Unbelievably bad gas mileage

[Chowbird]

This is such great info, thanks. Lots of mechanic friends around here, is it simply a case of retuning the carb, or is it major PIA diagnosis job? Your description is spot on. It is definitely Fat and Happy

 

[Rustytruck]

Carb got a big bog when you accelerate hard? How about pops and backfires?

I would throw a rebuild kit through the carb and adjust the float low. With the rebuild kit you will get a new power valve. Then you should be all good inside. Then I would look at the accelerator pump stroke. As set right now I am guessing that you getting a bad bog and need to lean it out some so look to reduce the stroke and reduce it until the the engine has weak acceleration and then go back up a step or 2.

With the Jeep and the kids driving your fuel is probably being eaten up with a mal adjusted choke system and a miss adjusted accelerator pump. The kids are running all over town and not driving long trips on the highway. If your running an electric fuel pump I would check fuel pump pressure and make sure the pressure is around 4 lbs. Early Jeeps bounce around allot with high fuel pressure and a float set too high you can over flow the float and add significant fuel.

352 motor peak torque is 295 ft lbs @ 2000 rpm when new and your pulling all of that at freeway speeds. Slowing down to 55 mph should help the mileage numbers.

 

[jckkys]

It seems a little hard to believe anyone would transplant an FE into a CJ5. And if that choice wasn't odd enough, the FE chosen was a 352. The same bore and stroke as a 351w plus 100 more pounds of cast iron. With the same outside dimensions and weight, it could have been a 428. The statement that 352s and 390s are POS engines is incorrect. The '60 performance 352 had more HP and torque than the '71 Boss 351 Cleveland.The FEs had too much air flow to work well as small displacement low performance applications. The 351w does this better with less weight and less bulk. The other problem is length. CJ5s have a 83.5" wheel base. That's almost a foot shorter than early Broncos.

 

[Chowbird]

Yeah, I realize that was an odd engine choice. I bought the jeep about 8 months ago from a shop that restores old jags and specialty cars They must have had this engine sitting around and done it as a lark. It's a great looking CJ with an original hard top and heat. Big pluses for living in MN. Like I said, it runs great, drops a bit if oil on the ground from either the valve covers or the rear main seal, but the fact that it starts, runs drives so well I grabbed it at a pretty low price. Now I realize why they got rid of it. It was their shop truck for sure. It's four wheels strapped to a gigantic engine. If I can get the mileage under control, it'll be a pretty fun ride. Would you guys suggest a different carb all together,nor just rebuilding the Holley 2 barrel. Believe it or not, the shop gave me an intake manifold for a 4 barrel.

 

[jckkys]

The 2bbl Holley is likely a 2300 model, that came in 350 and 500 CFM. Hopefully its not the 500 CFM, that's anything but an economy carb. The 4bbl intake would let you use a 4100 that was OE on 352s. The 480 CFM 4100 is the best choice in a 4bbl. A 450 CFM Holley 4 bbl like the List#4548 is a good economy carb. Another big factor in fuel economy is good ignition timing.

 

[pcf_mark]

Also pull the dipstick and see if the oil stinks of gas. Or if it is way over full because gas is leaking into the pan. If it works like the 289/302/351 the fuel pump diaphragm can tear and leak raw fuel into the engine.

 

[AxlesUp]

my brother's dodge had that problem for a while. turned out his mechanical fuel pump had ruptured and was pouring fuel into the oil. guess the heat from the engine was burning most of it off.

My friends 39 Jag had that problem as well. turned out his fancy stainless braided fuel lines were dry rotted and cracked on the inside were you couldn't see it and the fuel was evaporating through the braids.......

moral of the stories is that mileage that bad is caused by something fairly minor that is easily overlooked.....


PS:

I almost forgot.

clearly the problem is the body and frame


Edit:
LoL page two gets me again...........

what they said about the black smoke. you are running really rich.

 

[roostracing]

for what its worth, i have a Holley 600cfm 4 barrel on my 302, 5.5 lift, 4.56's, on 36 super swampers, through a C4...i get an average of 10 mpg going 60-65 mph.

Something is definitely not right on your rig. That motor isn't helping but i'm almost positive its your carb.

5.6 mpg has to be the record for worst gas mileage ever on a EB.

 

[Chowbird]

Yeah, I'm realizing my carb is hopped up on caffeine like Kramer. I am really thankful to you guys for helping me out with the diagnostics. I'll have to look into which is the best option:

Retune or rebuild the current Holley 2 barrel

Ditch the Holley and install a carb that's more
Economical

Ditch the 352...a road I'm not fond of undertaking.

Install the new intake manifold and put on a 4 barrel that is better suited for the engine.

Maybe all the carbon on my garage floor will fossilize into diamonds.

 

[Rustytruck]

start by rebuilding the carb.
http://www.jegs.com/i/Holley/510/37-1543/10002/-1?parentProductId=

Then tune it and run it and see what the plugs look like.

Check for vacuum leaks everywhere. Adjusting a carb to cover vacuum leaks will not help you at all.

Then its all about the timing system. If its not working properly everything else is a moot point. Every thing from improperly set, to a slacked timing chain to the centrifugal and vacuum advance systems sticking and not working properly.

Its a jeep its not supposed to run and drive properly. That's why Ford made the Bronco to smooth over the the jeeps bad issues. Swapping the Ford motor was a step in the right direction but it was a very bad step using a 352. But it should be able to make the motor run properly. I would also borrow a real gps to do your calculations.

 

[jw0747]

It seems a little hard to believe anyone would transplant an FE into a CJ5. And if that choice wasn't odd enough, the FE chosen was a 352. The same bore and stroke as a 351w plus 100 more pounds of cast iron. With the same outside dimensions and weight, it could have been a 428. The statement that 352s and 390s are POS engines is incorrect. The '60 performance 352 had more HP and torque than the '71 Boss 351 Cleveland.The FEs had too much air flow to work well as small displacement low performance applications. The 351w does this better with less weight and less bulk. The other problem is length. CJ5s have a 83.5" wheel base. That's almost a foot shorter than early Broncos.

You're talking about "performance" 352 and 390. Those were equipped with 4bbls and other performance items. I was around in the 60s and those stock 352 and 390 engines equipped with 2bbls were junk. 2bbl is not enough carb for those engines. My brother had a '67 2dr Fairlane 500 w/ 390 2bbl and it was sharp looking but ran like a POS. Mustang with 289 2bbl could outrun it.

 

[Chowbird]

Haha! . Yeah I'm on it. Here's to double digits!

Incidentally,my Bronco cruises happily all day long, topless, carefree, and in-tune. The CJ is like its younger brother that just can't get his act together.

 

[67RT]

Could be you have the wrong speedo gear on the cable. Is it a stock rear tank, because I think there are only 12 gallons.


http://classicbroncos.com/forums/showthread.php?t=249180

I agree and bet your miles are not logging right. DId you change gears? 33" tires and different gears can tilt the speedo/odometer. Run a tank using your GPS and log miles that way?

 

[hardwork]

Another common problem we used to see a lot of was the vacume advance diaphragm
leaking so there is no vacume advance . this will loose power and destroy gas milage

 

[TheGanzman]

Any chance someone is f*cking with you and siphoning out some of your fuel? We did this to a fellow street racing buddy (a New Brunswick NJ cop!) of mine back in the '70's when he put a 427 big block into his '69 Chevelle. For three months he thought he was getting ~4mpg!

Of course, after we had our laughs all around, his mileage JUMPED up to ~7mpg; but that was back when the skinny pedal was just an "On-Off Switch, LOL...

 

[Chowbird]

Haha! . That's awesome, Ganzman. Sadly, that's not the case. The gas cap is a locker. Why, I'm not sure, it came with the jeep. Someone's gonna have a rude awakening if they try and suction gas out of that echo chamber. Kinda like Geraldo and Al Capone's vault.

 

[jwhit]

my 1970 with 4.11 gears 302 3 speed with 33tires 9.6 mpg is my average if i dont dog it
not much highway driving since the 4.11 gears kill me

 

[Timmy390]

my 1970 with 4.11 gears 302 3 speed with 33tires 9.6 mpg is my average if i dont dog it
not much highway driving since the 4.11 gears kill me

That's in line with my rig. Same setup only I dog it and get around 7MPG

Tim

 

[TheGanzman]

Funny stuff - I'm here in a hoity-toity guard-gated community in Kalifornia - my wife owns an AMG63 and I own 3 gas-hog SUV's - our BEST mileage is ~15mpg! I wake up WEEKLY to a burning cross in our front yard, LOL. We're not too popular with our Prius-owning neighbors, doncha know...

 

[DirtDonk]

...not much highway driving since the 4.11 gears kill me

You must not have a tach. That or you're thinking in terms of modern cars that loaf along with the engine at 1200 rpm at 70 mph. Your Bronco was geared that way from the factory and was perfectly capable of zinging along at 55-60 with those little 27" tall tires at probably 2700 rpm or so.
Or are you strictly talking about fuel economy?

As far as engine life is concerned, with your 33's your engine is probably spinning around 2500 rpm at 70 and perfectly happy to do that. You can run your engine at 3000 rpm all day long and it won't even be breathing hard. Besides, it takes a bit of rpm for our old carbureted engines to push the ole' 4000 lb brick through the wind anyway.

I took many a long freeway trip in mine with the same gears and 32" tires and never hesitated to run anywhere between 55 (the legal limit back then) and 75 knowing that I wasn't abusing the engine at all.
Yes, it's not as efficiency or conducive to a 200k mile lifespan, but it had almost that and purred like a kitten when I stopped driving it.
Never made much of a difference in fuel mileage. At a steady 55 I could get 17, while at 70 I was getting 12. Certainly a difference, but even the 12 is still what a lot of people are getting anyway.

Just saying don't hesitate to take it out on the freeway if you need/want to.

Paul