Occasionally when working on older machines, it’s common to come across a carburetor that is no longer serviceable. But finding an exact replacement can sometimes be very difficult, or prohibitively expensive. If a new OEM replacement isn’t an option and a search for a used part isn’t working out, there is little choice but to find another model of carb that will work.
There are companies that manufacture new carbs, designed to work for a variety of applications (think Edelbrock or Holley for example); however, you’ll need to know your engine’s CFM (cubic feet per minute) requirements in order to pick the right one.
Just because an alternative carb will bolt onto the manifold and the diameter of the air horn looks right, it doesn’t mean it’s right for the job. It needs to be rated for the correct CFM of air intake and have the right size jets to supply a correct air-fuel mixture for the entire operating range of the engine.
- Read more: How to overhaul your own carburetor
- Read more: Carburetion. It’s all about the air to fuel ratio
To determine the engine’s CFM requirements, you’ll need to do a little math. This will determine the size of carb adequate to provide enough air flow for 100 per cent volumetric engine efficiency. Here is the formula to use when figuring that out.
CFM = (cubic inch displacement x maximum r.p.m.) divided by 3456
For example, here is how that equation works for a 350 cubic inch engine with a maximum r.p.m. of 4,500:
(350 x 4,500) divided by 3456 = 455.72 CFM
If you only know the displacement of an engine in litres, multiply that number by 1,000 to get cubic centimetres, then divide by 16.39 to convert to cubic inches. For example, a five litre (5,000 CC) engine converts to cubic inches this way:
5,000 CCs divided by 16.39 = 305 cubic inches.