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I am in Aus so we are on different time frames so that is why it takes a while to respond.  I feel bad that people are arguing like this on my thread.

My BBK seizure happened about three years ago so I will go on what I remember.
It was on full premix, the oil pump was disconnected.
It seized on my first tank of fuel. I remember being conservative with my oil mix so I think it was 40:1 but could have even been 30:1.
I was using 98 octane fuel and full synthetic oil (I think it was Motul)

I had done a couple of plug chops and the plug seemed a fine tan colour, no indication of what was to come.

I don't have the piston any more but here are pics of the barrel.

I had a #90 main in, standard air cleaner without snorkel and was on full throttle when it went.
I remember Scootercraft not sending the piston they said they would in the kit I ordered (they sent a Anthea piston with a Stage 6 barrel). I remember as I installed it had very little slack between piston and bore.
This one locked up solid so the kick starter wouldn't budge.

Ok I nipped up my 50cc Minarelli piston and cylinder (12mm pin) and need to replace it.  I'd like to go 70cc and need to hear from people who have done it.

I have blown one up previously (70cc) when Scootercraft sent me the wrong too tight piston for my stage 6 kit so I want to be careful this time.

My concerns are this.  I have heard I should go aluminium instead of steel cylinder for cooling.  Do I need to ditch the oil pump feed and go premix, if so, what ratio?  Ideally I'd like to leave the pump in and add oil to the fuel tank as well.  Also jetting, where do I start with a main, I will have a 19mm Mikuni with a 70# main standard for the 50.  Any other concerns issues I should watch out for? As usual any help is appreciated!!!

Flywheel and rotor have never been touched.

I got the Mikuni as it is the factory one so I could eliminate any anomalies that might surface with an alternate. I am starting to fear that the induction through the crankcase is not strong enough to create the vacuum which means something sinister beyond a crook carb is in play. I still seem to have compression (via feel of the kick starter and finger over the plug hole) and I haven't had smoke pouring out the pipe or an oil leak that may be an indication of a crank seal. Heck I'll get the new carb in a week or two and if that isn't the solution I'll pull the motor out for an autopsy.

Mar 15, 2016 7:00:46 GMT -6 cyborg said:

Carb still off?,,check the reeds,,,behind the carb in the reed block on the cylinder,,,a cracked or chunked (broken) reed petal will create such a condition

Hi, thanks, I had the reed block out yesterday. I am running carbon fibre reeds and they are in great shape. I held the block up to the light and there are no gaps ...