Rochester Carburetor Troubleshootingby John Stevens J.D.
Rochester carburetors were quite possibly the most widely used carburetor in the twentieth century, with over one million units produced by the Delco Carburetor Company, a division of General Motors. Rochester carburetors are known for their durability and simplistic design, yet even Rochester carburetors will eventually develop problems. Thankfully, the most challenging part of repairing the carburetor is first identifying the likely causes of the problem.
Cold No Start or Stall
If the engine will not start due to a carburetor problem when the engine is cold, the cause is most likely a defective accelerator pump. In fact, this is one of the most common problems with Rochester carburetors. The accelerator pump is particularly vulnerable to vacuum leaks, backfires through the carburetor and dirty gasoline. If the accelerator pump is in good condition, the problem may be with a choke that either will not close because it is caught on something, or because the choke linkage requires lubrication. It the engine does start when cold, but stalls after a few seconds, check whether the idle screw on the driver's side of the carburetor needs adjustment. If the idle screw is not screwed in far enough, the engine simply is not being supplied with enough gasoline to idle. Another cause which can be traced to the carburetor is a float level which is set too low.
Hot No Start or Stall
If the engine will not start when the engine is hot, the choke may be stuck in the closed position, typically due to a lack of lubrication. However, chances are good that the problem is caused by something other than the carburetor, such as a large vacuum leak, a lack of compression or a fuel supply problem, such as a kinked or clogged fuel supply hose. If the engine does start when hot, but stalls after a few seconds, again, the choke may be stuck in the closed position. If the choke is in the open position, the carburetor's power valve may have blown, or the idle jet within the carburetor may be clogged with dirt.
Hesitation or Stalling During Acceleration
If the engine either hesitates or stalls upon acceleration due to the carburetor, the carburetor's accelerator pump nozzle may need to be cleaned or the secondary throttle plates may not be closing all of the way. Occasionally, the carburetor's idle jet will become clogged with dirt, which will cause the hesitation or stall. Finally, check to ensure that the vacuum lines which connect to the carburetor are hooked up correctly.
Hesitation or Stumbling Under Heavy Acceleration
If the engine hesitates or stumbles only upon heavy acceleration, inspect the accelerator pump for the presence of dirt and replace the pump if necessary. The problem might also be caused by sticking metering rods or a defective power valve. The float level may also be too low, which prevents the carburetor from storing an adequate supply of fuel for heavy acceleration.