The Carburetor Shop LLC
204 East 15th Street
Eldon, Missouri 65026
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 my 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”.