Holley 750 Questions

Hi guys I recently bought a 79 Trans Am from a friend the engine is a Pontiac 400 bored .030 over has a lumpy cam and 6X heads( he said they have been bored out). My problem is there is a Holley 750 vac. sec. and idling it fumes and shows black smoke. I tried to jet it down from #72 jets to #70 helped a little. My question is should I keep jetting it down or get a Holley 650? Which carb will be better in the long run? Does any one have a Holley 750 carb on there 400 and which jets do you use? Thank you.
           Dave  

I have a holley 750cfm on my regal with a pontiac 350 and it perform's great.
Have you set the floats and if so and at what level.  
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Yes, the floats were adjusted to when the car is running and I take out the sight plug the fuel just barely comes out. (fuel doesn't come out until I rock the car).  What jets do you have in yours? What type of 750cfm is yours (vac or manual)?  
Thanks for the reply.  

The factory size #72.
Right now I have a vacuum secondary on it.
How many turns is the idle mixture adjustment screws turned out.  
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Hi thanks again. I'm asking for help cause I don't know for sure If the problem is the jets are too big or maybe the power valve is shot the carb is duel feed inlet newer Holley which should have the power valve protection in it(check valve). I don't know what the jets should be I don't know if #70 jet is still too big. I tried taking the power valve out and when I suck on it it does close. I watched the video on the Holley website and it says if you turn the air fuel mixture screws in the engine should shut off if it does the power valve is ok mine does not shut off so do you think my problem is indeed the power valve. Any help is greatly appreciated thanks.
           Dave  

That is very possible.
How many turns is the idle mixture adjustment screws turned out.
Have you pulled the site plugs out and recheck the float level.
When you turn the mixture screws in does the black smoke go away.  
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I started out at 1 and a half turns but I have turned in almost the whole way but back out maybe a eight turn. The engine seems to run a little better but did not shut off when screwed in the whole way like they said it should. I pulled out the site plug out and the fuel is right below the hole until I shake the car. Do you know what primary jets you have in yours?  

I have #72 jets in mine.
Ya you probably have a bad power valve has it back fired through the carb.  
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What rpm does it idle at? It's possible that it is idling high enough that it is starting to pull fuel from the main circuits instead of the idle circuits. When this happens the idle speed screw is usually turned in enough that the primary throttle plates are open far enough to uncover the transfer circuit. Often times drilling a 1/8" hole in each primary plate will allow more air in with less throttle opening which in turn allows the idle screws to fully control the mixture at idle speed. You might also need a power valve with a lower vacuum number.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

Ok thanks for the help. I guess I'll try a new power valve. I wasn't sure if #70 or #72 jets were too big but if that is what you have I'll try a new power valve first. Ever since I got the car it smoked black smoke so maybe the guy that had it before me had a bad back fire.  

The idle is set at 800 RPM's. The vaccuum at idle is 14" according to Holley's website you take half the vaccuum to decide the correct power valve mine is a #65 so It should be close. Thanks for the info.  

No problem let me know if that works.  
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Hi guys well I bought a new power valve and same thing. I called Holley and the Technicion told me maybe the gasket was not centered on the power valve I don't know how it could'nt be but I thought I would take it back apart to see. Everything was good so I fired it up and same thing. The technicion also said it could be a warped base plate. I found another carb but not sure how good they are Rochester 7028280 4bbl Quadrajet Carburetor he said it was still in the box for $50.00 any suggestions or does anyone know any thing about this one please let me know thanks.
             Dave  

Have you check the spark plugs to see if they are fouled.
Also what type fuel pump do you have on the engine stock or aftermarket and do you have a fuel pressure gauge.  
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Hi yes the plugs were bad I put new Ac in. The fuel pump is on the engine/(I think it is stock). No I don't have a fuel pressure gauge but the carb worked good when the other guy had it.