How can I tell if carb running too lean?  As far as I know the jets are original but not sure.  Of course AIR pump gone but all else original to my knowledge.  Mixture screws on carb set as per emissions sticker.  Looking for better performance here.

Thanks
Neil

Check yer sparkle plugs. If they (insulator tips) are a bright, white color, with no/very little tan color, you're probably runnin' a bit lean. Are you using a Holley, or Quadrapuke carb? It's easy enough to jet the Holley up a size or two, but the Quad takes metering rod changes to do the same thing.

Thanks John,

I have a holley and the plugs do have white tips - no or very little tan.  What do I ask for, bigger or smaller jets?  How do I know what size I have now? Should I go up or down one or more sizes?  Is there a kit with several different sizes?  Can this be done with the carb on the car?  What do the mixture screws do on the side of the carb?  Can I use them to enrichen the mixture?

Thanks
Neil

 

Questions,Questions,Questions,!!!
Go for a larger size jet to 'fatten up " the fuel. The Holley fuel bowl(s) need to be removed to change the jets. Take the fuel inlet line loose first, and be sure to put some rags or lots of paper towels down to catch the fuel. Then, take one of the bottom screws out of the end of the bowl, and use a soda can with the top cut out to catch the fuel from the bowl. Then remove the remaining three screws, and gently pull the bowl off the carb. (You want to try and leave the metering plate stuck to the carb, but it's not always possible) Then, take a wide screwdriver and unscrew the two jets sticking out at the bottom of the metering plate. Look carefully at them, and you will see a number on the side. It will be a '68', '72', '55', etc. This is the jet size. You want to go up one or two sizes. So, if you have '70' jets, get two '71' jets, and two '72' jets. Then put in the smaller of the the set, and see how it works. You can go up (larger) if you need to, but it's best to take it a little at a time, and drive the car to see how the changes work.
Now, this info has been for the single inlet Holleys (vac. secondaries). If by chance you have a double-pumper, the same procedure works on the rear of the carb, but is rarely needed for street cars, as most double-pumps are over jeted on the rears, anyway.

You have to know what size jets you have first, in order to know what to get. Holley has a kit that has a selection of jets in it you can get from Jegs, Summit, etc. I would also suggest you get some extra bowl and plate gaskets, because there'll be at least one of them that tears. You need the # off of the choke horn to ID the carb, to get the right stuff.(3310, etc.) If you need more info, AfterShark works at Holley, and he might have some specs or other tech. that would be of help.
The mixture screws really only adjust the air/fuel ratio at idle. They don't have much effect above that.

What heat range of plug do you have, how do you rate the overall performance of the car,GM dropped the compression from 11:1 to 9:1 in 71 to meet new emissions standards, and unleaded gas, they also built them to run a little lean, the 70 LT-1 came with the same holley as did the L-88, 71-72 dropped down to a 750 holley, they also installed a solenoid on the right side of the engine bolted to the intake manifold next to the distributor to limit vac. advance to high gear only, its controled by a TCS switch on the cover of the four speed, if the solenoid is still there get rid of it, run the vac.adv. line from the dist. to the ported outlet on the holley, your engine will respond better a little on the lean side better than the fat side, I suggest that you leave the primaries alone and jet the secondaries, if you have the stock jetting go two size's out back, you also might get a little better performance by having the yellow spring in the vacuum seconday dashpot, use a vac. gauage to set your idle mixture go from side to side several times to attain the highest vac. reading, hopefully you still have the stock Holley, they are a good street carb. anips38534.9193981482

Thanks for the help guys.  Carb is a Holley "4801" on the air horn and "6335" on the forward metering plate.  TCS system long gone and vac adv line run to port under the forward fuel bowl.  Plugs are AC R44TS.  If I try jetting up 2 sizes is there anything else that needs to be adjusted along with the change (like different plugs)?

Thanks again
Neil

Sounds like you have the stock carb, the correct # should be R4801A,the plugs are the correct ones, when I make a change I only do one at a time, as far as the rear jets there isn't any thing else to adjust, my plugs look like yours,a little on the lean side, but the car runs good, I tryed jetting, didn't seem to help, went back to stock up front two over out back, at the time I had a slite high speed miss, thought it might be lean the jetting didn't help the miss, it went away when I replaced the point setup with a performance HEI, you might get a few more horses by shaving back the ground electrode on the plugs to the middle of the center electrode, kinda like a poor mans Splitfire plug , by grinding the ground strap back it unshrouds the spark a little, I understand it might be good for maybe 5HP.