Atlas Intake Manifolds and Heat insulators

Hi
I decided on and received my new Amal Aluminum Premier 930's and have what may be a stupid question but will ask anyway. Should every effort be made to ensure that the inside diameter of the carb, manifold, phenolic heat isolator, and gasket (if used ) all line up so that aren't any dams along the way leaning out the mixture? Reason I ask is that the inside diameter of the isolator that I received from RGM is a good 1.5 mm small than the bore of the carb and manifold. Open it up with a Dremel? Thanks

The insulator is the same size as the standard inlet ports, 28.5 mm (1 1/8”). On the early commandos with the same size ports and 30mm concentric carbs the manifold tapered down from 30 to 28.5mm. Your manifolds should ideally do the same. The heat insulator goes between manifold and cylinder head.
Yes, there should be no step in the intake .

P.S. In my answer I assumed that you are using twin carbs, the ports are standard and it isn’t a single carb with the insulator supposed to be between carb and manifold in which case open out the insulator to suit.

its a 1967 Dunstall Atlas that I’ve been told came with 930 concentrics. That said sounds like stock Atlas manifolds wouldn’t be quite right. It came with finned manifolds with balance tubes and an o ring recess machined into one face. Didn’t realize that manifolds were tappered but interestingly the bores in the head matched the manifolds and the carb. It was just the spacer causing inconsistency. What’s was the purpose if the taper? Speed up the atomized fuel?

The Amal concentrics don’t flow as well as a plain manifold or port of the same size at full throttle so it is usually to have a slightly larger carb than port. A tapered manifold provides a smooth transition from carb to port. A step disrupts the flow along the wall, reducing the flow.

Have you a copy of Cycle World’s road test of the Dunstall Atlas Sept 1967.

hybridracer said:

Have you a copy of Cycle World’s road test of the Dunstall Atlas Sept 1967.

Click to expand...

I don’t. I’ll have look out for it. Thanks for all the info