The Stovebolt Site Forums The Shop Area Engine Shop Expected gas mileage

I just got my truck on the road. It has a 235 bored 30 over with a mild cam (254 I think, don't recall off the top of my head), dual 1bbl Offenhauser manifold with 2 1bbl Model B rochesters, Fenton headers, T-5 with overdrive and 3.73 limited slip rear.

I'm getting 10.5 mpg with about 400 miles on the engine, with the A/C running full blast. I thought I'd do better than that. Do I have a problem with the Model B's? Or is this the best I can do? (If it is, the trip to Kansas City is going to cost a bundle.)

There's a tech tip over on the inliner boards written by a guy with a setup pretty much like yours. He was getting pretty low numbers too until he reworked his twin B's. There are several things that need to be done to get the two B's to play nice together. He gives a step by step modification list of what he did. His mileage now is around 20 mpg as I recall.

Isn't a 235 bored 30 over more like a 239 or 240? or do I have my formula messed up?

If you think the model Bs may be running rich, pull the plugs after a road trip. They should be a light tan colour. If they are black then it is running rich. A friend of mine had this problem and he finally switched to a different type of carb that Langdon recommends.

Truckernix, the trouble with the Langdon's setup is that it's over-carburetion for the 235. Two 2bbl's is too much. The machinist that did my engine told me he would not warrant his work if I used two 2bbls because the gas would wash the cylinders dry.

Thanks, Waldo. I'll head over to inliners and start reading.

You're running two unmodified Rochester B's on an engine that used to run right with only one of them? Of course you're going to have problems. There needs to be some major changes made in main jetting, and power valve calibration. Another option would be to set up your throttle linkage like the Buick and Hudson systems used to use, where only one carb handles idle and low speed to midrange throttle positions, and the second carb opened quickly for full-throttle operation. Hudson called it the "Twin-H" system, and it was used on their racing models, IIRC. Some Buick straight 8's also ran two carbs, with one used for high-speed only.
Jerry

Your machinist needs to stick to a subject he understands. Over-carbureted engines don't get too much gas- - - - -they run terribly lean! Of course, as soon as the power valves open up due to very low manifold vacuum, they waste gas, but they generally don't get rich enough to wash down the cylinders. Too big, or too many carburetors make an engine try to run on mostly air, with very little gasoline. The lack of power makes the driver push the gas pedal open further, which drops manifold vacuum. If the carb has a vacuum-operated power valve, it goes full rich and stays there!
Jerry

Hotrod, who would know how to do that right? I sure don't.

I can set up a dual-carb system, but I need my dyno and a 4-gas exhaust analyzer to do it right. For starters, try putting in main jets that are about 25% smaller diameter, and go to VERY light springs for both power valves, so they stay closed almost all the way to wide-open throttle. Since you're flowing air through two venturis instead of one, and supplying gas from two main jets, it's necessary to cut down on the total fuel volume available. You might also try raising the float level slightly, to help the venturi vacuum get the gas "over the hump" from the float chamber to the airstream. Just don't raise the level high enough to drip fuel from the venturi nozzle at idle.

A better choice for carburetion would be a matched pair of Carter carbs jetted for a 216, but the manifold stud spacing would have to be altered, or adapter plates would need to be fabricated. Carbking has suggested similar carbs several times for 2X1 setups.
Jerry

Where you buy smaller main jets and lighter power valve springs? I can get rebuild kits at NAPA, but those aren't going to have specialized parts like you refer to.

I can tear a carb apart and put it back together, but I don't know the first thing about jetting and power valves and the kinds of tweaking that you're talking about.

Check your IM's.
Jerry

Speed is a big factor in these old pickups. They were not aerodynamic. Expect fuel mileage to drop with speed. Running two carbs is and a mild cam is not in the interest of fuel economy and you weigh as much as a big Mercedes. Of course, your engine isn't broken in yet. It would surprise me if you ever get up much past 15 mpg if you run 60 or 70 down the highway.

I used the term "Langdons" because I couldn't remember the type of carb that they used. I don't think that they were two barrels and they were the same type used on the Pinto. Anyhow it is nice to hear that it is on the road!

The Langdon's I know is stoveboltengineco.com, and he sells 2bbl webers.

My problem is that I wanted to look as original as possible, so my setup is the same as the hot rodders of the 50's would have setup the motor - 2 Model B's on an Offenhauser manifold. I've got enough money in the air filter housing and linkage that I'd be loathe to change the setup unless there was no way to get the Model B's working as I'd like. (They work fine now - no hesitation, plenty of power, smooth throttle - gas mileage is the only problem.)

The tech tip at Inliners looks quite useful. I'm going to print it out and study it carefully. But I do need to know where one can find the non-standard size jets and springs and the like.

One sneaky trick I've been known to use on round track race car carbs is to re-thread the main jet holes on different carbs to accept Holley jets. (the same ones fit 2-bbl and 4-bbl Holleys) Those jets are available in just about every diameter known to mankind, and once the carb body has that thread, you've got a wide variety of mixture options available. They're CHEAP, too!
Jerry

It's all about the drop in flow per carb you get when going 2x 1bbl. It's not half, but the drop is still considerable.
You can tweak the 235 RochBs but they will never run as lean as the smaller 216 version.

I also started out with 2x 235 RochBs, then tried 2x 235 Carter YFs.
But no tricks or tweaking got the carbs to match the requirements of the 235 and make it run as well as it could.
2x 235 carbs are great for a 261 bored and cammed for racing...!

After confirming my experiences and talking to 'Carbking' I understood what was going on and now use 2x216 Carter W-1 and Stromberg bxov-2 carbs from 1948 on both my street 235 and 261.
Power, fuel economy and response from the throttle is ideal and I get around 20 mpg on the freeways.

I use 2x1 McGurk intakes, which always used 216 carbs, even for the 235 - the only 2x1 235 intake that got the carb size right for street use.
You can have a new repro Offenhauser 216 intake bored at the head ports for the 235 diameter.
That's what I did on my previous 235 setup.
Or you could also use the 216/235 adapter alignment rings from Buffalo Enterprise, which are the same cost as boring the intake ports.
That's what I use with the original cast 216 intake.