vaccuum problem

[John Sanford]

I am new at the engine building, running thing. I know an engine creates vaccuum and I guess the engine has a certain route or way that the vaccuum has to run and their has to be so much of it (PSI wise). I am already having a spongy break problem. I have a 350 with a vaccuum secondary carb 4 barrel. the trans is a TH350. Not the original engine that came in the car. The ignition is HEI. The cam is hydraulic with about 290* duration and about .500 of lift. Some people have told me this is why I have no brakes. Is this true? The car has a booster (power brakes.) I have also been told that maybe the vaccuum from the carb secondary is robbing it. I do not control the headlights by vaccuum. The headlights are manually raised and lowered. I have one rather large hose coming from the intake going to the booster. People say I need to put a reserve cannister in between those two to achieve better braking. I have one small hose coming from the base of the carb or the intake (hard to tell) going to the vaccuum fitting on the distributor. The only other hose I have is coming from the top of the intake going to a stainless line that comes up in between the engine and the firewall. Im guessing that this is maybe from the trans? If I take any of these lines loose you can hear the intake sucking vaccuum and the engine tends to run a little higher rpm (this is while idling).Basically the qestions I am asking are
1. Will a reserve cannister maybe work to help the brakes?
2. Does it sound like I have my hoses routed right?
3. is their supposed to be a leak somewhere or is it supposed to be sealed like I have it now?
4. Why is there a vaccuum fitting on the distributor/what is it's purpose?

The car had a big leak when I had been driving all weekend. I just noticed it tonight and fixed it, basically sealed every thing off. I noticed two things when I did this. When I backed the car into the garage it had a little more torque, and second it tended to idle lower and sound like it was struggling to run. I'm confused. Please help! I appreciate it! its a '74 stingray.

[David H.]

Re: vaccuum problem

If that is your (advertised) duration; 290; then that is about the limit for street. If you have low compresion pistons they will add to the weak condition at low engine speed. Cams like this rely on low rear end gears and usually a standard transmision. Try advancing your spark timming a little and setting the idle speed up to at least 1200 rpms for a road test. If you have a straight runner intake, not dual level as stock, this will add to the idle problems as well. David

[Eric Fairclough]

Re: vaccuum problem

With that cam you need a vacume reserve for power brakes. When i went to a 486 lift cam my power brakes did not work at all. When i bought a vacume brake reserve can it works perfect. Eric

[Werner R.]

Re: vaccuum problem

one thing you didn't tell us is what is the vacuum reading at idle. how many inches. i was running 8.5 and had no problem with the brakes unless i was at a very long stop sign, and then after idling a few minutes i lost braking ability. the vacuum canister is a good idea to create a vacuum reserve at long stop lights, etc, but i have never used one.
setting the idle at 1,200 or higher will definately improve things, but will affect the timing by activating the mechanical advance. somthing to keep in mind.

[Doug Flaten]

Re: vaccuum problem

It sounds like your hoses are routed correctly. You may want to disconnect the hose from the booster and plug it to see iff the engine rpm changes. If it does, that may indicate that the booster is not holding vacuum and could be the cause of your weak breaks. A Mighty-vac vacuum pump is also handy to test the booster and distributor vacuum motors. You may want to repeat this on transmission line also. Plugging the vacuum line on the distributor should change the rpm. The rpm with the distributor disconnected and plugged should be lower. If not, the vacuum diaphram may be leaking. You may be able to adjust the carburetor and distributor timing to give you sufficient vacuum for your power brakes. There have been dozens of posts recently regarding timing and vacuum advance. You can do a search of the archived messages.