Inliners International Forums General Discussion Engines Dist& Vac Advance

(stock 235 in a 59 chevy truck)

Can somebody give me a quick overview of Vacuum Advance? What is it? How does it work? What role does it play when setting timing?

Also... my dist. has a large knob just below the cap. The shop manual shows it as a Grease Cup. What is it for? Should it be screwed in all the way?

Thanks

Vacuum Advance is needed to adjust the distributor timing in relation to the load/no load put on it in increasing or decreasing acceleration of rpms of the engine. A vacuum mechanism is usually attached to the distributor in a way that it moves the breaker plate, usually with a vacuum line from the carburetor.It will advance the timing on acceleration and retard it on deceleration such that the plugs will fire at the correct time during compression/power stroke resulting in the best performance/economy of the engine at all speeds.Of course all said, it's not a perfect world so timing adjustments have to be made or at least checked periodically. And if you add perfromance options, well that's even more issues.That's why many,many people are switching to newer electronic ignitions making all this easier and simpler-NO POINTS !
Regarding the "large knob" on 235's: It should be filled with chassis grease(lithium is ok) and turned in periodically.In my case whenever I think of it.I don't know a time frame for it.If you remove it you'll see it contains a small stiff tension spring with a bakelite plug on the end that rubs against the distributor shaft.It all acts as an upper lubricant device for the distributor shaft and really doesn't need alot of maintenace.Just keep it filled.Sorry this took so long to explain.Hope it helps.

thanks CATDOG.

The problem I'm having is that I did a tune up and now the engine runs and idles great but once I get up to speed it stutters & sputters until I ease off the gas. New plugs, checked for correct point gap, timing shows the ball at the mark so that looks okay.

Are you talking about a Petronix type set up?

Dear Memo;

If the distributor moves (rotates) when the engine speed changes your advance is working.

You might replace the HV coil wire and see if that helps.

Good luck John M......

Memo,Pertronix conversion is one to use although I've heard of some problems with the associated coil.Tom Langdon(How many times does Tom get promoted here? :rolleyes: ) at www.stoveboltengines.com has a nice conversion and some members have machined up there on from a newer HEI distributor.

Usually the vacuum line should be disconnected from the distributor and plugged while checking the timing. Your vacuum advance is working, that's why the engine smooths out whent you back off on the throttle, as this increases vacuum and advances the timing. The timing being right on the timing mark may not mean a great deal if the harmonic balancer rim has rotated. Try advancing the timing a little at a time and accelerating up a hill in high gear. If it pings, back off the timing a couple of degrees. Also, you have advanced it too far if the engine kicks back when you try to start it.

The vacuum advance advances the timing during part throttle operation and does not advance at full throttle operation when under load, thus preventing pinging.