Test-drive the car and repeat the adjustments until the spark knock is eliminated. The maximum is about 14 degrees of vacuum advance. If too much advance is added, the engine will either start to knock or ping or perhaps it may surge slightly at very light throttle opening with high vacuum. To make adjustments to vacuum advance sensitivity, you remove the vacuum hose from the nipple on the vacuum canister.
Hook your HEI up to the ported part-time vac port on the pass side front metering block port. No vac at idle, so you can set your initial timing and idle speed without any vac intterference.
On this port, you only get vac advance when you hit the gas. Plugging your vacuum advance into a direct source will allow it to engage at idle, which is good for a number of reasons. Much like cruise conditions, engines run leaner at idle than they do under load. Again, this means the mixture burns slower and needs an earlier spark to optimize the burn. That vacuum line operates a mechanical device which moves the internals of the distributor to advance the ignition timing.
The harder the engine is asked to work, the more the ignition advances. Skip to content Natural sciences. I would just start with how it came out of the box rex. Start off by checking and confirming your total advance with the vac canister disconnected and the line to the carb plugged. Take that number, and subtract the total mechanical from earlier from it to get your vaccum advance number.
As a general rule of thumb, we usually look for a vaccum advance to take the total timing to between 45 and 50 ish degrees when active. At the end of the day, give your engine what it wants. Set a certain degree of timing to begin with, then take the vehicle and test it. If you notice any Detonation loud pinging then you have too much for your engine.
I understand that I can hook up a spring nn The front side That would return the gas excelerator back to full idle position.
Is there a name for that spring? Hi Jeff, thanks for all of the great articles. On this article, could you verify that you are talking about manifold vacuum or ported vacuum? Thank you. What does the letter F represent on vacuum advance distributors with an arc, followed by another arc stating Dwell? I am trying to help someone set up a electronic distributor for a Chrysler cc Solex carb engine and the closest thing I could find in Ducellier distributor specs is for the 2.
Your email address will not be published. Your Website. Save my name, email, and website in this browser for the next time I comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. Summit Racing Equipment. Author: Jeff Smith Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age After graduating from Iowa State University with a journalism degree in , he combined his two passions: cars and writing.
Smith began writing for Car Craft magazine in and became editor in In , he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since , Jeff has held various positions at Car Craft including editor , has written books on small block Chevy performance, and even cultivated an impressive collection of and Chevelles. Now he serves as a regular contributor to OnAllCylinders.
Great article! Many people do not understand this and you explain it well! Jeff says: March 28, at pm. Richard Petitpas says: March 29, at am. Ken Chorney says: March 29, at pm. Scott says: March 30, at pm. Ted says: August 25, at pm. Robert Fortier says: September 5, at am. Ross says: September 8, at am. Thanks Ross. Steve Koen says: January 31, at pm. Jeff Smith says: February 1, at pm. Steve Koen says: February 7, at am. Bill says: March 15, at pm.
Jeff Smith says: March 15, at pm. Steve Koen says: April 29, at am. Harry Day says: September 22, at pm. Matt says: August 12, at pm. Jeff Smith says: October 17, at am. Matt says: August 22, at pm. Ross York says: May 5, at am. Michael Gillean says: August 29, at pm. Ant says: September 27, at pm.
Randy H says: October 16, at pm. Nick Rice says: February 16, at pm. David says: March 5, at am. Ted says: March 22, at am. Ted, kindly ignore what Matt says, he knows what he does not talk about. Before emissions manifold vacuum was used. Manifold vacuum is situated below the throttle plates and maintains vacuum at idle and off idle.
Tony says: March 27, at am. No carburetor has used venturi vacuum in over 50 years. There are three forms of vacuum that has been used on carburetors. Manifold vacuum : vacuum at idle and off idle. Ported vacuum : vacuum off idle only Venturi vacuum : vacuum that increases as engine speed increases Venturi vacuum was used before distributors had mechanical advance as it was a increasing vacuum signal as the engine ran faster and faster.
Ron Skye says: July 1, at pm. Chet says: June 29, at pm. Paul says: October 14, at pm. Jeff Smith says: October 15, at pm. As I said earlier, in most circumstances you hook it up to the ported vacuum nipple on your carburetor, which is usually one that is a little higher in location than the other ones because ported vacuum comes from above the throttle plates, not below like manifold vacuum does.
There are other times and circumstances where a vacuum advance might be hooked up to manifold vacuum. One reason might be for an engine that has a cam profile that's a bit too radical so it gives it idling issues, especially in gear if the stall converter doesn't have enough stall RPM in it. Another time to do this is if you have an engine run-on problem, or what's commonly called "Dieseling", which is when your engine sputters and continues to kind of half-ass run after you've shut the key off.
I'm not saying that your idle itself is set too high. Your engine might barely be able to idle at all. Some performance engines use a degreed balancer. The timing is read from the degreed numbers on the harmonic balancer. Reading total advance with this arrangement does not require a dial-back timing light.
Centrifugal or mechanical advance works based on centrifugal force. As the distributor shaft spins faster the weights pivot outward, which moves the eccentric-shaped shaft in the center arrows. As the weights move the shaft, this advances the position of the spinning 8-pole piece, advancing the ignition timing. This MSD illustration shows how different size bushings can fit over a pin that rides in the advance slot.
Each new MSD distributor is shipped with a selection of different bushing sizes. You can alter timing by changing bushing diameters—use a thinner bushing for more advance, or a larger one for less advance. Factory distributors have a fixed slot length and pin. The slot must be modified to change the amount of advance.
Inside this vacuum advance canister is a small diaphragm that is connected to a link on the advance plate arrow that mounts the magnetic pickup in the distributor. As vacuum increases, it pulls on the arm, which rotates the plate and advances the position of the magnetic pickup to add timing. This vacuum gauge reads both manifold vacuum and positive pressure.
The needle points to just below 10 inHg of manifold vacuum. Mild street engines will idle around 12 to 16 inches of manifold vacuum. This engine has a big camshaft and struggles to idle at RPM at 9. As each of the lugs on the shaft line up with the magnetic pickup, this creates a signal that the distributor uses to trigger the coil and put voltage into the spark plug.
Some vacuum advance canisters can be adjusted by inserting a small Allen wrench into the nose of the canister. Turning the Allen wrench clockwise will increase spring tension working against the vacuum applied to the diaphragm. That will decrease total advance and also slow the rate of advance versus vacuum. Not all distributors have the mechanical advance weights in a convenient place. Ford and Mopar distributors like this one place the mechanical advance underneath the mount for the ignition pickup, requiring disassembly to access the components.
This makes MSD distributors for these engines more desirable as they are much easier to modify. This graph is representative of a typical mechanical advance curve on an engine. This curve starts at 10 degrees because this is the amount of initial timing. This curve can be quickened slightly but is a good place to start.
Total mechanical advance is determined by the length of the slot in the distributor advance plate while the rate steepness of the curve is determined by the weights and springs. I had to basically replace the whole top of the engine…. When it came to the Distributor I was in a jam.
I feel that was the best Distributor ever designed, one compact unit… And yes uses stock AC Delco parts. What a god send.
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