The Stovebolt Site Forums The Shop Area Engine Shop fuel pump flow and pressure during cranking

OK- - - - -here goes!

[b]FUEL PUMP TESTING

Here's a procedure I learned back in the late 1950's to evaluate the performance of a mechanical fuel pump. It works on anybody's pumps, even non-GM ones! Mechanical fuel pumps are simple devices, a reinforced rubber diaphragm operated via a lever linkage from a lobe on the camshaft. They provide a one-way flow of fuel from the gas tank to the carburetor when they're working properly and they're usually pretty reliable. Unfortunately, since virtually all the pumps that are available for stovebolts are made in some offshore sweatshop by unskilled slave laborers, the failure rate of even "new" pumps is pretty high.

1 Obtain a vacuum/pressure test gauge from either a local parts store or online. On average they cost less than $20.00 and they have a lot of uses other than testing fuel pumps. For example:

www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html

2. The engine will run for a minute or so on the fuel in the float chamber. Disconnect the fuel line at the carburetor and "dead head" the line into the hose on the gauge. Be sure to clamp the hose firmly to the fuel line to avoid a gasoline spill.

3. Start the engine and idle. The gauge should indicate somewhere between 2 and 4 PSI. Here's the important part- - - -when the engine is shut down the pressure should remain for several seconds, maybe as long as half a minute. This tests the outlet check valve in the pump which prevents fuel from draining back and emptying the gas line between the carb and the pump. Too-high pressure is caused by the pump manufacturer installing too stiff of a spring above the diaphragm when the pump was manufactured. I have seen pumps that produce 8-10 PSI or more and cause the carburetor to flood.

4. Plumb the fuel line at the carburetor to a catch container, using a piece of runner fuel line. Run the engine at idle for 30 seconds. A good pump will produce at least a pint (2 cups) in 30 seconds.

5. Move the gauge to the inlet side of the pump and attach the hose securely to prevent air leaks. Idle the engine. There should be at least 10 inches of vacuum on the gauge, and it should also remain for a short time after the engine is stopped. This checks on the integrity of the inlet check valve in the pump.

6. Move the gauge to the upstream side of the flexible line from the pump to the vehicle frame. Repeat the run test. A pinhole in a flex line can leak air and reduce or stop the fuel flow, often without causing a gas leak.

7. If desired, vacuum test the metal line from the engine to the gas tank. The vacuum should be close to what's developed at the pump, all the way to the gas tank, and should also remain for a short time when the engine is stopped.

If the engine won't idle for a short time on the fuel in the float bowl, you've got other problems besides a fuel pump in the mix. A turkey baster can be used to fill a float chamber through the vent tube that most carbs have in the air horn. It's that brass tube with the bologna-sliced tip that sticks up above the choke plate. Good luck!
Jerry

[/b]