Sorry if my question may be "stupid"...

my Edelbrock carb ( 1406) is connected to the distributor per the "timed vacuum port". I read in the Edelbrock lit that this port is for emission control cars. If emission controls are not an issue , is it possible to use the "full vacuum port" instead? which the difference?Any difference in terms of performance?If I'll use the full vacuum port , how the rearrange the other vacuum hoses from the carb?
My car comes from California . please look at pics as reference
thanks in "advance"...

[IMG]local://upfiles/6949/2C9F90691D2E4C69842A1B79004D0CE3.jpg[/IMG]

[IMG]local://upfiles/6949/A4C9CA41E9C9466FB155221D01538424.jpg[/IMG]

 

Only as a reply, Aldo; I always thought that the vacuum advance was supposed to be hooked up to a "full vacuum" port on the carb. I've never had anything that had all the California emissions mess, so for me to give you any advice would be crazy as far as I'm concerned. But, just in general, why you "wouldn't" want a full vacuum port hooked up to your vacuum advance, I have no idea.

 

I'm with Texas, don't know about the "timed vacuum port". I always go with full vacuum. I don't know if there is any difference between the two. [&o]
As I recall, the accessory vacuum port is of larger size. (1/4 in. or 3/8. The dist. port is 1/8 in. )

 

Hi Aldo, I'm with Jim and Dave on this one. Since your not running stock induction, ignition, or exhaust then I would use full manifold vacuum for the advance. The distributor appears to be a MSD Street Fire HEI, if it is then it has an adjustable vacuum can. Pull the vacuum line off the can and gently insert a 3/32 in. allen wrench in the port to turn the adjustment screw, this changes the spring tension on the diaphragm and will let you fine tune the advance.

 

I am with the rest of the crowd on this one. Full manifold vacuum for the vacuum advance. Timed vacuum sources are for pollution control devices. Yours are gone, except for the catalytic converters on the exhaust.
78buckshot - Great info on being able to adjust the vacuum advance on that distributor!


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The timed vacuum port was above the throttle plates (butterflies) and allowed for a retarded spark at idle which apparently was good for emissions. As soon as the port is uncovered, manifold vacuum is directed to the vacuum advance.

Don't know if this site has been posted before but a ton on info: http://www.lbfun.com/Corvette/Tech/vettetech.html

Scroll down a ways and there is a lot of info on timing/advance.

 

Hi Aldous, there are no stupid questions on this forum, so just ask anything anytime.
For maximum performance out of your Vette you want the vacuum advance can connected to full manifold vacuum.
In the early 70's in order to meet early emissions requirements GM retarded the timing and used a "timed vacuum port" or what is called "ported vacuum"
What's the difference?
Ported Vacuum come in to play only at steady part throttle, GM did this by moving the hose from the vacuum advance can to a connection on the carb that is above the throttle plates. With this connection you get no vacuum advance at idle, and full vacuum at part throttle.This was all part of the AIR (Air Induction Reactor System in conjunction with the air pump Etc)

Manifold vacuum is what you want and it is just a matter of moving the vacuum advance line to a location below the throttle plates, look at the Edelbrock lit and you'll find where that is.

We are talking about tuning your car here so lets go step by step in order to get it right.
First hook up a dwell meter and make sure the dwell is steady thru the entire rpm range. If not replace the bushings in the distributor, since if this is off than nothing else will help.
Than set the initial timing with the vacuum hose disconnected and plugged at the can end. Lets say you have 16 degrees at this point.
Than reconnect the vacuum line to the can, at this point you should now have 32 degrees timing at idle. That's 16 initial plus 16 from the advance can=32.
Now as you increase rpm the next thing that comes into play is the centrifugal advance, it will add about 20 degrees total.
So at a steady speed of 60 mph you have
initial timing of 16 degrees
plug 16 more from the vacuum advance=32 degrees
plus 20 degrees more from the centrifugal advance=52 degrees.
That is the maximum that you want to run at. You may have to set the initial timing lower to get this number or you will have to adjust the vacuum can.
GM put strong springs into the distributor at the factory so you will not get full centrifugal
advance untill about 5,000 rpm so you will have to change the springsto get full centifugal advance at 2,500 to 3,000 rpm.
At idle you have high manifold vacuum and also at steady cruise you have high manifold vacuum, at the moment you floor it, you have no manifold vacuum then when you reach steady cruise your vacuum again comes into play.
Hope thishelps, PG.

 

Thanks to you all.
OK , things starts to get harder for me. I have to study...
I'll study the tech lit on http://www.lbfun.com/Corvette/Tech/vettetech.html( thanks 73sharkfor this very useful link)

PG , I'll follow your stepby step procedure with my mech ( i don't have the proper tools : dwell meter, timing light...)

Just the last info about. If I move the vacuum advance hose to the full vacuum port , what should I do with the hose related to the headlights actuators that are now conneted to the full vacuum port?Should I connect everything together at full vacuum port? Or should I move the headlights hoses to the timed vacuum?

I hope to be able to tune the advance of my car soon. I suppose that a certain lack of power at low rpm ( 1500-3000) is due to this. I felt my car's power-torque is not very progressive accelerating slowly up to max rpm. If I push hard on gas pedal , everything OK instead.there's also a kind of "kick in the ***" at 4000rpm ( all of sudden power peak)

Any other advise about proper vacuum advance is welcome , before I get to my mech

To tune my MSD distributor, just an allen wrench? or need to buy springs or other? I think my distributoris an MSD , but can't see any label on it

Nice sunday to everybody

 

Hi again, 73shark posted the best link for information, go there and scroll down to "Distributor curve specs.pdf."
73shark you posted more in depth links once before, can you show
Aldous where thay were again?
Aldous, I'm impressed with the amount of time and effort and great results you have put into your Vette this far. So get a dial back timing light, a vacuum guage and a dwell meter and you won't have to pay someone else to do this for you.
My car is a 74 automatic, my manifold vacuum for my head lights and the automitic transmission comes of a "T" fitting directly from the stock manifold yours may be different,
I put my foot in my mouth by telling someone else that they had no wiper relay before I realized that there Vette was a different year than mine so let me go step by step with you. PG.
PS> It's 3 AM here, what time is it by you?

 

Hi PG,

I spent the last 3 hours ( after reading your reply with the step by step procedure) studying the tech lit at the link supplied by 73Shark ( thanks again) and the MSD tech bulletin
Ok. I have understood now how to tune my engine advance.
I need to get the proper tools now, unless my mechs have them ( mechs here are quite cheap : for fitting my 4 bilsteins 25Euro =35$, a cocktail is 5 Euro here, just as reference)

I look forward to check soon the advance settings of my car . I suspect to have it not tuned for "performance". Until i get the proper tools , I'll open the distributor and check the springs and bushings used, just to have an idea of the possible settings.

about the vacuum hoses , I'll move everything to the carb full vacuum port ( needa T adaptor), or I may connect the headlights hose to the timed port ( but I have to accelerate the engine a bit to actuate the headlights, I think)

Now is 10,30am here , 4,30am US east coast time

PG, I'm going totally mad for my Corvette. I want to learn more and more about Vette and motors tech in general.The problem is that this is taking time to my job and to other things. Not a real problem , indeed...