Iron_dog

Guys, I've read many articles and posts about the two arguments but it's not clear to me the relationship between carburator and ignition.
In other words if I change one of these in some parameters, how does the modification reflects to the other?
These are the modifications I would like to do in my setup:

Carb:

1) adjust mix screws, car too rich
2) adjust secondary spring (maybe)
3) replace primary rods/jets (increase primary metering area)

Ignition (I follow Lars paper):

1) change initial timing rotating distributor
2) recurve distributor

Thanks

cv67

Looking back Id say curve the distributor first get the timing right where you want it. Then go after the carb.
I fooled with mine foreeeever looking for the power thought the curve was on. Wasnt til I took it in somewhere and was shown the bushings werent what they were supposed to measure out at (msd)...no way it woulda been right. 100 well spent with the old timer. Did his own mods to the bushing holes/weights.
Car runs NIGHT and day different. Now some subtle tweaks on the carb motor has really come alive.

my 76 ray

I noticed you have a '74. I would add one thing to the above.

Get your dwell set properly first, then timing, and finally adjust the carb.

Iron_dog

Thank you, but I've a Pertronix Ignitor 3 installed, so no issue with the points!

7T1vette

The carb provides the fuel/air mixture, in correct proportions, for the needs of the engine at any RPM and with any loading. The distributor provides the spark and spark 'timing' at a given engine RPM and with modifications dependent on loading.

The distributor determines "loading" based on the level of vacuum detected by the vacuum advance can connected to the distributor's advance mechanism. The carburetor determines "loading" based on the throttle cable/linkage movement it receives from your 'foot'.

The adjustments to the distributor are [mostly] independent of the carburetor's settings. The adjustments to the carburetor are dependent upon the distributor's actions. Thus, you should do any distributor work first (ie, recurve for performance or economy, per your choice, select and adjust vacuum advance can per those same requirements, and set timing accordingly). Then, the adjustments can be made to your carb.

jim-81

good stuff 7T1Vette...

Iron_dog

Thanks, that's what I wanted to hear! So if I change the jets I don't have to change the timing, right?

scottyp99

One thing that I have noticed a lot of people have a hard time understanding is the relationship between centrifugal advance and vacuum advance. Centrifugal advance advances as the rpm of the engine goes up, and the vacuum advance advances as the vacuum goes up. They both act independent of the other, neither one has any effect on the other. The centrifugal advance advances the spark timing as rpm goes up, until it reaches it's limit, so that as the engine spins faster, the spark gets a little more lead time (which is what you are doing when you re-curve the distributor, you're not changing the amount of centrifugal advance, just making it hit its limit earlier, 2500-3000 rpm, as opposed to maybe 5000 rpm on a stock distributor) Vacuum advance advances the timing as the engine vacuum goes up, so that when you put a load on the engine (vacuum goes down) the spark advance retards, and when you let off, say while cruising down the highway or sitting at idle (vacuum goes up) it advances, for good fuel economy. Remember, the two systems can go up and down without having any effect on each other. For example, at idle, no centrifugal advance, full vacuum advance (If you are using full manifold vacuum for vacuum advance, which you should). On the highway, full centrifugal advance, full vacuum advance. Punch it on the highway, say you want to pass someone, full centrifugal advance, no vacuum advance. Sorry to be so verbose about this, but as I said, I have noticed many people have a problem wrapping their heads around the whole concept of centrifugal and vacuum advance, and it's important to have it straight if you are going to be monkeying around with it. Also, as others have said, get your ignition set up with a good curve that brings the centrifugal advance all in at 2500-3000 rpm, set it at 34-36 degrees at full advance, and purchase a vacuum advance canister that will allow for about an extra 16 degrees of vacuum advance, for a total of 50-52 degrees while under low-load cruising. The stock vacuum advance can be set to give more than 20 degrees of advance, to work with the stock ignition timing, which will be too much once you have optimized the ignition curve. An easy way to tell what your vacuum advance is giving for advance is to measure timing at idle with vacuum advance disconnected, then hook it up to a full manifold vacuum source, the difference is what the vacuum canister is giving for advance. Once you have all of this squared away, then start monkeying around with the carb. Guys, if I have missed anything, or messed up and said anything backwards, (nobody's perfect!) Please square the OP away, will ya? Thanks, and good luck,


Keep the shiny side up!
Scott

scottyp99

No, re-read 7t1vette's post carefully, he's saying the exact opposite: optimize timing, then monkey with the carb. Re-curving a distributor sounds intimidating, I avoided doing it for years, but it turns out to be easier than falling in love, once you know the basics. And you will definitely notice some power and fuel economy, too, it is by far the best "bang for the buck" mod you can do to your engine.


Keep the shiny side up!
Scott

scottyp99

Speaking of carefully re-reading posts, I see that you are sporting a Pertronix Ignitor 3 distributor. It probably has a good curve right from the factory, but you need to check it to make sure. Does it have an adjustable vacuum advance canister? If so, you will need to learn how to adjust it properly. Remember: ignition first, then carb! Write it on the back of your hand with a Sharpie if you have to!


Keep the shiny side up!
Scott

LB66383

Not so sure. He said he has a Pertronix Ignitor 3 installed, didn't say distributor. It's probably their trigger unit inside the stock Vette distributor. So he'll have the factory curve with "electronic points." Still needs to optimize the vacuum and centrifugal advance.

7T1vette

You are correct, IronDog; changing the carb jets does not affect distributor timing.

scottyp99

Oh, I get it, he's asking if he changes the jets, does he have to change the timing to compensate for it. I thought he was asking if he could just change the jets to compensate for not recurving the distributor! LOL, this whole post makes a whole lot more sense to me now! Sorry about the misunderstanding,


Keep the shiny side up!
Scott

Iron_dog

Thats right, I have the stock distributor with pertronix inside. One think I sill have to understand is how its adaptive dwell affects timing... dwell is fixed with points but not now...

Super6

Vacuum Advance Canisters are stamped with an identifying number near the advance rod.

"B20" starts to come in at 5-7 inches and is all in with 8 degrees at the distributor (16 at the crank) at 11-13".

"B22" starts at 7-9" and is all in with 8 degrees (distributor) by 14"-16".

"B28" starts at 3-5" and is all in with 8 degrees by 5.75"-8".

"B31" is the Crane adjustable can. At least, the one I tried was a B31. The info on the other canisters is from an article by Lars, titled "Distributor Vacuum Advance Control Units - Specs and Fact for GM Distributors". Good reading. Another good one is "Timing and Vacuum Advance 101" by John Hinckley.

If you have a healthy cam, you can't really tell what you have as far as total vacuum advance just by taking the timing reading difference with the distributor vacuum plugged in versus disconnected. Some cams just don't produce much vacuum at idle. You need to match the vacuum canister to the characteristics of your cam and engine.

Iron_dog

Guys, today I did the first measurement with a timing light with advance, I set the advance to zero and looking at the mark on the crankshaft I read 2 degrees advance... what does it means? Shouldnt I read zero???

7T1vette

It means that your timing is set at two degrees advance. Whatever it reads on the scale....when the light is set to "0" on the dial...is the timing setting from the distributor. If you put the light on the damper mark and adjust the dial so that the mark is on "0" on that metal scale, then whatever you read off the timing light dial is what the distributor timing is at the time.

Super6

You should aim for around 10 degrees (with the vacuum advance disconnected and the vacuum source plugged) and the engine running (warmed up) around 750rpm. This may be too much or too little on your engine depending on cam and compression. At the first sign of any rattling when you step on the gas, back the timing down.

scottyp99

I've never used a dial back timing light, my old regular timing light works just fine with the timing tape I have on the dampener, so I don't know anything about them, but I would imagine they must come with some sort of directions, have you taken a look at them?


Keep the shiny side up!
Scott

Iron_dog

This is the problem, I cant reach the 0 on the metal scale... the light display goes from 0 to up so my understanding is: I move the dial until I read 0 on the crankshaft, but I dont reach it... maybe I connected the timing light wrong??? Note: vacuum advance IS connected, the purpose is to read the setup somebody put, not to tune it (for now)