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Vintage Mustang Tech Q&A: 1970 Fuel Issues
Beyond the Basics
Fuel Pump Issues
I have a 1970 Mustang with a stock 302ci two-barrel V-8. The fuel pump died the other day and I replaced it with a new one from the parts store, but the engine still doesn’t get any fuel and won’t start. I can spray starting fluid into the carb and it fires right up, but of course dies a few seconds later. I pulled the fuel line off of the carburetor and nothing came out when I cranked the engine, so I don’t think the carb is stuck. I changed the fuel filter as well. Any ideas would be helpful.
Jordan (last name withheld)
The fuel system is very simple on a small-block. The most common culprit is the diaphragm in the fuel pump. It goes bad, and when it does you should do what you did and just change the pump. Usually you are on your way. The second thing we look for is an obstruction in the fuel lines or carburetor. Change the fuel filter again (if you have varnish in the gas it can clog quickly). Drain the fuel out if you can, and put fresh gas in the tank.
You can take the line off of the carburetor and run it into a small bottle or jar, turn the motor over, and see if you are getting fuel. If you are, then you can carefully put a length of fuel line to the carburetor and fill it with fuel. Fill the line (a foot or two might work), hold it up, and let gravity feed the carb. If it starts or sputters you know it isn’t the carb. If you don’t get any fuel, my suggestion would be to unhook the lines from the carb and fuel pump and carefully blow super-low pressure through them—3 to 5 psi max. Open your gas cap and have someone listen for air escaping in the tank (or bubbles), and listen for the carb fuel line to be clear. If the lines are clear, you can confirm your fuel pump is operating properly off of the engine by running a line into a gas can and routing the output into a jar or back into the gas can. Give the pump arm about six to seven pumps and you should get fuel pumping through.
Now is where it gets tricky. If everything above is working, you can go get an inexpensive electric fuel pump (5 to 7 psi for a carb), temporarily hook it inline, and see if the engine will run with the electric pump. An inexpensive fuel pump block-off plate is also required and available at most auto parts stores or by mail order.
If it runs with the electric fuel pump, you may have a problem with the fuel pump eccentric located on the end of the camshaft. On your 1970, it is a single piece (early 1970s had a two-piece unit), and is mounted offset to push on the fuel pump arm every revolution of the camshaft. A dowel pin drives the timing chain gear and runs through to the fuel pump eccentric to hold the eccentric in place. Sometimes on an engine rebuild, the pin gets inserted a little too far into the camshaft and just barely holds the eccentric in place. If the pin lets loose, the eccentric can move enough to prevent the fuel pump arm from moving its full travel (if you notice the arm will move about two-thirds before any “pumping” occurs). If that has happened, it means the front of the engine needs to come off. Not fun.
I know the electric pump test is a little money, but it is cheaper than a front engine teardown. You need to confirm it is anything but the camshaft eccentric before you have to dive in.