[SuperHart]

There is no vacuum to mechanical conversion kit for Holley carbs. Mechanical secondary carbs are usually designed for racing applications while vacuum secondary models are considered street carbs especially for heavier vehicles. Because the Cobra is a lighter vehicle you could get away with a mechanical carb but with a price, greater fuel consumption because of the secondary squirter; every time you mash the throttle there goes another 30 cc of fuel. Final thought - although many of us think we can get a lot more out of the engine with a mechanical secondary carb, think about why nearly all factory high performance engines came with vacuum secondary carbs. Could it be that they know something we don't? Just my opinion, but if you already have a vacuum carb, don't waste your money going for a mechanical carb. The difference, if any, won't be worth the expense. Tune the carb you now have. It will give you some fun and perform better in the long run. You don't say exactly which carb you have but if you chose a carb based on your engine displacement and performance level it should be quite close right out of the box. You also don't say whether you richened or leaned the jets and by how many steps when you "jetted to your elevation". Holley states that if you have to change the jets more than 8 numbers you have the wrong size carb. The Holley web site contains a good article outlining how to tune a Holley carb for individual applications. You might like to download it and read it over before making any significant changes from stock. Finally, always write down all changes you make so as not to forget should you need to backtrack. Have fun and don't be in a rush.

How To Choose A Carb
Another How To Choose A Carb
How To Tune Your Holley Carb