Hot Rods Chevy 235 gets shitty gas mileage

Here are the vitals: 1951 Chevy AD 3100 pickup with a 235 6 cylinder. Engine has a hot cam, dual Rochester carbs on a dual-carb Fenton intake, dual Fenton split exhaust, stock 3-speed trans, 1980 Blazer K5 rear axle. Truck runs well, plenty of grunt for a 6. It gets about 9 miles per gallon! The only hint of a problem is a heavy stench of fuel when I get hard into the pedal. Yeah, these hotrods aren't supposed to get great gas mileage, but 9 miles/gallon seems way off the mark for a 6. Even if I baby the pedal, the gas mileage sucks. What do you think? Not sure what size jets I have, should I start there?

Thanks in advance.

 

Your truck is not that heavy, but that would be about normal mileage on say a 1960s F150. What are the rear end gears? I run a 4:11 in my 72 F250 and would love mileage that good!

 

first thing you need to do is be sure the speedometer/odometer is reading accurate so you can be sure your MPG figure is accurate.

I had a buddy who claimed 23 MPG from a 350 powered 48 Chevrolet and he did not even have a working speedometer in the car. I always figure at least half of peoples MPG claims are pulled directly from their butts.

 

I believe the rear gears are 3.73.

 

Driving style might have something to do with, also. Shift early, it helps a lot.

 

This is a good point, I will check it. But the stench of fuel is an issue that I want to solve. That can't be good.

 

First you gotta figure out where that stench is coming from...carbs leaking?

 

No visible signs of leaking. But maybe the fuel is evaporating off quickly at the leak?

 

I'd think if you are smelling gas you have too much gas going somewhere at the wrong time. That could mean that either the jetting is wrong, the power valve (s) are stuck open, the float level is too high or some other carb related function is off kilter.
Then you get into the "is the timing set correctly?" thing. Along with that is the advance in the distributor working correctly
As Jseery mentioned previously. What gear ratio> driving it around town and not on the road? My daily gets great mileage on the highway but sucks at gas mileage in town.

My guess is that you just stuck two stock carbs on the engine without having them or setting them up to run as a pair on and engine that normally runs one of them and they are dumping more fuel than the engine needs.

 

Certainly not a gas mileage setup.

The economizer or power valve uses rising vacuum to taper off the fuel as the engine accelerates. It's a trick to get them right so you don't ping, but you're never rich either.

You can use an open vacuum line and your thumb to diagnose this, but I don't have time now to tell how.

(& if you're running stock circuits on both carbs this is an issue!)

 

if you are looking for mpg 235 is not it tuned right in a light car and your pu is probably lighter than a sedan 11-14 is probably as good as it gets. remember old school tech plus these were built when gas .10-15 per gal. then you add cam dual carbs and you are probably pretty close on the mark tom

 

Wide band O2 sensors kits are down to around $150 and make tuning much easer. You only need to save less than 50 gal of fuel to pay for them.

 

can you set up a video camera under the hood and film it?

Where is the gas tank?

 

Yep, and the smell may or may not be part of the problem. Still got the in cab tank?


Posted using my chicken and 2 cigs

 

Tank is indeed in the cab. It's tight though. Made sure the sender gasket is good, fill hose is good, everything in the cab is tight. It only stinks when I accelerate hard.

 

As said before, check your point gap, initial timing, and vacuum advance. Every 235 I've ever owned, including the one in my current truck, likes more advance than the book calls for. I run mine just below the "ping" point on hard acceleration. There's a tech tip over on inliners.org on how to sync up dual rot-pots.

 

Thanks, I'll check out that link. Electronic module ignition, no points. Timing is spot on, tweaked for max vacuum. Vac advance is working like a champ. I suspect the carbs/carb setup.

 

Back in the day My Mom's Nova had a 250 six, and My 55 Chevy had a v8, I never paid much mind to mileage, the gas crunch came and We were all just glad to get gas, but if memory serves the Nova got about 20 mpg, nowadays gas is costly,and lately My rods with sbc's get 20 mpg, and that's with multi-carbs, I'm not trying to be rude but I would change out to a 283, there is a guy in My town with a 59 Chevy He has kept the 6 in His car, it runs fine but I'm sure the car would get better mileage and need fewer rebuilds on the motor with an 8 rather than a 6, just a thought. Von Doc

 

While engines affect gas mileage (especially if they are not running correctly), the big issues with mileage are: weight, frontal area, rolling resistance and rear axle gearing. Hard as it may be to believe, most of the modern improvements in mileage stem from addressing those issues. Computer control of the engine and transmissions are also now major factors because of the precise control it provides. With carbureted engines there is little you can do to improve mileage over getting them running correctly.

 

Do you have progressive linkage on the carbs? Or do they both open at the same time?

 

The plain simple truth is that is about all your gonna get. That set up is from the days when 15 mpg was considered good.
Yes, there were claims of over 20 mpg at times. Most of my friends that ran them "in the day" tell me 15 mpg was the best and most were worse. A 52 Chevy with P/G was a 4-5 mpg car. Makes your truck seem great.

 

I learned to drive in a '62 Bel Air wagon with a tired 235, 3spd w/OD and 4.11 rear gear. Seems to me it got in the low 20's for highway milage.

Blue

 

Not sure what to tell you. From my experience is that i can pull great gas milage out of my 235. I run tripple rochester b's on a progressive linkage, dual exhaust, t-5 and the stock rear end in my 58 chevy truck. In town i get about 10-12 and running on the highway at speeds from 70-75 i get 18-19 which is great for those engines. The t-5 made a huge difference in gas milage wise.

 

The linkage is not progressive. Both carbs spray in unison.

 

There is you answer, the T5. That overdrive IS going to make a huge difference.

 

LOL

 

The worst possible setup for gas mileage IMO.

It's because the two matched carbs are never perfect, and never perfectly matched. That makes it very difficult to get the ideal mixture all the time, and you end up with overall rich running, once you raise the mix enough to mask the lean spots in the performance curve.

A progressive setup uses only one carb for all ordinary running so the ordinary mixture is much easier to make accurate.

It's easy to balance out the carbs at WFO throttle, but making them right, and keeping them right in the normal operating range is the trick.

To me, this makes most all non-progressive setups basically "race only" and not suitable for a driver unless it's only an occasional driver.

 

Makes sense. I'll mod the linkage to progressive. Thanks!

 

That is your main problem, way too much fuel most of the time. I bet if you switch to progressive linkage your gas mileage will go way up. It should also run much better and the gas smell may go away. You could also go to electronic ignition like Pertronics and that will help also. Let us know if any of this worked. Brian

 

Problem is this is a straight 6, with a log type manifold, if the carbs are set up progressive its going to lean out 3 cylinders and run like garbage most of the time.