Carb Idle Circuit

[jord]

I cannot get any adjustment on my two idle mixture screws; carb was just rebuilt by a rebuttable source. Was trying to increase my vacuum which is around 15. Installed a new manifold gasket and manifold. Any suggestions on how I can find a possible vac leak. Called the rebuilder and he say's I should be able to kill the engine by turning both screws in and with this still no change. Car idles and runs and still stumbles. 56 H engine 

 

[Lee H]

If your idle speed is set too high, the throttle plates will not be in position to draw from the idle circuit. It will start drawing fuel from the main circuits. Open each mixture screw two turns, then reduce the idle speed throttle opening to where it will just barely tick over, and try adjusting the screws again. I think the spec is in the low 400 rpm range.

[carbking]

As Lee mentioned above, if the throttle positioner screw is too far in (throttle plates open too wide at idle), there will be insufficient signal to allow the idle circuit to work as designed.

 

I don't own a 12 cylinder Lincoln (but have been around a few carbs  ).

 

Holley suggests setting the idle mixture screws at 3/4 turn each from lightly seated (a freshly rebuilt engine or a worn out engine will require more opening); start the engine and warm to normal operating temperature, then  set the throttle positioner screw to give the desired idle speed (Holley gives no spec for idle speed). Now try adjusting the idle mixture screws. Turning them in SHOULD stall the engine. If the engine is in good operating condition, you should end up with the idle mixture screws around 3/4 turn. A freshly rebuilt engine, maybe 1 full turn. If the engine is so worn it uses more oil than gasoline, you might need to go between 1 1/4 and 1 1/2 turns on the mixture screws. This is assuming factory issued idle mixture screws with the short taper. If replacement long taper screws have been substituted, then the adjustments will be more turns.

 

Vacuum leaks may be chased using an UNLIT propane torch. Acquire one of the bench tank holders so there is no danger of the tank upsetting. Acquire 10 feet or so (or whatever is necessary) 3/16 inch vacuum hose, and a 6 inch piece of 1/8 inch steel tubing. Neck down the nozzle on the propane tank to the 3/16 inch hose. Attach the steel tubing to the other end of the hose. Start the engine. Now crack the propane control so there is propane, but not a lot. Use the steel tubing as a wand to pinpoint the leak.

 

Try adjusting the carburetor as in paragraph 3 before you start looking for vacuum leaks.

 

This is the least messy method of which I know.

 

Jon

[jord]

Thanks, Lee and Jon,

Jon could explain the propane set up I am not understanding the process. I think my idle may be high will go back and try again.

[jord]

Found the process on UTube, neat idea. 

Thanks guys.