'72 LT-1 Holley mod; tuning suggestions?

[Larry M.]

Today I changed the jets in the Holley carburetor on our '72 LT-1, from the 68 primary, 73 secondary used on '72 LT-1's only; to the 70 primary, 76 secondary used on '70 and '71 LT-1's. Post-change, and prior to any other adjustments, the car now has lower idle speed, backfires through the carb at 25-30 mph when accelerating, and the engine begins to stumble under load at approximately 3500-4000 rpm. I'm obviously on the right track to great performance gains!
The engine has a hydraulic L82 cam, a NAPA/Echlin VC-1810 vacuum advance, Pertronix Ignitor module, timing set at 10 degrees advanced, smog equipment removed. Prior to the jet change, the engine ran fine (and yes, I'm seriously considering kicking myself).
Before I begin going through the usual tuning/adjusting/troubleshooting procedures, do any of you have any suggestions for a starting point to restore driveability (short of a jet change reversal).
Thanks in advance.
Larry, # 10731

[Mark #28455]

Put it back to how it was

Before you did the change, was there any evidence the engine was running too lean? Were the spark plugs sooty or any other symptoms? If not, you should put it back.

Does the L82 cam idle with a higher vacuum reading than the original LT1? If so, the leaner jetting would have been better as the engine would have provided the carb with a stronger signal than original.

What is your total timing (with the vacuum advance temporarily disconnected) and at what RPM is it all in?

Mark

[Mike McKown]

Re: '72 LT-1 Holley mod; tuning suggestions?

I think you have a vacuum leak. The jet change by itself wouldn't have caused your problem. There's nothing wrong with your jet selection.

[Joe L.]

Re: '72 LT-1 Holley mod; tuning suggestions?

Mike and Larry-----

I totally agree; there has been some other problem generated during the jet change, be it a vacuum leak or something else. There's no way that a modest change in jetting that has been described is going to create a problem of the nature described.

[Larry M.]

Re: '72 LT-1 Holley mod; tuning suggestions?

Thanks gents. I guess I have a bit of 'duplicate effort' work ahead of me.

[Mike McKown]

You may have used the wrong

metering block/main body gaskets. There are a number of different configurations. This may be your trouble. Don't know. I've never used the wrong ones so I don't know what the symptoms might be but you'd have a problem of some kind.

[Larry M.]

Re: You may have used the wrong

Mike, thanks for the suggestion, but I only removed the float bowls from the metering blocks, not the blocks from the body, so I only replaced the float bowl gaskets. I appreciate your thoughts!