Camarobuild18

Hey there everyone, im looking to benifit others with this thread, and hopefully it will.
Im sure theres others that could also help me with this so any corrections would be helpful. Id also like to give kudos to a GM engineerer on hotrodders forum for bringing this up also. Ok so you either stuck a carb on your GM Small block or you've just gotten a thirdgen with a carb set up already on SBC. Ok so your carbs acting up or you think it is? Engines gotta pinging noise, wont idle, gets hot, shuts off down the highway. Didnt know things like this could cause such things huh? vacuum can be your best friend, vacuum leak could cause alot of these, this is relatively simple explanation of how things work. HEI Vacuum advance distributor.

The reason why they put the vacuum advance on these engines is so they ran more effieciently, it takes more advance to burn a lean fuel mixture, at idle, and vacuum is the best at this point.(if cammed up) If not massage this out and do a timing light inspection. Fast burn heads take less timing...
But again lean mixture takes more time to burn than a less potent rich mixture. When you accelerate, this mechanism shuts off do to vacuum loss, and your carb is shooting rich and your mechanical advance is a kicking in through the whole rpm range. Neat how a vacuum can save gas huh?? The total vacuum for stock SBC (for timing advance) is 15 HG, and it should be hooked straight to your manifold(right below the carb) cause theres 2 different types of vacuum. One is manifold vacuum, and the other is 'ported vacuum' which you would see going near the top of the carb. Ported vacuum is not a good choice, for loss of vacuum will occure.
Getting a new totally jetted out carb sounds great! But could also shoot to much of a rich mixture for idle-able effiecient car, and most just set the carb to idle it at like 1000 rpms. That will not due! Your engine could predetonate with a rich mixture, aka spark knock, since the idle is sitting at 1000 rpms, and your vacuums at its peak, this could ruin an engine, and cause rod failure, ring failure due to richness, wash the cylinder walls down, create a whole lot of difficultys. Well if you have any other questions, PM me ill gladly try and help the timing issues, and trouble shoot with any one.

This is my disclaimer, i dont know every SBC, and i hope to have helped anyone.

Anyone got anything on those vacuum plates that sit under the carb? Do those help vacuum at all? That would also be helpful. i am a wee bit curious of these too. Again any corrections would be helpful!
-Alex

Camarobuild18

hey i thought this would help anyone needing to see what i seen to start this particular thread it tells you alot right here... Note there is alot of thoughts and opinions on how to run vacuums. Theres ported and manifold vacuum. These are not my words, but a GM engineer... http://www.hotrodders.com/forum/dist...1-a-59033.html

Apeiron

I don't know where to start.

jchaussee

I have a vacume gauge. ( mity vac) and would like to know how and where I check for vacume, and what numbers should be. Also why so many ports on a carb, yet most are blocked off.

Camarobuild18

i have a mighty vac too there pretty good tools. It depends also on how you run it, i would run your lines the normal way and right from the carb id check the numbers on the mighty vac to get the number your aiming for, if the cars idleing (which in most cases it cant cause of vacuum problem) should be atleast 15 HG. Im geussing you have a holley?

Camarobuild18

if you can run a vacuum ball, those are better than a can, cause off its structure plus smaller, GM started using these cause the can would rust.

jchaussee

Is a Qjet. I don't know how the vacume line SHOULD be run. It was the way it was when I bought it. I have a vacume ball, but there is nothing going in or out of it.

Camarobuild18

well i say if you have emission standards to run ported vacuum. I heard thats what it was for.. I will post a diagram cause i have no idea as to how they ran vacuum lines. I was thinking and worryed of any vacuum secondaries that might also be blocked off, bc you did say they were all blocked off. And my opinion and from what ive heard sbc likes manifold vacuum, cause ive heard that you get better gas milege, and engine temps are at a low 190, rather than an elevated 225 degrees on ported... just some thoughts. Above is a link you could look into, ill also bring up diagrams of vacuum setups which will take awhile..

Camarobuild18

ok i found this, four barrel diagram im gonna post more pics of diagrams, this is for emissions which most of us have so its 'ported style'. Hope this solves most your troubles... Alex

Camarobuild18

here is a diagram of emissions setup for four barrel qjet, i dont think this is detailed enough but i could draw up my own for decisive vacuum on sbc camaros. ill have to do some more info hunting, again this thread is lacking and im really sorry, but trial and error are two good ways to start...
i have a good idea. Run a line to your vac advance dizzy, from the vac ball, to the might vac. Pump it up to 15 HG... Also run a port vac from the middle of the carb, (there is a port thats bigger than the rest, it should be the vacuum port. Look for anything on the carb that could be idle friendly) then test run it... Trouble shoot, PM me if so desired.

jchaussee

I shall give it a shot. Mine is wuite a paine in the ***, because it is a carbureted motor in a originally TPI car, with no emmisions left on it, no more ac, no more vapor canister. But I would assume that the principle apply to any SBC with simular setup. But I assume that many of us who own third gens have a simular setup as mine and the vacume info tends to allude a lot of us. People just plug stuff and call it good.

Camarobuild18

its just a prefrence, many people dont get alot info on this as it is a dumb founded subject, not many can comprehend and rather just plug it all up... Thats why im trying to make it some what easier! >.<
And also get the info around...
If what i prefered doesnt work, bare with me bc i have some good ideas on some manifold vacuum... Remember it might need the dizzy retarded a lil bc its at 15 HG!

Ok well heres something for those with a Holley in mind, you WILL have to run manifold vac. Because with some holleys like mine there is only one place for a vacuum.. I have drawed up my own lil diagram, there are places in some intake manifolds that will allow you to run a vacuum line through it... My TBI engine has one for the booster running towards the front of the engine. I Am not an expert at vacuums, this is just a simple emissions free four barrel holley vacuum system... It is a lil choppy for i am not an artist but i could try and draw up another.

BillyBLKPearl91

if the pic was a little clear i would say man you a a saver becouse this is what i been trying to find for weeks the way it looks make a lil more sense to me because you have the engine to show how it really goes the pic on the top is a lil confusing post the pic up a lil more clear please thanks

Camarobuild18

i should sketch up another one except not as screwy. Sorry there guys.

HawkZ28

iTrader: (2)

KR, knock, spark knock- that is more prone to occur with a LEAN mixture, not a rich mixture.

A rich mixture is more resistant to detonation. Fuel also acts as a coolant in some cases.