4.1 mpg?

[84prspctr]

would a bad vacuum advance cause it to be this bad?

 

[84prspctr]

It feels like it might be in the secondaries on the highway, and does not have much pep at full throttle

 

[Bextreme04]

would a bad vacuum advance cause it to be this bad?

You could have the distributor mechanically locked out and be full throttle the entire 25 mile drive and it STILL shouldn't be that bad

 

[Bextreme04]

It feels like it might be in the secondaries on the highway, and does not have much pep at full throttle

Get and AFR gauge. I would almost guarantee you are pouring fuel into the engine all the time.

Or you have a big fuel leak from somewhere.

 

[RecklessWOT]

Small engine, big tires... sounds believable on the 4.1 mpg. Math doesn't lie.

35s aren't small, but they're not HUGE either, a 350 can turn 'em. If anything that will just lower his final drive ratio in a sense. I get 15 or so with 33s. Only thing I can think of is if his rear ratio was already 3.08 or something really tall and the converter can't lock up with the big tires even just cruising on the highway. It might sound stupid, but try going 80-85 for a long distance and see if that makes a difference

 

[84prspctr]

At 85 it’s turning close to 4000 rpm

 

[84prspctr]

Vacuum advance isn’t doing anything when i put a hose in and suck on it. Feels like it’s losing the vacuum

 

[84prspctr]

Is there a way to tell which vacuum advance is on it now? Is there supposed to be numbers on it somewhere?

 

[Hutch8671]

I feel your pain, I've got an 86 k10 short bed with no lift and 31- 10.50 x 15 tires. I recently swapped out the stock 400 small block for a new crate 383 stroker from blueprint engines, has 10 to 1 compression, decent size cam, aluminum heads.... appx 450 hp with 465 ft lbs torque and a dual line holly 750 ( not a double pumper ) with electric choke and by my rough estimate it's getting around 5-6 mpg on the highway running 70 mph. Has a turbo 350 as well and I don't remember what gears at the moment but can go look if needed my question is does that sound right for the mpg cause it's killing my wallet ( but it's fun as hell to drive).

 

[Ricko1966]

Yes a vacuum advance that doesn't work will kill your gas milage. Yes you should be able to get numbers off the old one,but considering it's not the original engine and not the original carbureator who knows what else has been changed and what distributor is in it. I think you'd be better off to start fresh, I believe it's crane that has an advance kit that comes with springs,weights vacuum can and instructions on how to get your timing as close as you are going to get it. I'll see if I can get you some part numbers. Okay It's crane 99600-1 go to their website,see if you can find install instructions,that will make what I'm telling you make more sense.

P.S. you are dealing with at least 2 issues,with a bad Vacuum advance,the vacuum leak is signaling to your carburetor that the engine needs fuel from the enrichment circuit, and you don't have enough advance to operate efficiently and probably not even enough advance to burn your air fuel mix completely.

 

[mibars]

4.1 mpg (57.3 L/100 km) I don't believe it's possible it's actually burned, the amount of heat to reject would be immense. Maybe it's not firing on all 8 or running crazy rich? Any smoke?

 

[GTX63]

Back to basics. At 4.1 mpg, either your app is wrong or there is a very basic, mechanical issue, ie a fuel leak. Or someone is in your driveway at 2am siphoning your gas.
A stock 350 cannot burn that much fuel under normal driving conditions without symptoms. There would be at the very least, an extremely rich condition that you could smell if not see.
Or, you are running 5.13 gears, pulling a loaded 32' RV up a mountain into a 30mph headwind, both ways, in 4WD low, all of the time.
In other words, imagine pouring a quarter gallon of fuel down the carburetor in 60 seconds, or at approximately 60 mph. There would be obvious signs.
Now, the OP says he is running 1/3 throttle at 65mph.
Again, for a stock gen 1 small block Chevy to experience that sort of fuel consumption, there is something else going on.

 

[Ricko1966]

Okay he has mystery gears, no speedo, non working vacuum advance, at the very least,a vacuum leak which will make it run rich, pulling fuel from the enrichment circuit,because with low vacuum conditions the carb thinks he's in the throttle all the time. Who knows what else is wrong.We have pages and pages of guys saying you cant get better than 10 out of a good running square. You look on fuelly and at random the K's that I looked at,don't get stellar milage. Lets quit arquing about what his milage REALLY IS or couldn't be etc. and help him fix his problems.

 

[mibars]

I missed the posts from 2nd page, with no vacuum advance part of the combustion will happen in exhaust manifold, that's how the fuel may burn without creating usable work, just heat the exhaust.

 

[geocrasher]

At that kind of mileage and if you drive it regularly, an AFR gauge and edelbrock tuneup kit for the carb would pay for itself pretty quick.

At that kind of mileage, FiTech EFI would pay for itself rather quickly too. 4mpg seems way low to me as well.