Triumph Repair: Timing off?, chevy nova ss, gt6 mkiii

Question
I am a mechanic and have a 1972 GT6 MkIII and am having a hard time with the engine timing. Everything is stock with 116,000 miles, new tranny. All new tune up parts set to factory dwell @42 degrees. (I have experimented with 30-45 degrees as well just to see if it helped my problem) Vacuum advance is disconnected while setting timing. If I set the timing with my scope to 4 ATDC as specified the engine runs poorly and is a DOG when taking off from a stop. Also coughs and sputters and has black exhaust soot on the rear bumper. However, if I run it to 12 degrees BTDC it idles smooth and runs like a scalded cat, smooth and fast off the line. My worry is that I may be detonating on the piston though all I hear is what appears to be standard valve train noise not pings or knocks. I had a Chevy Nova SS that would only run well at 25+ degrees (6' was specified) advance with no problems, could this be similar? OH, carbs are stock and seem to be fine, air filters good and clean too. Thanks

Answer
Hi Dave,
If you have the vacuum retard and advance unit on the distributor this may be causing the discrepancy in the settings. Also the centrifugal advance may be at fault.
You didn't tell me at what RPM you are checking this.
Here are some specs that might help.
Centrifugal = 2 to 6 deg. max at 1000 RPM
9 to 16 @ 1750
13 to 18 @ 2400
18 to 23 @ 3600
24 to 28 @ 5000
Vacuum advance = 20 in hg = 22 deg. max
14 in hg = 13 to 17 deg.
11 in hg = 8 to 12 deg.
3 in hg = 0 deg
Vacuum retard = No retard below 2.5 in hg
15 in hg = 8.5 to 11.5 deg retard

These figures were fine at the factory but unreal on a used older car. I have faced what you are now many times over the years and all I can tell you is what I did.
I first would put one grade lower fuel in the car than what you intend to run and first check to see that the marks on the front balancer are correct. Number one on TDC with the plug out and with a rod feel for true TDC and look at the pulley markings. Then get the engine oil up to operating temp. (A little more running than coolant temp) Set the timing so you are just a little short of the quick throttle opening pinging at lower RPM. Then put the higher grade fuel in and check what total advance is with no load. I would not check at anything higher than 3500 RPM on customers cars as the condition of an older car might cause engine damage. Some, I worried going that high.
Also, you may want to read my tech tip on ignition on my website. http://mg-tri-jag.net/tech6.htm
Let me know how you do with it.
Howard