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Twin Rochester trouble
#1377302 Wed Sep 16 2020 04:40 PM
Joined: Apr 2010
Posts: 17
New Guy
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?

Re: Twin Rochester trouble
chevyfiftynine #1377304 Wed Sep 16 2020 05:02 PM
Joined: Nov 2002
Posts: 2,204
Carburetion specialist
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.


Last edited by carbking; Wed Sep 16 2020 05:03 PM.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
Re: Twin Rochester trouble
chevyfiftynine #1377305 Wed Sep 16 2020 05:04 PM
Joined: Feb 2004
Posts: 22,576
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.

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Re: Twin Rochester trouble
chevyfiftynine #1377315 Wed Sep 16 2020 06:08 PM
Joined: Dec 2008
Posts: 1,597
it should only have one carb and two never run properly
Except for the Hudson and about 30 British L6 engines.

Re: Twin Rochester trouble
chevyfiftynine #1377373 Thu Sep 17 2020 01:33 AM
Joined: Dec 2017
Posts: 954
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.

Last edited by Dragsix; Thu Sep 17 2020 01:34 AM.

Re: Twin Rochester trouble
chevyfiftynine #1377529 Fri Sep 18 2020 05:00 PM
Joined: Apr 2010
Posts: 17
New Guy
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.

Re: Twin Rochester trouble
chevyfiftynine #1377539 Fri Sep 18 2020 05:18 PM
Joined: Feb 2001
Posts: 6,959
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.

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Re: Twin Rochester trouble
carbking #1377552 Fri Sep 18 2020 06:24 PM
Joined: Mar 2005
Posts: 1,447
Shop Shark
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.

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Re: Twin Rochester trouble
chevyfiftynine #1377557 Fri Sep 18 2020 07:07 PM
Joined: Dec 2017
Posts: 954
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.

Last edited by Dragsix; Fri Sep 18 2020 07:14 PM.

Re: Twin Rochester trouble
chevyfiftynine #1377671 Sat Sep 19 2020 08:45 PM
Joined: Nov 2002
Posts: 2,204
Carburetion specialist
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.


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
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