Reproduction 1115355 vacuum adv.

[Stuart F.]

Re: Reproduction 1115355 vacuum adv.

I've only had one car that had the phenomenon of an oily substance getting back into the vacuum advance. It was a 72 Nova SS with the 350ci 200hp and a quadrajet carb. I had modified the carb to get full time vacuum advance by drilling a second port below the butterfly into the rectangular vacuum chamber. I had also eliminated the transmission controlled solenoid in the line (well sorta). Actually, I connected it to a manual switch so I could pass the emissions test in Illinois. At any rate, I was still experiencing some erratic VAC operation and I discovered there was an oily substance in the VAC. It seemed to be settling/draining back into the line from the carb, so I made a new line with an upward loop in it which seemed to prevent that. Therefore, whatever accumulated in the line before the loop was drawn off at the next engine start rather than drain all the way back into the VAC.

I discussed this phenomenon with several engineers at my division and at Chevrolet, but never did get a satisfactory answer.

Stu Fox

[Clem Z.]

Re: Reproduction 1115355 vacuum adv.

I've only had one car that had the phenomenon of an oily substance getting back into the vacuum advance. It was a 72 Nova SS with the 350ci 200hp and a quadrajet carb. I had modified the carb to get full time vacuum advance by drilling a second port below the butterfly into the rectangular vacuum chamber. I had also eliminated the transmission controlled solenoid in the line (well sorta). Actually, I connected it to a manual switch so I could pass the emissions test in Illinois. At any rate, I was still experiencing some erratic VAC operation and I discovered there was an oily substance in the VAC. It seemed to be settling/draining back into the line from the carb, so I made a new line with an upward loop in it which seemed to prevent that. Therefore, whatever accumulated in the line before the loop was drawn off at the next engine start rather than drain all the way back into the VAC.

I discussed this phenomenon with several engineers at my division and at Chevrolet, but never did get a satisfactory answer.

Stu Fox

that loop was GM fix on a TSB to keep gasoline out of the vacuum can. some cars even blew the cap off from the vapors getting ignited after the diaphragm ruptured

[Stuart F.]

Re: Reproduction 1115355 vacuum adv.

Clem;

That's interesting. I don't know the time frame involved other than 1972, but maybe they stole my idea, Ha! no wonder they never got back to me.

In 1983, I came up with a fix for my new Z-28 H.O. too. When you accelerated hard it would bog out and starve. I used a temporary fuel hose from the pump to the carb along with a clear inline fuel filter and noticed when I blipped the throttle hard, the fuel would drain out of the filter. I studied the fuel circuit and discovered the return to tank line and the main fuel line were very close to the engine at several points (line ran up the driver's side and crossed over to the right side. Car was designed for the V6 fuel pump position). I decided to try a restrictor in the return fuel line using a Holley carb jet threaded into the return fuel hose. It worked like a champ. This too I discussed with our engineers and some from Chevy. In the 1984 H.O., the problem was rectified by restricting the fuel flow in the return line. No kidding!?!

Stu Fox