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Joined: Feb 2011
Posts: 1,173
T
'Bolter
I think this is the problem.
First off disconnect the vacuum advance hose or line from its vacuum source. Then set the timing at 5 degree's before TDC. You said initial timing set at 25 degrees. Initial always means that the vacuum line is off when setting it.
Things that happen and can happen.
With the Vacuum advance connected to the intake manifold yes it at idle will jump up to full Vacuum advance, but when you drive and the manifold vacuum goes down or rather positive pressure goes up then that retards the spark, especially for going up hills or pulling a nice,
load. The important thing is initial timing is either 5 or 10 degree's, 10 is for higher octane fuel back in the day. I think you've been setting the initial timing too much before reconnecting the vac line.
One thing that can happen on old Chevrolet V-8's is the damper weight can slip on the rubber bonding material and give an erroneous timing reading, but that normally would be I think in the retard condition, just something to be aware of.
Does the mechanical advance seem to be free?

Last edited by Truckrolet; Mon Sep 19 2022 03:42 AM.

Kicking self for selling off my Taskforce trucks.
Still looking for an LCF or conventional big bolt in decent shape.


As of 10-26-2022, A 55.2 Taskforce long bed now the work begins
Joined: Feb 2004
Posts: 25,334
H
Kettle Custodian (pot stirrer)
This is a brand new engine with a fairly radical cam that needs a lot of initial advance. No stock distributor advance curve or ported vacuum system is ever going to work right. Both the centrifugal and vacuum advance, plus the initial timing are going to have to be custom tuned to the engine, plus there's no way to predict how the oddball electronic ignition is going to affect things. It has an abominable reputation for doing strange things, and failing completely without warning.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
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WAG MORE- - - - - -BARK LESS!
Joined: Sep 2011
Posts: 2,676
L
Lugnutz Offline OP
'Bolter
This morning I set the initial timing to 18° with RPMs at 600. I did not connect the vacuum advance hose. I kept the hose to the carb plugged.
My drive to work was delightful. I had plenty of power. No pinging.

Joined: Feb 2000
Posts: 4,566
J
'Bolter
The first pdf link is a break down on a stock 283 distributor I just worked on for a fellow member, this is with all stock parts. The numbers are at the distributor, double them for crank degrees. The second file is after changing the springs and adding a little heavier weight. Also pay attention to the vacuum advance, they each have starting and ending points with X amount of degrees advance. 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

Attached Images
'62 283 distributor.pdf (19.94 KB, 5 downloads)
'62 283 distributor altered .pdf (25.06 KB, 5 downloads)
Joined: Apr 2005
Posts: 11,310
Grease Monkey, Moderator General Truck Talk & Greasy Spoon and HiPo Forum
Lugnutz, if your happy with the performance it’s time to lock it down and move on to other adventures. Very interesting thread for sure.


Martin
'62 Chevy C-10 Stepside Shortbed (Restomod in progress)
'47 Chevy 3100 5 Window (long term project)
‘65 Chevy Biscayne 4dr 230 I-6 one owner (I’m #2) “Emily”
‘39 Dodge Businessmans Coupe “Clarence”



"I fought the law and the law won" now I are a retired one!
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Joined: Sep 2011
Posts: 2,676
L
Lugnutz Offline OP
'Bolter
Originally Posted by Justhorsenround
Lugnutz, if your happy with the performance it’s time to lock it down and move on to other adventures. Very interesting thread for sure.

Yes, I really do want to move on. However, I had a very strong feeling that the engine was not performing as it should when I was driving it with the vacuum advance. I was right.
I will tinker with the timing a little bit but keep the vacuum advance disconnected.
I have so many other projects that will take priority that as long as it runs pretty well, it will sit on the back burner for a while.

Last edited by Lugnutz; Mon Sep 19 2022 02:37 PM.
Joined: Dec 2017
Posts: 1,183
D
'Bolter
I dont think so. Some motors are full manifold vacuum to the vacuum advance, others are ported vacuum to the advance. A 235 is ported vacuum and typically there is no vacuum at idle, hense no need to disconnect the adcance to set the timing. Others are full manifold and the advance must be disconnected to set the timing.

For a small block chevy, 38 to 40 degrees total timining is about the range you typically see. This is set using a combination of mechanical advance and initial advance. A typical chevy distributor has 12-14 degrees of mechanical advance, none of which is in play at idle. 12-14 at the distributor equated to 24-28 at the motor. To get to say 38 degrees total, you set the timing, with the vacuum advance disconnected and plugged, at 10 degrees before TDC. That gets you the 38.

Now, once your vacuum advance is hooked back up, and connected theoretically to full manifold vacuum, it will add what ever the can was made to add. Different GM can numbers equate to different advance totals and different levels of vacuum. So its not unusual to see the vaccum advance add a few degrees, in this case 19, at idle.

As you put your foot into the gas pedal, the vacuum drops and the vacuum advance is not adding any timing to the equation. This happens pertty quickly which is why that much advance at idel is not much of a concern. Throttle opens, vacuum goes away, vacuum advance goes away. At WOT there will likely be 0 vacuum so it does not come into play at all. In between, various levels of advance will play into th4e equation depending on the amount of vaccum applied to the advance diaphram. The vacuum advance is helpful at part throttle and crusing so its good to have. Here is a little light reading on the subject.

https://www.motortrend.com/how-to/1607-tune-your-vacuum-advance-for-better-drivability/

http://www.langdonsstovebolt.com/tech/vacuum-advance-and-why-you-want-it/

Last edited by Dragsix; Mon Sep 19 2022 08:37 PM.

Mike
Joined: Jan 2014
Posts: 2,345
J
'Bolter
I don’t think I ever saw a clear answer to the question about whether this carb was set up for ported vacuum or full manifold. But I may have missed it.

Joined: Dec 2008
Posts: 1,785
P
'Bolter
The bigger the cam, the more it needs ported vacuum.

Joined: Feb 2004
Posts: 25,334
H
Kettle Custodian (pot stirrer)
I seem to remember some 283 2 barrel engines with a center-drilled carburetor mounting stud with a smooth upper section, and a vacuum line attached that put full manifold vacuum on the distributor at startup. That system is definitely NOT appropriate for the cam this engine is running.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

WAG MORE- - - - - -BARK LESS!
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