I just acquired a 1971 LT-1 and have been chasing some carb issues. I think I've resolved an idle-bog problem by replacing the primary needle setup and setting the floats, but I'm getting some hesitation at WOT when running through the gears. I replaced the diaphragm that opens the secondary butterflies and have been playing with different springs to see how the car reacts.

Following some instructions from a number of posts that I've read hear, I started by using the lightest (white) spring and taking a spin to see how the vehicle reacts. The white spring resulted in some backfiring (?) through the carb that was audible in the cabin, and a definite stumble as soon as a got on the gas. The smaller yellow spring eliminated the backfire, but the stumbling remained as soon as I got on the gas. The larger yellow spring seemed to be better, but would bog down a bit as soon as I got on the gas -- it would hesitate for a second or two, then pull like a freight train.

Should I continue to work through the springs progressively until the problem is eliminated, or settle on a particular spring that everyone else uses and look for other parameters to adjust? I inspected the plugs after a few pulls and they are definitely dark, which (I think) means that I'm running too rich.

One other note -- the secondary diaphragm did NOT have a check ball, and I'm not clear if I need to have one or not. I understand that this feature helps to attenuate the opening of the secondaries, so could be an issue. Not even sure where to find a replacement, either.

I'm still learning as I go here, so any advice to offer is greatly appreciated. I've already figured a bunch of stuff out by searching the forum.

 

A lot of people don't understand the function of the ball, so they remove it, causing problems as you describe. Some carbs use the ball, and others do not. To see if yours should have a ball, look down into the hole where the ball would normally be located. If the seating surface for the ball has a little slot in it (it's real small, so look closely), there should be a ball. If there is no slot, but rather a small orifice hole with no ball seating surface and no little slot in that surface, there should not be a ball.

The ball is contained in all Holley rebuild kits, and it's just barely smaller than the hole it goes into.

The tall yellow spring should work well if the ball is in the hole. Backfiring through the carb is an indication of a lean condition caused by excessively fast opening of the secondaries (which have no accel pump shot to prevent a lean-out upon fast opening). You could go to the purple spring, but that should not be needed in most cases.

Lars

 

Thanks -- I checked earlier when I was swapping out a spring and I didn't see any sort of slot/groove where the ball would seat. I am going to install a quick-change kit this week and will verify for sure when I crack it open again.

And what if my setup isn't supposed to have a ball? Should I try the purple spring, or swap out the secondary vacuum kit for one that has the ball?