Hi Michael - I have been following the thread and would like to make a few comments.
The thermostat only cycles once, it opens after the coolant reaches 135/140F and remains open for the rest of the operation, cooling is controlled by the air flow.
All radiators are not created equal, a poorer quality radiator will not dissipate the same BTU's. The cooling capacity of the radiator is governed by the construction of the core. In your photo the bonding between the fin and tube looks suspect.
The WOT spec must be achieved, if the engine does not come close to the specified RPM than you cannot use the jetting charts supplied. An overloaded two stroke engine will run richer than desired, thus the lower EGT reading and higher coolant temp.
In the video it looks like you have a ground adjustable 3 blade wood propellor. I would suggest you should carefully check this over. If the RPM loss is a result of movement in the blade clamping it must be corrected. Ignoring it could be catastrophic.
The PSRU appears to be a 'B' box and I would be very surprised if that prop does not exceed Rotax limits for mass and wieght. Most 3 blade adjustable props are in the upper limits for a 'C' box.
If it was mine I would reassemble everything, check the clamping, balance, pitch and tracking on the prop with it set to pull 6250 static. This should produce about 6500 on climbout and close to 6800 WOT. I would be surprised if this does not correct your cooling issue.
Dennis
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