hello all,not sure this is the right place for this post.A friend of mine just bought his first RV. he has always owned travel trailers and set them up in permanent camp grounds.bought a 1979 class c holiday rambler on a ford chassis. Has the 460 with a C 6 transmission with 3.73 gears. also has the "tag" axle.all emissions have been deleted by previous owner and a after market holly carb and K&N filters. duel exhaust.. runs great!!! this thing got 1.9 mpg driving 60mph for 50 miles on the highway!!i cant imagine that is what it should be considering my 1982 gets 8 mpg with my heavy footeverything is in order under the hood. only thing i haven't checked is the timing or looked at the size of the carb. it is a holly double pumper.. no sign that it running rich tail pipes are clean as to be expected and no fumes.any one have any ideas??

If it were that bad it would be running very rough and belching out black smoke out the tailpipe. Either that or it is leaking fuel badly. It is possible the fuel pump is bad and it is leaking into the crankcase!

How did you determine it used 25 gallons in that 50 miles?Had a 1975 F250 460CCI. (385 motor)It got 5 mpg Due to emissions engine had been detuned (timing ******** 8*)R E T A R D E D is blocked?Timing was SET BACK 8* as in opposite of Advanced!Changed cam/crank timing gears to pre-detuned, 4 bbl Carter and set of headers.Mileage went to 10 mpg regardless of how I drove it or loaded itOn Edit:Yes ........mine was a P/U not a Class C but Ford 460 (385 motor) was same whether it was in a truck/RV/boats and 1972-1978 were detuned.

I just went out and pulled the oil dipstick.. right on the full mark and no gas odor or taste either!! no signs of a fuel leak anywhere i looked for that when we got it home..i know it used that much gas because it was on E when we got it.. went to the nearest station and put 24 gallons in it.. or 50 bucks at 207.9 a gallon.. drove 50 miles and ran out of gas!! tank was bone dry i even laid on the ground under the gas tank and had my friend shake the rv to listne for gas to slosh around in the tank

I also had a 1976 f250 with a 460, it got 10 or 12 MPG going to work or carrying a big old pickup camper. Did it ever shift into high gear?

Are you sure it does not have two tanks and you filled one and then ran on the other one? I had an old class A like that once, when it coughed you had to manually switch tanks.

just purchased fuel milage horrible | Good Sam Community

21 Replies

Acdii
Explorer
Jun 29, 2016
Look down the barrels of the carb. If was really using that much fuel, you would see it going through the venturis, especially the secondaries. Rev it up a few times while watching, then rev it to about 2400 RPM, which I think with the C6 is about 50 MPH depending on the rear gear. If you see a white cascade of fuel, that is the problem, if running correctly, you should see very little fuel being sucked in, a fine mist maybe, but that would be it. Also pull the plugs that are easy to get to, if its really burning a lot of fuel, there would either be heavy carbon deposits, or they will be spotless. I used to rebuild those motors, and I can't imagine one using that much fuel that didn't have a blower on it and a pair of quads, or ram air.
patperry2766
Explorer II
Jun 29, 2016
Had an old 82 Ford that I inherited from my dad. He let it sit around and decompose as well. It had low miles but he kinda let it sit with minimal upkeep.

It would run fine, but when I would accelerate, the air would start blowing from the defroster. Once I let off the gas, the air would return to the vents. The controls were vacuum operated and the hose was cracked enough that it would operate fine at idle, but under load it would malfunction.

Cheapest fix would be replacing ALL the vacuum tubing and work up from there. Based on the vehicle's age, it should really be done anyway.

X2 on what the previous poster said. Ethanol fuel in BAD on all fuel lines unless it is designed for it. I had to replace the lines in my weed eater & blower this year because the fuel eventually disentgrated them and turned them into a jelly like substance.
bsinmich
Explorer
Jun 29, 2016
The newer gas is very hard on the old rubber lines. I got less than 4 mpg taking my GMC to Florida and with new gas lines got over 9 mpg on the return trip. That was one of the good things that came from thatbtrip. The rest was a fiasco.
Sport45
Explorer II
Jun 29, 2016
I don't know if your aftermarket carburetor has one, but the Ford OEM carbs of the day had an "Economizer" valve. This sensed manifold vacuum to adjust fuel flow. When they worked, they worked fine. But when the diaphragm fails the vacuum hose just sucks raw gas out of the carburetor bowl.

The test to see if it is good is as simple as disconnecting the vacuum hose and checking to see if any gas comes out.
tpi
Explorer
Jun 28, 2016
I'd check the timing, cylinder contribution, and make sure the vacuum advance works. Possibly the air/fuel mixture or at least have someone follow and insure you're not getting black smoke or a fuel leak from vent etc. Pull a spark plug or two and "read" them.

1.9 MPG is almost physically impossible from something which is running sort of right if you're not climbing steep hill. Perhaps the tank is hard to fill fully and gives different shutoffs on each fill.
mabynack
Explorer II
Jun 28, 2016
I worked on a 351 from about the same era that someone had removed the emissions from and it had a similar problem. The emissions are engineered into the engine and you can't just remove them without causing problems. The person who modified the 351 had drilled out the plugs that were inserted into the carb adjustments and backed the needles all the way out. His thinking was that more fuel meant more power. he had also disconnected the vacuum advance from the distributer. The guy had also removed the thermostat to make it run cooler. All of that resulted in an engine that got miserable mileage and had no power.

I would say it's time to take the motorhome to a good mechanic who understands older engines. The 460s were fuel hogs, but they should be getting closer to 8 mpg.
ol__yeller
Explorer II
Jun 28, 2016
Those Holly double pumpers are very thirsty. I had one years ago on a '76 F100 with a 351 Cleveland Cobra jet engine. It would pass anything except a gas station.
Chuck_thehammer
Explorer
Jun 28, 2016
check that the distributor is installed correctly .. NOT 180 degree out.

next check that the camshaft and crankshaft are in time.
check that the Vacuum advance is NOT stuck RE-Tard-ed (stupid editor) will not let me spell the Re-Tard-ED word., and is receiving vacuum.

stuck float in carburetor

dragging brakes. one or all.

and I understand bad gas mileage.. my 1965 Impala SS 409 with 4:11 gears got 8 on a good day... with a 4 speed transmission and (2) 4 bbl's.

but this thing has REAL problems... get working on it.. Thank You.

and AS STATED already.. duel Tanks.. Check them...
Grit_dog
Navigator II
Jun 28, 2016
I'm in the couldn't have burned fuel at 2 mpg. Maybe 4-5mpg up hill only. Don't think it could pull enough air without a blower to process that much fuel without burning pig rich and then some.
Gotta be leaking somewhere, or did you verify it really was out of gas? Ie, put a few gallons in and it fired right back up?
Interested in the outcome.
gbopp
Explorer
Jun 28, 2016
Sport45 wrote:
jking006 wrote:
I just went out and pulled the oil dipstick.. right on the full mark and no gas odor or taste either!! no signs of a fuel leak anywhere i looked for that when we got it home..

That's a new one, don't think I've ever heard of a taste test for gasoline. :)

One time I stopped at a GMC dealership with a Pontiac. The transmission was not shifting properly. The mechanic pulled the transmission dipstick
and tasted the fluid. He said there was no problem with the fluid.
He found the shifter linkage was the problem.
I wouldn't have believed it if I didn't see it.