Demon carb Woes

Hi all .... I wonder if anyone will be able to help me. I have had a 650 d/p Speed Demon on my mustang for the past 3 years ... recently the gas mileage went to double what is was (using twice as much gas !!) and the plugs were fouling up. I took it off and stuck a Holley on which has been fine for the last 6 mths. or so. Today I stripped the carb, thoroghly cleaned it, rejetted it to 68 primary and 76 secondary, put a new 4.5 power valve and reassembled with new gaskets .... the only thing now is the car started but ran like cr@p ... lots of smoke out the back and sounding like it was on 6 or 7 cylinders ... the back float level was a bit high so I adjusted that and also raised the front as that looked a little low .... no better. When I looked at the gas level in the front when the engine was stopped, I could see it dropping and hear gas running/dripping down into the intake manifold .... I thought it must be the power valve so I replaced it with a new 8.5 ... everything else looked OK in there .. put it back together .... exactly the same .... anyone any ideas ??

Thanks for the reply ..... The power valve I first put in was a brand new 4.5 with a new gasket ... looked OK, the gasket too, and the second was a brand new 8.5 with another new gasket. I can't see what I have done wrong *so wrong that the car won't run* I wouldn't mind if I just had to spend time jetting, tuning etc. but this obviously isn't the case

SORTED !!

Hi all .... I have solved my problem. When I rebuilt the carb I used new, but leftover gaskets from various Holley rebuilds. I used a correct gasket on the secondaries, but the gasket on the primaries was as per the attached photo .... original red and the one I used blue ... as you can see the blue one has a cutout at the top of the D and this must have been what was not sealing .... running fairly well now just needs some time setting up idle etc.
Just a quick point on the roadtest the revs didn't drop much between gear changes just wondered if it was the 8.5 power valve which is letting in too much gas (engine makes 10 at idle) I will change to the 4.5 tomorrow and try that. The linkage and return springs are all OK ... the butterflies snap back quickly but the revs drop slowly.
Cheers from sunny Spain
I shall be back on this forum a bit more often in the future as I have finished bulding my AC Cobra 427 replica and my next task is a full rebuild of my 65 f/b including rewiring (40+ yr. old brittle wires ) fitting rack and pinion, retrofit air con etc. etc.