Starts Hard

carburetor technical help: hard start troubleshooting

How To Start Your Carbureted Vehicle?

A carbureted vehicle is started differently than today’s machines. Press down on the accelerator once, then let up. Crank the starter. The vehicle should start in a second or two. What this does is allow the choke valve to close and sets the fast idle screw on a higher step on the fast idle cam.

Accelerator pump not working correctly.

When cold and you step on the gas, the accelerator pump circuit will squirt some gas into the cold intake to get it started. This is easy enough to test. Simply look down the carburetor when cold, press on the gas and see if the pump squirts gas.

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)

Updated on 01/12/2021

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