This is a big misconception, the size of an SU vs engine size.

An SU is so designed that an engine will only use as much is required. The piston, needle (mixture) will lift for the required amount of fule/air mixture for an 850 or 1275. It is not like say a nikie, holley, weber or such were they force fuel into the engine with a pump.

For instance I ran twin 1 3/4 on a mild daily driven 1330!!!!! no problems.

Anyhow the single 1 3/4 ran absolutley fine from idle to over 7500rpm.

Just get the needle right and your laughing.

PS. I put that 1 3/4 SU that cost $15.00 from the wreckers plus recondioning on a standard 1.5 inch cast manifold.

Agreed rupert....but
you need air speed across the carby, i find that you won't get this from a twin
1 1/2 or 1 3/4 inch su setup. i much prefer a smaller throat with more response

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