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Almost certainly a sticky carb float, resulting in overflow as designed when that occurs.
 

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Yes, depending on the year (and VIN) of your Shadow either the float bowl tops need to be removed and needles and seats cleaned or replaced and possibly faulty floats replaced or the carbs themselves need to be removed to access the needles and floats in the base of the carbs.
 

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@Jim Walters has pretty much covered it.

Anything that causes fuel to dump from the overflow is, while working as designed when what's not supposed to happen actually happens, not normal.

Depending on which part of the carb is at fault (and I always jump to the floats, first, but it can be otherwise) the fix is different. But the fix will lie in determining what's wrong with one carb or both carbs (and it could be a different problem in each one if both are involved) and fixing it.

There are lots of topics here about servicing SU carbs, and there are reference books out there about same.
 

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What is the VIN of your car?

Have you recently made changes to the fuel pump? Too high a pressure will force those floats down and the gas will pour out the overflows.
 

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Discussion Starter · #10 ·
Yes, depending on the year (and VIN) of your Shadow either the float bowl tops need to be removed and needles and seats cleaned or replaced and possibly faulty floats replaced or the carbs themselves need to be removed to access the needles and floats in the base of the carbs.
It’s a 1975 Shadow LWB
 

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Not that it's likely to be critical here, but can you give us your chassis number? It's going to be LR?----- where the ? will be the character for the 1975 model year, or H for a home market car, and the dashes all digits.
 

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1972 --A
1973 --B
1974 --C
1975 --D The 3rd space in the chassis number for a 1975 car would have a D followed by the actual chassis number.
 

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Discussion Starter · #13 ·
Not that it's likely to be critical here, but can you give us your chassis number? It's going to be LR?----- where the ? will be the character for the 1975 model year, or H for a home market car, and the dashes all digits.
LRD20890
 

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The reason we ask is your car will/should have the HD8 SU carburetors. The float chambers on these cars are really easy to open and inspect. Just remove the fuel feed, overflow pipe and remove the central bolt and the lid of the float bowl will lift off. The lid contains the needle valve assembly. You can also lift out the float and check for leaks.
 

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The reason we ask is your car will/should have the HD8 SU carburetors.
And I'll hair split on this and say that the reason chassis numbers (for cars of that era) or VINs/Derived Chassis Numbers (for cars of that era) are absolutely vital is so that we can actually determine what equipment a given car left the factory with.

Unlike most automobile manufacturers, Crewe would use up existing supplies of thing X or thing Y before switching over to "the next generation." They were very much "waste not, want not" in philosophy, particularly during the very cash-strapped 1970s. Thus, even cars of the same model year might have different equipment based on precisely when they were produced.

This can be seen, vividly, in the booklet that documents, Chassis Number & Production Changes through SY Series. A car of the same model year produced in April might have an entirely different {insert type of part, say carb here} than did one produced in March. The same thing can be seen, but with a bit more digging, when searching the spare parts manuals and IETIS equivalents, where what your car has will differ depending on its production sequence number.
 
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