Stage 0: Serienabstimmung
Setup |
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RD 250, Typ 522, '75
Carburettor type 522 |
Rd 250, Typ 352, '73
Carburettor type 361G1 |
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Like on the left hand except of
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Result
- Performance 16 kW of the rear wheel
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.
Stage 1: Simple modifications to air box and compression
Measures
- U.S. air filter, original air box
- 1.3 mm squish gap instead of stock 2.25 mm
- Other air filter box lid with a greater admission
- Exhaust whistle shortened
- Compression 10,9
Setup of the carburettor Type 522
- Main jet left #125, on the right #130
- Idling jet #22,5
Result
The 2 kW of increased performance can clearly be felt while riding.
Mill down cylinder head
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.
What is the squish gap
Result
Stage 2: Open air box, higher compression, smaller squish gap
Measures
- Open air box with a rough YPVS filter mat
- 0,9 mm squish gat (stock cylinder)
- Cylinder head volume increased by 1 cc
- open series air filters with Y-rubber
- PowerDynamo ignition/dynamo
- Exhaust whistle shortened
Set-up of 521 javelin carburettor
- Main jet racing cylinder left #180, right #185
- Main jet stock cylinder left #170, right #170
- Idling jet #25
Result
Performance with stock cylinder 20-21 kW at the rear tyre
Pic: open air box
Flatten upper cylinder seal face
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.
