The Stovebolt Site Forums The Shop Area Engine Shop Gas mileage.

I have posted about this as a discussion. But I am getting desperate. Last spring I went to Stovebolt HQ and in my 1954 3100, I averaged 20 mpg. It has a 235 with 2 Rochester b carbs, 2.73 rear end, and t-5 transmission. On the way back I experienced an electrical glitch. Made it back then it completely shut down and would not start but after fiddling it did. That should have been my first clue, turned out to be a loose ignition switch wire. But in the meantime, the hei distributor was rebuilt. On my trip to KC for the truck show, I averaged 15 mpg, which was a shocker, I have made the trip 8 times and averaged 20 mpg each time. I just filled up yesterday and my around town mpg was 10 and usually, it is 14. Any suggestion to what I might check. BTW the distributor was never removed.

I bought some carb cleaner and sprayed a copious amount in it without the engine running. Not being able to leave things alone I started the truck and set it at high idle and sprayed both carbs. The front carb did not leak but the rear one leaked out around the gap between the intake manifold and carb base. Any significance in that?

Ron, gently tighten that carb base and see if it stops. You may have to pull it and put a new base gasket on it.🛠

When I notice the leak I tried that. It even looks like there is not a gasket on it. Thanks.

check the vacuum advance

Not saying this is your problem, but certainly possible:

One of the design issues with the Rochester B is casting warpage due to too few screws attaching the air horn to the bowl. There is an internal vacuum passage to operate the power valve. When sufficient warpage occurs between the air horn and the bowl, the passage is sufficiently exposed to reduce the existing vacuum in the passage below the threshold required to keep the power valve closed. I have yet to see a Rochester B that this phenomena did not eventually occur.

When it happens, gas mileage is significantly reduced. When It first occurs, black smoke does not appear at idle in the driveway, as there is sufficient vacuum at idle to maintain the threshold.

If this is the issue, there are two solutions: (1) straighten the castings using a special jig which can be fabricated, and heat; (2) simply replace the B's with something without this issue.

Generally, the castings last about 5 years after straightening before the issue reoccurs, but it eventually will reoccur.

And the vacuum advance mentioned in the above post is certainly a possibility.

Jon.

In 1952 the B1 was redesigned with bigger bosses, thicker flange and larger/longer screws to prevent deformation and "warpage".
But you said between manifold and carb base. That is a funky place to leak gas. It's not totally fluid at that point....should be vapor'ish. Leaking gas there is some kind of clue, but not to me.
Jon addressed the leak at a different spot, maybe he will advise a leak as you describe. Almost sounds like sunken float. Gas overflowing and pouring out....from the wrong places.
Also it seems like gravity would take the gas into the manifold instead of a sideways leak. Must be a big hole. BUT a leak there would have vacuum and negative pressure on the crack. Are you sure it's not leaking down from above at area Jon describes?

When I first noticed the leak I was spraying carb cleaner in it. I had noticed a stain on that carb before but ignored it. What carbs would you recommend. When Mtneer Mike mention the distributor It reminded me that was the only thing that was messed with when I had the electrical issues and new parts were put in and that was not the problem. Now for the question, keep in mind I have a mini HEI. should rotate whe you increace the rpms?? It is rock sold will not turn by hand when the engine is off and when I start it and rev it up it does not move.

The carbs you have are fine. Good carbs.
Dizzy should not move. Advance is done by centrifugal and vacuum. Mostly inside.

The older motor's original distributors do rotate (235, etc) Your HEI is not like that.

How much trouble would it be to go back to a one carb setup? With your other improvements over stock, if you go to a single carburetor, I bet that all of your woes will disappear, your gas mileage will improve, and your performance will not suffer one bit.
With your 2.73 rear end gears, you will never reach high enough RPM to ever utilize the supposed benefit of having 2 carburetors, in my humble opinion. (I can be humble if I try real hard.)
Carl

I agree with Carl's humble opinion and raise one humble.

Well, Carl that is an idea but the 2 carb set up has worked well for 9 years and their has to be a reason for the change in gas mileage. Then the cost of all the parts, the ones I took off I gave away.

How do check that the advance is working?? That may be the answer. Like i mentioned the only real change was the replacement of parts in the distrutor. And they were not needed, it was a loose ignition wire at the switch.

Have someone hit the gas and see if it advances

...click...

That procedure doesn't work on the mini-HEI. The distributor housing doesn't move. Take the distributor cap off and apply some vacuum to the advance unit. Either use a "Mityvac" hand pump, or just attach a long piece of hose and suck on it like a soda straw. You should see the pickup coil move, and stay put as long as the vacuum is applied. If it doesn't move, or moves and then returns slowly when the vacuum hose is pinched off, you've got a hole in the advance diaphragm. Back when I smoked, I'd puff cigarette smoke into a vacuum chamber to check it for leaks. There are much better ways to do it!
Jerry

With your HEI the above video is useless. Hook up your timing light and point it at the BB hole. Start the truck and rev the engine in short bursts, If the advance is working you will see the BB move significantly. My personal vote is for a complete carb rebuild of both carbs by a professional who will give you a warranty. Although for the $ you will pay for that you could probably go back to 1 carb. Cheap fix, disconnect the offending carb or remove it and put a block off plate over the hole.

Took to the shop after trying to check it myself and the said it is ok. I am taking it back when they get time to get it in, they think it is the rear carb. I bought carb kits last year but I did nothing with it.

My bad

Didnt read entire thread
My video is of my electronic ignition

Carry on

Nine years is a long time to expect 2 carburetors to stay in sync. It is my understanding that when using 2 carbs, it is imperative that they be identical. It sounds like you had that state of identicalness in the beginning and it remained that way for several years. However, there should be no expectation that 2 carburetors would wear in an identical and equal manner long term.
The unequal wear can be in the throttle shafts or it could be from one of the carbs having the dreaded casting warp, which causes internal issues which are difficult to see.
There is merit in the opinion that 2 carbs is one too many for these engines.
Its hard enough to troubleshoot a carb issue when one is only running a single carb. Running a dual carb setup multiplies that difficulty X 2.
Carl
Still humble

Carl - the two Carters on my non-Chevrolet shop truck have been untouched and trouble-free for 19 years (knock on wood )

They have some fuel stains, but start and run perfectly.

Jon.

Not meaning to start an argument/discussion, but, the dual Rochesters used on either of my old 261 engines have also been running fine/untouched for over 20 years. They have very minor fuel stains over those years

Clean/new gas always, clean gas tank and fuel lines, clean air filters, and clear-view fuel-filters in two places. We balance and adjust the carbs quickly using the vacuum gauge adjustment/balance technique (every few years).

As I have always posted, those two Rochesters were/are "too much carburetion" for the engine - the truck is no longer mine. I now have a single larger-than-stock 2-barrel from a GMC 302 (with adjustable jets) on my 1960 261. No fuel stains on this carb (5 years of operation).

Without the smell of gas and fumes, I would not be interested in old trucks. It all reminds me of my yute. As cousin Vinny says. Another thing, it was a long, long time before cars and trucks did not leak oil on the driveway. I have a 2002 Chevy and a 2011 Ford and neither do.

"Without the smell of gas and fumes, I would not be interested in old trucks."

Hmmm, smell of gas and fumes?
I can hardly remember - maybe for one year, back in 1969?

Not that kind of fumes.

Gas mileage is a big concern for 2 reasons, I am retired now and the price of gas has gone up by 50%. And you get used to 20mpg average on long trips it is hard to accept the change. Hopefully, the issue that the shop mentioned can be corrected. I have had this set up for around 15 years with no issues until last year.