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Joined: May 2005
Posts: 260
5
'Bolter
Originally Posted by Joe H
You can limit them pretty easy and pick different ones for tuning. Here is a great list to keep around, https://www.crankshaftcoalition.com/wiki/images/2/25/Lars_distributor_vacuum_advance_setup.pdf

Lars does publish great information and makes it easy to understand. This pdf appears to be 20 years old and sometimes his papers are modified when copied and published on various websites. Contact Lars directly and he will send you the most current information. He does add and update his papers from time to time. I highly recommend the information he publishes for questions about timing and quadrajets.

Mark

Joined: Mar 2010
Posts: 10,289
5
'Bolter
My question is, why are you running a 2 barrel carburetor on a hopped up engine? I am a big fan of the Rochester 2GC carburetor on a stock V8, but yours is not stock. Something else for you to think about. I do know that you do like to think.

Joined: Sep 2011
Posts: 2,676
L
Lugnutz Offline OP
'Bolter
Originally Posted by 52Carl
My question is, why are you running a 2 barrel carburetor on a hopped up engine? I am a big fan of the Rochester 2GC carburetor on a stock V8, but yours is not stock. Something else for you to think about. I do know that you do like to think.
I searched for a decent Rochester Q-jet (was highly recommended). Couldn’t find one. I also couldn’t find a correct 4 barrel intake for a 283 that had the correct oil fill tube unless it was a Rochester 4Jet manifold. I have one of those in my shop now. Carb King told me to stay away from Edelbrock carbs, so I did. Sometimes you just go with the stuff you already have at hand. Sure, I can change the intake to any of the numerous aftermarket SBC versions, but the sacrifice is that I then have to change the valve covers and next thing you know the engine looks just like a 350 SBC. I wanted the 283 vintage appearance.

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Last edited by Lugnutz; Tue Sep 20 2022 02:16 AM.
Joined: Feb 2000
Posts: 4,566
J
'Bolter
Ported or manifold vacuum doesn't really matter which one you use as long as the engine is happy at idle. My 250 runs on ported vacuum because the W-1 Carters are set that way, it idles better on manifold because of the camshaft thats in it, I live with it. You just have to decide what's best for the engine you have.

Joined: Mar 2007
Posts: 1,910
Crusty Old Sarge
Dragsix is spot on about initial timing. Often a degree or two at the crank can make all the difference, vacuum advance comes into play while under load or at mid RPM as mentioned.

The attached link (again by Lars) has some great information regarding GM vacuum advance canisters and part numbers

https://www.crankshaftcoalition.com/wiki/images/e/e4/Vacuum_Advance_Specs.pdf

Last edited by TUTS 59; Tue Sep 20 2022 04:44 PM.

Craig

Come, Bleed or Blister something has got to give!!!
'59 Apache 31, 327 V8 (0.030 over), Muncie M20 4 Speed, GM 10 Bolt Rear... long term project (30 years and counting)
Joined: Dec 2008
Posts: 1,785
P
'Bolter
Vacuum advance STOPS under load, if not quickly enough it will cause knocking at part throttle.
Ported vacuum only connects when the throttle disc passes over the port in the carburetor throat. It shuts off when engine vacuum is lower than the spring tension in the can.

No vacuum advance at all?
At any throttle opening except idle and WOT vacuum advance provides better part-throttle acceleration, keeps spark plugs cleaner, reduces engine temperature, improves gas mileage, and lowers oil temperature during cruising.

Vacuum isn't the only way. During the Civil War, Harley-Davidson owners adjusted their manual spark control (left handlebar) to reach 45 degrees, and marked the control for 36 degrees to find it when you want.
For best effect in light throttle cruising or cold weather, roll the advance all the way, just remember to retard it back to 36 when accelerating. Ford Model T: same way, drive with both hands (levers on the steering mast control spark and gas).

Joined: Sep 2011
Posts: 2,676
L
Lugnutz Offline OP
'Bolter
I just talked with the man who rebuilt this engine. He recommends that I use vacuum advance if possible. He wants my mechanical advance to be “ALL IN” at 3000 RPMS. I’m going to look into the possibility of an adjustable vacuum canister for the distributor, but I want some data first.

I like experiments that collect data. I’m going to buy 6 feet of vacuum hose so I can run it from the carburetor vacuum line to a vacuum gauge mounted inside the cab. I will then drive around and measure vacuum numbers. I will record numbers at light acceleration, medium acceleration, WOT, and highway cruising speed.

Correct me if I’m wrong please, but mechanical advance can be measured in the driveway at different RPMs, correct?

Last edited by Lugnutz; Tue Sep 20 2022 06:43 PM.
Joined: Dec 2017
Posts: 1,183
D
'Bolter
The best way to sort out mechanical advance is to pull the distributor and give it to someone with a Sun or other brand distributor machine and have them set up the distributor for those specifications. The other way to do it is with a timing light, tachometer, degree tape on your balancer. Disconnect the vacuum advance, start the motor and sort out where your at idle (shoud be at your intial setting, say 12-14 degrees advance. From there, bring the rpm up 200 rpm at a time, and sort out whre you are at. so for example, if the car idles at 800 rpm, and the initial advance is 12 degrees, run the motor to 1000 and see how much mechanical advance came in. Do that at say 1200, 1400, 1600 until no more advance adds in. so from idle to 3000 you should see about 24 degrees add in until the advance weights are fully extended. Now you can graph the advance curve.

If the advance is still coming in after say 3000 rpm, either the mechanicl advance is all gummed up or you need to change to some different weights and or springs to get it where you want it.

If you are lucky, you get it all in by 2800-3000 and you dont have to do anything.

The vacuum avance, before you do anything, get your baseline initial and mechanicl set. Then, hook up the vacuum advance and see what the motor is doing.


Mike
Joined: Sep 2011
Posts: 2,676
L
Lugnutz Offline OP
'Bolter
Originally Posted by Dragsix
. . . . . The other way to do it is with a timing light, tachometer, degree tape on your balancer. Disconnect the vacuum advance, start the motor and sort out where your at idle (shoud be at your intial setting, say 12-14 degrees advance. From there, bring the rpm up 200 rpm at a time, and sort out whre you are at. so for example, if the car idles at 800 rpm, and the initial advance is 12 degrees, run the motor to 1000 and see how much mechanical advance came in. Do that at say 1200, 1400, 1600 until no more advance adds in. so from idle to 3000 you should see about 24 degrees add in until the advance weights are fully extended. Now you can graph the advance curve. . . . . .

I did this a few months ago.
These numbers are for the remanufactured distributor.
MECHANICAL ADVANCE with the vacuum line plugged.
570 RPMs = 12°
750 RPMs = 12°
1000 RPMs = 15°
1300 RPMs = 23°
1500 RPMs = 24°
2000 RPMs = 27°
2500 RPMs = 32°
3000 RPMs = 35°
3500 RPMs = 40°

Last edited by Lugnutz; Tue Sep 20 2022 08:53 PM.
Joined: Sep 2011
Posts: 2,676
L
Lugnutz Offline OP
'Bolter
These numbers are for the remanufactured distributor.
MECHANICAL ADVANCE with the vacuum line plugged.
570 RPMs = 12°
750 RPMs = 12°
1000 RPMs = 15°
1300 RPMs = 23°
1500 RPMs = 24°
2000 RPMs = 27°
2500 RPMs = 32°
3000 RPMs = 35°
3500 RPMs = 40°

The numbers below are from TODAY with the 1967 Corvette distributor. Looks like the Corvette dizzy has less mechanical advance than the remanufactured dizzy.
Corvette dizzy is “ALL IN” at 3000 RPMs.
650 RPMs = 12°
1000 RPMs = 20°
2000 RPMs = 27°
3000 RPMs = 34°
3500 RPMs = 34°

Just drove truck with a vacuum gauge connected to the carb. See pics. I ran vac hose inside the cab to a gauge hanging on my dash.

Here’s the data.
Idle 650 RPMs = 11–12 inches Hg
Light throttle 15-20 MPH in the neighborhood = 20 inches Hg
Hard WOT = 4-5 inches Hg
Highway cruise 50 MPH = 15-16 inches Hg
Moderate throttle acceleration = 12 inches Hg.
Of course the vacuum jumped way up to 23+ when I let off the throttle between shifts and when foot off throttle to slow down for a stop light.

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Last edited by Lugnutz; Wed Sep 21 2022 12:15 AM.
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