Hi all

On my 276C/1B the horizontal bore isnt 1" perfectly round its oval on the side which bolts to the barrel is this normal? there doesnt seem to be any wear marks more like the casting was off. The air inlet side looks fine.
This has been in boxes for years so unsure how it runs with this on.

If the carburtter is overtightened on the two studs and the engine gets really hot this distortion can occur. Normally you will get away with it running ok. My term for it is egg shaped and have seen lots of them over the years. It appears to have Velo mac written on it and the bottom nut under the body looks to be the taller type and not the BSA short type?

Hi John, i think you are correct as on the flange there is defo signs of being overtightened. Im not sure the bike has been in boxes for years, as far as i am aware the one that it had on when it was running was a 276/014r but this has corroded considerably to the point of being unuseable. If this is the taller type will it still work on the m20?

I think most MAC's had a 276 with a 15/16" bore as opposed to the M20 1" bore (John will probably confirm) And the bottom nut, slide, main jet are different, but very easy to swap from your old carb or buy new from Amal or Hitchcock's. Or look up "Joe the carb" (in UK) for a complete refurb. At the least, the flange will need checking and truing for flatness. Ron

email (option): ronpier@talk21.com

As usual Ron is spot on Velo Mac had 15/16 of an inch where the air goes in and 130 main jet and 6/3 throttle slide where M20 had one inch and 170 and 6/4.

So the carb is definately a 276C/1B this is stamped in and it measures a perfect 1" bore except from at the end which is clearly distorted, i am going to attempt to ream this to a better shape. Unsure why it has velo mac wrote on it though

OK, I wouldn't worry too much about the oval bore at the flange end. The bore in the cylinder barrel is usually bigger anyway as the same barrel is used for an M21 which has an 1 1/16" carb.

Most important is to get the flange flat if it's been distorted, by taping a sheet of W&D paper to a surface plate or piece of plate glass. With water, circling motions and some patience it's soon done.

Ron

email (option): ronpier@talk21.com