I've just fitted a new NOS air filter to my DT400E as the original disintegrated as soon as it was touched!

The new filter came sealed in a bag and was pre-oiled. Having been out for a reasonable run with a brand new spark plug fitted, I checked the plug condition on return and it has sooty deposits.

This is sometimes attributed to the air filter, but with a new one I wouldn't have thought this would be the cause. However, I wonder if the oil in the filter has perhaps thickened in the time since it was made and is restricting air flow?

Should I take the filter out and clean it and do they really need to be oiled for UK conditions?

Also, the bike wasn't keen on being run at a constant part throttle opening, is there anything that can be done to improve this? I seem to remember it being mentioned in a road test, so maybe that's just how they are.

No thick oil never did them any harm I oil mine with 220 viscosity gear oil

is the main jet the standard size for the bike?

Perhaps the choke isnt seating, is that a possibility?

RH crank seal passing and burning gearbox oil...is that also a possibility?

Thanks Airhead for the quick reply.

Haven't dismantled the carb to check on main jet.

I'll have a look at the choke to make sure that it's seating correctly, though the lever seems to "click" off ok.

Hope it's not the crank seal, though being 34 years old I guess it could have hardened. Is replacement a big job?

I think it can be done with the engine in the frame mate but the drive pinion and clutch would have to be removed

has the gearbox got a breather pipe in the top on that model?

Can you smell petrol if you remove the gearbox dipstick?

With extended use does the gearbox oil level change?