Does it clear up if you leave it running for ten mins? When you first start it will dump fuel in for the "cold start" cycle and ignore signals from the lambda sensor (o2 sensor) in the exhaust so it will smell of fuel, but once up to temp it should read the temperature from the blue temperature sender and the oxygen level from the o2 sensor, adjusting the amount of fuel being injected, alongside the boost sensor and WOT switch signals.
I've had annoying issues like this before, my current GT for example gobbles fuel (240 miles to 45 litres) and misfires 3 times on a warm restart, then it's fine. Probably the o2 sensor but I've never bothered looking into it, just lived with it. If your car has sat for a while I'd say check things like your earths are clean and flexible, not crusty and hard/ corroded. Possibly water has got into the ECU and causing odd running? Have you got another G40 ecu you could plug in and try?
Also check the usual things, blue temperature sender (next to the black one on the front of the engine next to the distributor, as you look at it.
I doubt it's the valve, the spring would keep it closed, the tappet (technically it's called a "hydraulic lifter" in this specific case) is what sits between the cam lobe and the top of the valve, so if it's not filling with oil, it won't be filling the gap between the valve and the cam lobe properly, causing it to open less than required...... thinking about it now I've written it, maybe it is the tappet? Whip it off and have a look. Last time I replaced them they were about £8 each from a parts store, that was 10 years ago though.... Pretty sure they're the same on all model polo though so should be easy enough to replace.
