As the title, will they fit, i know they are narrow for their size?

No problem. 2.4 MK's are like 2.2's from other manufacturers.

Yep, tubeless and soft and they were still OK (for me, maybe I just don't hit corners hard…)

They'll be alright, but its pushing it really. The lower the pressures you try to use the more it'll try to roll off the rim. You may find you need to run quite high pressures to stop it from pinchflatting the tube too

It's not pushing it at all, these are small 2.4s and 717s are rated up to a genuine 2.3 (the website says 2.1, but the rims state 2.3) so you'll be absolutely fine. You can run 2.35 Kendas- which really are 2.35- without the slightest issues never mind these little skinny 2.4s.

From personal experience I'd have to disagree with some of the stuff above. I've run the MK 2.4 and 717 combo (40psi too) and found the back rolled rather unsteadily in the lateral sense (not very technical language I know!) This were folding though – do they do a wire bead version too?

I regularlyrun 2.3 verticals without a problem though…

I have run a 2.2" Bonty tyre on a 717 rim, and found it rolled around a bit with higher pressures. Never came off though.

It's fine, people worry about this shit too much

I use 2.2 Bontys and they are fine on 717s. I tried 2.35 Kendas and they were terrible for folding off the rim when cornering …..

It's not going to fall off the rim. It might squirm and fold over if you do corner hard at low pressures though.
IMHO there's not a huge point in running wide tyres unless you're putting them on wide rims and running them softer than a viagraless octogenarian.

Ah, I should qualify my comments slightly, I ran a 2.4 Supersonic on both my 717s and my Traversees (26mm), and there was no difference other than in volume, no rolling on unsteadiness or other unpleasantness that I could detect in the brief time I spent with the tyres before deciding they weren't very good and selling them.

Cheers for the responses guys but i changed tack! I have Conti Explorer 2.1's on my e5 and they are framkly gash compared to my old 2.35 nevegals on the last bike. Despite being an xc bike i ride it hard on the dh and need the volume that a 2.3/2.4 tyre brings, especially with 14 stone over it straight lining Garburn. I have actually pushed the boat out and gone 2.35 Swamp Thing front and rear now, Super Tacky front, 60a rear with a Ghetto tubeless kit.

2.35 Kenda's here, with 16st & no probs.

"have actually pushed the boat out and gone 2.35 Swamp Thing front and rear now"
They'll be fine, as they're only slightly bigger than 2.1" Kends

It can't physically roll off the rim, but because of the angles you're forcing the wider tyre edge upto the narrower rim you may well find that it pinches the tube more easily, so you may need to run more pressure, to the point where you maybe better off dropping a tyre size.

2.35" Kenda on XC717s seems bonkers (to me at least)

Nah, you shouldtry it, works absolutely fine. Again, there's a difference between my 717s and Traversees because the tyre's higher volume on the wider tyre but they work just fine on the 717s.