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Hagerty Employee

Troubleshooting a spring no-start on a car with a mechanical fuel pump

So, your car ran in the fall. You take it out of storage after a winter snooze, and crank and crank and crank it, and it won’t start. What’s up? If it’s a carbureted car with a mechanical fuel pump, it may have a priming issue.


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another issue with the older pumps is that today's ethanol contaminated garbage gas attacks the type of rubber used in the older diaphragms. there are companies out there offering rebuild services and kits to replace the vintage rubber material with an ethanol tolerant material

Intermediate Driver



Thanks for another great article.  


But I do take issue with one thing  that you wrote:  You had said "it’s different in a carbureted car with a mechanical pump whose fuel pressure depends on engine rpm."  The pressure delivered by a mechanical pump simply the force delivered by the spring / the effective area of the diaphragm.  These don't change with speed.  Admittedly the peak volume the pump is capable of delivering is speed dependent.  But mechanical pumps are sized so that they never reach peak volume.  If the carburetor demands less fuel, the diaphragm just doesn't retract enough to allow the lever to fully re-**bleep** the spring.

Intermediate Driver

Ha ha - I typed "re-c o c k the spring" and it printed as "re-**bleep** the spring".  Funny world we live in!

Community Manager

It's amazing how random people on the internet have ruined the internet for the rest of us 🙂 

Intermediate Driver

Priming bulb brilliant. I have a Cal-29 sailboat with an atomic 4 30 hp that I prime at the beginning of each voyage. Now I will do the same for my 1971 triumph TR six dual weber at the beginning of each new Season. With the fire extinguisher nearby of course. Love reading your detailed articles. Let’s keep them on the road. PS had a 72 BMW 2002 for 20 yearsLoved it. German engineering is quirky. Rubber donuts on the driveline, valve adjustments and loved to be thrashed


The best product I have found for adding a "fuel filter" on the inlet side of a mechanical fuel pump is Holley HydraMat.  If any part of the HydraMat is touching fuel, the fuel will flow to the pump.  I found it to be a game changer.


Hi Rob, I enjoyed your article as I  always have , but you lost me at "If no liquid pumps when the pump is attached but it did when it was tested alone, you may still have an air leak you haven’t found."


At that point, nothing that belongs to the car is hooked up. You still have the small containers for input and output.

Pit Crew

Great article. Just like to add some backyard mechanic advice. Besides using your eyes for a visual check of the fuel system use your nose to pinpoint possible leaks or seepage not seen. A strong raw gas smell in the area of the trunk or fuel tank might be the tip off of a rotten hose or corroded steel fuel line. Same goes for the engine compartment. There should not be a raw gas smell except coming from the carb if fuel is flowing. I am old enough to remember sight glasses in the fuel line leading to the carburetor. If it wasn't filled with amber liquid you had no fuel. It also acted as a sediment or water indicator. In that vein of thought using a cheap clear plastic inline filter temporarily spliced into the fuel line above the pump will tell you if the pump is working. If the engine will not start the first thing I do is check for spark. Then with the coil disconnected look in the carb to see and smell if fresh gas is flowing. If not disconnect the fuel line from the carb and carefully crank the engine to see if fuel is coming to the carb. I connect a length of fuel line to the car fuel line then poke it through an appropriate sized hole in the lid of a clear fruit jar to capture any gas. This system was put in place after starting a fire in the engine compartment of my K10 Chevy pickup when the aforementioned disconnected coil wire threw a spark that ignited the spilled fuel that I was attempting to catch with a shop rag. The truck did not burn and I did not get hurt but I did have my heart rate go to about a thousand and had to replace some burned wiring and vacuum lines.

New Driver

My Sunbeam Alpine has a little handle on the fuel pump that allows you to pump it by hand.  Super handy to fill the carb if it's been sitting for a while.


And all this time I thought the mechanical pump was located at the front of the car because that's where the cam is! Seriously though in my experience a carbureted fuel system with no leaks will gravity feed to the pump which is low on the block; the pump only needs to lift the fuel the two feet up to fill the bowl(s). I laugh at the cars I see with 15 psi pumps (usually chromed!), and regulators to get it back down to 3. 2psi is all you ever need for this. But the chrome never hurts....