1. Home
  2. Technical
  3. Starts Hard
Starts Hard When Cold

Improper starting procedure used - Most classic cars are started cold by pressing down on the accelerator once, then cranking the engine. If the thermostat is working correctly it will close and the fast idle cam will increase the RPM.

No fuel in gas tank - Add fuel. Don't assume the fuel gauge is working correctly.

Choke valve not closing sufficiently when cold - Realign the choke valve or linkage as necessary. If caused by dirt and gum, clean with automatic choke cleaner, or spray carb cleaner. DO NOT use oil to lubricate. Oil will collect dirt. If parts replaced re-adjust to specifications.

No fuel in carburetor - Remove the fuel link at the carburetor. Connect hose to fuel line and run into container. Remove the high tension coil wire from the center tower on distributor cap & ground. Crank the engine. If there is no fuel discharge from the fuel line, check for kinked or bent lines. Disconnect fuel line at the fuel tank and blow out the line. Reconnect the link and check again for fuel discharge. If none, replace fuel pump, then test the fuel pump for proper pressure. Most carburetors need around 4 lbs of pressure, but check your motors manual.The pressure varies greatly between carburetors.
NOTE: New fuel pumps are especially suspect. It isn't unusual for a new pump to put out too much pressure. Either get a pump that works correctly, or install a fuel regulator.

If fuel supply is OK check the following:
Inspect fuel filters. Replace if dirty.
Remove air horn, or fuel bowl and check for a bind in the float, or sticking float needle.

Vehicles that have been sitting may contain gas that has turned in varnish, coating the inside of the carburetor.

Hard Starting After a Few Minutes

Look down the carburetor after turning the engine off. Is the fuel dripping out of the venturi. If so check these things:
  • Are the gaskets sitting flat. They sometimes need to be trimmed.
  • Gas cap should be vented.
  • Any vent line going back to the gas tank should be clear and with no kinks.
  • Float bowl vent should be open.
  • Clean the small holes in the venturi. They get clogged easily.
  • Test the accelerator pump circuit to make sure fuel isn't getting past the check ball in the main discharge. Test by filling the float bowl with fluid, hold the check ball down and pump the accelerator pump. Fuel should not get past the check ball. If fuel is getting past the check ball, then use a brass drift punch and tap the ball lightly to seat it. Too hard and you will get the ball stuck.
Hard Starting After a Few Days 

Could be caused by percolation. Percolation is when the engine gets hot and then when you stop, the fuel boils over. This has become a too common problem because of the lousy gas we now have. See our percolation page.

Evaporation - Make sure any vent tube on the top of the carburetor is not blocked off.

Leaking - Take the carburetor off the engine and sit it on top of your work bench and place a hand towel under it. Fill the float bowl with fluid and leave overnight to see if any fuel is leaking out of the clean out plugs. There are a couple of small plugs on the side of the carburetor that can leak.

I have heard of fuel pumps siphoning off the gas when the needle & seat are worn. You can disconnect the fuel line after stopping the engine, then connect it back up after a few days. If the engine starts, then your problem is with the pump, or the needle & seat.

At idle, or after turning the engine off, look at the venturies to see if any gas is dripping into the throat. This could be caused by blocked idle holes on the venturi. (the very small holes on top of the venturi)

Can't find what you need? Contact Us