Really bad milage

[Surfdaddy]

Throughout the history if this truck the mileage has always been about 10mpg or better even with a 10ft cab over camper on it. I have all the mileage and trip data from when it was new. I have been noticing what I thought that the mileage was getting a little worse since all I did was drive it around town. Yesterday I drove to the swap meet and had a full tank. I was going between 55-63 mph and I could truly watch the gas gauge go down. I drove about 100 miles on the tank and it was empty, (really empty since I ran out of gas with 1/8 left on the gauge and two blocks from my friend’s house). The question is what would cause such bad mileage. I have a petronics ignition, the timing is right on what the book calls for, new plugs and wires.
Granted it’s a V6 and totally stock with 410 gears and 28 inch tires but I would have thought it would get better than 6-7 mpg. The idle mixture is a little rich and I have the screws turned out about 2 turns from bottom. But according to the book there is no adjustment once you are off the idle circuit.

[Clyde65]

Sounds like you may have a hole in your fuel line some where. does it loose fuel sitting? if it does it is between the tank and the pump, if not after the pump.

[b2948kevin]

I know that this is really basic, but any way there could be a leak in the fuel lines somewhere? Maybe it just leaks under throttle somehow? I would imagine that the engine would have a hard time running if it was rich enough to actually see the needle moving.

Just some thoughts. I am sure others will have better suggestions.

[TR65]

My 65 with the stock 283 and 3.73 gears got around 10 mpg. At highway speeds they are rolling billboards and geared way too low.

Same truck with with LS2 and good gearing gets an honest 17 mpg. Same drivetrain and gearing in my vette gets and honest 21 mpg due to aero difference I guess.

I think you would see some good improvement with better rear gears and an OD tranny.

TR

[HEI451]

Check the oil level in the engine. If it magically made the oil level rise all by itself, there is probably fuel in the oil from a leak in the fuel pump diaphragm.

Also, the fuels are now more oxygenated than before, with the percentage of ethanol up from 10 to 15 percent. We have seen an overall mileage loss of between 1 to 3 mpg with the new formulation.

[1Bad62Pro/Street]

I don't know much about the BBV6's or the gas mileages and never known anyone around my woods have one.
On these engines they claim Oil and fuel consumption is low.
I would check for leaks first. Then go from there.
Check the links below:

Spark Plug Gaps:
If you are running the very desirable GM HEI or Petronix system, you need to increase the spark plug gap. The reason is the HEI and the other one produces more voltage and amps. I would recommend starting out at .045 for the HEI and .040 for the other one.
You will notice a much smoother idle after ditching the points. Take it out and run it through the RPM range. Lug the engine some and then wind it out to max RPM's. Does every thing feel and sound ok? Increase the gap by .005" until the idle starts to suffer and drop back .005". After that is all done, install another new set of plugs two heat ranges colder and try expanding the gap once again. It is unlikely the plugs will carbon foul which indicates either a rich mixture (too large of jets) or the plug range is too cold to burn off deposits. The idea is to run the widest gap possible without misfires and a smooth idle. The wider the gap, the smoother the idle quality and better high RPM power will be. To top that off, milegae will increase, too. Those air/fuel adjustment screws on the front of the car is for the idle ciruit only. Those screws do not lean out or richen the mixture at all. What they do is increase or decrease the amount of air and fuel equally to the idle circuit. To adjust: turn screws in or out to get the best and highest idle, then back in to get a lean drop of 20 RPM's. A 50 RPM drop is ideal, but your carb may not like it. This does nothing to hurt fuel ecomony or performance, but does clean up idle emissions.

http://www.6066gmcguy.org/performance.htm
http://www.6066gmcguy.org/gmcv6a.htm
http://www.6066gmcguy.org/gmcv6b.htm
http://www.6066gmcguy.org/GMCplaid.htm
http://www.6066gmcguy.org/351v6.htm
http://www.6066gmcguy.org/401v6.htm
http://www.6066gmcguy.org/478v6.htm

[Rich 5150 69]

Know anybody with a exhaust gas analyzer, you could dial in your carb.

[awbrown]

I fooled around with a truck when I was going to tech school and of all the things I tried actually richening up the carb made mine get better gas milage .not to say yours is , you can try it either way ... That being said Does your truck have a vacuum advance ? check to make sure it is in fact working ,that will suck a pile of milage out of it if it is not .. and make sure it goes to manifold vacuum... on the carb a rebuild cant hurt first , then start changeing jets and drive it for a week with each change ..... I would still adjust the idle mixture with the engine hot choke off til it pulls the highest vacuum reading ....

[Surfdaddy]

I checked the lines and carb and no leaks. I got about 110 miles to the tank. I really should fine a gas analyzer since it would help with the carb tuning on the VW also. I don’t know if the vacuum advance is working or not, though now that I think about it, I think the timing stayed close to the same when I had the light hooked up and revved up the engine. I’ll check that again this weekend. The truck ran a lot better at idle after I adjusted the voltage regulator to kick out 13 volts. Still runs pretty rich at idle and is not as smooth in gear as I would like though it’s still pretty smooth. That could be because it’s an automatic and there is a load on it at idle. I need to get a vacuum gauge too. I also think that running 60 on the freeway is close to max of what I would like to run with these gears. I should have bought taller tires when I put it back together

[awbrown]

yes , also if it does not advance when you rev it up check the centrifical advance in the dist. sometimes the weights and shaft get stuck and that wont help gas milage any either........

[awbrown]

Did you ever get this problem fixed????

[Surfdaddy]

I adjusted the idle mixture but that was as far as I got... I was driving the VW squareback that I finished for my daughter and the motor lost the rear mian seal right when I got on the freeway. The only way you know something not right is when the clutch starts slipping. So I had to put the truck on hold until I finish rebuilding the motor. Should be finished with it in two more weeks. Whohoo going from 60hp to about 90!!! Then comes some serious truck time. I still haven't fixed transmission yoke seal on it yet either. So 1st thing is the seal/yoke and then the carb, timing, vac advance, and engine pan leaks. I hope to get back to at least 10 mpg maybe 12 if I'm real lucky