pullin-gs

Yesterday, I set out to go over my timing "end-to-end".
Car is a 1963 w/300hp (original block).
Distributor is setup with a B1 vacuum can: (8-degrees @ 16-18hg)
Engine pulls 15hg @600rpm.

Car is warmed up. Connected up timing light.
I started process by disconnecting can, and plugging vac line.
All vacuum hoses leak-free. I also plugged PVC inlet on carb.
I dialed in my timing light to 36-degrees.
I ran RPM on car to point cent. advance was "all-in", and timed car to 0-degrees.
I brought engine back down to 600rpm and dialed light back down to zero.
***OK...this is where things got funky****
Timing mark was reading 16-degrees....kind of high (supposed to be 6-8).
No big deal I suppose. Timing is now dialed "all-in" and the numbers are what they are.
OK, now I connect unplug vacuum line and connect the vacuum can.
I take timing reading (and verified multiple times): Vac-advance adds in 17-degrees (dialed in 17 on timing to bring mark back to 16)! That is kind of high for a B1 that is rated for just 8.

Thoughts? These numbers dont add up at all.
20 degrees mechanical advance, and rotator plate vac-mounting point is setup to double advance of any give vacuum can?

PS: Based on the symptoms I'm seeing (off-idle bucking when pulling out at low throttle...carb is fine), I thought my marks were off. So I verified TDC with the mark on the harm. balancer. TDC is fine.

65-StingRay

From what I read - problem may be your vacuum can. It could be worn out or the lever arm is pulling in too much vacuum. Lars (C3 forum) has a good paper on the subject of limiting the pull on the vacuum can lever.
I agree there seems to be too much vacuum once your vac line is hooked up.
There are a lot of other things that could be not right here but this is my best guess. Good Luck.

65-StingRay

Tiros

I think the B1 8 degrees is distributor degrees, which is 16 crankshaft degrees. The initial extra advance is probably a good thing if it starts up ok. To much vaccum advance leads to chugging or I call it "burbling" at cruise.

Stan's Customs

I agree..16 degrees on the mark (crankshaft)...8 degrees at the distributor.

A can with 8 degrees at the crank, would give you the readings you seek...although not really necessary.

SWCDuke

For one thing the B1 is a boat anchor. It doesn't pass the Two-Inch Rule. Yeah, I know what the parts books say. If a manual trans you need a 15" B22, and a 12" B26 if PG.

If the centrifugal curve is OE you have to rev the engine to over 4600 for all 24 centrifugal degrees to be in, and you have to set the initial at 14 to get the near ideal 38 (NOT 36). This is why the later 327/300s beginning in '66 got 30 degrees centrifugal, so all you needed was about 8 initial.

If you want to grind out the slot to get 3 more distributor degrees and then use some lighter springs, the engine will perform better as long as the CR is not so high as to cause detonation. Chances are your engine has no more than 9:1 if it was rebuilt. (Measure the head gasket thickness to get an idea.)) From the factory the true CR was somewhere between 9.5 and 10, which is no where near the overinflated advertised 10.5:1.

On a '66-68 327/300 with the appropriate VAC as specified above (converted to full time vacuum advance if OE is ported) with lighter springs to bring in the 30 degrees centrifugal at 3000-3500 with 8 initial, total WOT advance would be 38 and total idle advance would be 24, which is about ideal, and none that I know of with this spark advance map setup detonate, and they all are stronger across the entire rev range, especially the low end.

A 327/300 (manual trans) should pull 17-19" @ 500 in neutral, so I wonder if you have an OE cam.

Duke

pullin-gs

Excellent points all. I had'nt a clue about degree rating of vac-can is at the distributor. Makes sense though.

Anyway, it looks like my timing is close after all....at least close enough not to be causing the problems I am seeing:
Off-idle light-throttle (when pulling-out from stop) engine bucks. Heavy throttle no problem. All other speeds/load engine runs great. No detonation either.

Engine has been rebuilt (no idea what the inards are though).
Carb (rebuilt 2 yrs ago) throttle pump is working fine. Even off idle.
No play in linkage, and AFB rod bushing is tight.

When I advance the timing even more, bucking problem is much better, but then I get detonation.

If I turn back timing to 16, and then I pull the vacuum advance, engine behaves much better. Bucking is gone.

Powershift

Believe what SWCDuke says. The current vac can and your engine vacuum are to close.......which can/will cause erratic operation of the vacuum advance. I would replace it with what Duke suggests.

You can run with it disconnected it for awhile. This should hopefully reduce detonation and the bucking/surging.

Initially (original engine) your engine should pull around 18-21 in HG vacuum at idle.

Larry

pullin-gs

Good advice. I'm going disconnect vacuum canister and plug vacuum line, and look for a new canister.

Jam421

OP…..Moroso Advance Curve Kit # 72310 as the one I used. Here’s a Mr.Gasket recurve kit: https://www.summitracing.com/parts/mrg-928g
I was told not to use the HEI recurve kits. In the Moroso kit the spring tensions are listed as follows: Copper-Light / Silver-Medium / Black – Heavy.

The vacuum advance canister should be available at most auto parts stores especially NAPA.
Here’s an Accel Canister for non-HEI ($34) : https://www.summitracing.com/parts/acc-31034
Here’s a Mr.Gasket Canister ($18): https://www.summitracing.com/parts/smp-vc24a

What matters when designing the curve is when/how you want your timing to come in. My SBC 383 combo with Copper/Silver springs showed me full advance around 3,000rpm. This was fine for idling, around town + really gave my car a kick in the pants with the go pedal. Two copper springs brought full timing in a bit sooner around 2600-2800rpm.
*As mentioned vacuum advance MUST be disconnected with port or vacuum hose plugged to properly install the advance spring kit.

Bluestripe67

Another post from the graveyard....4.5 yrs old! Dennis

vettebuyer6369

You are answering a post from 2016.