header tube size vs carb jetting

[68gts340]

trying to help out a neighbor kid, here s the problem ; just removed a set of hooker comps, 1 5/8 tube, put in hooker super comps, 1 3/4 tubes, ....now it is running rich, today i went through the primary side , new power valve, checked all the passages , and found nothing. it s runing a 3310, 750 cfm vac secondarys, the car has been sitting about three years, last time i helped him jet the carb and the plugs were nice tan , a little on the dark side ,but not bad, the motor was fresh then so we decided to run it like that for breakin,,,,, now the plugs are black , but dry.........my question is this ..can the tube size affect jetting that much? or should i dig into the carb some more ? i tried the idle air adj and it will stall out the motor closing them, but it runs best at about 3/4 turns open.. i know it needs jetting down, just wonder if i am missing something ? any thoughts on this?
part two;.. next step he wants to try an 850 double pumper, i need a starting point for the jetting. it is a used carb and the jetting isnt as new, also this has a rear power valve, do you recomend square jetting it? i dont have a lot of time with this car so any help will sure be appreciated... thanks
one last thought... it a good chance it has last years fuel in it ..

 

[Mrmopartech]

68 GTS,It would start the post off even better if we all knew what motor and size and ignition and all the in,s and out,s of the car we are dealing with.Because if its a truimph Spitefire,the pipes are way to big,just messin with ya,Mrmopartech

If its a small block,you should have stayed with the 1 5/8 tubes as it will kill the bottom end and its driveablity.

 

[rumblefish360]

I'd call it reversion. The heads are sucking the exhaust back in due to the velocity drop of the larger tube. Also, at the same time, the large tube could be empting the chamber so much, the next charge coming in is rich. The running rich problem is likely due to the same thing. Like a 2 for one deal.
Also, tight centerline cams, 110 or less, will idle rich because of the overlap. Once the engine spins a little, it'll start to work as intended.
Best thing to have is an air/fuel ratio gauge. You can get a cheap one, or a basic Edelbrock unit that has light to indicate rich or leean.
Once off idle, the lights level out.
(Been there done that)

On large carbs, a square jetting is fine to start. It's the primary hassle ratio jetting first, then fine tune with the secondarys.

 

[68gts340]

thanks guys, its a 406 chevy, msd , cam is about 256 / 245 ? duration @.050 about 550 lift i really cant remember exactly, 12.5 pistons, actual comp is about 11.5... 4000 stall, victor jr. bracket racer only... i just didnt want to waste time looking for a problem that isnt there ....thanks again.....