Last week I bought a new summit racing 600cfm carb to replace my 500cfm eddy. I had read great things on the annular boosters and I do feel the extra power. My problem is that I have always ran manifold vacuum but this carb doesnt like it for some reason. I have the mild isky cam. bored 0.60 292 with mummert intake and headers. Pertronix conversion in dizzy. original t98 trans. when I hook it up to manifold vacuum it wants to die when you blip the throttle quickly. It revs nice but then rpm drop super low. I have tuned with vacuum gauge as well. Now on to timing. with the eddy I had to retard the timing a bit to use manifold vacuum. have you guys had to do this? Whats you timing method when using manifold vacuum? 

???

Most vacuum advances can be tuned.  Original ones can be shimmed.  Aftermarket and replacements can usually be adjusted using an allen wrench through the vacuum port.

Ported and manifold vacuum work exactly the same everywhere but idle.  To switch between the two requires idle mixture and idle speed adjustments.  No change to initial or mechanical timing should be required.  Assuming initial and mechanical are tuned well, you adjust the vacuum advance if any issues arise with it in use.  Vacuum advance is only an economy feature, so you do not want to change initial or mechanical to compensate since those are more vital to the performance of the engine.



Lawrenceville, GA

Its confusing because when manifold vacuum is connect the base timing at idle is greater. So would I need to retard timing at this point?

Charlie is right on. First, I'd try running the ported vacuum, and I'm assuming the carb has one of those ports. Might just solve it outright. Different cars, motors, some seem to like one and some the other. I've never figured out why. In general, no real evidence, my automatic cars like the ported more, and the stick shifts don't care as much. This is just for off idle/throttle opening street driving. In either case, your WOT performance is just the initial and mechanical advance. Once that's good, you can tune the vacuum advance like Charlie says. More advance for gas mileage, less if you've got a part throttle ping. I've had motors (not y blocks) that liked as much as 50 degrees total at a 60mph cruise.

Edit: your post came up while I was typing. You set the initial with the vacuum advance disconnected, and the hose plugged. Where it goes when the vacuum advance comes in is where the tuning starts.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

If your base and mechanical timing is tuned well and vacuum advance causes problems, you tune the vacuum advance.  Do not change any other advance parameter.

The difference between ported and manifold is with ported, there is no vacuum at idle and with manifold, there is vacuum at idle.  Because of that, it is normal for the initial advance to appear to have changed when it has not, with manifold vacuum in use.  That additional advance increases idle rpm, which is why you have to adjust idle speed if you switch from one to the other, and idle mixture requirements, which is why you have to adjust the mixture.  If it adds too much advance at idle, then you have to dial the vacuum advance back.



Lawrenceville, GA

I think I am getting it. Thanks you guys. I dont know for sure which dizzy I have but i know i changed the points out for the pertronix. How do I set the vacuum advance back some? I know i do not have the adjustable canister because I have tried with several different small allen wrenches. Are the adjustments under the cap? I have read so much on people stating you should always use mainfold advance on older engines.

If it has no adjustment, then you will need to get one that does.

Most replacement advances are adjustable but it requires a pretty small allen wrench.  The vacuum canister of these will be entirely stamped steel.
Originals unscrew and can be shimmed.  See here for pictures:  http://forums.y-blocksforever.com/Topic125745-2.aspx




Lawrenceville, GA

thanks however I cant get that link to work for me. Do you have links to the adjustable canisters?

you should be able to get one from any auto supply store. It will look like this and only use a rubber hose connection. This shows were the allen wrench adjusts it. Sorry for the small picture. I've used both manifold and ported at times. When I had manifold vacuum I would set the initial at 10 degrees with the vacuum line disconnected, then connect the line and recheck and adjust the vacuum advance so it was at 18 degrees at idle. Every engine will be different, you will have to find what is best for yours.