Jim Smart
July 23, 2012

In this new age of electronic engine control, the humble carburetor seems lost to the old technical manuals, vintage swap meets, and dusty attics. So why an all-new performance carburetor from Demon when fuel injection has the obvious advantage? Because there are plenty of people who want the simplicity and crisp response of a performance carburetor, one they can tune themselves without a laptop or an exotic software program. They also want performance and fuel economy, both of which have been well thought out by Demon’s designer, Larry Tipton.

Larry has spent most of his life designing and engineering fuel systems. In fact, he began his career with Carter, a company many of us are familiar with, and has designed and engineered fuel systems for nearly half a century. 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?

When we asked Larry about his inspiration for an all-new carburetor design, he commented, “In carburetor design, as with any complex product, you need to keep the complete design in mind at all times. Nonetheless, one of the first challenges was to provide the new carburetor with instant throttle response. To this end we quickly settled on 13⁄8-inch primary throttle bores. To ensure that the primary venturii responds immediately and with authority, we decided to equip it with the strongest possible vacuum signal at the primary nozzles. So we adopted triple-stack venturii, a highly effective innovation that intensifies the vacuum signals and transfers them very quickly to the primary nozzles, giving great effect immediately at off-idle.”

Larry added, “Since its beginning, Demon Carburetion has specialized in racing carburetors, or for street-strip and muscle cars. Later, the Demon Six Shooter and Demon 98 carburetors and their attendant intake manifold systems were designed for small, specialized markets. But the company never had a carburetor specifically engineered for the popular performance markets, especially a lightweight aluminum, die-cast model. It was this niche that inspired the 625 Demon. We felt there was an opportunity to improve the appearance and function of the current AFB-style carburetors in the market today.”

The basic architecture of the Demon is loosely based on the Carter AFB/AVS carburetors with metering rods and jets along with a piston-style accelerator pump. This is where similarity ends. What makes the 625 Demon more advanced is long needed improvements—butterfly-style secondary transition air valve, spread bore design with “right sized” primary and secondary throttle bores, a main body designed to keep heat away from the fuel for improved performance, and a no-brainer adjustment system that makes engine tuning a snap.

When the engine is fired and at operating temperature, it is time for initial adjustments before the first road test. Adjustments depend largely on ambient temperature, humidity, and elevation. Idle mixture screws located on the front of the 625 Demon are strictly for idle air adjustment only. They have no effect on off-idle (open throttle) performance. At idle (approximately 600-800 rpm), you should have 17-22 inches of vacuum, depending upon the cam profile. One at a time, slowly turn the idle mixture screws clockwise until engine speed begins to falter. Then slowly back out counterclockwise until the idle is smooth. Do this on both sides. Then do it again one at a time until you have a smooth idle.

If you have a distributor with a vacuum advance, this is the time to check the vacuum advance function. When you check ignition timing, you’re looking for two things—centrifugal and vacuum advance function. As a rule, vacuum advance typically hands off to centrifugal advance as rpm increase. Total ignition timing at 3,500 rpm should be no more than 36 degrees BTDC. With the vacuum advance connected, goose the throttle and watch the timing mark. If advance is aggressive, you need less vacuum advance. If it is sluggish, you need more.

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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.