Re: Revs fine with no load, Boggs when clutch engaged.
I assume since you have Grubee you have CNS carb, but which version? The one with the red plastic air filter is known for clogging, the foam insert gets loaded with blowback from the carb and chokes out your air, so you need to remove and clean the foam insert thoroughly and regularly (I recommend changing to a cone style high flow air filter and this problem will go away). Also there are 3 jets in the CNS, the main jet, the idle jet and the enrichment jet (choke). The idle jet is very small and clogs easily, you need to make SURE ALL the jets are clean. Finally, you should check the fuel filter in the petcock and make sure it isn't clogged, best just to remove that bogus filter and run and in-line filter instead. '
Also, contrary to advise from the manual, the 16:1 break in mix is WAY too much oil. I like to break in with 1st gallon 20:1, 2nd gallon 24:1, then 32:1 from there on. If you have 400+ miles on the motor with a break-in mix then I would move right over to 32:1 fuel mix and stick to it (quality standard 2-stroke oil, air cooled type, NOT marine type)
Thats a bunch of general stuff off the top of my head....
PS and make sure your head is still torqued down properly.....