The Stovebolt Site Forums The Shop Area Engine Shop 235 vacuum advance

I was wondering how much should my distributor move while I give it gas? What is the best way to check and see if my vacuum advance is working properly?

Timing light. Watch the BB leave the window when you rev it a little.🛠

Ok that's what I thought. I did that exact thing with the light and as soon as I gave the truck some gas, the BB left the timing window. I just didn't know if you should see the distributor move a bunch or not.

Use a delay flash timing light, set the speed to about 2000 RPM with the idle screw, and dial the ball back to the pointer. With the centrifugal and vacuum advance combined, the total advance will be somewhere in the neighborhood of 40 degrees at cruise RPM. The distributor housing will turn a noticeable amount when the vacuum from the carburetor comes up as speed increases.
Jerry

I'm not seeing real noticeable movement when a move the throttle linkage, that's why I was asking.

Check the small bolt which holds the advance mechanism to the distributor mechanism. These wear out as well the hole they are in, reducing the action of the diaphragm of the vacuum advance. Apply vacuum to the advance with a mighty-vac to make sure the diaphragm isn't punctured.

So I pull off the vacuum line and checked it with a hand vacuum pump. It held vacuum just fine. So I reinstalled it to the vacuum advance and it won't hold any vacuum. So I'll be ordering a new one. I need to turn the distributor a little anyways to get the cap locks parallel to the motor.

So I'm going to have to pull the distributor to replace the vacuum advance anyway. When I go to timing it again, what is your best recommendations? This is a 1954 3600 chevy with a 235. I overhauled this thing 12 years ago. All I did then was set the octane selector to 0, adjusted the distributor until the BB and pointer lined up, tightened the distributor clamp down and called it good. It seemed to run really well like that, but I'm always up for better advice for tuning it.

You'll probably have to drop the distributor shaft into the engine on a different gear tooth if you want to make the cap clips align to a preferred position and still get the timing set right, or walk the wires around the cap one position. The BB/pointer position is a good way to do initial timing, followed by a road test for finding the final sweet spot. Doing the vacuum gauge thing is a poor substitute for having a good timing light, and setting the total timing advance at cruise speed with a delay flash light is the best way I can think of. Have fun!
Jerry

Ok I'll make sure to use the BB again for initial timing. My manual, and other places, say to get the motor to tdc and use the u/c mark on the flywheel, then drop the distributor in correct orientation.

#1 cylinder TDC (U/C mark on the flywheel) on a compression/firing stroke, as described in the Shop Manual.
http://chevy.oldcarmanualproject.com/shop/1954truck/54ctsm1232.html

How did you determine #1 on firing/compression stroke?
(the crankshaft rotates twice for each firing stroke - you have a 50/50 chance the U/C mark is a #1 firing stroke ).

You do it just like that shop manual says, I have that exact manual . I mean if you watch the valves to determine when the compression stroke is, then bring the u/c mark to the timing pointer. Correct me if I'm wrong.

You are correct.

Or I could do like you said on another person's post, put a cork where the plug goes and wait for it it come flying out lol!! That actually is probably easier than pulling the valve cover off.

Yes - the cork should be in your tool kit.

On my 38 216, I checked to see that the distributor moved around when I moved the throttle. I thought this was a good test for the vacuum advance. It has always lacked guts. Anyway I put a new vacuum advance on that I had sitting around and it was like a different motor, with a lot more getup and go. In both cases I timed it correctly so the vacuum advance must have been faulty.