Your engine needs more advance under part throttle but the mechanical advance can't provide it. You can get by with mech only advance if you have an agressive advance curve and jet the engine lean in cruse mode.
If not, it will load up when cruzing and when you punch the throttle it will smoke a bit and then "clean out".
When crusing along the plugs will run cooler because of the retarded spark. This will form deposits on the plugs. When you punch it the plugs will warm up and burn off the deposits, or most of them.
With todays fuels maybe it's not such a problem anymore.
I had mine a little off and when driving on the freeway for about a hundred miles. I pulled in for gas and my engine almost quit! I revved it up and it belched smoke.
I re did the advance and leaned out the primary jetting and it worked very well. I could cruze all night and when I punched the throttle it accelerated cleanly.
The problem with running a vaccum advance is that the stock ones have too many degrees of advance to work with a re worked mechanical advance.
They tend to ping just off idle. Once the vaccum drops and the vaccum advance moves to retard, it stops pinging.
In this instance you need a vaccum advance can that retards sooner under fairly high vaccum and does not have too many degrees of advance. (stiff spring, limited travel)
The L88 Corvette distributor uses a good one.
I think it's about 12 degrees advance.
Check my web page for suspension info:
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer
[This message has been edited by davidpozzi (edited 07-02-2000).]