Club Cobra - Got Advice??

Here's the deal. Several months ago, I decided to lay my car up for a short while and perform several mods that I thought could only improve my car's performance. The planned mods were a new set of ceramic coated Sanderson shorty headers to replace the restrictive stock exhaust manifolds (my car has underbody exhaust), swap in a DN 5 speed trans in place of the 4 spd, and remove and overhaul the rear differential. That's it, I made no other changes to the engine whatsoever except for some new wire looms!

The header swap required a trip to the local muffler shop to have the head pipes refabricated to match up with the new header collector locations on both sides. This resulted in a much more free flowing system as the new headers required no bends in the pipes to flow around the chassis. I also had to have the ejector tubes relocated/rewelded to a point further downstream on the pipes. Again, this mod appeared to be a sure improvement over the old exhaust system.

At this point, I should also mention that this is a Weber carburetored engine that previously ran extremely well with no flat spots, pulled strong through the entire rpm range, and never needed to be tweaked or retuned in the past two years. From the moment I put the car back on the road (3 or 4 weeks ago), it was obvious that the car was substantially down on power throughout the rpm range and has lost the "fuel injected" feel it used to have. Not only that, but it has developed a wicked backfire as well.

I know Webers are extremely sensitive to flow changes, and I believe I need to install larger main jets to compensate for the increased exhaust flow. The smallish 125 mains were adequate with the small tube factory mainifolds, but I should be running 135s or larger.

The fact that I now have a wicked backfire baffles me. Experience tells me that I have an exhaust leak in the system combined with an over-rich mixture. I do have a minor exhaust leak at the passenger side collector flange that I haven't been able to completely eliminate to this point. I've tried copper gaskets, dead soft aluminum gaskets, and hi-temp RTV. The backfire typically happens after I've accelerated through 3 gears and back off on the throttle to drop into 4th gear. I used to be able to lug the engine in any gear and never backfire. And believe me, I've had plenty of exhaust leaks through the years. Also, all indications are that I should actually be running too lean with small main jets installed, not too rich.

Guys, sorry for the long story, but any suggestions would be greatly appreciated.

Mike :MECOOL:

Btw, I've completely checked out the ignition system (MSD 6AL, Mallory Unilite, Blaster II, 8.5 Superconductor wires) and everything checks out okay. I've also tried several different timing settings. Fuel pressure and float levels also appear to be okay.