Looks like your onto something there TLRMARK, I pulled the reed valve out at the weekend and one of the petals has snapped off and was lodged in the rubber boot. I think we may have found the problem. Ive ordered some new petals that should be here tomorrow, ill let you know how I get on.

Glad you found the problem, had the same result as you with my montesa 315 10 years ago when I submerged it.

Edited March 5, 201412 yr by tlrmark

Yep that was the problem alright. Running like a dream now. Little bit smokey perhaps but hoping thats just all the wd40 and carb cleaner thats been through it burning off!! Thanks for all your advice everyone much appreciated.

Good find then. Being a bit of a geek, the only operational theory on this I can think of is the fact that at lower revs, the crankcase pressures blowing back through a broken reed at low throttle slide opening would actually push (blow) the fuel backwards making for a difficult pick-up of fuel for the next stroke if intake vacume. The choke would help override this lean condition.

The fact that the hydro effect of the water entry just blew a reed petal is good, because if the water made it up top side of piston a bent rod is assured, or worse!

And thus, a lesson for all, DUH!

We did not have this issue with old piston port motors, yes, I have swamped a few, Lineaway!