I finally got around to swapping intakes on my vette, I had a loaner carb on there (Thanks again Lars) so as long as I'm changing the carb, might as well do the intake (Edelbrock proformer). I went to Advance Auto to pick up all the parts, lots they don't have, next trip to Auto Zone, again not all parts in stock. Finally a trip tp A&A what they don't have I can pick up at the warehouse (5 minures away) so I hope I will finish up tonight. I also got an adjustable vac. advance canister as the one on my distb. was bent. This came with new springs to change the rate of vac. advance. Has anybody used these, or have any feed back. Why would hevey or kighter springs be better? How do I detirmine the best combo. seems like lots of trial and error.

Thanks

 

It's all trial and error. Mainly trial to see what you like.

 

Swap done car runs poorly. It now has a stumble under light load, (this existed prior to carb rebuild, not present with laoner carb) I even had a mechanic "set the carb" after I was unable to get it to run smooth. Any suggestions, the carb was rebuilt by a reputable shop, it runs rough under part throttle and light load, fine at WOT. What could be the cause?

Thanks
Leo

 

In addition to your question, send the post thread address to Lars as an email and and ask him to read it and comment. It sounds like a fuel flow deliver problem with the carb. Filter etc.

Did you make th echanges in the distributor with the vacuum advance and springs? If so, get yourself an advance timing light and draw out what th eadvance curve looks like. Plug up the vacuum advance and see what RPM's it starts coming in at and at what RPM's it is fully advanced at.

 

Leo -
You've changed 2 things (distributor setup and intake/carb), so it's tough to tell which one is causing your problems. As I tell people, most "carb" problems I see are actually ignition/distributor related, so let's start there...

First, I have never seen one of the adjustable vac advance units that's worth a plug nickel (here comes the hate mail....!). A vacuum advance unit has 2 parameters: The LENGTH of its advance (how many degrees it actually pulls in at its maximum limit), and WHEN it pulls this in (how much manifold vacuum it takes to START the pull and how much it takes to achieve the FULL pull). If you have a properly curve distributor (are you running points or HEI...?), you should be getting a maximum mechanical advance (vacuum disconnected) of 36 degrees, all in by about 2500 rpm-or-so. You need to verify that this is happening using an adjustable timing light. The correct operation of this advance curve is key and critical to all subsequent tuning. Once you have verified that you are, in fact, getting a good, smooth, quick advance curve that pegs out at 36, you now need to isnstall a vacuum advance unit that will supplement this curve at light throttle to produce a combined total advance of not more than 52 degrees. That means, you need a vac advance unit that pulls in 16 degrees of crankshaft (8 distributor degrees) advance. You want this to occur at relatively light throttle so you don't get detonation with your aggressive mechanical curve. So throw that adjustable one in the trash or hang it ono your pegboard and pick up a fixed unit that has these parameters. If you have a point-style distributor, you want to buy part number VC680 from your local NAPA store. If you have HEI, you want part number VC1838. This will produce a perfect, known curve that will give you the characteristics you want with none of the hassle.

Once this is all verified and set up, it's time to move on to the carb. If you're still having trouble, let me know what type of manifold you installed, and give me the carb number off your carb. We can compare a few of the numbers and the jetting to the Loaner Carb that was running good for you, and get your original carb set up and running just as good. I can give you several things to check before you tear the top of the carb, so keep me posted on your results with the timing.