Why is my GS750 hissing?

Q. I have an old Suzuki GS750 and it’s running very sweetly since I cleaned and synced the carbs, but it’s just developed a bit of a hissing sound (in time with the engine) from the front of the engine somewhere around the carbs or cam cover along with a smell of petrol when it’s running.

Is this more likely to be a crack in the intake manifold or an improperly adjusted valve?
Lozstone, MCN forums

A. An air leak from the exhaust close to the engine can sound like a tapping valve, but a hissing noise is definitely an air leak from the inlet side. If you can smell unburnt petrol too it’s going to be a crack in an inlet rubber somewhere.

For this sort of noise a length of petrol pipe or a similar hose is a good stethoscope to home in on it. Or you can move the bike outside and spray a little WD40 around the noisy area, if it gets sucked in the engine speed will change.