When you say 'pre monobloc carbs', I presume you mean Amal '76's' or '276's' - in which case, they will be normal road going carbs with needles in the slides. That is what I have on both my 500 DT engine and the 600 DT. They are on methanol and both are running 600 main jet, .109 needle jet (with a trimmed down needle), and a 3 cutaway slide. There is no provision for altering the slow running jet, so the mixture can only be controlled with the pilot air screw.
The main consideration when running on methanol and in anger is that the mixture should be on the rich side ( lean mixture will result in holed pistons!). Having said that, the magneto needs to give a good,strong spark to reliably ignite it! The mag timing is fixed at 36 degrees BTDC.
As has already been said, the whole fuel system has to be opened up to cater for the increased consumption with methanol. If you have top feed float chambers, they may not be getting enough fuel through the float needle to keep the carb supplied - bottom feed chambers usually feed more fuel.
When the DT models were produced, for speedway use, there was the option of 'track carbs' - they were similar to the standard carb but had modified choke blocks and ran without a needle in the slide - the idea being that initially the slide cutaway controlled the mixture until 3/4 throttle, then the main jet took over - the throttle was more like a tap (on or off!).
Going back to the fuel leaking from the carbs - the fuel level should be just below the level of the bleed hole in the side of the choke block.
Hope some of this helps,
Regards,
Eddie.