I have a '71 LT-1 and originally set my timing advance (per Lars) back in May to 36 degrees advance (with vacuum advance plugged). I didn't do anything special other than to ensure that I had approx 36 degrees advance coming in before 3000 RPM. Car has been running great, and I didn't notice any pinging whatsoever.

Now, I had some time on my hands this past weekend and decided to learn a bit more about my distributor (not original - see https://www.corvetteforum.com/forums...-question.html). I noticed that the springs looked a bit rusty (they were different sizes), so I purchased a Mr Gasket 929g kit and decided to re-curve the distributor. I followed Lars instructions more closely this time...putting a single weakest spring from the kit on the distributor to make sure full 36 degrees of advance was coming in (around 2k). After playing for a bit, it seemed like a black/silver spring combo worked best to get all 36 degrees of advance coming in by 3000RPM. I was much more thorough this time around.

However, the car doesn't seem to be running as well with this timing configuration. The idle isn't a smooth, and it doesn't quite have the top-end that it had before. Frankly, I really have no idea how much advance I was pulling before as I never really bothered to check it past 3000RPM and never removed the springs.

Do you guys see a problem if I revert to my original springs and just make sure I have no more than 36 degrees coming in before 3000RPM, since this config seemed to work quite well? Will there be problems with revs > 3000RPM if more advance happens to be coming in?

Thanks in advance.

 

If it ran better before, put it back like it was. (You did remember to re-connect the vacuum advance?)

 

LOL, I did remember to connect the vacuum advance. My primary concern is ensuring that I'm not damaging something with my original config. The car seemed to run fine, but I never really did determine if I was getting > 36 degrees of advance beyond 3k RPM's.

 

the springs you use do not change the total mechanical advance. The total mechanical is limited by the stop bushing and the shape of the weights. Install the old springs and see what rpm is required to get the full 36 degrees.

 

I re-installed the original springs today and here are some results:

16 @ 600 RPM
24 @ 1650 RPM
30 @ 2670 RPM
36 @ 3400 RPM

Just for comparison sake, I swapped out some springs to see if it made any differences.

Results for one stock + 1 929g gold spring:
16 @600 RPM
24 @1500 RPM
30 @2500 RPM
36 @2750 RPM

Results for 2 929g gold springs:
16 @600 RPM
24 @1250 RPM
30 @2100 RPM
36 @2400 RPM

For all of the above, if I blip the throttle, the advance pushes past 36 (to nearly 40, I'm guessing). Should I retard my timing a bit and re-test? Does any of the above shed any light on which spring combo is the best setup?

 

Keep in mind that those numbers are for mechanical advance ONLY, with the vacuum advance not connected. Once you reconnect the vacuum advance, the engine WILL advance beyond 36 degrees in many situations.

With stock springs in the dizzy, your numbers are reasonably with some 'safety' factor. Performance should be best with the spring combo that achieves max mechanical advance a bit BEFORE 3000 rpm.

But, I think leaving the stock springs in it might be best for your engine and location, based on your discussion. You've done adequate testing and it runs better there. So be it.

 

can you see the advance stop bushing, you may need something like a dental mirror?
the original was plastic/nylon and maybe gone or damaged.
did the spring kit you bought contain the replacement brass bushing and if so, are you will to pull the distributor to install it.

 

Here's a picture of my distributor - I don't see any advance stop bushing unless I'm missing something obvious:

Edit - I think it's hidden in this picture (beneath), so I need to check.

 

1970 Chassis service manual lists the following:

0 @1200 rpm
12 @2000 rpm
20 @4600 rpm

W/ RPO K66
0 @950 rpm
2 @1200 rpm
12 @2000 rpm
20 @4600 rpm

I realize you have '71 so there may be differences but....Going off a combination of the chassis manual and Lars' papers try setting yours up like this:

0 @1000 rpm
36 all in by 3000 rpm

 

I did determine that the advance stop bushing was missing, so I used one from the kit. Not sure it made much of a difference as I just re-checked with original springs and the curve turned out to be about the same. I am getting 36 mechanical just around 3k RPMs and it will jump past the mark if I blip the throttle. Should I be concerned about that or just leave well enough alone? I will lug it around a bit at low RPM's to ensure I'm not getting any knock at WOT.

 

what is the initial with that 36+ total?
are you using a dial back timing light or have a degreed damper?

 

I'm using a dial-back timing light. Initial w/original springs (plus bushing) is 16 at 600 RPMs.

 

Just to keep it "apples and apples", this measurement of MECHANICAL advance needs to be done with the vacuum advance disconnected and that vacuum hose plugged.

If your vacuum advance was connected, any timing numbers [for the purpose of establishing a mechanical advance curve] are meaningless.

 

Correct. No vacuum advance (hose plugged) in those tests.

 

Quick update - after doing some research I determined that I purchased the WRONG kit for a points distributor. I ordered up a 928g kit and compared with the 928g kit -- the springs in the 928g kit are just a tad shorter. This would explain the sloppy idle that I was getting with the 928g kit as the weights were't being held tightly at idle.

So, I found a few posts (from Lars, of course) and determined that the 928g gold springs were the ones to try. After tinkering around for a bit, I was able to produce the following curve:

13.5 @580
24 @1500
30 @2250
36 @2700

Perfect, right? Driving around, the car was responsive and suffered no sloppy idle issues as before. However, now she's running HOT in stop/go traffic (new 180 thermostat). The original config was very reliable - she would run up to just about 210 that back off as the thermostat opened up and stop/go traffic never a problem. With the new springs/curve, she's running past 210 at idle and slowly returns to 210 after some cruising. I'm thinking that this curve is retarded just a bit too much as detonation is occurring just a bit to early (?) and causing more heat?

Thoughts?

 

This is a comment/thought....

Do you go 36 all in by 2500-ish regardless of mechanical advance at idle? Or do you go with NO mechanical at idle and 36 all in by 2500-ish?

36 is supposed to me initial plus mechanical. If you have 13.5 @ idle then what's your initial? Place rubber bands around the weights and recheck your timing, I'll venture a guess and say its 0 or even retarded which may explain the increase in heat.

 

Just yesterday I rechecked timing at my 71 (not lt1) and I've set it at 34@2500. It runs beautifully. No overheating no nothing. just lovely driveability. I've set it like that for security sake. It purrs like a kitten. lovely idle and she sings above 3000.

 

Thanks for the feedback. Was gone over the weekend, but had a thought - what if my vacuum advance isn't working properly? I have a 360 12 can and checked my timing again today and compared connected vs disconnected (vacuum plugged). I'm getting NO difference in idle timing with the can connected - does that suggest that the can is faulty? I checked the (manifold) vacuum feeding the can and am getting roughly the following:

15 @600RPM
20 @1000RPM
22 @1500RPM

Thoughts?

 

Update - I DID determine that the can is bad, so that might explain why I'm chasing my tail a bit here since I'm getting zero vacuum advance at idle. Does anyone have recommendations on a replacement can? I see that Summit has an adjustable (31034) - would it make sense to go with something like that with my setup or a standard can? If the latter, any suggestions on brand/model?

Thanks in advance.

 

I just went through the same issue with my 67 tripower. Turned out the vacuum can was bad (held no pressure, I could blow right through it.) Replaced it with a new can that worked according to my manifold pressure(rule of 2) and it is now back to normal.