Yes, need lighter spring. Be nice to see 16-18 initial and limit your centrifugal to 16-18 for a 34 total with vacuum advance pluged all in by 2500-2800. You will like that if you can get it.

 

This Friday, I'll finally have more than 20 minutes to work with...I'll have all day...and I'm not quitting till the timing is nailed, and I get a reading of the header temps with my IR gun. They currently temp at around 520 deg...with header wrap.

 

Kevin,

The reason you were not getting vacuum when idling is because the port you were using is ABOVE the venturis and is open to atmospheric pressure, therefore no vacuum. The other port would appear to come from BELOW the venturis, i.e. the inlet manifold, where the "pressure" is below atmospheric due to the closed venturis. This negative pressure is called vacuum.

The reason GM used ported vacuum, i.e. ABOVE the venturis, is that with no vacuum advance at idle, the timing would be RETARDED from its optimal setting. When an engine runs retarded, it runs hotter and this tends to burn up any unburnt fuel in the exhaust manifold (to a degree) but its main purpose is to reduce the amount of NOX or HCs being emitted IIRC. This was all part of early (read CRUDE) emissions control.

By connecting your vacuum advance to a port that comes from the high vacuum inlet manifold (with closed throttles) you will have an advanced timing situation at idle. Normally, an advanced timing condition would lead to pinking or preignition under load, but at idle, the engine is not under load and can happily tolerate this advanced timing.

However, as soon as you crack open the throttle, atmospheric pressure will nullify the inlet manifold's vacuum and the timing will be retarded to the static timing mark, around 12 degrees in your case. This will eliminate any chance of preignition and your car will pull away smoothly.

As the revs increase, the centrifugal timing will come into play and the timing will advance, up to the limit of the advance mechanism. You should not get any pinking with this extra advance because of the speed at which the engine is running. Also, you will not be getting any vacuum advance with the throttle open.

When you reach a cruising speed above the point where maximum mechanical advance is engaged, you will back off the throttle and then the timing will be further advanced due to the vacuum advance coming in. With very little load on the engine, it will tolerate 50 or more degrees of advance with no ill effect.

As soon as you open the throttle to overtake etc., the vacuum advance will drop off and the timing will be retarded to the combined static and mechanical combination and therefore minimise any pinking that a sudden load situation would otherwise cause.

You should always obtain your vacuum source from the inlet manifold, the only down side is that you MIGHT emit a few extra nasty emissions at idle. However, by keeping your car in tune and checking it on a regular basis, you are most likely emitting FEWER emissions than another old car that's driven by a non caring owner.

Hope this helps.

Regards from Down Under.

aussiejohn

 

Sounds like you've got it down, now! I told ya you'd wonder why you had so much trouble with it, once it clicked. It all sounds about right. 18 degrees from vacuum is about right, 22 degrees from centrifugal advance is about right. Lighten up the springs in the distributor so that the centrifugal advance is fully advanced by 2500-3000 rpm, (like you said) and you should be in good shape. Something to watch out for, though, is that if you go too light with the springs, it will start advancing at idle, which will cause problems. With the vacuum advance disconnected, if you start to see less than 22 degrees difference between idle timing and fully advanced timing, it means your centrifugal advance is already advancing at idle, which is not good, it can make for really weird idle tuning problems. When watching the timing mark with the timing light, slowly give it some gas, and your timing mark shouldn't move until you have increased engine speed at least a couple or 3 hundred rpm. And, that's about all there is, as far as the basics go.


Keep the shiny side up!
Scott

 

OK, so, SUCCESS! I had to play around a little with those little plastic limiter bushings, but I got it to 16 deg initial, no vac no mech advance, then I had to replace the springs twice, as at first, the mech advance started to come in as soon as I quickened the idle up on the carb...backed off the idle again, did one gold, and one of the OLD springs, and VOILA, mech advance comes in full at 2,700RPMS...from there, I had to reduce the amount of mech advance, which took me a while to get, but now I'm rolling with 34 deg timing at idle with the vac plugged in, and the advance is right at 34 deg at 2,700rpms.

One thing I have YET to do, however, is drive it. I'm not gonna lie...it's hot as ***** out right now, and the AC feels wonderfull, I've sweated out about a gallon, and am beat...and I didn't even work that hard, lol. I'll drive it tomorrow, maybe. Thanks for all the helps, guys!

 

OK, so, NO SUCCESS! I decided to take it for a drive after all, once I cooled off, and, being the guy I am, I checked everything all hooked up, before heading out. Good thing.

So, my initial with no vac at idle is 16 deg. The new timing curve kit limits the advance, so that, with two silver springs, I am getting 26 deg at around 2,700 rpms. However...what I DIDN"T check was, how this all worked out WITH the vacuum plugged in. With it advanced to 16 deg initial, with vacuum I am looking at 34 deg total at idle...which seems fine...until I rev it. For some reason, with the vac plugged, when the mechanical advance kicks in, my timing goes WAY high, like, close to 50 deg high...and I heard what must have been pinging....sounded AWFUL, like metal beating on metal, at those higher rpms...so I didn't hold it long enough to get a good reading.

I'm gonna put of those pics of where I have the vac hose plugged into, now, also. I'm assuming that is the problem...are there some areas that make constant vacuum, regardless of rpms?


Help?

 

After some searching, I found this image, and my vac hose is plugged in the same spot.

https://www.corvetteforum.com/techti...D=86&TopicID=3

 

The only thing I can think of is that your vacuum advance can is sticking, and not backing off the timing as the vacuum goes down. I've never seen or heard of this happeneing, but it's all I can think of. Is it an adjustable vacuum can? Maybe it's adjusted wrong.


Keep the shiny side up!
Scott

 

How would I find out if it's adjustable or not?

 

If it looks like this:

http://www.junglee.com/ACCEL-31034-A.../dp/B00062YHRO

It's adjustable.


Keep the shiny side up!
Scott

 

Nope, looks like that MINUS the square part to wrench on.


I just double checked my numbers to make sure, and braved getting a reading at full mech advance, and with vac plugged in, at 2,700rpms, timing is at 45 deg. It seems to only ping if I go PAST 2,700rpms, even though the timing remains the same. I'm thinking this might be the reason the guy I paid had the vac hose plugged to the port on the passenger side, rather than the drivers side...

 

If the dizzy "can" is stuck, is there a way to unstick it, or does it need to be replaced?

 

Is it possible I did something wrong when I replaced the base plate, weights, and springs recurving the dist, which could cause the vac can to be stuck, or getting stuck?

 

The new timing curve kit limits the advance, so that, with two silver springs, I am getting 26 deg at around 2,700 rpm

--------------------------------------
Kevin ....these guys have all been telling you that you need to be at 34* all in.....you are still only at 26*.

You need to go slightly heavier on your spring rate I believe (Lars said to try the "gold" springs).

Get 34* all in with vacuum plugged at 2700rpm, then adjust idle accordingly. You're more worried about base timing and the final readings with vacuum applied than need be at this point in time.

Lars' paper CLEARLY states not to worry about base as long as you are in the window.

I'm certainly no expert....but this isn't rocket science as these guys have stated. I think you're just over thinking things bud....keep it simple, and work the process one step at a time as Lars says.

Once you are all in at 34* at you desired rpm you're almost home. Set idle and road test, if she's "pinging" back her off a bit.

Good luck my man....we're all pulling for ya!!!

 

The 45 degrees is normal with no load. You have initial, mechanical 34 degrees all in @ 2700 plus your vacuum advance. Many cars have 50-54 degrees advance in this situation. Run premium fuel, your where you want to be. If premium don't eliminate the ping you either need a different vacuum can or back initial timing down a couple degrees.

 

The gold springs are heavier, and therefor, will prevent me from having all my timing in as early as 2,700rpms. What limits the total advance amount are the little plastic pieces that go in the weights...they inhibit the range of motion the weights can move out to, reducing the total amount of timing capable. Why I need to limit that advance, is so I can run a higher initial timing, without running TOO high mechanical advance timing. In other words, with no recurve kit installed, I was getting something like 20 deg, or so, from the mechanical advance...which works FINE, if I'm only going 12 deg initial timing, as that puts me at 32 deg, at around 4,000 rpms. Now, I COULD have simply swapped springs, and had 32 deg at 2,700rpms...but that would have required continuing to run 12 deg initial, which I was suggested, both by folks here, and by Lars papers on timing, that 16-18 is where I want it. None of this, however, is my issue. My issue is, my vacuum advance does not back off, no matter how high I rev the motor. Which boggles my mind.

 

Normal with no load. How does the car run driving, any ping? If not run it.

 

I feel like such an idiot. The fact that it was not under load never freaking came to me. Frankly, after hearing that noise, I was too afraid to drive it...Not for nothing, I got one of them budget builds, lol.

I slapped my vac gauge onto the manifold port, and got about 12-13hg at slow idle, and 20hg at 3,000rpms. Normal?

Over on the venturi side, it was nothing at idle, then around 10hg at 2,500rpms, and then progressively higher, the more rpms.

I am using premium fuel, and it's not more than 2 months old, or so. I filled it up after I got it out of the garage, right after winter.

I fiddled around some more, and I think it got it JUST right, but i have YET to check it with everything hooked up, namely, vacuum...to check for pings. It's at 15 deg initial, it makes 19 deg vac at idle, for 34 deg total at idle, which is about as close as I could get to what you said, without pinging...though I still have yet to drive it, to see if it pings. I have another 19 deg mechanical advance, coming in at 2,700rpms, resulting in 34 deg again at 2,700rpms.

Before I hop in this thing to drive it, as I'm worried I won't be able to hear the pings, due to the really loud exhaust...a question...is it more likely to ping under load, with timing too advanced? Or is it more likely to have WAY too advanced timing revving it while in neutral, resulting in detonation? And last, doesn't that mean revving a motor is bad for it, even though everyone does it?

 

Taking it for a drive now. No hills really near me, but I figure I can simulate one by getting to about 30mph, shifting into fourth, and flooring it, right?

 

You have 15 initial, 19 mechanical for a total of 34 @ 2700. All good. You also have 19 vacuum at idle. Add them together your at 53. That is surpassing the 52 max recommended. (there is your ping above 2700 in neutral) You say your vacuum goes from 12-13 at idle to 20 @ 3000. If your vacuum advance is continuing to add vacuum you may be well surpassing 53 degrees causing your ping in neutral. This should not affect ping while accelerating but could cause ping at cruise if your vacuum advance continues to advance past 19 degrees, again your surpassing 53 degrees total with advance. Solution is to limit vacuum advance to about 16 degrees for 50 total with vacuum. You have efficent heads that allow 34 degrees total so I would drop the total with vacuum to 50 degrees to be safe.

This is a quote from Lars:

"How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal
advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control
unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit at cruise. If
the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance
unit can only be allowed to pull in 16 more degrees of advance."

You want your vacuum can to start @ 2HG below idle vacuum and pull 8 distributor degrees at 2 HG below your max vacuum. The below numbers are for points distributor. If you have HEI let me know.

A VC1675 also known as a B13 vacuum can will start advancing @ 9-11 HG and get maximum 8 distributor degrees (16 crankshaft degrees) @ 16-18 HG.
This is perfect for your combination. It is the vacuum can used on:

1968 327 Camaro Powerglide
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

If you can't find one go with
VC680 Also known as B1 will start advancing @ 8-11 HG with a total of 8 degrees maximum distributor advance (16 degrees crankshaft advance) @ 16-18 HG

Used on:

1959 – 63 All Chevrolet
1964 Corvette exc. FI
1964 Impala, Chevy II
1965 396 High Perf.
1965-67 283, 409
1966-68 327 exc. Powerglide
1967-68 All 396
1969 Corvette 427 High Perf.
1969 396 Exc. High Perf.
1969 Corvette 350 TI
1969-70 302 Camaro
1970 400 4-bbl
1970 396 High Perf.
1970 Corvette 350 High Perf.
1973-74 454 Exc. HEI


Buy one, they are cheap. Leave your initial and mechanical as is, replace vacuum can with the one above, your golden. If you have HEI let me know..