Stage 0: Serienabstimmung

 

Setup

 

RD 250, Typ 522, '75

Carburettor type 522
250 cc

Rd 250, Typ 352, '73

Carburettor type 361G1
 250 cc

  • Main Jet # 105, idle #22,5
  • needle 5L3, clip position 2 from top
  • Nozzle 175O8
  • Slide cut 2.0
  • air filter german version
  • Compression 9,7
  • squish gat 2.2 mm
  • tiny exhaust mod

Like on the left hand except of

  • Hauptdüse #95
  • needle 5I4, plip position 3 from top
  • Schieber Cut 2.5

 

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

•Performance with series top-hat 18 kW 
•Performance with the racing cylinder 17 kW 

 

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.

 

 

Attention:
With tuned head the engine runs best with 18 degree befor TDC.
 

What is the squish gap

The squish gap is the smallest distance between pistons and cylinder head.

 

 

Result

Pic: Original cylinder 18 kW, 0-100 km/h: 9,31 sec, 1/8 Meile 10,7 sec bei 107,6 km/h
Pic: Original cylinder 18 kW, 0-100 km/h: 9,31 sec, 1/8 Meile 10,7 sec bei 107,6 km/h

 

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

To reduce the squish gap further to 0.75 mm the top-hat seal surface must me milled down.

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.

Fig.: Milled down head with 12 cc volume on the left and with 11 cc on the right
Fig.: Milled down head with 12 cc volume on the left and with 11 cc on the right

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.

Bild: Serienzylinder, 20kW, 0-100km/h: 7,56 sec, 1/8 Meile: 10,06 bei 112 km/h
Bild: Serienzylinder, 20kW, 0-100km/h: 7,56 sec, 1/8 Meile: 10,06 bei 112 km/h
Bild 3: Racingzylinder, 19kW, 0-100km/h: 8,11 sec, 1/8 Meile 10,43 sec bei 109,4 km/h
Bild 3: Racingzylinder, 19kW, 0-100km/h: 8,11 sec, 1/8 Meile 10,43 sec bei 109,4 km/h