Mixing & Matching – Retro Fitting A CV Carb On A Shovel Head – Part One

Published In The October 2014 Issue Of Cycle Source

Article By: Will Ramsey – Faith Forgotten Choppers – www.faithforgotten.com


Will Tech

I’ve been asked many times about the CV carburetor set up on the last couple bikes I built. So, I’ve decided to write a two part tech series that will illustrate how I mounted the carburetors on both Dead at 19 and Tight & Shallow. This first article will address modifying the manifold to accept a CV carb, while the second article (next month) will illustrate the fabrication and design used to securely mount the carb to a shovelhead. I’m pretty much sold on retro fitting CV carburetors on shovel and pan motors… I like the crisp midrange response, the adjustability, the ease of kick starting, and I just love the look of them. Now these carbs were never intended to be mounted to a shovelhead motor; therefore, we need to fabricate an adaptor to mount them to an existing manifold. There i is an adaptor you can buy that will bolt to an S&S manifold, but I find that using that set up places the carb too far from the motor. And since it is a “bolt on” set up it just looks like an afterthought to me. This tech will show you how to machine and weld an aluminum adaptor for a CV carb that looks as clean as it performs.

Photo 01

It all begins with a stock S&S manifold for a shovelhead and a short piece of 2.5” round 6061 aluminum.

Photo 02

After facing the aluminum in the lathe, the center is roughed out with a large drill bit. The final bore will be 1.600” so go ahead and use the big drills.

Photo 03

Whatever stock remains, after drilling, will be removed with a boring bar. Make sure you pay attention to feeds and speeds because surface finish is important. You want a good smooth airflow through the manifold.

Photo 04

The inner step is machined .500” deep to allow the CV carb to index against it.

Photo 05

The outside of the adapter is machined to match the outer diameter of the S&S manifold. In this case 2.375”.

Photo 06

The outer step is machined to allow the isolator boot to slip on tight.

Photo 07

A parting tool is used to precisely cut the length of the adapter. This could be done in a band saw as well.

Photo 08

The new adapter is returned to the lathe and held in the opposite direction so that a clean 30 degree weld chamfer can be cut on the outside edge.

Photo 09

On the inside diameter a 15 degree taper is cut to avoid a sharp step between the adapter and the existing manifold… It is most important that the largest diameter of the taper match the inner bore of the manifold.

Photo 10

Using a vertical band saw the bolthole flanges are cut off the S&S manifold. Take care not to damage the machined surfaces that mate to the heads.

Photo 11

A hand held grinder or a belt sander can be used to grind a weld chamfer onto the manifold.

Photo 12

The two pieces must be cleaned with acetone before being fit together. The fit up is critical and should be done to assure the inside diameters line up perfectly. After tacking the two pieces together the final weld is made using an inverter TIG machine and purified argon to ensure a dense porous free weld.

Photo 13

The weld seam is turned back down and polished in the lathe.

Photo 14

After a final buffing the new manifold is ready to bolt on. Tune in next time for the solution to the second dilemma: How to support a CV carb when mounted to a shovelhead…

Leave a Reply

Your email address will not be published. Required fields are marked *