Many models of Rochester carburettors were used in various motor vehicles made by General Motors. If you have had to rebuild your Rochester carburettor for any reason, it will require tuning in order for it to work properly for your vehicle. Rochester produces many models, and many different models were used in the making of these vehicles, but some general tips may be able to help you to determine how to tune your Rochester carburettor.
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Adjust the Idle
After you have successfully rebuilt your Rochester carburettor, you will need to tune it. To do this properly, you will need to make sure the engine is completely warm prior to performing the work. You will know the engine is completely warm when the choke butterfly on the carburettor is in the wide-open or vertical position. For an engine that has had no modifications, the idle should be adjusted to 0.020 clearance between the throttle body and the throttle plates when the engine is at idle. If the engine has had modifications done to it, it will be necessary to adjust for these modifications in order for the carburettor to work properly for the vehicle.
Increase the Idle Air Supply
During the tuning process, you may need to increase the idle air supply from the carburettor. This can be done by drilling one small hole in each of the two throttle plates. If this is necessary, you will need to begin with a hole measuring no more than 0.060 inches. From there, the hole can be increased if necessary, but the hole should not exceed more than 0.125 inches. Performing this modification does not enhance the fuel mixture of the carburettor, only the idle air supply. This can help if you are having problems with acceleration when you are stopped and need the car to accelerate.
Increase the Fuel Mixture Supply
Part of tuning your Rochester carburettor may also involve increasing the fuel mixture supply that is delivered to the carburettor. This becomes necessary when the engine has been tuned slightly higher than specified or when you are using ethanol. The first thing to do if this modification to the carburettor is necessary is to measure the idle tube diameter. To increase the idle fuel mixture, you will want to drill the idle tubes wider. The first drilling attempt can be made at a 0.020-inch increase. If this is not sufficient, each subsequent attempt should be made at no more than a 0.001-inch increase in size. The tube should never be drilled more than 0.005 inches in order to make the modification work.
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