Since I have received helpful repsonses from posting some of my vette issues in the past I would like to post my most recent frustration .  I am thrilled to have the original rebuilt 350/350 back in my 70 vette.  Unfornately my dreams of installing, tuning and speeding off into the sunset did not occur.  The trouble is we just cannot seem to get it tuned perfectly.  We have set and reset the timing, etc.  When we put the engine back in the car we replaced all hoses, fuel lines, and have most other components rebuit.  We have rebuilt the original Rochester carb, we have rebuilt the distributor.  It seems to finally be running pretty smooth but still pings when accelerating at a low rpm.  My qestion, and hope, is that it could be a simple case of the use of low octane fuel.  I have been running the midgrade gas, 89 in my state, but maybe I need to only use high test.  I have driven about 500 miles since rebuild.  I know the the high compression engines were designed to run on high grade fuel.  My 350 has 11:1 compression.  Anyput would be appreciated. 

I know the owner's manual for my 74 specifies that it needs at least 91 octane.  I would try that and see how it does - if nothing else, it's an inexpensive effort.

Before you go to extremes, run a tank of 93 octane high-test. (Check at the pump; many stations are calling their 92 octane "high-test"). Remember that they were not adding ethanol to the gas in the 70s and that may be part of your problem. Depending on your exhaust system, you might even try a can of tetraethyl lead if the pump hightest doesn't do it.

Also we installed a new original style exhaust this past spring.  As soon as possible I am going to fill up with the highest octane available.  I think I saw 93 today.  Now, I shouldn't add lead if when they rebuilt the motor they put the hardened valve seats in correct?  What about octane booster?  Or is that an octane booster?  Thanks again!

Try retarding the timing slightly if you're still pinging with the higher octane fuel.

I agree with Chips...try the highest octane you can get, and retard the timing. You have a high compression engine...it won't run very well on lower octane fuels without "pinging". Also, the advance curve may need to be slowed down a bit.
Forget about timing lights...the only thing they are really usefull for is seeing where it is once you get it running right. That way, you have a "baseline" to start with from then on.
Check out THIS THREAD ,and THIS ONE. There is some very solid info one timing an engine to run without pinging, or being hard to start hot.

TIming brings up another question.  The owner's manual says to set timing at 8 degrees while the shop manual says to set it at 4 degrees.  When set at 8 it pings awful.  Tonight we were messing with it and retarded it to 6, test drive, still pinging, then 4 degrees, still pinging but a little less.  Now it is at 2 degrees and seems better.  I went and put 93 octane in it but it only took 9 gallons.  I drove around a bit to try to run it through but it got dark and I got hungry and frustrated.  So tomorrow I will start again.  I also have another issue of incosistant idle.  First the choke doesn't seem to function, replaced with new when installed rebuilt motor.  Once it warms up seems to hold idle fine except every now and then it will die with the clutch in or in neutral.  Then will not hold an idle until I turn it off and restart, then fine.  I hate intermittent problems.  I am about to seek a higher power--a corvette mechanic.  But those are hard to find.  Thanks again for all your input, it is appreciated.  Will check one thing at a time.  Maybe the carb needs to be rebuilt by a professional or replaced. ......   And reading all that timing setting informatio makes my head spin.  Springs and weights are beyond my knowledge, plus the guys at the machine shop just rebuilt the distributor but it seems to make setting the timing more difficult.cms1970stingray2006-09-21 18:29:14

If the carb is running a bit lean at light and mid throttle.  A lean condition will contribute to pinging.   You may also try running with mechanical advance alone, and not use the vacuum advance.   This along with higher octane fuel may help.    11:1 does usually take a higher octane.    You can try an octane booster instead of trying to just fill with higher octane.  The fuel in the tank will lower the octane rating of the fuel added, creating something in between the two.

The engine running after shut down sounds like the carb may be running to lean. See if you can find someone to check your air fuel mixture at the exhaust & try & get it at 14/1. If you have a dyno around you they could solve your problem as it could be a combination of a few things all mentioned in the above. Any car part store has octane booster. My old engine was 12/1 & I had to add 1 or 2 cans of 108 octane booster to every tank.