Briggs and Stratton 375 Sprint mower engine - carburettor blocked?

While the engine will start, it cuts out when the priming fuel is used up after about 3-5 seconds.

The manual says the carburettor cannot be adjusted, but I think the main jet is blocked. How do I unblock it? The manual is no use.

BTW, it is an oldish Sprint, Model 9x900 where x is B, C, D or 8.

On the identification panel it says it is a TBS158U1G1RA:EM but thus model does not appear on the B&S website