///////////////Just wondering if vacumn running from fitting on intake manifold will give you the same results for distributor advance as coming from carb fitting?////////////// ----------------------------------- Certainly not! Two very important terms here: Manifold vacuum and Ported vacuum. Manifold vacuum exists at the fitting you mentioned because it is below, or beyond the throttle plates of the carburetor. It is at its maximum at idle speed and drops off sharply when the throttle is opened. Ported vacuum, where the vacuum advance connects, is above the throttle plates, and should read (nearly) zero at idle speed, but increases sharply as the throttle is opened. ----------------------------------- July 2005 What vac advance port to use on a carb..//// The ported vacuum is correct. Use of the manifold vacuum will give you full advance at idle and declining advance as the engine works harder-not what you want. ------------------------- Wide open throttle is zero vacuum everywhere unless you are severely under carbureted or have an intake restriction. Ported vacuum is the ideal spot for your vacuum advance. If you run full advance at idle emissions and idle quality will suffer. No gain in performance as ported vacuum reacts instantly to throttle opening. There is no more or less vacuum at the port just that it is above the throttle blades rather than below so it is not active until the accelerator is moved past the idle position. If you are experiencing a ping or detonation at cruise or high load low speed situations you may have too much vacuum advance. I had that problem with my original dual point. The car would surge at cruising RPM's, pre 700R4. I have an adjustable on the Delco now, I don't know if an adjustable from a Mopar distributor might work on the Prestolite. You can limit the amount of advance by shortening the distance the vacuum can's arm travels. I'll get into it if that's something you need to do. From a performance standpoint the vacuum advance neither helps nor hurts, strictly a driveability thing. It will also help you run cooler at cruising speeds as a retarded engine will run a little hotter. P.S. the original AFB was ported vacuum. ------------------------ Yep, it will for sure. The intake manifold connector will give it full advance at all times when the car is running. It also holds the diaphragm in the fully retracted position, so if your engine is producing the normal 16 to 18 inches of vacuum it can damage the diaphragm very quickly, and advances the timing to full advance all the way from start to cruise, which is not correct. Especially when starting the engine, you do not want timing advance. The correct carburetor fitting will only give vacuum or advance as the throttle is pushed down, in varying amounts, as the vacuum advance was designed, which is correct. Unless your shop manual specifically tells you where to put the vacuum advance line, always put it to a carburetor fitting. Note: Some carbs have more than one vacuum fitting. To find the correct carb fitting easily, block the carb fittings off with rubber caps or pieces of hose with screws in them, start the engine and let it idle, remove the caps one at a time. If you can feel vacuum with your finger while the car is idling, or the engine changes rpm when you remove the cap, that is the WRONG fitting. Normally you will find the correct connection on the base in the front of the carb, near the air mixture screws, or on the passenger side, (choke side), in the area of the carb in front of the choke. Since the correct connector will have no vacuum until you increase the engine rpm, the obvious test is the increase in vacuum will be able to be felt with your finger. Also, automatic chokes normally have a fitting or fittings going to the intake manifold to draw heat from the intake to warm the choke. That fitting normally has no vacuum, and is a very big fitting, has threads, and is very obviously NOT the right connection. The correct connection to the carb you are looking for is about the same size or diameter as the connection on the vacuum advance and does not have threads. I have seen earlier carbs with metal lines and compression fittings on both the vacuum advance and carb ends, especially early Ford, Jeep, Chrysler, but it's very easy to figure out where those lines connect. Hope this helps. Sonny ----------------------------------- Should not be manifold vacuum. If it were, you'd have the advance plate at full advance at idle (high vacuum) and when you stepped on the gas (drop manifold vacuum) the timing would retard. Then the timing would advance again with the centrifugal weights. It would act wierd, start hard, and not run well. Carb vacuum taps into a port drilled right in the venturi ring of the carb. Bernoulli's principal says that as cross section in a tube decreases, the velocity increases, and the pressure decreases. This is how the carb gets the fuel into the manifold. The engine doesn't suck the fuel in, the outside ambient air pressure pushes the fuel into the low pressure are right at the venturi rings of the carb. This same area (at the narrow point of the venturi rings) then can be tapped for a vacuum signal that tells the distributor approximately how much air (mixed with fuel) is going into the engine. That can be calibrated to get the distributor to advance the timing to make sure the spark has enough lead time to light the fuel/air mixture in the cylinder. Since the burn rate of the fuel and air is fairly constant, you need to light it earlier to get the pressure buildup from the fuel burning process to peak at exactly the right time to put the maximum push on the piston to get the most work out of it. Hope it helps. Jeff (love that Bernoulli guy ) Rice ----------------------------------- //////////very good explanation! Now another one-- what if the carb has no vacumn port? Does that mean I can only run mechanical advance? This is a racing carb with no choke. Running another "chokeless" one at the moment that idles great and only takes 50 or 60 seconds to warm up in 50 degree weather before going on my merry way. I would never have believed it and thought it would be a bitch before installing it. //////////// ----------------------------------- ep, that's what I would recommend. There is a kit available to put springs and weights in the distributor that will, "kinda" give you incremental advance. I say kinda because they are normally just in and out of full advance at a pre-determined rpm, but it's STILL a better situation than full vacuum to the vacuum advance. Some race cars used magnetos, and the newest hi-tech racing cars don't even use a distributor. > This is a racing carb with no choke. Running another "chokeless" one at the moment that idles great and only takes 50 or 60 seconds to warm up in 50 degree weather before going on my merry way. I would never have believed it and thought it would be a bitch before installing it.< It sounds like you have a typical racing carb that is set up way too rich for the street and shouldn't be a problem if you make your living on the street in the highest RPM ranges all the time, are not worried about the 5 mpg or less, and the mean temperature always remains above 50 degrees. A carburetor that is set up correctly would have a hard time starting and running smoothly until it was warmed up. Also, running an engine too rich on the street has other negative consequences especially if you have to cruise around for a while looking for the other guy willing to lose his pink slip. ----------------------------------- this topic makes me want to ask... I put an edelbrock 1406 on my 63 avanti to replace the afb which wouldn't run right. It's got two vacuum ports. The manual that came with the carb listed one of them ( not the one I'm using) as "timed". I'm still not totally happy with the way it runs at lower RPM's. Could this be some of the problem? ----------------------------------- Use the port on the passenger side. ----------------------------------- > I put an edelbrock 1406 on my 63 avanti to replace the afb which wouldn't run right. It's got two vacuum ports. The manual that came with the carb listed one of them (not the one I'm using) as "timed". I'm still not totally happy with the way it runs at lower RPM's. Could this be some of the problem?< Very possibly Jeff. Check the vacuum ports to see which one only gives vacuum when the throttle is opened. That is the port you want. If that doesen't help, you probably need a combination of timing and carb tuning. Good tuning is somewhat of an art if you don't have the high tech machinery, never was an exact science, just take your time, make one change at a time, test it, note the difference and on and on. Remember of course that some changes effect some of the settings that you were satisfied with! See what I mean, no science here. Have you checked very thouroughly for vacuum leaks around the top of the engine? It wouldn't hurt anything to get on a tuneup machine that has exhaust gas analizer capability. Hopefully we can get enough feedback from everybody trying to do tuning to see what really works. Sonny ----------------------------------- Sonny makes a good point about vacuum leaks - you can check with a can of wd-40, or even Gunk engine cleaner. With the engine running, spray around the carb base and other potential vacuum leaks - if there is one the idle will change when the spray hits it. Used to use that test after installing a carb on my 49 Harley, the o-rings for the intake manifold were a bitch to seat correctly. ----------------------------------- I use an unlit butane torch. RPM jumps...found it. No clean up. Jeff (sometimes no eyebrows ) Rice ----------------------------------- -----------------------------------