RD 250, Typ 522, '75
Carburettor type 522
Rd 250, Typ 352, '73
Carburettor type 361G1
Like on the left hand except of
Down-to-earth, it is disappointing how less an adjusted stock RD250 performs. This model is a RD250, type 522, BJ 1975, with 24 kW at the crankshaft according to the car documents. Even if we assume 20% loss through transmissions chain, gears and tyre, these would only be 16x1,2 = 19,2 kW at the crank. Comparisons with machines of colleagues confirm the measured values as right.
The 2 kW of increased performance can clearly be felt while riding.
To increase the compression further, the cylinder head must be milled down until the little edge between seal surface and combustion chamber disappears (blue lines in the picture).
Fig.: Edge to mill down in the cylinder head (blue line) at a 350 cc top-hat here
The cylinder head then has 11 cc of a pure head volume instead of as before 13.9 cc. With the original head gasket and the original top-hats 1.3 mm squish gap can be realised. The volume of the head can remain as it is.
Performance with stock cylinder 20-21 kW at the rear tyre
Pic: open air box
By flattening the upper top-hat seal surface the groove disappears so that now flat seals with different thickness can be used. Top-hats after 1976 RD250C are already flat. The tightness gets much better with the new seal. By self-manufactured head gaskets from sheet copper as of 0.1 mm upwards the squish gap can be varied. 0.7 mm 1.0 mm have been proved as optimal.
At 0.75 mm squish gap the limit of knocking combustion is touched (compression 1:13). It is advised to mille out the head volume be by 1 cc.
Vorher bei der Serie war die Quetschkante 2,2 mm. Der Unterschied ist sehr deutlich spürbar. Schon im Leerlauf wird der Motor wesentlich quirliger. Die Verdichtung beträgt jetzt gemessen ab UT 1:12 statt wie in der Serie 1:9,7. Der Motor läuft damit problemlos.