Expert: Brad Sears - 5/10/2010

Question
QUESTION: The choke is located on the intake manifold.

ANSWER: Ok, you have a Rochester 4MV carburetor. The choke vacuum pulloff should get it's vacuum from a vacuum port on the throttle body of the carburetor. There should be a vacuum port port coming off the front lower left of the float bowl (center section of the carburetor) when looking at it from the front. It should be almost directly under the accelerator pump. This should be ported vacuum and there should be zero vacuum there at 700 RPM with the vacuum line to the distributor disconnected. This vacuum should increase as the throttle plates are opened. Now, as far as I can find this is the port that should be connected to the distributor so that the engine idles with only the cranked in 10 degrees of timing and then increases timing as the ported vacuum increases. The engine should idle at 700 RPM if a manual transmission and mixture should be set at 1/8 turn rich from peak RPM. The way that I set the idle mixture screws is to back them out, one at a time, until the engine looses RPM. Then, one first and then the other, turn them in slowly until I get peak RPM and just drop off slightly on the lean side. Then back them out till peak and the 1/8 turn more. Then reset the idle speed and fine tune the mixture screws finally setting the RPM. Let me know if we are on the right track, but this is how I would set this engine up. I do not know if you are the original owner, but back in the day when these cars were routinely coming into the shop the street corner "experts" were moving the distributor vacuum line to full intake claiming they got 400 more horsepower. When we got the car in for a tune up inevitably the owners were complaining of lousy idle and poor economy. We changed them back to ported vacuum. An engine at full throttle cannot take the advanced timing and the vacuum advance is manly used to provide an increased timing under light load and lean mixture. Heavy load require more burn time and a slightly retarded timing. Let me know how you make out.
Brad

---------- FOLLOW-UP ----------

QUESTION: Actually the car has the original 600 Holley (actually 580cfm) single pump. The vacuum port for the distributor is located on the passenger side of the throttle body and the choke vacuum is in located under the front bowl. I have an old Chiltons repair manual that covers this car, but does not cover the idle issue. The breakdown of the Holley in this book shows the distributor vacuum on the passenger side of the throttle body here mine is located. I believe that full intake vacuum is at this location, and that vacuum decreases with RPM. Correct? I'm thinking that I need to have the carb rebuilt (inconsistent idle speeds with vacuum disconnected and plugged) and go from there.

Answer
Sorry for assuming that it had the Rochester.You have the 4150 series Holley. That carburetor has three vacuum ports on the right side right front corner. The one most towards the front is the choke vacuum pull off. The next one is the distributor port and that should be ported vacuum and have no vacuum at normal idle. The last one back is the PCV connection. What we did see in that era on this carburetors and a few others was a build up of varnish around the throttle plate area that slightly restricted air flow meaning that the mechanic had to adjust the idle screw in a little to get the engine to idle. That would uncover the ported vacuum slots or ports in the throttle body and allow vacuum to be seen at this port. It also uncovered more of the idle mixture transfer slots in the throttle base at the throttle plate closed position and made idle adjustment a bear. Most of what the old Gum Out on car carburetor cleaning procedure did was to clean this area of the built up varnish and allow the carburetor to be set up properly. The 700 RPM idle setting was critical to keep the vacuum advance from advancing the spark at idle. 10 degrees advanced is enough for that engine at idle. Keep in touch, this has been fun, more than the usual question.
Brad