Posted on: March 9, 2009

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If you have a dual carb-equipped car such as an Alpine I through III, an Alpine V or an Alpine GT, there might be some fine points you have missed in the course of tune-ups. Dual carbs are no great secret: all they take is some care and TLC. But if you neglect them, you will find your vehicle is running like a Mack truck and sounding like a boiler factory when it’s idling.

One thing to look for is poor throttle synchronization. With an older car, the passage of time causes damage and wear on the throttle linkage. It’s absolutely imperative to make sure both throttles are opening evenly and at the same time. Make sure they both open fully, and toy around with the linkage between the carbs until you have them opening at exactly the same instant. It takes a little attention, but it pays dividends. The Alpine V Stromberg linkage is particularly prone to problems here.

Another thing that is often overlooked is ensuring the various components are making a tight and leak-free seal. The Zenith carbs on the Series I through III often worked loose, or the manifold would vibrate free just a tad. It might not be obvious but it would be enough to cause a manifold leak with constant problems. The Stromberg manifold is much shorter and better angled to avoid this, but it still bears checking out. Check all carb and manifold mounting bolts with a wrench and snug them down securely but not TOO tight, as both carb and manifold are aluminum and can be warped.

Also, the workshop instructions on adjusting strangler (choke) controls are suspect. Both Zenith and Stromberg (actually the same people!) have backed off from the “drill in the throttle plate method” in recent years and have simply stated that the best way to adjust your choke is by ensuring that it advances the idle to the “reasonable” speed. With the Alpine I through III, 1000 rpm is probably a satisfactory idle speed. With the Series V and GT, 1500 rpm is probably good. You should try to find the speed at which your car will idle evenly under cold conditions with the strangler out. Due to wear on the engine and carbs, no two engines will have exactly identical solutions, but the above values give you something with which to work.

Balancing dual carbs is a tremendous shibboleth that is actually not at all the arcane secret it’s supposed to be. Use a Uni-Syn or similar tool if it’s available. If not, use a 1/4 inch rubber hose to listen to the hiss at the air intakes. (WARNING: The PSW tool kit method of comparing piston rise simply isn’t accurate enough. Why spend $7.95 for something that’s not good enough? What you’re trying to do is to ensure that both carbs are drawing evenly. It doesn’t have to be exact, just in the ballpark. Try it a couple of times to get it worked out and you’ll find it’s really not that hard. I recommend you take the air cleaners off and run the engine hard for a twenty minute run to get the engine warmed up nicely, then work fast. The closer to actual road conditions you are, the more accurate your work will be.

Dual carb setups are more sensitive to throttle shaft wear than are single carbs, and it’s important that your exhaust be in reasonably good condition. Double-check your linkage when you are finished to ensure that the throttles are precisely even and open simultaneously. That is, once you’ve balanced the carbs and set mixture strength, go back and make sure both carbs open and close at the same time.

You also should balance your carbs at fast idle. That is a fine point even the workshop manuals leave out. Pull the strangler out, check the balance, and toy around with the linkage to make sure both carbs are drawing evenly.

Watching for these few points should improve your idle quality considerably and should improve your car’s response, mileage and performance. Dual carb setups are superb and are a fine solution to the problem of fuel metering, but they do require some owner attention to detail at tune-up time!

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