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Super Chevy 10-08-2004 02:54 PM

vac advance help.
well i just got my el camino on the road and i took it on a 600 mile trip to ohio for a car show, the engine is a 305 ho, i have an edelbrock 600 cfm carb on it and i had my vac advance hooked up to the ported source, i tuned the carb, got about 15 inches of vacuum, and it ran good at first, then it started to stumble at higher rpms, around 3000, it would really hesatate, i retarded the timing a bit, (there was also a lil detonation at full throttle) and switched the vac advance to the full time source, and now the hesitation problem is gone, but when it idles, it idles fine at about 700 rpm and then drops and wants to stall and right before it does the rpms climb back to normal, but then continues to want to stall, what is causing this?, do i need to retune the carb?

oh and what is the proper way to set the timing with a vacuum gauge?

thanks in advance
if u need more info on the engine just let me know.

jameskirk1 10-08-2004 07:32 PM

With the vaccum advance hooked on "full" instead of ported, the timing will wander and cause the idle speed to raise and lower, affecting the vaccum level, affecting the advance amount...

First, you should move the vaccum can back to the "ported" port...Then set your initial advance at idle around 12 degrees before TDC or so with a timing light...You will then need to readjust the idle mixture. Set your idle adjustment with a vaccum gauge...Then flog away and listen for the dreaded rattle after the engine has fully warmed up...

Let us know how that works out...:thumbup:

DoubleVision 10-08-2004 09:33 PM

I used to run vacuum advance on a ported source, as I didn`t see the need for it at idle, then BOBCRMAN told me to move it to a manifold source, the car went from a slug to real responsive after I did so, vacuum advance is needed at idle because idle and off idle mixtures are lean, and leaner mixtures burn slower than rich ones do. set it to 12 degree`s btdc with the vacuum advance off and the line plugged, after the timing is set, lock down the distributor and reconnect the advance. Afterwards I would reset the idle mixture with a vacuum gauge and set the float.

76redstinger 10-09-2004 07:57 AM

double vision is correct

plug in to manifold vacuum source, not port vac

port vac is from the emission/gas milage last 70's

it will hamper performance


batman09 10-09-2004 09:30 AM

Both DV and 76 are correct.Hook it up to full manifold vacuum.Ported was designed for emissions.Your car will be much more responsive with full.FORGET PORTED EVERYONE>>>DON'T USE IT!!!!!

Super Chevy 10-09-2004 06:38 PM

i already have it on full vac, but now i have idle problems and when i start it i get greyish white smoke when it first fires up, after that there is no smoke, and if i cruise at low speeds in 4th gear, rpms around 1500, there is a quiet pop that comes from the exhaust and the kinda shakes when it happens, like it hesitates and then cures itself. i have no clue whats goin on, i have yet to retune the carb, and i have no timing marker so i cant time it to a specific number, so i would like to know how to do it with the vacuum gauge if it is even possible to do it that way.

thanks for your help

jimfulco 10-09-2004 09:07 PM

You can get a steady-speed miss if your vacuum advance is advancing too much. I replaced a bad vacuum advance can on my '72 Nova with a parts store brand & it started missing just driving down the street. I looked at the amount of travel allowed by the can & it was almost twice that of the original GM can, which allowed 15* advance. I soldered up the slot so it wouldn't move so far, & it worked OK, but I bought a new GM one soon afterward. I read somewhere that the HEI cans allow more advance, but I have not checked one myself.

Super Chevy 10-10-2004 12:17 AM

hmmm, ill have to look into that, i was thinking of getting a recurve kit and an adjustable vac advance, the one i have now is an old one and it may be causing some problems.

killerformula 10-10-2004 12:29 AM

this is absolutely stupid. What is your compression ratio, and why has nobody asked!?!?!?!?!


Super Chevy 10-10-2004 01:17 AM

the compression ratio is stock for a 305 HO, which i was told with the 58cc head was around 9.5:1.

rifraf 10-10-2004 02:38 AM

Ive voiced my opinion on this subject many times,with oposition,there is no right answer because there are to many variables that must be determined first,without saying someones way is wrong,I will say this,most of the aftermarket distributors are set up to be plugged into ported vacuum,the one that has near nothing at an idle,and more at a rev,I will also point out that most non emission carbs,holley,carter and BG demons acomodate this feature,in my opinion,because its a better way. If you choose to use my way,first plug the vacuum guage to full manifold vacuum like off the manifold,unplug the vac advance,then with the distributer movable and the motor warmed,fire it up and at an idle turn the dist untill you reach the highest number,subtract it by 10 percent,this is the number you want to retard your timing to,clockwise.If you can achive 20,retard to 18,if you get 15,go to13.5,and so on. Now this is assuming the gas you have is up to par,run it and if it detonates at all,retard it a hair till it stops.This is the only way I will do it and if you would like,Ill tell you why,keep in mind your overall mechanical curve and what your vac horn pulls,these vary,some are adjustable. I have had the best luck at about 32 degrees toatal.

Super Chevy 10-10-2004 03:22 AM

thanks rifraf, i noticed a huge difference when i switched over to full vacuum, so i dont know if i want to switch back. but i can still use your timing method if i dont switch to the ported vac source right?

quick question, what should i have my idle set at to do this or even to tune the carb, any thing below 700 gives me a jumpy reading and less overall vacuum, i dont know what the lobe sep is on the cam but its pretty lopy at idle, should i just adjust the idle to where the reading is steady? when i tuned the carb the last time, 15 is the highest amout the needle went to, it didnt stay steady there.

thank you for your help!

rifraf 10-10-2004 08:47 AM

Well,here you have one of the many symptems of running it that way,reason being,when the motor is at its most efficiant place,the vacuum will be pulling its hardest,as you have it now,as soon as you aply any load whatsoever,or the cam makes it load up a bit,your advance is going to drop,how much? depends what your horn pull is. This is where the guy at the light,being you,has the rig that keeps learching forword,the bigger the cam,the worse the learching,If you graffed your vacuum,it would look like a rollercoaster ride,the retard drops rpm worse yet and signals the power valve to open,making things real interesting. Want to know any other ill afects of running it that way,it makes power valves go bazerk?Maybe thats why guys who run em like that are so fond of carters.I would advise running no vacuum to you before running it off the manifold After spending countless hours tunning,I can tell you with all honesty,you power feel is relavant to how much advance you have in and when,you should be able to have all the power you feel now in a more user friendly package,its realy pretty simple to set yours up the way I discribed. Oh yea,you can use my method like that,but it will be wacked,so,it would be best to use a light,or go by ear.

Super Chevy 10-10-2004 05:12 PM

well, it looks like im just gonna have to figure this one out for my self, i have ppl telling me one thing and ppl telling me another, so ill have to try both ways, and see what happens.

rifraf 10-10-2004 05:35 PM

Either way,you need to know what the dist is doing so you have something to refer to,you need a certain amount of advance and how you run it will make a difference no mater which,a timing light,a vacuum gauge,and some degree tape for you balancer and you can go to town on it,make sure the tape is for the right diameter. Do that and your ready to dial it. Before all that thou, have you started from scratch like I suggested? You might be surprised,it wont take very long to satisfy your curiosity,before you go into re curving it.

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