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?
