The 85 cc D.R. is a 50mm bore version of the 75 cc D.R. cylinder. For use in the Piaggio APE50 tricycles, however, it has been rotated by 1mm at the cylinder base compared to the DR75, leaving the same dimension at the top of the cylinder. So the steering angles were reduced to 152°/98° (exhaust/overflow) to allow the cylinder to be used even earlier in the speed range.
An improved acceleration is not to be expected on a Vespa V50/PK with original gearbox compared to a DR75, because the cylinder finds its end of revs very fast. The gears have to be shifted through quickly to reach a halfway acceptable acceleration.
This characteristic can be used with an extended gear ratio to turn a fifties into a durable long distance tourer. Use a longer primary (3.00) and a 19/19mm carburetor with intake manifold and a 10.7:1 compression ratio cylinder which runs very well with the high torque banana exhaust from BGM (BGM Touring) or an original banana. PK models can very well use the original exhaust if not coked.
After the obligatory adjustment of the spraying (main nozzle min.74), check the pre-ignition (the standard pre-ignition of 19° is maintained) as well as the use of the spark plug with the correct heat value NGK B8HS or Bosch W4AC) is a durable and powerful touring package laced.
TIPP: Piston rings please always carefully install.
Some pistons have trapezoidally profiled piston rings (so-called key toner rings), these only fit in one installation position. If the ring is not free, please loosen it carefully and reinstall it upside down.
TUNINGTIP: Simply lift the cylinder with a 1mm foot spacer and turn the cylinder by the same amount on the upper sealing surface. This lifts the control angles to the same level as the DR75, making the cylinder much more turnable.
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