On the test bench: GSF donation motor “20PS +” part 2

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The trains were installed quickly and we are looking forward to the first runs!

The engine came to us with jet nozzles: without a needle, with a 50 mm auxiliary nozzle and 150 mm main nozzle.

The needles are the crux of the vote when it comes to tuning. Picking the supposedly right one from a seemingly endless offer of carburetor manufacturers is often quite difficult or, better, very time-consuming.

A small selection of needles from various manufacturers are always available on the test bench.

Small selection of Keihin needles

Small selection of Keihin needles

Experience has shown that rotary vane motors of this type feel quite comfortable with a DGL needle. The DEK needle installed as standard in the 35 mm Keihin is usually a bit too bold in the lower partial load range.

All that remains is to check the ignition.
Since the motor housing was welded to the outside of the overcurrent ducts, the ignition base plate had to be machined - unfortunately exactly at the point where the markings for ignition setting are attached to the base plate in the unchanged condition.

gsf wsm2012 018

So that we can control the ignition point, we have to mark the top dead center, the TDC and the desired pre-ignition value.

As always, we determine the OT by means of a reversal measurement.

For this purpose, the piston is prevented from overcoming top dead center with a piston stopper.

Piston stopper for reversal measurement

Piston stopper for reversal measurement

We mark the pole wheel with a scriber and then transfer this mark to the motor housing at the first “stop”. The markings should of course be exactly in line. Here, in the picture below, the camera's angle plays a trick on us.

OT mark 1

OT mark 1

After the pole wheel has been turned once against the second stop, we also transfer the marking from the pole wheel to the motor housing.

gsf wsm2012 007

With a scriber you can now determine the exact position of the top dead center between these two markings on the motor housing.

gsf wsm2012 009

Ok, now we know where the piston is at the top, but we're aiming for an ignition point of 18 °. This means that we now have to set a corresponding marker.

We also need the circumference of the pole wheel. This value can be simply measured or determined using the calculation method (D *?).

The pole wheel has a circumference of 529mm. If you divide this value by 360, you get the value for 1 °. This value must then be multiplied by the desired value for the number of degrees.

In our case 529/360 * 18 = 26,45mm.

This value is then marked on the circumference of the pole wheel, starting with the “0” marking.

gsf wsm2012 010

During the subsequent check, the 18 ° marking should be congruent with the OT marking on the housing.

The control is carried out with a stroboscope lamp.

As you can see, the ignition point is around 2 ° next to our desired value of 18 °, i.e. 16 °. We reserve the right to adjust it for now and let the engine run as it is - you have to start somewhere.

gsf wsm2012 013

When warming up the engine, it is unfortunately noticeable that third gear jumps out under load. Mmmmh .. stupid. Unfortunately, the problem cannot be remedied in the short term, so we drive the tests in fourth gear. The gearbox is practically the lever against which the engine has to work. This lever is of course much longer in fourth gear, so it “brakes” more than in third gear, which is usually used for measurements. The peak power on the graph with the dynamic measurement, power versus speed, will therefore be significantly smaller than would be the case in third gear.

As a final check, we may try a static run, performance against km / h and hope to be able to hold third gear in position at least once with the shift handle.

Ok, first try ...

1 GSF Christmas donation engine 2012

As suspected, the spray is too bold. The performance drops badly, especially in the pre-reservation area.

We change the auxiliary nozzle to 45 ...

2 GSF Christmas donation engine 2012

Well, something positive is happening in the pre-resso area - now we're going to 145 with the main jet

3 GSF Christmas donation engine 2012

We haven't found top performance by changing the main jet. The curve becomes much smoother and the engine starts more willingly in fourth gear against the load of the roller. These are subjective impressions that no graph can depict.

What remains is the change in ignition timing to 18 ° ...

4 GSF Christmas donation engine 2012

The ignition brought it again. More power in the pre-resol and peak area with more torque. 18HP in fourth gear with the longest transmission brake is okay.

Finally, a run over all four gears ..

GSF Christmas donation motor 2012 kmh

Well, almost 20 horses in third gear ...

The difference in fourth gear is nice to see in this run. After all, the longer translation swallows almost two and a half horses.

In terms of spraying, we would leave the engine like that for now. The remaining fine adjustments should be adjusted according to the geographical conditions of the new owner on the road.

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