Airbox is standard, but no flap, didn't have one when I bought it.

Tank vent working?

The breather hose on top of it? Yeah I can smell petrol through it, if that's what you mean

Open your petrol cap, and blow through the breather hose/spigot. Should be nice and easy, if it feels like blowing up a balloon, then your tank won't be releasing the fuel.

It does have some pressure to it when doing that, like I have to blow hard.

But it was releasing petrol fine when I manually operated the fuel tap?

bikemonkey wrote:It does have some pressure to it when doing that, like I have to blow hard.

But it was releasing petrol fine when I manually operated the fuel tap?

If the breather is knackered then it will eventually vaccumn lock.

I don't think this is the case though as your only getting it in certain gears.

It sounds like airbox / flap in those gears to me but difficult to see without seeing it.

It started after a crash?

Yeah, I had a right side lowside a few weeks ago.

Rode the bike home in it's slightly mangled state, but didn't go above 60mph or fast at all for obvious reasons.

Then after I got the bike fixed and rebuilt I went to go to a relatives down a dual carriageway, accelerated out in the right hand lane then it just bogged right down and wouldn't let me get past 60, with all the symptoms I've explained.

What do you think I should try next?

Been out on the bike again to get to my mums, and in 3rd at 50 mph in started doing the crap power delivery thing again.

I'm beginning to think I should fit the rubber flap I bought ages ago and never fitted, seeing as its windy and I think it could be an airflow thing.

As Andy666 said you should not have to blow hard through the vent hose with the tank cap open. Any restriction is bad, it will eventually stop fuel flow. The smell of fuel is not an indicator of good venting.

New tank vent? Or just take it apart first and see?