The Stovebolt Site Forums The Shop Area Engine Shop "ported vacuum" ( and other myths)

An update:

A while back the topic of "ported vacuum" for the distributor advance was being kicked around.

My own difficulty was with a 1972 Chevy 250 six which had had it's original "Transmission Controlled Spark" system & Monojet carb replaced with a Rochester "BV".

The problem was that the distributor was getting a "full vacuum signal" (and thus full advance) at idle, and as the throttle was opened, the vacuum dropped and the timing retarded. Essentially, the stuff was working backwards from the way it should; I had "vacuum retard" instead of vacuum advance.

I looked at the base on a spare BV carb and noticed that the vacuum port into the carb throat from the 1/8" pipe tapping was drilled on the "manifold side" of the throttle butterfly, so naturally, anything connnected to this port would have full vacuum @ idle, and less than that as the throttle opened.

I studied the operation of the vacuum advance on the 235 on my '55.1, and the vacuum advance doesn't operate until the throttle is opened.
So, reasoning that the advance should function the same way for any engine using the B-type carb, I determined that the 1/8" hole through the carb throat was on the wrong place!

Experimenting on the spare carb, I filled the existing hole with J-B Weld, and when that had cured, I drilled a new 1/8" hole through the throat, at the top side of the pipe tapping, which comes out above ("air cleaner side")of the throttle butterfly.

After smoothing the inside of the throttle bore w/ emery paper, I put the "improved throttle body" on the truck, and hooked a vacuum gauge into a tee on this line.
After re-timing the engine (sticker says "4 BTDC")
I got it running, and at idle pulled no vacuum, no advance. :
As I opened the throttle, the vacuum began to rise, and the ditributor advance began to do its thing.
So far, so good. So I opened the throttle quickly, and the vacuum jumped to 10 or 15 inches, and the engine responded smartly.

So, I've been driving it this way for about two weeks, and the performance is greatly improved, although not yet perfect: if you floor the accelerator while under load, the engine begins to "break-up and stumble" as the revs get higher :confused: ; so I'm going to go through and tune up the whole engine and see how we're doing.

Prior to making this "improvement", the truck had been a real "dog", even for a six; no power, heating-up on a pull, using way too much gas (my rear bumper is black from excess richness in the mixture!).

I'm planning on doing an "exhaustive" vacuum gauge analysis on the 235 vacuum advance system when I get the brakes done on the '55.1; the weird thing is , my inspection mechanic has a '65 Chevelle with a 230 six, and the Rochester BV, and his vacuum system is just a steel line from the brass fitting on the carb throat to the distributor, and his works just fine.

Did GM vary the position of the distributor vacuum port relative to the closed throttle plate on the Rochester "B" carbs and their variants?

Yes, GM had differant versions of the Rochester B carb with ported and manifold vacuums. The B on my stovebolt is ported. It doesn't matter which you use as long as the distributor curve matches. Some are meant for ported and some are meant for manifold. Ported vacuum only is in effect when your foot is not on the gas pedal. Once the throttle is opened slightly it becomes manifold vacuum.

PS- timing always advances with vacuum and retards with less vacuum. In other words, as the load on the engine increases, it drops the vacuum and the timing needs to retard. Your problem is probably the transmission controlled advance distributor curve. You need a distributor from a manual transmission truck to match your carb and you should be set.

Josiah,

Thanks, I think I know where I can get such a distributor for cheap!(Free, that is! )

I'm really resisting the idea of putting that TCS system back on the vehicle- too many gizmos to get out of whack!

Will let you know what a different distributor does for things!

If an ignition system is designed to operate with ported vacuum, you can get some bad results using straight manifold vacuum.

Like 4onthefloor said, ported vacuum becomes manifold vacuum after the throttle is opened slightly. When the throttle is closed, there is, essentially, no vacuum.

At idle, there is no need for additional advance. Piston speed is slow, but the fuel mixture still burns at nearly the same rate. Using additional advance may make the egine idle somewhat rough and the typical response is to adjust idle mixture rich to smooth it out. This shortens the life of spark plugs, carbons up the combustion chamber, and can contribute to poor off-idle response.

Upon opening the throttle, manifold vacuum is allowed in and vacuum advance begins to be phased in. If the throttle is opened a lot, manifold vacuum will remain low and, consequently, vacuum advance will remain low (or none). As RPM rises, mechanical advance will phase in, regardless of vacuum. As RPM comes up and the throttle is reduced, vacuum and vacuum advance will increase.

At cruise condition, you will have maximum manifold vacuum and maximum vacuum advance.

If you hold the throttle wide open and wind up the engine to several thosand RPM, vacuum will be coming up with RPM. Guess what would happen if you suddenly release the throttle to idle position? a HUGE vacuum spike. This is hard on Vacuum advance cans (if you tend to replace them often, check your vacuum connection). However, the ported vacuum saves the day by blocking the spike from the advance can.

Now, all this is for systemd DESIGNED to work with ported vacuum.

YMMV

Banjo & Josiah,

Well, what you fellas are tellin' me makes sense; the situation I "inherited" when I purchased the truck sounds like it was a case of a "ported vacuum (TCS) distributor" & advance hooked up to an older carb that furnished manifold vacuum to the brass distributor fitting.(Previous owner probably thought:"A vacuum line is a vacuum line.")

So, I think I've created some sort of "ported vacuum" by moving the location of that hole into the throttle body to the upper("atmosphere") side of the throttle plate; it mightn't be perfect (which is why I "operated" on the spare carb), but I'm gettin' closer!

Soon's I get a little "mad-money", I'm going to get a distributor & vacuum advance for the earliest 250 application I can find, and hook that up to a Rochester BV that I personally removed from a 230/250, and see how I'm doin'.

The way it was before my "elective surgery" to the throttle body, she'd spike to almost 25" of vacuum if I closed the throttle while going down the highway; that'd turn lots of things inside out!