Coming to this conversation a bit later, but I thought I would add my two penny's worth.

Get the head and barrel skimmed professionally, using glass isn't the best option. I know a lot of people will say 'I do it all the time and never get an issue' (I have done it myself, when I can't get to use my milling machine). I know, If you do if correctly then it can do, but it will never be as good as in a proper machine shop. It's not just about having a flat surface, its about having a slight rough finish for the head gasket to grip to.

Check the squish band measurement, if it less than 1mm then this could be the issue, although I doubt this is the issue as lots of different heads and barrels have been used.

If you don't have the carb heating pipes connected to the carb, then they should be at least connected from one banjo to the other. so they is always a flow of water from the head to above the thermostat, ie bypassing the thermostat, to bleed air.

Check the timing is correct?

As stated before replace the rad cap. I would change the thermostat as well.

If the reed valves are not sealing/ closing, then replace them. If you are using standard reed valves then use the stock 6.8mm gap. If you have changed them for Boysen reeds or other after market, then you can open up the stops to give better performance, but they must still seal when closed. If you hold then up them up to the light and can see day light then either the reed block is damaged, or the reeds are knackered.

DT125R FORUM

BEng1968