That could be as simple as changing the clearance of the backing plate to the shroud. The folks that disassembled them didn't mention any differences. Either way...not a big deal.
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There are some bad MK2's out there. I have had 3 15 Mk2 and 3 21 mk2 and now i have mk3's.s. The first 15 Mk2 was very weak and stalling all the time. Others have been very strong and 21's are bit stronger. MK3 is slightly more powerfull and erconomics are better.
Bottomline is that if you have a good MK2, then add washer and i am sure you dont have stalling problems. I have read reports that Mk2 is more powerfull than Boss so you might have bad unit.
FWIW I’ve noticed with BF Mk II the polish you use can effect if it how easily the machine stalls. Seems to me like it just works well with Rupes polishes. Tried some Sonax Perfect Finish & felt like the machine was stalling more.
I dunno if the polish makes the pad more grabby or what. Both were with Rupe’s yellow pads.
The thing is, TBH, either way you get great results. May seem like the machine is stalling a lot, but it still delivers. People are doing a lot of cobbling together various pad/polish combos - so there are a lot of opinions. Most of them work well, but it seems to me like if you just simply use it as designed it works best.