The Stovebolt Site Forums The Shop Area Engine Shop Using multiple Rochester series B carbs

A thread came up on one of the other Chevy boards as to the use of multiple Rochesters (and why I use other than Rochester B's) so posted this information. Thought some here may find it of benefit.

Jon.


There are many ways to "skin the cat"; and others will have different procedures. Since we have not used others' procedures, will not comment; as I do not have sufficient data to comment.

We used the following procedure to do several dual Rochester setups, which worked very well; FOR A WHILE! I am just lazy enough to not wish to redo things every 3~5 years, which is the main reason we quit doing the Rochester B series carbs.

This procedure gave excellent driveability, fuel economy, and power for a few years.

Jon.

Procedure

Dual Rochester B carburetors inline Chevrolet 6

Rochester built 141 different type B carburetors to be used on engines from 153 CID to 292 CID. These carburetors have different main venturi sizes (air flow), different fuel jets (easily changeable), and different air jets (requires machine work to change).

The first step (and the very more important step) is to determine WHICH carburetors to use. We would suggest for a basically stock 235 or 261 for street (normal driving, cruises, an occasional trip to the drag strip) that MATCHED TAGGED 216 CARBS ARE USED! For the primary strip vehicle that sees some street use, MATCHED TAGGED 235 CARBS MAY BE USED. For the race only vehicle (not licensed, no street use) MATCHED TAGGED 261 OR 292 CARBS SHOULD BE USED! For a basically stock 292, MATCHED TAGGED 235 CARBS SHOULD BE USED. For optimal results, choose the carburetors to fit the airflow requirement of the engine, and then modify (if necessary) the intake mounting flanges to accommodate the proper carburetors. If the manifold requires modification, the better way is to have a heli-arc welder fill the holes, and then redrill and retap new holes at the proper position. Mixing throttle bodies with bowls to “cheat” on the above is apt to meet with unusual results.

If you choose not to use matched tagged units, invest in a good sets of number drills from 1 to 60, 61 to 80, and fractional metrics; and be prepared to invest a LOT of time in measuring and fabricating to match the carbs you choose.

Once the above choice has been made, one should look for matched tagged carbs. When selecting your carbs, pay close attention to the clearance of the throttle shaft to the throttle body. Measure this clearance with a dial indicator. 0.005~0.006 inch is optimal; 0.007~0.009 is acceptable; over 0.009 plan on machine work to either install bushings, fabricate a new throttle shaft, or both. Order repair kits specific for the tagged number (not generic kits). At a minimum kits should contain gaskets, fuel valves, accelerator pump (leather pump if you live in an area with deathanol), power valve, and power valve actuator spring (critical), and new throttle and choke plate screws. These will probably be unavailable locally, so get them coming while you start the work.

Completely disassemble both carburetors, but keep the pieces of each carburetor separate (a couple of shoe-boxes work well for holding the disassembled carburetors). If these are your first Rochester B carbs to rebuild, set up a camcorder on a tripod, and record the disassembly. Talk to the camcorder (ie “throttle plate down edge to….” etc.). Remember to file the under side of both throttle and choke valve screws to remove the factory “staking”. Otherwise, you have an excellent probability of breaking at least one of them. Easy outs should be renamed “not so easy outs”!

Clean the castings.

Now time to remove the warpage of the airhorn (top casting) and the bowl (center casting). Don’t waste time looking for used cores that aren’t warped, my guess would be 99 percent plus are warped. A procedure (there may be others) is as follows: acquire 4 pieces of at least 3/8 inch thick steel plate and several large C-clamps; machine these plates to fit flush on both the tops and bottoms of the airhorn and bowl castings (there are a number of projections on the castings which you have to cut out the plate to fit); lightly clamp the plates in place on both castings; measure the clearance from the plate to the mating surface for each casting, and tighten the clamps to reduce the clearance by 0.003 inch. Place in an oven and heat at 350 degree F. for 1 hour and allow to cool naturally. Repeat as necessary until castings are true. Place the plates and clamps where you can find them. On average, you get to repeat this procedure in three to four years (this is the major reason we quit doing Rochester B’s)!

Once the castings appear to be true you need to check the internal vacuum passage. Locate the vacuum passage in the bowl casting (the vacuum passage is located just in front of the accelerator pump). Note the “sealing ridge” on the bowl casting mounting surface. Assemble the bowl casting, the bowl to airhorn gasket, and the airhorn. Insert the airhorn mounting screws, and snug, but do not tighten to torque specification. Disassemble. Now observe the gasket; there should be a light ring where the sealing ridge around the vacuum passage fit against the gasket. This is critical. The ring indicates a seal for the vacuum passage, and gives some assurance that the power system will perform as designed.

Determine if you require chokes. I personally would use chokes on BOTH carburetors, even if you live on the equator, but others may differ in opinion. If you decide to use a choke, then use it on BOTH carburetors, not just one. If you use a choke only on one, THE CARBURETOR WITH CHOKE WILL PERFORM BETTER at idle to midrange than the one without the choke.

At this point, you are ready to reassemble the carburetors. ACQUIRE A SET OF SPECIFICATIONS FOR THE TAG NUMBER CARB YOU ARE USING!!! Different carburetors use different float settings, and the float setting is critical to plus/minus 1/32 of an inch!

After reassembly, install the carburetors; and synchronize using either a set on manometers or a “Unisyn” (generally available from a good motorcycle shop).

Our experience has been that if the above procedure is followed on a stock or mild street engine, that final calibration may be done by adjusting fuel jets only. We found no need to play with the air jets. We found that stock calibration worked pretty well most of the time, but occasionally slightly larger main metering jets were required.

The key to calibration is to recognize that the carburetor has four metering circuits (idle, off-idle, main, and power); understand the function of each; AND MAKE SURE EACH IS FUNCTIONING AS DESIGNED. There are books available on this subject. A good one is “Rochester Carburetors” by Doug Roe and Bill Fisher. Doug was the design engineer on the Q-Jet, and knew all Rochester carbs “in and out”.

REMEMBER: the most expensive carburetor you will ever buy, is the wrong one you attempt to modify!

Good Information!

I'm running a Zenith carb from a GMC 270 on my Chevy 261 now. If I wanted to go with dual Zeniths for street use, would I look for MATCHED TAGGED Zenith carbs from a GMC 228 OR 248?

M153 - if basically stock 261, carbs from a 228 should work very well.

Jon.

Im running two holley 1bbl's on my 235.With the glass float bowl,they look really cool and...once re-jetted,, they work just perfect!!
Marty

I'd like to see a picture of the Holleys!

Joe

No problem,Joe. Ill get you a couple pics tomorrow.(thurs)
Marty

I picked up a copy of Doug Roe and Bill Fisher's book on Rochester carburetors for a dollar at a flea market last week. Very interesting. The section of the Rochester B is only 5 pages long but it did help me with reassembly.

The kit I got was from Walker, number 15157. Don't buy it.

The great thing about the book is that it actually covers the whole fuel system from theory to practical application. It can give you ball park figures for sizing carburetors, fuel lines, etc. for various purposes.

The sizing chart actually indicates that two Rochester Bs (155 cfm according to the book) are really at the upper end of the size range for a 292 that isn't going to see 4000 RPMs very often.

I was thinking about a pair of Stromberg 97s, which have a slightly lower CFM rating, but the Rochester B has one advantage over any other carburetor that I've seen. I'm not sure that it is a decision maker, but the circular float bowl is specifically designed so that the fuel level in the float bowl won't have any severe variation no matter what angle the vehicle is parked at.

Frankly, I would love to go with two Webers, but I don't have the money, or the years that the tuning would take.

Carb King, even though I disagree with you about liking Rochester carbs, thanks a lot for your comprehensive info above. I have run as many as 5 on a GMC 270 on a Howard manifold, am now using 2 on my stock GMC 248, and have used them one at a time on numerous Chevrolets and GMC's over the years. My experience has been positive in all cases, easy to tune and bulletproof. I used a stock Roch. on 1953 and 1954 Chevy's that I drag raced and held records at numerous strips. I always kept a 5 gallon can of carburetor cleaner in my shop and cleaned my carbs quite often, and seldom had a "leaker". I will continue to use Rochesters as long as I can get them, exxcept on my 292 that has 3 Weber side draft carbs (a real chore to tune, but they look neat).

Chevman32 - if everyone liked the stuff I like, then it would be so expensive I couldn't afford it :big grin: !

Like I said above, we could make them work, and work almost as well as Carters, for awhile.

But my philosophy on working on something is evidently different than yours. We heavily modified the engine in my shop truck for performance and fuel economy. 15 years and 400,000 miles later when the rust gremlins destroyed the body for the third time, I gave in and scraped the truck. During that time, the carburetor went totally untouched, as did the engine. Normal maintenance (oil,lub, filters, belts, tires, brakes and electical) was all that was done other than drive it. Extended wheel-base van with 300 CID 6, 22 MPG at 70 MPH. This is my idea of how a carburetor (or a vehicle) is supposed to perform.

Fix it once, then forget it, and do something else.

However, from a professional standpoint, another issue arises: returns. If we do not feel that a carburetor will last our 1 year unlimited mileage guarantee, then we simply do not do the carburetor.

Oddly enough, we feel most other Rochesters (excepting the types AA, BB, R and RC) are excellent carburetors. We really do like the rest of the Rochester line. The 2G series, 4G series, and quadrajets are all excellent design; both working well and pretty much maintenance-free (well, some of the 1965~1966 Q-jets have some issues).

And opinions change with time. For years I have been advocating the use of the Carter YF, as they are an excellent carburetor, and cheap. Unfortunately, they utilize a diaphragm type pump; and the diaphragms to NOT like deathanol. Not too bad in a daily driver, but for a collector vehicle that is driven infrequently, the diaphragms dry out, and once having been in deathanol and allowed to dry, begin to deteriorate. I no longer recommend these carbs unless one can still buy gasoline.

But as I said in my first post, many ways to "skin the cat". Disagreement with respect on the part of all parties increases the knowledge base of all. I learn much more from those who disagree with me than from those who agree.

Jon.

Jon, Thanks lot for your comments, I have absolutely no problem with anything that works. I know of people that have had great results with stuff that won't work for hardly anyone else. I like good performing parts, carburetors, cams, etc. I know I had great results with the old Rochesters, can't explain why, I just know it has happened. I used to do a lot of carburetors for friends, I have had great luck with Carter AFB's for Chevy V-8's, but also have had a couple that I could not make run well. I have used many Holly carbs when I want to use a lot of fuel (they seem to do that well), but they also make plenty of power. I liked your comment about the expense of stuff, pretty wise statement. It's good to have a professional like yourself to give advice to the novices in our ranks, thanks a lot!

And many thanks to both of you from one of the novices. That's good info. I am still planning on running 2 NOS matching tag Rochester B 235 carbs on the 261 stroker that is my current project. My neighbor, the guy who will be doing the assembling, sez he can make them work well and not leak. BTW, a couple of years ago, he also suggested that I used 2 tag matching NOS carbs if I could find them. It didn't take long.

I bet they look something like this! http://rides.webshots.com/photo/2791114620092261295pSVaee
Only mine do not have the glass bowls.