Posted By: chevyfiftynine Twin Rochester trouble - Wed Sep 16 2020 04:40 PM
I have a '58 Apache stepside 235 which has been fitted with an Offenhauser manifold and twin Rochesters. This has led to two problems. The first is that the linkage doesn't return properly when idling so it runs too fast but if a stronger accelerator spring is fitted the pedal won't go down. The second is that it runs too rich and the plugs are always black. I've tried carb kits, smaller jets (48s) and now new carbs but nothing seems to work. I've read many times that it should only have one carb and two never run properly. Would it be possible to blank off one carb or is there a way to fix both these problems, short of converting it back to one Rochester?
Posted By: carbking Re: Twin Rochester trouble - Wed Sep 16 2020 05:02 PM
There are many threads on this forum concerning dual carbs, and also Rochester type B's for your reading pleasure; searching should fine them.

(1) Make absolutely positively certainly sure that the power valves are functional, and not open all the time.
(2) Make certain the carbs are properly synchronized.
(3) "Would it be possible to blank off one carb?" : NO!
(4) We have found more often than not, if two stock carbs are used on the stock engine, LARGER jets are required because of lower venturii air velocity. Too rich is probably power valves (or a pertronix conversion, without an alternator).

Linkage can be difficult or easy to work with, depending on the type of linkage, and the experience of the adjuster. Make sure that, if either carb has a fast idle circuit, that the choke on that carburetor is not preventing the throttle(s) from closing.

Posted By: Hotrod Lincoln Re: Twin Rochester trouble - Wed Sep 16 2020 05:04 PM
You can block off one carb if you don't mind one end of the engine running a little leaner that the other. That problem can be minimized, but not totally eliminated with some jet size tinkering Use a piece of soft aluminum sheet metal instead of a base gasket and dimple the center of it down so the throttle plate on the dummy carburetor doesn't hang up on it. Also, block off the fuel line to the fake carb. The mouth-breathers at a "show and shine" will still marvel at how cool your ride is, and you'll have a lot better overall performance.

Overall, it's not a good idea to run a carb that far off center, particularly if you want the heat riser to work as designed. Tinkering with the way an engine was designed is very seldom successful, unless you're willing to do your homework first. It always involves a lot more work than just bolting a bunch of mismatched parts together.
Posted By: panic Re: Twin Rochester trouble - Wed Sep 16 2020 06:08 PM
it should only have one carb and two never run properly
Except for the Hudson and about 30 British L6 engines.
Posted By: Dragsix Re: Twin Rochester trouble - Thu Sep 17 2020 01:33 AM
Read the article by K. Patrick Smith on the subject. Pat was Dyson Racing’s long time crew chief, now retired, one of the best mechanical minds I know, former president of inliners international, and ran a pretty fast GMC powered race car a number of years ago. It’s a little bit of work to get them to run right.

On the other hand, and as an alternative, get a pair of adapters from tom Langdon over at stovebolt and install a pair of 1974 ford pinto Holley/Weber carbs. They are a progressive two bbl, small primaries and larger secondaries, threaded input for the fuel lines. They run really well right out of the box. You can also use the actual Weber carb, it’s the DGEV 32/36, essentially the same carb just no threaded input, uses a brass nipple.

Tom Langdon may still a carter/Weber carb, little smaller then the Holley Weber, that can also be used and will also run way better Right out of the box then the two 1 bbls.
Posted By: chevyfiftynine Re: Twin Rochester trouble - Fri Sep 18 2020 05:00 PM
It seems obvious now that if the engine was designed to run with one carb, it's not going to run well by bolting on another. Same with the linkage - the spring could shut off the throttle when the carb was above it but with a carb on either side of it the spring hasn't got the power to pull them both down properly. I read all the technical info and the Langdon article but unless you're a real carb expert you could get into an awful mess trying to adjust the carb without really knowing what you're doing. What surprises me is that the dual carb set-up is so popular you would think there was a definite way to get it to run well, such as a fuel pump limiter, if one exists. Maybe the bigger jets would work - when I got the new carb it had a 58 jet, so maybe that might help. Thanks for your help - I'll keep experimenting with it.
Posted By: WE b OLD Re: Twin Rochester trouble - Fri Sep 18 2020 05:18 PM
I know some might chime in but I am running dual Rochester's with little trouble except for what I caused. I have 36,000 mile on the set up. I have driven to KC 10 times and to the east coast 2 times.
Posted By: Curt B. Re: Twin Rochester trouble - Fri Sep 18 2020 06:24 PM
Originally Posted by carbking
Linkage can be difficult or easy to work with, depending on the type of linkage, and the experience of the adjuster.

Can you suggest an easy to work with linkage for the Offenhauser and or Fenton dual intakes? IMO if the carbs are not actuated by a pivoting rod true syncronization is impossible.
Posted By: Dragsix Re: Twin Rochester trouble - Fri Sep 18 2020 07:07 PM
There are a lot of theoretical opinions on the one carb vs two carbs but not a lot of empirical evidence one way or another. However, one such piece of evidence that is available for review are the results of tests done in 1955 by the well respected Racer Brown. With a vette exhaust manifold, a McGurk 2X1 and a pair of Stromberg one barrel carbs, the power increase of an otherwise stock 261 motor was measured to be 14-18% more then stock. The article appeared in the May 1955 issue of Hot Rod Magazine.

Back in the early 1990s, pre internet, I received permission from the late Grey Baskerville at Hot Rod to publish the article in Inliners International's 12 Port News (and it was published). So this article has made the rounds over the years and to my knowledge, no one has disputed those findings, or done any additional testing.

Over the years, some automotive manufacturers have also added a second carb as a way to obtain a little extra power, like Hudson's compound carburation set up. I am no automotive historian, but I will bet there have been others over the years.

This business of opining that a chevy 6 motor will make no more power with two carbs then it does with just one carb is just not in reality.
Posted By: carbking Re: Twin Rochester trouble - Sat Sep 19 2020 08:45 PM
Mike - glad you posted the article so that everyone can read it. I have a copy in my files, and obviously have read it before.

Points that I would like to suggest about the article:

(1) My guess is the ability of the person doing this test is above most of us as far as what he can accomplish; so we can say that his results were maybe 99 percent of optimal for available parts at the time.

(2) The Stromberg carbs used (wonder how many OTHER carbs were tried before he settled on the Strombergs) were 216 carbs; two of which he considered optimal for for the 261. NOT a pair of 235 carbs on a 235. If a pair of 216 carbs is optimal, does anyone not believe that larger carbs on a smaller engine are going to be over-carbureted?

(3) Incidentally, he listed the Stromberg sales number 380269 which is not found on any Stromberg carburetor. For any wishing to duplicate the test, the Stromberg code number for 380269 that is STAMPED on the top cover of the carburetor is 14-22. There may be an engineering "status code" appended to the 14-22. So one might find a 14-22A or 14-22B. Same carburetor with a "tweak".

(4) It would have been nice had a test also been done where dual carbs were added with the stock manifold, but we don't have that so we have to draw conclusions from what we have. The addition of the Corvette exhaust manifold gives approximately a 5 percent increase alone, where the addition of the both the Corvette exhaust AND the dual carbs give approximately a 16 percent increase. Since we don't have the test results, I would GUESS that the additional of the dual carbs alone would give approximately the same 5 percent increase as the Corvette exhaust; and the 16 percent was the combination of both, as they complement each other.

The article does prove that some modest improvement IS POSSIBLE; but it does NOT follow that all of us are going to obtain the exact same results as the gentleman doing this test, even if we use the same parts!

For a number of years, I have been suggesting to those that wish to use dual carbs, to use the 216 carbs on the 235 (or the 261), but they are many more running 235's than 261's. I have been suggesting Carter W-1's for the reason that they are MUCH more readily available AND less expensive than the Strombergs. Also parts are much more readily available for the W-1's than the Strombergs. I have also suggested for many years that the Stromberg type B and the Zenith type 228 are very slightly better than the Carters, and significantly better than the Rochester B's.

So what is the bottom line for this part of the post? Placing dual carbs on the Chevrolet six (216, 235, 261) is NOT the BEST configuration, because of the cylinder head intake port configuration, for MAXIMUM improvement. Yes, the article did prove some improvement is possible on an otherwise stock engine (and Corvette exhaust) IF THE CARBS ARE CHOSEN WITH CARE AND INSTALLED AND ADJUSTED BY A PROFESSIONAL! I would submit that if the carbs are NOT chosen with care, and professionally adjusted, it is quite possible to actually LOSE power with the dual carb installation!

The one issue many do not consider, and this is true, when replacing anything that is defective with something new: just because whatever is new performs better than a defective original does not mean whatever is new is better than the original IF the original is in perfect working order.

As to the question about linkage:

Depending on the manifold and carburetors used, the carburetor throttle throw will either be parallel to, or perpendicular to, the length of the engine.

If the throttle throws are parallel to the engine:

(1) assemble materials (A) a 1/4 inch steel rod approximately 2 inches longer than the centerline distance of the two carburetors, (B) a piece of 1/2 inch brass rod maybe 12 inches or so (too much unless you make mistakes).

(2) Measure the inside diameter of the holes in the throttle arm for carburetor linkage, AND the thickness of the throttle arm.

(3) chuck the 1/2 inch brass bar in your lathe, and turn a diameter 0.005 (5 thousandths) less than the inner diameter of the hole. This diameter should be long enough to include (A) the thickness of the arm, a retaining washer, and a space to drill a hole for a cotter pin. Cut the 1/2 inch bar so that the 1/2 inch diameter is 1/2 inch long. Drill a hole in the small diameter for the cotter pin of your choice. Drill a hole through the center of the 1/2 inch portion of the brass perpendicular to the diamter that is 0.228 inch (#1 drill bit). Now drill another hole perpendicular to the first. This hole should be 0.133 (#36 drill bit), and tap for a 6 x 32 set screw.

(4) repeat step (3) as you need one for each carburetor.

(5) make a similar piece for the approximate center of the rod to hook up the linkage from the footfeed.

(6) Assemble the two fittings into the throttle arms of the carburetors, assemble the rod through the large holes in the fittings, with the third fitting also on the rod.

(7) Start the engine, synchronize the carburetors, and THEN tighten the set screws on the bottom of the fittings.

Once you have done a few, you will be able to machine the fittings in 10~15 minutes each.

If the throttle throws are perpendicular to the engine, the procedure is a bit more complicate, but certainly within the ability of any enthusiast with a few tools. The idea is to fabricate two brackets, one for the front of the front carburetor and the rear of the rear carburetor to hold a rotating rod. Make the necessary fittings to hook the rod to the carburetor throws, and adjust.

Since the above is "eye candy" by all means, send the parts off to be chrome plated, but if you do, adjust the clearances I have mentioned to allow for the thickness of the plating. Ask your plater the average thickness of the plating.

And if space in the engine compartment permits, I will continue to suggest 3 carbs on any of these 6 cylinders, because of the cylinder head intake port configuration.

Take a look at the post by panic, suggesting 3 short intakes rather than a log intake with 3 holes. Agree completely, but remember the choice of carburetors will be significantly different for the two different intake configurations.

Posted By: Hotrod Lincoln Re: Twin Rochester trouble - Sat Sep 19 2020 10:27 PM
Jon- - - - -here's a little music for your entertainment. It's very appropriate to this discussion.

Posted By: tclederman Re: Twin Rochester trouble - Sat Sep 19 2020 10:43 PM
Very nice/appropriate song, Jerry []

Thanks, Tim
Posted By: Dragsix Re: Twin Rochester trouble - Sun Sep 20 2020 01:21 AM
You are too cool Jon! Thanks for posting. I agree, a dual set up is not optimal, triples would be better but a lot of cars and trucks simply don’t have the room to fit a triple so duals. I ran early carter YFs for many years and loved them. But truth be told, the holly Weber 5200s ran a little better. If I was going to go back to a dual or triple single set up, I would definitely look at the carter W-1s.

Right now though, my 20 something son has me convinced to start looking at maybe working out a dual fuel injection set up. It’s intriguing. I have never messed with F.I. before but when a friend installed dual four style F.I. setup on his tunnel ram, and how that car now starts and drives, I was sold on at least trying to investigate the possibility.
Posted By: Curt B. Re: Twin Rochester trouble - Mon Sep 21 2020 04:24 PM
Originally Posted by carbking
If the throttle throws are perpendicular to the engine, the procedure is a bit more complicate, but certainly within the ability of any enthusiast with a few tools. The idea is to fabricate two brackets, one for the front of the front carburetor and the rear of the rear carburetor to hold a rotating rod. Make the necessary fittings to hook the rod to the carburetor throws, and adjust.


Read the above paragraph very carefully as I believe it strikes right to the heart of your difficulties. What is NOT said is that the linkage provided by the repop manifold vendors is an exercise in futility as return spring pressure is not equal, it puts excessive stress on the throttle shafts, and it won't stay in the same place for long. Using that information the solution I came up has resulted in the dual Rochesters on my Fenton intake not requiring adjustment or tinkering in over a decade. I also made one for Paul Schmehl (Stovebolt IT guy) seen in the second picture on his Offenhauser intake with dual Rochester carbs. I wonder how long he has gone without adjustment...

Regarding the Racer Brown article I cut up, modified, and flow tested several heads to investigate individual flow bench guy said that opening the intake runner up to the size of the intake rings was a serious velocity killer and not beneficial in any way. Likely other mods masked this defiency.

Attached picture Curt.jpg
Attached picture Paul.JPG
Posted By: Doc.Hall Re: Twin Rochester trouble - Mon Sep 21 2020 06:38 PM
This seems to be a constant occurring and fretful subject regarding two carburetors on a 235/261. If one insists on making water run uphill and helping politicians be honest then your task is pretty much futile. Your dealing with a subject that is impossible. Unless extensive modifications have been made to both the top end and bottom end of the engine you are trying to make a mule into a triple crown winner. Pages and pages on The Stovebolt have been dedicated to this issue. It will NOT work! A good running model B or Holly is as good as it will ever get. You are trying to make a very good farm truck into a Ferrari. The 235 will most definitely finish the twenty four hours of LeMans but about 20 + hours later. I have had two double Rochester's on both Offenhauser/Fenton type intakes. I think it added about 3 to 6 horsepower and about 24 hours of work each week. Too much money spent on linkage, jets and experts. You can put a 68 Pontiac DOC and get better performance if you want H.P. from a six cylinder. Doc.
Posted By: tclederman Re: Twin Rochester trouble - Mon Sep 21 2020 07:02 PM
This 1959/1959/60 261 has been running reliably with two 235 carburetors for over two decades. []

It has "off-the-shelf" dress-up and intake/carburetor parts.

The 1960 261 in my Suburban has a single 2-barrel GMC AA1 carburetor with adjustable jets.
It has very nice power/acceleration (at all speeds) and good gas mileage (except when I cruise between 65-75 mph).

I am proud to be a "shade tree" mechanic.
Posted By: panic Re: Twin Rochester trouble - Tue Sep 22 2020 12:38 AM
EFI using a TB works, but the manifolds are not great.
Individual port injectors and a throttle valve has the siamese intake ports obstacle.
Posted By: Joe H Re: Twin Rochester trouble - Tue Sep 22 2020 02:10 PM
I strongly disagree with the above statements about dual or triple carbs not working. They do work when done right. I did not gain a lot of HP, what I did gain was a big gain in fuel mileage at highway speeds, better drive-ability, better starting, less fuel smell after driving, and it looks really cool. I have a bone stock 250, and it runs better then it ever has, and I drove it with a stock intake for years trying to get fuel mileage out of it. With dual W-1 Carters, I went from 14 to 21 miles per gallon at 65 miles per hour. It gets even better at slower speeds. Did I gain more power? Some, but that wasn't the reason for the I went this way. I have a cylinder head flow bench, so if I wanted more power, I would hog out a head and run the engine at high rpm, thats not what I'm after. To top it all off I made the intake from scraps of steel left over from a project at work! I also added a picture of the triple Rochester Corvair setup I ran for awhile. I originally made the intake for dual Rochester Mono-jets, so the mounting holes and port openings are much bigger. You can see the spacers under the carburetors that adapt the Carters to the bigger openings.

Attached picture IMG_1033.jpg
Attached picture IMG_0141.jpg
Attached picture engine pictures 013.jpg
Posted By: Doc.Hall Re: Twin Rochester trouble - Tue Sep 22 2020 03:07 PM
That is a nice looking job but you are using it on a very different engine. The 235 head and intake has a poor circulation (swerle), if air was a rope it would look like a knot coming into the combustion chamber. Both the 230 and 250 were developed to remedy intake/exhaust problems. I agree that two Rochester's can be used, even a blind chicken can catch a worm every once in awhile. Most of us out there will agree that one Model B is enough to take care of. I can't remember the brand name carburator a buddy of mine ran in his roundy roundy 51 Chevy 235 automatic but he won his fair share. I'm thinking it was an Autolight. Anyway Yahoo, for the guy that has two model B's on a 235 if I'm reading it correctly. A 261 is adaptable due to the larger intake pull, it kinda straightens out the knots. It's to bad that we don't have the old dirt tracks around here anymore, remarkable innovations were accomplished by equally remarkable men. I'm talking the 50's and 60's here. Joe H,I have a 250 on a frame that I use to check carbs, don't do much carb work anymore your rig would be just right for my 34. Doc
Posted By: Dragsix Re: Twin Rochester trouble - Wed Sep 23 2020 01:57 PM
Curt, if I may be so bold to ask, during your investigation, what did you find were the better improvements to make to the 235/261 head?
© The Stovebolt Forums