erbssr: 3/2/2000 - 2:52:02 PM
Two questions guys----
1) What should points be set at on a Mallory Dual Point distributor?
2) Is it really necessary to use a Multi-Sync or other such device to set up dual carbs?

rumble seat: 3/2/2000 - 3:46:48 PM
1)Dwell setting is 26 degrees for each set of points or 33 degrees total. If you're not using a dwell meter for setting the points, set the gaps at 0.022 inch.
2) No you don't have to use a Uni-Syn or such, but it's probably the easiest way to get them synchronized. Most of us old timers used to use a short legnth of heater hose to listen to the hiss each carb made. We'd hold one end of the hose against an ear and move the other end back and forth between the carbs air horns to listen to their hiss. We'd adjust the idle speed screws in or out until they both hissed EXACTLY alike and at the idle speed we wanted. Then we'd tighten the dual carb linkage without disturbing their idle speed. Once this was done, we'd verify they both still had equal hiss.... rumble seat

32tom: 3/2/2000 - 9:07:36 PM
Holley makes a plastic syncro tool that works great. its very easy to match the cfms. the pot metal one with plastic tubes is junk. I'll try the hose trick to see if I did it right. we use the same trick to find exhaust leaks on late models where you cant even see the engine

Bob: 3/4/2000 - 4:30:06 PM
Now that I have my pinging taken care of I have some other questions: I use an Edelbrock 1404 4V (500CFM) with a MCF 3/4 race cam with 281 degree duration. What should my vacuum (inches of hg) be? I still have some surging at 40 MPH and below. But the car now has POWER now that it doesn't ping. Is 281 considered long duration? I used shorter springs in distributor but how do I figure exactly what my total advance is? At what RPM, is vacuum line on or off etc? Thanks for your help. Bob

rumble seat: 3/4/2000 - 9:50:28 PM
Bob: Vacuum varies with considerably with altitude. Rule of thumb, the vacuum on a flathead should be very close to what OHV makes in your area. Here in Denver, we're lucky to get 10 inches Hg with any kind of a cam. Lets look at determining total advance. I'll try to keep this brief as I can. Total advance is the total of static (or initial) advance, plus centrifugal advance, plus vacuum advance. Static is what you set by rotating the distributor on an 8BA engine and shows on the crank pulley. Centrifugal, sometimes called mechanical advance, is controlled by the distributor's internal centrifugal weights, springs, and stops. The vacuum advance is controlled by the engines vacuum, or lack of, and the distributor vacuum advance mechanism. Example? Suppose we had 8 degrees static, 15 degrees centrifugal, and 16 degrees vacuum. Total advance for this engine would be 8+15+16=39 degrees. Determining the amount of each type of advance? Fairly simple on your 8BA engine with an HEI or similar type distributor. PLEASE note the following does not apply to the stock 8BA distributor. If you have a dial type timing light, timing marks on the front crank pulley, and a vacuum pump, it's fairly straight forward to determine how much each type has. With the engine idling and the vacuum line disconnected and plugged, set the dial on the back of the timing light to 0 degrees. Point it at the timing marks as we've all done at least a thousand times. As we all know, this tells us how many degrees static there is. Let's say it's 8 degrees. Now it gets a little harder to explain. Engine still idling and vacuum line disconnected and plugged, keep the timing light flashing on the timing marks (8 degrees static advance showing) and increase engine rpm gradually until the timing marks stop advancing. Now, without decreasing rpms, turn the timing lights dial until the timing marks return to 0 degrees. Let it return to idle while you read the dial. This number minus 8 (which was the degrees of static advance) is the number of degrees the centrifugal weights advance. Let's say the dial reads 23 degrees. 23-8=15 degrees centrifugal advance. Now connect the vacuum pump to the distributor's vacuum line. Adjust the dial on the rear of the timing light so the timing marks show it's firing at 0 degrees. Keeping the timing light flashing on the timing marks, pump the vacuum pump to around 15 inches Hg. Do not release the vacuum pump yet. Turn the timing light adjusting dial until the timing marks show it's firing at 0 degrees. Read the dial. Suppose it reads 19 degrees. This is the amount of vacuum advance you have at 15 inches Hg. So for this engine you have a total of 8+15+19=42 degrees advance. Simple huh? I'm certain others have a simpler and better way to do this, but this is the way I check my engines. The engine surge you have is usually too much vacuum advance combined with out lousy gas. Many distributor vacuum advance diaphrams come with an internal vacuum diaphram adjustment. Remove the vacuum tube from the vacuum cannister and insert a small allen wrench inside it and into the diaphram. You'll feel an allen head screw inside. Turning this outward decreasess the number of degrees of vacuum advance. Turn it all the way in and count the total number of turns. Record it for possible future use. Now turn it back out the same number of turns so you're back where you started. Turn it out an additional full turn and road test. You may have to turn it out a few turns before the surge fades. Remember, the more vacuum advance you can run the better mileage... a lot better!!! Usually 5 turns out is about as far as they will turn. AGAIN, PLEASE note this is NOT for the stock 8BA distributor since those have a different type of vacuum controlled distributor.... rumble seat

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