Jim Smart
July 23, 2012

Original Autolite distributor vacuum advances are tuned with shims. Replacement advance units call for an Allen wrench. If you want a more aggressive rate of advance, subtract shims or turn the adjustment counterclockwise to reduce spring tension. If you want a slower rate of advance, add shims or turn the adjustment clockwise. Do this in baby steps. Add or subtract shims one at a time or turn the adjustment one-half turn. Then observe the rate of advance and how the engine responds.

During the road test, try different driving regiments. Try normal throttle through 1-2-3 upshifts, gently leaning into the throttle with each upshift. You should experience a steady application of torque through each upshift without hesitation or spark knock (rattling or pinging). When you punch the gas, response should be quick without hesitation. The secondaries should open smoothly without a nose over or flat spots. There should be no spark knock under hard acceleration. When you return to idle, deceleration should be smooth without surging or stalling.

Inspiration for the new 625 Demon came from the desire to conceive a carburetor unlike any before with real-world improvements—dare we say the perfect carburetor?

Baseline Adjustments

New 625 Demon carburetors are already factory adjusted and tuned for a positive installation and start-up experience. However, every application is different. With the throttle linkage connected, make sure the throttle opens fully and closes properly without binding or sticking. You don’t want too much spring tension nor too little. At wide-open throttle, throttle plates should be parallel to one another and 90 degrees to the baseplate. At idle, there should be no play in the accelerator pump linkage connecting rod. The accelerator pump should begin its function the instant the throttle opens. Otherwise, you’ll experience hesitation issues.

At idle, the closed throttle position can vary between carburetor sizes and applications. Observe the transfer slot with throttle closed. It should be approximately .020-inch, just barely visible with the throttle closed. Accelerator pump tuning should look like this with a .15-inch gap when adjusted at the center hole. For a more aggressive pump shot, move link to #1 hole. If the shot is too aggressive, go to #3 hole. The most stroke you want is .30-inch.
Accelerator pump tuning should look like this with a .15-inch gap when adjusted at the center hole. For a more aggressive pump shot, move link to #1 hole. If the shot is too aggressive, go to #3 hole. The most stroke you want is .30-inch.