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 power valve bantam

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nigel breeze

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Number of posts : 348
Registration date : 2007-12-23

PostSubject: power valve bantam   Wed Nov 15, 2017 6:15 am

Evening Alan, i did originally fit a rotax junior barrel to the bantam cases. Inspired by a new Zealand bucket racer who got 30rw bhp from a suzuki gp 125 engine i got hold of a gp 125 crank 50.6 stroke and adapted it for use in the bantam crankcases with the thought of making it a rotay valve engine. then i changed my mind...   Then having had 3mm or so removed from the top of the 54mm stroke rotax junior barrel... i changed my mind again and opted for the cagiva mito 125 power valve barrel as it was already suited to the 50.6 stroke crank and had a power valve...I have set up the bantam cases to accept a variety of barrels by changing the base flange/spacer to suit various stud patterns. This entailed cutting off the neck of the crankcase and having some blocks of  aluminium welded in its place giving a flat/ larger  surface to build off.... whilst the engine is up and running my main problem was sealing the suzuki crankcase i fitted inside the bantam crankcase to house the crank and have a void between the two for water cooling. This was mainly due to the welding involved distorting ,causing the surfaces to be uneven and leaky..I Have now dealt with this... ( o  ringed) crankcase compression test proved to be good and no water leaking into the central crankcase area. I have tried the bike up the road recently and found that whilst it flys it requires loads of slipping clutch to get going and due to its having only a standard 4 speed gearbox seems to  fall out the power band between gears ( at least between 1st and 2nd its a short road!). So ive installed the power valve, removing the blank i made , and is kept in a normally open position but i can via a small lever now close it for pulling away. i Havnt tried it yet but it should help at least pulling away. It seems now with the revelation of changing clutch basket sprocket sizes and variable size crankshaft sprockets i may be able to change  gearing to keep the thing accelerating and not falling on its face out the power band between the gears on a standard gearbox.
 whilst i have managed to get 4 transfers and a boost port into a cast barrel i havnt tried to as yet to get some sort of power valve. My 5 port is deemed illegal and also my cagiva barrelled 4 speed  125 engine so no point wasting any more time playing with those for now...

my interest has been renewed in the stock 175 bike proposal... minimal tuning ect... will be interesting to see the actual rules finally
submitted... regards nigel breeze .
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