing pressure into the tank, it would take awhile for gas to start coming out near the clutch bell crank.

 

[Lumpy Grits]

You have ck'd all fuel line clamps/connections etc, to be sure there is no way the pump could be drawing in outside air as it sucks fuel from the tank?
How old is the FP you are using?
LG

 

[Torxhead]

You have ck'd all fuel line clamps/connections etc, to be sure there is no way the pump could be drawing in outside air as it sucks fuel from the tank?
How old is the FP you are using?
LG

The flexible line from the gas tank to the hard line is a good place to develop cracks as it is hard to get at. With a V8 I would assume that you have dual exhausts and finding a location to run a gas line and keep it cool might be tricky.

 

[Lumpy Grits]

Where I live(Mojave Desert), it doesn't take long for rubber fuel hoses to crack and weep. I'm sure the dang Ethanol has plenty to do with that also.
With rubber fuel lines, what I have found to give exceptional long service life are the lines rated for FI. I also use the FI style hose clamps, which don't 'dig' into the line like the worm-screw type will.
I also double clamp all my flexible lines wherever possible, whether they be fuel, water or air.
LG

 

[Posi]

QUOTE=Lumpy Grits;246155] The CJ has always run fine on the road and trail, when the HA jumper is in use.
LG[/QUOTE]

The high altitude GND wire on my '7' sure helps above 6K' ASL.
I have had my OEM, carb'b '7' above 10K' ASL with little issue on a few cool days. Power loss was noted.
LG

I usually do have to up the idle some.
LG

I can tell you that changes are going on from the increase in idle rpm and throttle response at altitude.
LG

Clear as mud

My guess would be you "did" have power loss as you mentioned, not advancing the timing by 7° would do that. More likely your engine didn't run all that good as well because you didn't follow the factory high altitude adjustment procedures as I mentioned earlier, and as documented in the factory I.S. bulletin below for 84-86 CJs.

Read adjustment procedure #2
http://www.civilianjeep.info/84-86fsm/n-i.s._notes.pdf

 

[Lumpy Grits]

Posi.
Thank you for that link.
They only refer to '84. I do know the spare MCU I have is labeled for Ca. 85-86 yrs only. Were there changes made from the 84 mcu?
I have had this spare MCU in place for over 20+ yrs, after my OEM one had a 'stroke'.
LG

 

[Posi]

All 84-86 CJ computers are the same.
If you have an auto cal emissions their all the same through 84-86.
If you have a manual cal emissions their all the same through 84-86.
same goes for the fed emissions. J10 and 89 yj use the same fed manual computer as well.

If you look at the beginning of the bulletin it states: covers the period of Jan 1-1984 to Dec 31 1986

Plus, this I.S.bulletin replaces all individual I.S. bulletins from 1984 through 1986.

 

[Lumpy Grits]

:ty:
I will try the timing adjustment, the next time I use HA.
Guess I been lucky all these years. Ignorance is bliss, somedangtimes
May I ask-How did you come about finding these notices?
I ask because I see this notice has a 1987 copyright.
LG

 

[Posi]

I just did a search on "CJ7 service bulletins", I had to look through some weeds to find it.

I really hope you can make the CO trail ride next year it would be interesting to see how the HA option works (I'm thinking set up properly it will work good). I image there isn't too many CJ owners that have a stock setup like you to allow them to take advantage of the HA option.

 

[Lumpy Grits]

I would like to see Colorado again. I was born there, and I've always called it home.
Your search 'foo' is much stronger than mine. I did spend a fair amount of time hunt'n also, and came up with NADA.
Here's the weird part-I just looked at my receipt(badly faded)for the replacement MCU. It does state for Calif models 85 & 86 CJ. The PN is to faded to see. I never recorded the PN in my maintenance/repair log book. Plus I can't see then label from under the dash.
Hate having to stress over SMOG for my 'toy' every 2 years. Plus the fact, it is now held to tighter emissions(changed in 2011)than when it was sold to me in '85. FK PRK.
If the Doc says it's OK for me to attend-I sure will try to be there. At 66+ years of age, I'm forced to deal with class 3 CHF. The altitude could well be an major issue. Might have to install nitrous to give me a boost.
LG

 

[mriplaybass]

The flexible line from the gas tank to the hard line is a good place to develop cracks as it is hard to get at. With a V8 I would assume that you have dual exhausts and finding a location to run a gas line and keep it cool might be tricky.

Yes, dual exhaust, but on the side where the fuel line runs the exhaust including the muffler is wrapped in header wrap.
All rubber fuel lines are new. We plan on replacing all the old steel lines this winter.

 

[Lumpy Grits]

Bass-Upgrade your fuel 'feed-line' to 3/8" dia. steel or copper tubing.
I'm positive, it will make an improvement. It sure won't hurt anydangthing.
Good luck,
LG