The Ford Barn - View Single Post - Idle Screw

drolston · 02-26-2019, 11:41 AM

Having to run the idle adjust screw all the way in to a high idle speed suggests that the idle fuel circuit, adjusted by the two idle mixture screws, is not providing fuel. At idle the air flow through the venture is not high enough do draw through the main jets. The idle circuit is designed to provide fuel for that very low rpm.

Open each idle mixture screw 2 turns from full closed, regardless of whatever other reference you have found. Get it running at the slowest rpm you can with the idle speed adjustment on the linkage. Then start closing the idle mixture screws 1/4 turn at a time to see if the idle speed picks up. if it does, back off the idle speed adjust to lowest idle, and go back to the mixture screws, this time, one at a time, to find the idle mixture adjustment that gives highest rpm, again backing down on the idle speed adjustment as you progress. If this fails, we need to look elsewhere in the fuel system.

Low fuel level in the carb float bowl can cause your problem. Start with checking fuel pump output. Disconnect the fuel line from the carb, and crank the engine WITH THE IGNITION OFF. You should get a strong spurt of fuel from the line of about a tablespoon for each cycle of the pump. If that fails, check the stroke of the fuel pump push rod at about .2 inches. If that is bad, the cam is worn and you need an electric fuel pump. If the stroke is good, try the pump with the input line a hose into a jar of gas. If the pump works okay with no obstruction to its' input, check the flexible fuel line for cracked or swollen, and fuel line from the tank unobstructed. On a '33 you can blow into the fuel tank to see if a stream of fuel emerges from the fuel line to the pump.

If the pump was good, remove the needle and seat from the side of the carb and look for a sticking needle. The rubber tipped ones get sticky in alcohol. I recommend replacing with the Grosse type that uses a steel ball.

If that doesn't fix it, check the float level adjustment to be sure it is not too low.

If that does not do it, the carb comes off for a cleaning and/or rebuild.

Good luck.

02-26-2019, 11:41 AM	#11
drolston Senior Member Join Date: Jun 2014 Location: Williamsburg, VA Posts: 1,627	Re: Idle Screw - Running Issue on a '33 BB Having to run the idle adjust screw all the way in to a high idle speed suggests that the idle fuel circuit, adjusted by the two idle mixture screws, is not providing fuel. At idle the air flow through the venture is not high enough do draw through the main jets. The idle circuit is designed to provide fuel for that very low rpm. Open each idle mixture screw 2 turns from full closed, regardless of whatever other reference you have found. Get it running at the slowest rpm you can with the idle speed adjustment on the linkage. Then start closing the idle mixture screws 1/4 turn at a time to see if the idle speed picks up. if it does, back off the idle speed adjust to lowest idle, and go back to the mixture screws, this time, one at a time, to find the idle mixture adjustment that gives highest rpm, again backing down on the idle speed adjustment as you progress. If this fails, we need to look elsewhere in the fuel system. Low fuel level in the carb float bowl can cause your problem. Start with checking fuel pump output. Disconnect the fuel line from the carb, and crank the engine WITH THE IGNITION OFF. You should get a strong spurt of fuel from the line of about a tablespoon for each cycle of the pump. If that fails, check the stroke of the fuel pump push rod at about .2 inches. If that is bad, the cam is worn and you need an electric fuel pump. If the stroke is good, try the pump with the input line a hose into a jar of gas. If the pump works okay with no obstruction to its' input, check the flexible fuel line for cracked or swollen, and fuel line from the tank unobstructed. On a '33 you can blow into the fuel tank to see if a stream of fuel emerges from the fuel line to the pump. If the pump was good, remove the needle and seat from the side of the carb and look for a sticking needle. The rubber tipped ones get sticky in alcohol. I recommend replacing with the Grosse type that uses a steel ball. If that doesn't fix it, check the float level adjustment to be sure it is not too low. If that does not do it, the carb comes off for a cleaning and/or rebuild. Good luck.