AMAL 376 monobloc Carburettor refurb

[David Smith]

On attempting to assemble a pile of monobloc parts into a 1 inch body for the KH I found that the various condition slides I had were too tight a fit, due I expect to distortion of the body due to overtightening, A one and five sixteenth second hand machine reamer from ebay held in a vice with the jet block less body rotated and fed onto it with plenty of oil removed the high spots and accumulated carbon/clag entirely successfully. The best condition slides are now a free moving fit with no movement or play being apparent

[simon.holyfield]

Sounds a lot easier than mounting it in a lathe!

[David Smith]

Simon
Better finish too!

Cheers


Dave

[nevhunter]

. Old diemetal bodied carbs are not worth investing a lot of time in generally so evaluate them carefully before you start spending. Rudge BSA and Norton used brass till the mid 30's in some models. New Jet blocks are in short supply and look as though they aren't easily made . I only oversize the part where the slide runs and it MUST be concentric with the part not machined. IF the body is brass you can soft solder a new thin sleeve in so it can be an "easy" fit. Nev

[nevhunter]

I SEE this relates to the monobloc NOW. . The concentric was always better than it. Sorry if there is confusion. Nev

[cmfalco]

A one and five sixteenth second hand machine reamer... The best condition slides are now a free moving fit with no movement or play being apparent

It's too late for this carburetor, but removing metal from a carburetor to deal with distortion isn't the best idea. I used Photoshop to exaggerate the distortion that is very typically found on old carburetors, often caused by overtightening the nuts against a relatively-soft phenolic spacer.

Since the carburetor is distorted, all the metal that originally was present is still there, just in a different place than it needs to be, and you need all that metal to restore the carburetor with the necessary 0.004" clearance with the slide.

Unfortunately, if you ream the ID to be round, or grind the mounting flange flat, the now-missing metal makes it impossible to restore the proper slide clearance. An oval slide might feel like there is minimal play with the new, larger bore, but the uneven clearance around it will affect the mixture in unwanted ways.

Instead, a tool like the following forces the displaced metal back to where it belongs.

[David Smith]

Well the amount of distortion and metal removal is very small and round unworn slides display no movement in any radial axis. Worn slides unfortunately do and are a waste of time, but the result is a serviceable body. The 0.004" clearance was a revised increased clearance as users were distorting and jamming the earlier tighter spec monoblocs I believe.

[simon.holyfield]

The 0.004" clearance was a revised increased clearance as users were distorting and jamming the earlier tighter spec monoblocs I believe.

That's interesting, where did you learn that little gem?

[David Smith]

Simon
https://vintagebikemagazine.com/technic ... cles/amal/
" A note about slides:
The is a lot of folklore about the slide clearance in Amal carburetors. To insure the slide will not stick in the bore from the mounting hardware being over tightened the slides are given .0035″ to .004″ clearance. The typical comment is that Amal couldn’t hold a tighter tolerance. This is far from the truth. The early versions of the Amal Concentric had very close clearance and they experienced slides sticking when customers over tightening the mounting hardware. The .0035″ – .004″ clearance is intentional and the low throttle open jetting was compensated for the extra clearance. "

[simon.holyfield]

Thanks David, that's an interesting read.