[Peti972]

alright guys i took my T/A in to class & had my teacher & class diagnose why my car doesnt jump like it's suppose to when i hit the gas..come to find out the vacuum advance canister connected to the distributor was no good..so My Dad & I got a new aftermarket vacuum advance that's adjustable...now my question is..how do I go about adjusting it? the directions says to advance the timing til i hear a "ping" or knock then turn it back..after that, it says using the allen wrench supplied, adjust the vacuum advance in 2 turn increments until i hear a ping then turn it back slightly...has anybody come across this be4?? I can tell I didnt adjust it right because it feels like the timing is advanced..but how do I go about doing it? because we also adjusted the idle too on the carb..any help would be very much appreciated thnx - Robert.

[Peti972]

nvm my teacher instructed me on what to do

[95ramairformula]

At full throttle there is no vacuum advance, its all mechanical advance.

[Peti972]

so what should my vacuum advance timing be? also what about my mechanical advance & what should they be at at which rpm?

[Ben M.]

Originally Posted by Peti972

so what should my vacuum advance timing be? also what about my mechanical advance & what should they be at at which rpm?

Depends on the engine (400 or 403 or 301?). A "general" starting point tends to have 34-38 total advance all in around 2500-3000RPM (subtract initial to get mechanical). Vacuum advance can add as much as 12-14 degrees as well, and if you have EGR operational it is not unheard of to have almost all applied when cruising at part throttle. Your initial timing setting also needs to be factored in (between 10-18 depending on engine). Engine specs and build all factor into these numbers and unless you know for certain the engine combination, tuning, tuning, tuning, and then tuning some more is the best way to determine exactly what your engine likes.

[Peti972]

my base timing at the crank is 12 degrees..it is a Pontiac 400 block...my vacuum should be at around 10 degrees? or should i just block off my vacuum advance & just put 19 degrees mechanical advance when i change the springs? is this possible? thnx in advance

["QUICK-SILVER"]

Originally Posted by Peti972

my base timing at the crank is 12 degrees..it is a Pontiac 400 block...my vacuum should be at around 10 degrees? or should i just block off my vacuum advance & just put 19 degrees mechanical advance when i change the springs? is this possible? thnx in advance

Peti972,
Not trying to be difficult, but 400 is not enough. 400 Pontiac base timing changed from year to year and varied by options and application; like, automatic or straight shift,
2bbl or 4bbl, dual exhaust or single, big car or little car (Bonneville or Firebird) even rear end gear ratios.
That said, factory base timing for a 400, from 71 to 79, varied from 8° to 18°.
With altered or modified engines, factory base timing is about only good to use for a reference point to start fine tuning.

IMO, with most low compression stock cam 400's Iv'e worked on, 16° to 22° base timing when tunning for performance and gas milage. Total advance (base+centifical) could go as high as 40°.

Centrifical advance depends upon how much your engine combo can handle and how fast it can take it. For instance, a 2bbl car with a 241:1 or 256:1 rear gear and automatic transmission is not going to rev quick. So centrifical advance would need to come up slower to match how fast the engine is reving up. Base timing on a car like this, would set a little lower (like 16°) to give the engine a little more umph taking off.

In comparison, a stout 4bbl engine with low gears behind it, would take off quick and rev fast, It could stand higher base timing and lighter springs on the centrifical advance so that timing would come up faster with the engine reving faster.

Vacuum advance has nothing to do with hot-rodding performance. It's there mostly for cruising and gas milage. After all other tuning is done, you hook it up and forget it. Timing with the vacuum advance hooked up, will be higher than your total mechanical timing. At WOT (wide open throttle) there is not enough engine vacuum for the vacuum advance to work, so the engine is running on total mechanical timing (base + centrifical)

Make sure your centrifical advance is working good, I wouldn't worry about springs just yet. Play with the base timing raising it up and down untill the engine feels the strongest. Always check it with your timing light and TAKE NOTES. Then start playing with the advance springs, try stronger and weaker till it runs best TAKE NOTES. You probably want be able to get it just right in one day. Take your time; drive it, feel it, watch gas milage and TAKE NOTES. You will figure out your best timing setting. The NOTES will give you something to look back at when you start making improvements.

The vacuum advance per your first post in this thread. The directions sounded right, setting it is not going to happen fast. You will have to set it, TAKE NOTES, go for a cruise and listen for a little valve ping under light loads. Engine good and hot, 55 or 60 and lean on the pedal some, like hitting a steep hill and trying to maintain the same speed. If you hear ping, you need less advance, if you don't, advance it till you do then back it off per instructions. It may take several trips and a bunch of stop adjust and go but when done this will be where you get the best gas milage cruising.

Play with it some, get to know it good, TAKE NOTES.
Most of all have fun learning.
Be careful, it can get addictive.
Happy Pontiacing
Clay

[Peti972]

i get what your saying thnx 4 the insight