My 79' Corvette is having a terrible leaving the line under heavy load.  When i attempt to doing even a burnour the car kind of bogs and is slow to leave the line.  The more throttle that i give it the worse that it leaves.  I currently have a 350 under the hood.  Has a comp cams hydraulic lifter cam with 224 230 duration, the lift is aroound .509 intake, and .512 exh, with 1.6 ratio crane energizer roller rockers.  I have Dart Iron Eagle 180cc heads with 64cc chambers, resulting in 10.14:1 compression ratio.  They are 2.02 manley valves with 1.6 exhaust.  The car has semi full tube headers leading to true dual exahuast 2.25" dia.  The intake is a edelbrock air gap, and on top is a speed demon 650 vacume secondaries.  I have a holley 110 fuel pump.  For ignition i have a pertronix HEI distributor, but no ignition box.  I have the timing set at about 34 degrees total timing.  The rear end has 3.55's, and the rear tires are BFG 275/60/15's.  I thought that this combo would have at least enough guts to spin the tires, but i just can't seem to get anywhere with this setup.  Any suggestions would be greatly appreciated.  

    Without further info, it sounds to me like it is not advancing the timing... I am assuming the HEI unit you have does have the vacuum advance can on the side... check the advance unit with a hand pump and see if it moves the rod that rotates the plate, also see if it holds vacuum, as it may have a pinhole in it. You might want to check your advance weights and make sure they are not stuck and move freely. lastly, make sure you have the advance unit hooked to the right vacuum source. Sounds like you have a sweet combo there, and it should have plenty of power off the line. Hope this helps!

Mike

Hello, My name is Bill, I'm a professional auto tech and racing engine builder in Lansing, Mi. First off, Check the secondaries, they may be opening too quickly, also check base timing, then mech. advance. Also you didn't say if its an auto. The stall speed may be too low for the cam, and if the secondaries open too quickly, its hard for it to recover. Check the basics first.  

I did check the vac advance canister and it holds vacume well.  The mechanical advance is also moving freely.  I also checked to be sure on the vac, and it is ported.  What else would you recomend checking?  

Bill, I am new with vac secondary carbs, and don't know the first thing about adjusting the secondaries.  And how do i know if they are opening too early?  The base timing is about 12 degrees and mechanical weights are moving freely.  The car is in fact an automatic, sorry, i forgot to put that in the original post.  Thank you for your help, any suggestions would be greatly appreciated.

JC

What trans and stall speed?

Sorry, forgot to put that in the first post, the tranny is a TH350, stock, and matching conveter.  Nothing special.  Just what came witht the car, about 65K ago.

JC

Depending on the exact carb, you either have a quick change setup or traditional. If it's a quick change, there will be two screws on the top cover, remove them and change to slightly stiffer spring. I'm really wondering about the converter. That cam should be with a higher stall converter, with lower gears (Higher numerically). Truthfully, until those two things are addressed, this is going to be the issue. Hope I can help.  

It's good to listen to the other carb and ignition advice here but don't expect your 'Vette to launch off the line with a matching (I am assuming standard) converter.

I expect that your symptoms are close to typical because I doubt your engine is being allowed to operate at an rpm where it is making good torque off the line.

With the transmission in "drive" and your foot hard on the brake, what rpm can your engine rev to without moving?

OK, the carb is a Speed Demon 650 vac, it has a single cap to get to the spring.  I can't quite try this out yet cause it just snowed here but i'll see what it does.  I held the brake and revved the engine in park and found that it would go to about 1800 or so.  Yes it is a standard converter, haven't done much to the driveline quite yet.  Thanks agan guys

JC

You've got to do it in drive! Is that what you meant? 1800rpm sounds about right for a standard converter and this will be a big reason as to why you're engine is "bogging" on take off. It can't turn at a speed where it is making enough power and therefore losing rpm.

Trying different secondary springs will help but you'll never fix this without a converter more suited to your engine's max torque rpm.

I'm guessing your engine would be more happier with a 2700-3000rpm stall speed with that cam. Your 3.55s are good. Contact a converter manufacturer with your engine, trans rear end and car specs and see what they recomend.

Ditto!

Thats what I said the other day.

You can mask the bog, but if the engines not getting to its powerband, your not going anywhere. You need a sloppyer converter.

M fault, I don't know what i was thinking when i wrote park, i did in fact do it in drive. 

What would be a good company to look at for a converter?  Do you think that this is the only problem? Thanks again

JC

I'm down under so what converter companies to suggest.. I don't know.

Once you change your converter, there will still be distributor advance changes/tuning and carb adjustments/tuning to do so don't discount what has been suggested to you already.

When the trans is out, it would be a good idea to have it checked out just to make sure the pump/clutches/bands/rear sprag is all in good shape. A shift kit for firmer gear shifts will help the trans last longer and slip less too.

Yup, he's right, you'll still have tuning changes to make after the converter issue is handled, but yeah I think this is the root of your problems.