Long time since I have been on here. Still working on the 75. I am having some issues I hope you all can help me with. First off here are the specs.

1975 C3
turbo 400 trans
stock gears in rear

1974 400 small block
Bored .060
Block decked .010
Scat Crank
6" rods
KB Flat top Hyperutectic Pistons
Speed Pro Cromoly Rings.
1/2 fill Blok hard
World products S/R torquer heads Milled .020 Swirl Polished
Manley valves and springs with roller rockers

cam specs:
Brand Lunati
Manufacturer's Part Number 60103LK
Part Type Camshaft and Lifter Kits
Product Line Lunati Voodoo Cam and Lifter Kits
Summit Racing Part Number LUN-60103LK
Cam Style Hydraulic flat tappet
Basic Operating RPM Range 1,800-6,200
Intake Duration at 050 inch Lift 227
Exhaust Duration at 050 inch Lift 233
Duration at 050 inch Lift 227 int./233 exh.
Advertised Intake Duration 268
Advertised Exhaust Duration 276
Advertised Duration 268 int./276 exh.
Intake Valve Lift with Factory Rocker Arm Ratio 0.489 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.504 in.
Valve Lift with Factory Rocker Arm Ratio 0.489 int./0.504 exh. lift
Lobe Separation (degrees) 110

Barry Grant speed Demon 750 vacuume secondary carb
Weiand double plane intake
Hooker headers
Full length 2" exhaust with Flowmaster Delta 40 mufflers
Petronix HEI flame thrower dizzy

Ok sorry so long already, just wanted you to have all the info. So here is the problem. the Machinist that did the work for me said the engine should push 500+ hp at the ground. I thought it was a bit high. I originally had a eldebrock intake, carb 600cfm. and stock dizzy and exhaust manifolds. Engine ran OK but could tell it was weak. I just recently added the headers, new intake, and carb, and converted my distributor to the petronix with the weakest springs. Now I have 300 miles on the engine. I have to have the air adjustment screws almost all the way in to make it run. I tried to set the initial timing with a vacuum gauge (pulling 17). Now when I load the engine it has a ton of power, but as soon as I back down it dies. power brake the car, and if I back off the throttle to fast it dies. what am I doing wrong here? I have the factory fuel pump, do I need bigger. Do I need to find a tuning artist, and have him go through it all? I am kind of upset at all of this figuring I have about 6k in the motor and I did all the assembly. I got my buns handed to me by a 2000 firebird at the track in a heads up. Ask me any question and I will answer it. Help me make it run. thank you all.

 

What are your timing settings, particularly total? Should be 35/36 without vacuum advance.

 

Is there a dyno tuner around you? Have that carb set up on the dyno.

But first, find out why you need to back out those idle settings to make it run. Do you have the right carb gasket on it? Leaking vacuum anywhere else?

 

im confused...do you mean you have to turn the mix. screws in to make it run or make it idle??.......what convertor??

 

sorry been so long. been terrible busy. this is going to seem like a dumb question, but how do you chect total timing. I am going out today to buy a new timing light. I checked everything this week I could without one. it ran pretty well with just the headers hooked up, but installing pipes again I get some detonation. I set the timing and fuel with a vacuum guage, and had -18 bar vacuum. I did notice that at an idle my clear fuel filter has no fuel in it. after a few seconds of half throttle it fills back up. is that a sign that the pump is on its way out. Still using the stock fuel pump. Also I have the Petronix Flame Thrower Distriutor Convesion kit. I installed the lightest springs, on the recommendation of a racing buddy. I will read more today on total timing and try and set at 34-36. Also for the one that ask, I have to have all 4 adustment screws on the outside of the carb, almost all the way IN to make it run. 1 (left rear) can be out 1/16 of a turn. rest are all the way in, just not tight. thanks for all the help

 

Check out this site, it was very helpful to me.

http://www.lbfun.com/Corvette/Tech/vettetech.html

 

It sounds like fuel starvation. If you have to turn the idle mixture screws all the way in you are running it as rich as possible.

It's possible that the factory fuel pump can't keep up with the demand for fuel from the carburetor for whatever reason (pump going bad, not enough flow, whatever).

cc

 

Turning the screws almost all the way in is leaning out the idle mixture, therefore when you get off the throttle the motor dies. You have practically shut off the fuel to the motor at idle.
Make sure the butterflies are closed and with the motor idling there is no fuel coming from the boosters. You should be at least 1 full turn from seated.

 

Ok Let me start off first by saying thank you to all of you. Secondly I would like to state that I think I must be mentally challenged. I have been working on this engine for 18 months. Never has ran like I wanted. This week I had my brother come over, and help overhaul the Speed Demon. When we pulled it off (for the 6th time since I got it) in the back of my head, I thought something looked odd about the base gasket. he overhauled and found they had put the big jets in the back and small in the front, and the floats were sticking. we cleaned out and redid the jets. when I when to install the base gasket, I found the damn problem. And here it is........After building cars for 20 years, I finally got bit. I installed a spread bore intake....and a square bore carb.....no adaptor!!!! I cant think of all the crap I have gone through trying to figure this out. I ran to the store this morning to get an adaptor to see if it helps. if it fixes it, then off to jegs to get a new intake. I cant believe I am this stupid......