on my 68 malibu w/ 327. This past summer i swapped out my edlebrock performer 750CFM for a BG Speed demon 650 Vac Sec w/ electric choke. It runs great, but there is always smoke coming out the back of the tailpipes when it is cold and warm. (Not a choke issue, fully open when warm) I have tried to lean it out on the idle mixture screws, but even at 1/4 turn out from bottom there is still smoke :mad: . It doesn't ploom, but there is a noticeable amt. my initial timing is something like 14 degrees (Suggested by BG tech guys) Could this be related to something else? Could a loose rocker/faulty lifter cause this? as i said, the car runs great, what could this problem be?

Did you check the compression? You might have a cylinder that's low. Also, did you check the spark plugs? Are they the right ones? What is their color?

You might have to put some smaller carb jets. I have a sb400 with a Holley 670...

There are so many variables. You just have to start eliminating some of them.

Good luck,
Theo.

A couple things come to mind,1st is you intial timing. Most mild to fairly stout perf hyd/solid cams need 16-18 deg btdc intial timing,your 14 deg btdc is still on the retarded side so get that squared away but whatch out your total does not get too high. (36-38deg btdc @ 3k rpms or so with mech advance only,not with vacuum advance hooked up)

Then what are the specs on the cam your running,is there enough cam timing/overlap to make your vacuum low enough at idle/low speed criuse that your power valve is prematurly opening up? This will cause an over rich mixture which is really the 1st thing the Tech at Demon carbs should have asked you.

Get a vacumm gauge and hook it up to a full on on idle vacuum source off the intake while the car is at a normal hot idle (auto trany in gear) and see what you have.

If you have let say 10 inches at idle then get a power valve that opens anywhere from apporx 5-6 to 8 iches of vacuum. I would opt for 5 or 6 to keep it a little lean for street use if you could get away with it but holley books do state to go 2 under your idle vacuum . Going only 2 under you idle vacuum for your power valve will get you into the power circut faily quick off idle which i think is good for optimal power but its' not best for all street setups and could waste fuel,etc. You can find out what's best for your setup by good old tuning/dialing in of your motor particular setup.

Let us know what you decide to do and how you make out.

Good luck.

Scott