Could be high idle, could be damaged clutch springs,mushroomed shoes,bad clutch bearings or freshly packed with too much grease clutch bearings if grease is what you use.Put your finger on the clutchbell & see how much force is moving the bell & if you can feel anything rubbing on it inside the bell.
I am not sure what bluggs down means exactly.What should happen is pinch as close to carb as reasonable & idle should rise slightly for a couple seconds,run stumbly for a couple seconds,idle should go up a bit more & die but not rev to the moon before it dies.Plug temp & type,exaust stinger size & length,head shimming,nitro % & motor design will change this slightly but only slightly & pinch test is only to get you close enough to tune it easier on the track.Knowing what exactly is happening may help.What the pinch test does is tell you how much fuel you have puddled up in the crank case.The longer it runs with thefuel line blocked off the more fuel you had as a reserve in the crank case.Make sure you rev the motor up & let it come back to an idle every time before you pinch.Think of it as simulating flying down the straight then shutting the throttle then getting back on the gas at whatever part of the corner you normally do.Too much extra fuel crammed into the case & the eng will bog when you get back on the gas.Length of time you tune the pinch test for is not important 3-8 seconds is normal-ish the important part is after you get the motor to feel good on the track then do the pinch test again & thats the time you use as your reference in the future.
Hope that is more helpfull than not but I work on real eng for a living & assume people know what I know & had the schooling I had so I'm not a real good teacher,sorry.
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