What did I do today? Realised that I am definitely not a mechanic, that's what.
3 months ago I replaced my 2bbl 500 Holley and manifold with a 4bbl manifold and new 575 Holley Street Avenger. Noticed an immediate power increase at higher revs, but low revs performance was rather shoddy.
The first thing I had noticed when I put the new carb on was the timing mark was way out on the balancer. I run a Pertronix Ignitor so really the timing should have been exactly the same, but here it was running at somewhere over 20 BTC . And when I dialled it back to the usual 6 - 10 degrees the car had no pick-up at all. So I set the timing about back to where it was originally and in my head I had talked myself into believing that the outer ring of the harmonic balancer had slipped on the inner and that I needed a new balancer.
Anyway, today I thought I'd just give the timing a couple of degrees shift to see if it made a difference, and this time I remembered to disconnect the vacuum advance hose first (which I foolishly hadn't done the first time), and straight away the timing at idle dropped straight down to 6 on the balancer. First thoughts were that there was something wrong with the vacuum advance unit for it to be so active at idle. Did a search on the net and stumbled across by accident a site where some guy had had the same problem as me. Turned out that he had the vacuum hose connected to the manifold rather than the vacuum fitting at the carburettor venturi, and by golly when I went out to check my set up I had done exactly the same thing!
Moved the hose to the right spot, and now pretty even power across the whole range! Feel like a bit of dunce, especially after leaving it that way for three whole months (luckily there was no evidence of pinging) - I really should pay someone who knows what they are doing for work like this......