Amal 275

[Greenfield]

Yes, I'm still having fun trying to get my B21 deluxe together. The latest issue is that the carb is correct ie an amal 275 and the magdyno seems correct, but when the carb is fitted, the edge of the dynamo hits the float chamber bolt, preventing it from being tightened and seating correctly. Both parts can be fitted but not together. Am I missing a trick here. You can't raise the carb or lower the dynamo and the only solution I can think of is to fit a spacer on the inlet manifold which will reposition the carb further away.

[militaryron]

You need the long throat version. Also although the 275 will work just fine, originally it was the 75 model. Ron

[Greenfield]

Thanks for that. Any ideas on what the measurements of the two carbs are? The throat on mine looks long but can be extended if I fit a spacer.

[militaryron]

I guess a spacer is your easiest option. Some of my Royal Enfield's use a similar type carb (276) for the same reason. I can measure one of those tomorrow.   Ron

[Greenfield]

The distance between the body and flange surface on my 275 is 49mm. A 1/4" spacer could well allow it to clear the dynamo. I'm interested to know what the throat length is on an amal 75 if anyone has access to one.

[militaryron]

Those 276 long reach carbs for the RE's  are only 47mm at the same point. Probably because the mixing chamber is bigger? Ron

[militaryron]

The 1939 Amal list quotes the following for a B21:- Type 75/145, 7/8", 120 main jet, 5/4 slide, needle position 3, 7 degree float chamber.

I've just realised and checked, both my Triumphs 3SW and 5SW have exactly the same carb and spec (except for a level float chamber)
The distance from the body and flange is identical 49mm.  Ron

[Greenfield]

After a chat with Burlens, it would appear that the main difference between the 75 and 275 models was the internal airways to allow for the addition of air filters.
During the discussion I discovered that the needle jet and main jet were wrong, so the correct ones are on order together with a set of gaskets. After trying out a range of spacers for the flange, it looks like a 1/8" tufnell spacer should be enough to prevent fouling with the dynamo.

[militaryron]

That is why I said the 275 will work just as well. It was around 1940 that military bikes went over to the 275-276 range so as to be able to filter the air as required.  Unless otherwise stated, it's usually a standard (No6) needle jet. I posted the rest of Amal's 1939 spec the other day.  Ron

[Greenfield]

For a 275 on a 1939 B21 Burlens recommends needle no5 position 3, needle jet 1065, main jet 120 and throttle slide 5/4. With a 1/8" spacer plus a gasket either side it moves the carb out 5mm which is enough to clear the dynamo. I can now reassemble the carb and take a look at the primary drive.