just purchased fuel milage horrible

[jking006]

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 foot
everything 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??

[Acdii]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.

[bguy]

If it has a 2 line fuel system I'd suspect a leak in the return line.

[stickdog]

Take your foot off the brake.

[Sport45]

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. 🙂

Others have given great advice. You should also check your hub temperatures to see if brakes are dragging.

[tatest]

To go through 25 gallons in 50 miles it has to be leaking fuel somewhere. It cannot be pulling that much gasoline through the carb and still be running, you would have large volumes of liquid fuel washing through the intake manifold and into the cylinders, a mixture too rich to usefully burn. "Runs great" says this is not happening.

Leak could be at the tank or anywhere from tank to carb inlet. 37 years old, there are a lot of gaskets and o-rings that could have deteriorated enough to not hold fuel under pressure. To say nothing about rubber or plastic segments in the fuel line.