VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 Rochester BC accelerator bog

Having disassembled, inspected and cleaned my 1954 Rochester BC 1-bbl carb from my 210 4-door sedan (standard shift), I was hoping the considerable lag in the acceleration circuit would improve. I could see that it had recently been done with a similar Niehoff kit by the previous owner, so I only changed out two gaskets. The accelerator pump (plunger) appeared to have a healthy and properly-flared leather ring but I still applied a light machine oil to the body.

Any sources of vacuum leak were addressed at reassembly. The bog occurs both from idle to power circuit and as well from the cruising to power circuits when under load and at speed. The only way to avoid stall when moving from idle is for me to have the mixture set a little rich.

For comparison, I previously owned a 1957 3100 pickup with the Rochester B carb (manual choke) and it was great - this one on the car is hopeless.

Any ideas out there? Has anyone purchased a rebuilt from any of the major 1949-54 suppliers?

[I'm sure GM and Rochester couldn't have designed this performance intentionally, but I must say their design is a bit too delicate. (At least the Solex carbs on my old VW's gave the venturi a visible squirt, using a true diaphragm accelerator pump.....)]

Thanks, Dave.

....and how are you setting the mixture a little rich?

I would say that you still have a problem within the accelerator pump and the system associated with the accelerator pump. The fuel flow circuits in the Rochester B, BC carbs are complex but not really delicate. The vacuum port in the base of the carb, must be kept clean and open in order for the carb to function properly.
The 1957 Truck shop manual is a good resource for learning how the Rochester B carb functions, internally it is about the same carb as the Rochester BC carb, however Rochester made improvements in the carb during its long application run in 216, 235, 261 engines.
This link is a Marine site but it had good info on the Rochester Model BC carb
http://www.marineengine.com/parts/technical_information/18-7071.pdf

Sit down with a cup of coffee & read this handy dandy publication.

http://chevy.oldcarmanualproject.com/booklets/5020mts00.htm

While you are drinking a drink and thinking about your problem, reach over and grab the throttle and give it a yank. Take a look at the vacuum advance and see if it reacts to this function. At least if it moves you can remove it from your check list.

That is a great publication. My 40 1/2 ton pickup has the Carter W1 carburetor, but I'm hoping to acquire and install a Rochester B carburetor to compare the perfromance.

I noticed in the trouble-shooting section, there is a reference to a problem of rough idle or hard starting when hot, which is a chronic problem with the W1 carbs. The Rochester manual identifies the problem as percolation or vaporization of fuel in the bowl from excessive heat, forcing fuel up the delivery tube into the carburetor throat. The fix is to drill a hole in the air horn at a location identified in the accompanying drawing, which evidently vents the bowl to atmosphere. I wonder if there's a comparable location for a vent hole on the Carter W1.

Mark Yeamans
VCCA #35724

Love these OEM publications, especially this one which was probably viewed as a filmstrip at the shop or dealership.

Page 19 is interesting to me. I don't remember seeing a circlip and strainer at the base of the accel pump well. Nor did I fully inspect the drillings/ports at the base of the throttle body.

For reference, mine was never fitted with a vacuum advance - at least a dashpot anyways.

Thanks on this topic.

Very interesting. Do I understand that there isn't a vacuum advance on your distributor?

You may be confused by the dash pot that looks some what like a vacuum advance. Your vehicle must have a vacuum advance...this vacuum advance device is attached to the distributor and has a small long steel tube that is attached to the bottom of the carb's throttle base. The dash pot shown in the carb booklet is used only on early powerglide engines and it's function is to prevent the throttle from closeing too fast. most Chevrolets with standard shift and trucks do not have the throttle closeing dashpot. Also many Rochester bodel B & BC carbs will not have the screen strainer and circlip. There were several mods to the Rochester model B carbs after this information was released. However most of the information applies to the entire series of Rochester Model B & BC carbs. Like MT said study it carefully, it will often get you back on the road.

The screen and clip was on the early "low cover" Rochesters only. Your 1954 will not have that feature.
Possible causes if it is the carburetor. Ball IN THE ACC. PUMP not seating. Shake the pump next to your ear. You should hear the ball bouncing around. The check ball in the pmump circuit missing or the wrong ball. DO NOT look at the early low cover instructions as they had two balls and in different locations. Incorrect bowl cover gasket. Two come in the kit. The more "solid" one is correct for 1952 and up.
The best place to look for the proper instructions is a 1952 and up Chevrolet shop manual. If a picture shows a screen the instructions are for a 195-51 early low cover unit. N.G.

I'm sorry and very embarrassed at the same time.

Does the accelerator pump work better if the small link from the throttle crank to the pump rod is attached?

(Good grief).

When I removed it from the manifold to give it another inspection, I found the link attached only at crank end, the tiny hairpin clip still right where I left it - in the groove. Reattached and fired it up tonight for a run. Running great now.

Thanks again for all the time and responses, guys.

Thanks for the reply and we all have done some "not too smart" things. Glad your problem has been solved.

It takes a good man to find one of there "I been there and don't know why mistakes" and have a good laugh after it is fixed. Good to know what happened.