kdclaw3

how do you check to see if a vac advance canister on a dist is good? this may sound stupid but i sucked on it and i can pull air easly through it, and it does not advance. should it have a seal?

vejatabul

yes. if you suck on it(yikes! not too hard!) it should have pressure behind it pulling it back. i used to have the part number for the vac canister that only pulled 10 degrees. i stoped running advance on carb motors years ago. started locking them out, loved em every since....oh yeah so anyway your vac canister is crapped out, buy a new one or lock it out or what you feel comfortable with. good luck, hope i helped.....

kdclaw3

thanks, i figured it was bad... i feel so stupid for asking...

Bunker82

I haven't tried sucking on it because I thankfully have a Mighty Vac which makes it really easy to test. If it is a street driven car, you want vacuum advance for sure (incase you have thought about ditching it). Better mileage, cleaner plugs, etc. I was just diagnosing a possible "ping" and disconnected the vacuum advance to test and it is a BIG difference. I was happy when I got that hooked back up! For a new can I recommend the Crane adjustable one. It lets you adjust the RATE of vacuum advance and limit the AMOUNT. Most aftermarket cans don't let you limit the amount. Look at the tech article on this site by Damon on mechanical HEI's. It shows you a good setup, and he recommends the Crane can as well. I think it is also a good idea to hook up the vacuum advance to a manifold vacuum source rather than ported. Great off idle tip-in improvement, and smoother idle too on my engine. Great thing is it is easy to go between ported and manifold vacuum depending on what you like best, but I know I am pro manifold vacuum. Lots of recent discussion on this board about vacuum advance if you do a search and are interested in leanring more about it. Good luck.

vejatabul

yeah my car was a street car, and it got 18mpg on the freeway with a th350. i accedently locked out the advance and found out when i was breaking in the cam. it ran so great at 32degress all the time, at idle, at 6000rpm, every part of the rpm band. it ran a 7.99 1/8 like that. with a 1 1/2 flowtech shorty header. not everyone should do this. i also ran a double pumper and a 3200 stall. the crane canister is a good peice.

camarodragon

hey bunker can u tell me the difference of putting the advance on the manifold port opposed to the other port?

Bunker82

I am not sure if you are asking how to tell which port is ported or manifold vacuum or if you are wondering about how different the motor will behave between the two.

To tell if a port in question is manifold vacuum or not just put your finger over it at idle. If you feel it sucking (and if idle speed went up when uncapping it to check) then that is full vacuum. With a ported vacuum source you shouldn't feel anything pulling on your finger at idle.

Possible benefits are mentioned in my post above: smoother idle (after you lower the idle speed since it will go up a bit once you hook it up to full vacuum and this can help to cover up more transfer slot too), and better off idle tip-in response and power. Might even run a little cooler at idle. As I said before, it is easy to switch back and forth so you can choose which way you like best. To really take advantage of this you will want to run an adjustable aftermarket vacuum advance setup like the crane kit mentioned. Just to note that the ONLY difference will be at idle and off idle. After that (higher rpms) ported and manifold vacuum are EXACTLY the same.

HTH

camarodragon

thanks man, but what would be the benefits of having it ported instead of at the manifold bec i already have mine on the vacuum one.

Bunker82

As far as I am concerned there would be no benefit from running off of ported vacuum. I have heard that if you have a crazy cam that the vacuum advance will be going in and out and make a lumpy cam's rough idle even rougher. In my experience all the advantages are for running of off manifold vacuum.

Edit: yeah there is an advantage to ported...emissions :lala: But if you are not worried about that then leave it they way you have it.

camarodragon

so there is no WOT benefits to the ported vacuum and its only benefit is making my car sound like its got a cam in idle right? lol.
and ya i dont give a **** about the environment right now bec we r using up the last of our gas resources ne ways.

Bunker82

This is a common misconception, though I don't know why. At WOT vacuum will be like 0.5" of vacuum either ported or manifold (unless you are severely under carbed). Needless to say your vacuum advance "shut off" a while ago. Nope, it won't make a lick of difference.

camarodragon

by the way is a 600 cfm carb to small for a 355 in ur opinion? and for some reason im having lots of problems starting the engine up lately

Bunker82

Carb size depends on a lot of factors, like a REALISTIC assesment of how many RPM you will run the motor to, intake manifold design, if it is a double pumper or vacuum secondary, etc. Just a heads up, the "formula" you find all over to calculate CFM is for single plane manifolds ONLY. The typical dual plane manifold will be better off with a bit more CFM. GM engineers were not that stupid when they put 750CFM carbs on 302s and such. If you have a daily driver 355, in all honesty I think a 600 would do well. Sure if you had like a 750 vacuum secondary you'd get a little more top end, but really how often are you in that rpm range? In my 383 I rarely get above 4000 rpm! With a 600 you'll get better driveability and better mileage too (and with gas prices that is a big plus). Maybe in the future you can get a bit bigger carb but I wouldn't be too worried about it. As far as startup problems I suggest you start a new thread and and list the symptoms and if anything has changed on your car recently. Mine has gotten hard to start but that is because it is getting cold and I have no choke!

HTH

camarodragon

thanks for all the advise bunker im new to this forum site and i dont know a lot about cars i just picked up my camaro like month ago and is my first car, ive been trying to learn everything i can from friends and everthing but u really seem to know what ur doing. is there like a friends list on this thing i can put u on? an by the way i know it prob sounds stupid but whats a double pumper and vacuum secondary i hear these two terms thrown around a lot.

Bunker82

No problem man. That is what these forums are all about, helping other people and learning new things yourself. I started with this site when I got my car back in '97! My Camaro is my first car and although I may get another car in a year or two, it'll be another 3rd gen no doubt. These things are great. Read what you can on these boards since there are people who know a LOT more than I do. Use the search button at the top for any question you have that is automotive to get opinions and experiences from members. And make sure you use a search before posting a new thread or else people will get all pissed. I never buy a thing for my car anymore without running a search on here. Saves money and makes you a lot more happy at the end since you didn't get suckered into buying crap parts. I attribute most of my knowledge to these boards. So read, read, read. Some stuff that gets posted is BS but you'll learn to recognize names of people who know their stuff.

I don't know if I'd say that I know what I am doing, but I am happy to help people by sharing what I have found that works and what has not. I don't know if there is a friends list on this thing or not, but you can always use a PM (personal message). There is a PM button after a person's reply to send one to them. Or email of course.

Not knowing what a vacuum secondary or double pumper is not a dumb question. I bet there are a few of the fuel injection people who don't either (jk) We all have to start somewhere. The difference is in how two of the four barrels on a carb are opened. On a vacuum secondary, the secondary 2 barrels use the engine's vacuum to "know" when to open. Vacuum is a good indication of how much load the engine is under (high load = low vacuum like at WOT vacuum is virtually zero). So using the vacuum to control when the secondaries open is a good way to only open them when the engine needs it (high load). A double pumper is also known as a mechanical secondary. It is basically a carbon copy of the primary side. It just uses a link that opens the secondaries based on how far open the throttle is. So if I have the gas pedal say 3/4 down the secondaries are just starting to open regardless of vacuum. In general vacuum secondary carbs are better for a car that is driven on the street, and allow you to "over carb" a little without causing any real trouble, and usually get better mileage. Mechanical secondaries are usually for racing and typically don't have quite as nice street manners and tend to be less fuel efficient. They also need to be more precisely choosen than vacuum secondary carbs. I think Holley's ( and Demon's) are the only ones that have truely mechanical secondaries and of course they make vacuum secondary carbs too. All others (Edelbrocks and Q-Jets) are vacuum secondaries. And usually people recommend vacuum secondaries for automatic trannys and mechanical secondaries for manuals. Well that is just a brief intro to the two. If you have a Holley, a good book to get is Super Tuning and Modifying Holley Carburetors by Dave Emanuel. He gives a more thorough description and goes over how carbs work in general.

HTH

camarodragon

ya man im pretty sure i have the double pumper type of carb, also when u tune ur carb do u screw it in snug an then unscrew 1 1/2 turns..because thats what i was told and after i cleaned and readjusted my carb it was running awesome and the car was dipping like no tom.. however the next day i cant start it and i still had that 1 and half unscrew turn adjustment on it and i cant figure out y it wonbt start, the engine just keeps turning over and then the carb spits out gas and thats the end of it. lol... hey by the way have u taken ur car to the track?

Bunker82

Which screw are you talking about? I assume the idle mixture screws, one on each side of the carb right? If so 1 1/2 turns is usually the starting point, but you want to get a vacuum gauge or use a tachometer that hooks up so you can read it under the hood to get it just right since every engine is different, but that should get you in the ball park. Get a Holley book for a detailed procedure. Most parts stores have the Haynes book on Holleys and that can get you started. If the carb is spitting out gas I don't know what to tell you other than it is probably getting flooded and won't start. Do you have a choke? Electric or a manual choke? When I had my Q-Jet I accidently set my electric choke too tight and it would not start. Stunk like gasoline too. I loosened it up a little and then it started fine again.

Naw haven't run it at the track. This was my first engine rebuild and so I worry I will throw a connecting rod if I revved it too high! This is also my only car so I can't afford to blow my engine, hehehe.

Later

camarodragon

lol well b4 u get ur new car u should rev it high and see what ur engine can do...but ya i was talking about those screws and i toke the choke off i dont really need it.