Gas Mileage Conundrum

When I first drove the irritant from Santa Barbara to its new home in the mountains, I was getting about 10 mpg @ 65-70 mph. Not a problem, I knew I was pushin the brick pretty hard, it was hot as hades out, and all I had was a tshirt and no top on the jeep. I wanted to get her home. Only took about a week to recover from the sunburn. The temp gauge barely came off the cold peg the whole run.

When I got her home (about 3500ft altitude), I checked the thermostat, and sure enough, it had a 160 degree stat in it. I replaced with a 195. Guage now reads slightly below midpoint, even on a 100 degree day, being driven up and down the mountains at 50-55mph. The jeep runs great. At the time I changed the thermo, I noticed that the choke, which has the little tube pinched off, and electic disconnected, was always half closed. So I went ahead and loosened the 3 screws, cranked the choke wide open, and tightened 'em up. I have checked my odo after replacing the speedo cable in a vain attempt to correct an insane speedo, and although the speedo still has a mind of its own, the odo is accurate.

Filled the tank a couple of days ago, didn't drive it but 30 miles since. Gauge reads half full. Filled up tonight, put in 5.2 gallons, worked out to about 5mpg. Hummm.

When I picked up the jeep the PO told me he had a new tank installed, and that the guy who installed it fought the fill hose so much, he caused a leak in the tank. He had a friend look at it and they patched it with JB, and he hadn't had a leak since. I pulled the skid plate when I redid the bushings, and saw no leaks at that time. Checked again tonight with a flashlight, and everything looked dry. I pulled the plugs in Santa Barbara before I drove it home, and they were a little black/sooty, I just chalked that up to running a little rich, and figured changing to the correct thermo and opening the choke would help that little problem. I haven't checked the plugs since I made the changes, but I will in the morning.

I know there can be issues with living at altitude, I live at 3500, and run up and down from about 1500 to 6000ft pretty regularly. So tody I bought a vacuum gauge, hooked it up to a little capped nipple on the same fitting that my pvc hose goes to at the manifold. I'm at about 20hg steady as a rock, moving to 21hg when fully warmed up, still steady. So it appears no vacuum leaks. Running a DUI, rebuilt Carter YF, no smog hooked up other than to the evap canister.

So my first question is, how do I adjust the idle mixture properly? Just so I can eliminate that as a problem.

Second Q, I have read that sometimes the main jet must be changed with drastic altitude change, how difficult, and what size jet should I get, if necessary. Would a 3500 rise in altitude cut my mileage in HALF, when beng driven at saner speeds than the trip from Santa Barbara?

I'm really stumped on this one, the only other thing I can think of is i've got a thief coming on the property. Pretty ballsy, what with all the weapons around. But come to think of it, my wife and I went into town for about 6 hours the other day, locked the pit in the house, jeep just sittn' outside. We live in the country, pretty isolated, one neighbor.

The only other thing I can think of is when I was doing the bushings, I noticed a line from the canister (I think) to the fuel tank was broken. I checked it pretty carefully, and it appeared dry as a bone, so I didn't worry about it. After all, it was broken when I got 10mpg on the trip up.

I apologize for the book length post, I wanted to give all the info I could, gas is expensive.

Any Ideas would be appreciated...

 

10 mpg? With a 258? That's darn low, unless you have a lot of hop-up stuff on the engine. My CJ-6 with the same setup as you, except a full hard top, got around 14-15 mpg on my long commute from Santa Monica to Irvine.

Assuming you have the stock 1V Carter YF carburetor, the idle mixture screw is at the base of the carb. Turn it all the way in with your fingers or a small screwdriver. Don't crank it down - this is a needle valve, not a screw - just fully seated. Back it out maybe 2 or 2.5 turns. Adjust the idle speed with the idle stop screw on the throttle linkage. Turn the idle mixture in slowly until the idle just starts to stumble, them back off maybe 1/8 turn. Readjust the idle speed. This is termed "best lean idle" and is the setting the factory advises.

The idle mixture setting only works at idle - once you pass about 1000 rpm, the main circuit takes over, and that's controlled by the main jet. IIRC the YF is a control rod carburetor, using a tapered rod seated in the main jet and connected to the throttle. You can adjust the jets on a YF, but I haven't done it - I'm more familiar with the 2V and 4V Motorcraft and Holley carbs. I think the rule of thumb is 1 size smaller (0.001") for every 1000 feet of elevation gain?

Somebody will comment on the YF that knows more, or maybe ask on the F4/V6 forum. The YF was used on a lot of differnt cars, including the F4 and 232/258 Jeeps.

Also, I thought that the YF used a thermal choke (with the choke stove in the exhaust manifold), not a thermo-electric.

hth!

 

Went down and messed with the thing this morning. 1st, pulled the plugs, they looked pretty good, a nice tan. Gapped a little tight, DUI recommends 055, I gapped 'em at about 050 with a gauge.

Turned the orange screw under the capped vaccuum port (attachment 032) by hand until seated, out 2 1/4 turns. Slowly turned it in, waiting for idle to stumble, the thing had no effect, until lightly seated, then the idle rose a little bit. If i turned it OUT quite a ways, then it would run rougher. Shut off the motor, removed the screw, and took a photo of it (attachment 035) I'm assuming this is the idle jet screw, as it is at the base of the carb. I have no idea what the green screw is just above it in attachment 032, so I left the green screw alone.

Another little problem, which can be seen in attachment 038, I thought this red screw was the idle adjustment. as you can see, there is nothing for the screw to operate against, so IT has no effect.

So the net result is i'm a turnin the screws but nuttin' is happening. Kinda like trying to win an argument with my wife.

I'm heading over to the Gasman's place this afternoon, he has a 72/73 postal for spares, we're gonna try that carb, although, i'd really like to keep the one I have on the jeep now as it is a clean looking reman. Supposed to be fresh, according to the PO.

What am I doing wrong here? Besides thinking I might have the skillset to be messin with carburators, that is?

 

The only way you'll learn is by doing.

The red screw is the stop for the fast idle. It's only active when the choke plate is closed. Not applicable in your case (since the choke is disabled).

The idle stop will affect the idle speed. If your idle speed is too high, then you're running on the main circuit and the idle mixture will have no effect. You need to idle down to 500-800 rpm. The proper idle speed will be listed in the manual, but that's the range. The only way to set it is with a tachometer - you can get a test instrument at Pep Boys or such. Not sure what they sell today, since my stuff is old (even measures dwell ) - but there should be a cheap instrument available, maybe combined with a multimeter.

Read more about the YF here: http://trailforge.com/TSM/Section_04.pdf The gist of this is the same as a '75 - Jeep calls the idle stop the "curb idle screw".

Follow the flexible throttle cable to the carburetor - the idle stop screw will be on that linkage. You should easily be able to stall the engine by backing off on the idle stop screw.

 

Just now drove the irritant up the hill from the Gasman's. The plan was to switch my carb with the one from his postal parts jeep.

Before we switched 'em, we took a closer look, as Steve has the proper "antique" measuring devices to do a proper tune up. I remember the basic tune up stuff that I learned with in the foggy past, tach/dwell meter, timing light and tried to buy same at 3 different FAPS yesterday. I was told they were "antiques" and they would have to try to order me some. However they did have a real nice OBD II code reader they were willing to sell me for $150.
Such is life in the sticks.

Anyhow, we did a basic tune up, timing was advanced to about 10-12 degrees, probably helping to account for attempting to get the thing to run cooler with the 160 degree thermostat. We set the timing to about 5 degrees. Next up, Steve noticed that he could shift the carb on the manifold by hand. So we tightened that up. Why a vacuum leak did not show up with my vacuum gauge, I have no idea.

We moved on to setting the idle. I had figured out that I was using the wrong screw, and after hooking up Steve's tach, we set the idle to 650rpm.
It was running over 800. So much for my "ear". Once we got the idle set properly, Then the idle air jet screw had an effect, just like it should. Big relief. I really didn't want to replace the carb.

For the cherry on top, when we went to replace the air cleaner housing, we noticed that the little "thermo flap" in the snorkel was frozen in a closed position. We figured this was pretty close to having the choke partially closed, which would really hammer the mileage. We couldn't get the thing to open up, no matter what we tried, so we took a look at the housing on the postal. It was frozen open, which was fine, but the rest of the housing had so many butchered holes in it, we decided to go with my housing, and cut the snorkel with a hacksaw just behind the frozen flap. So the jeep wound up with a 4" shorter snout. It broke both of our hearts to modify something on this stock jeep, but ya gotta do what ya gotta do.

Its only about a 15 mile run up the hill from the Gasman's to my place, not much of a drive to test with, but the jeep did seem to motor a little more smoothly up the hill in 3rd. Not so much chugging. And on one downhill section the popping from the exhaust when off throttle was gone, so all in all, I think we may have found/corrected enough small problems to have an effect. Time will tell. I'm gonna run a couple of tankfulls through the thing, and I'll report back the result, I promise.

You sure do learn by doing, and it really helps to have a friend with a better memory than mine, plus the proper "antique" tools. All in all, a good experience, I'm gettin back that feeling I had when I was a kid, just learning about cars, some of it is coming back.

Thanks for your responses, everybody, and you'll be hearing from me again with more problems soon, 'cause I drive a Jeep!