Technical Information Sheet for the Restoration of the BMW R75 and Zündapp KS 750.
Monthly available on our website.
H - P Hommes
Misfire at high revolution speed
The automatically adjustment to
advanced ignition by the centrifugal timer may cause misfires above a certain
The stop of ignition can only occur on one side of the cylinder.
The cause is the incorrect adjustment of the ground plate.
|By turning the armature, a voltage is
generated. Durch die Drehung des Ankers wird eine Spannung erzeugt.
At their max. strength (stärke) a contact should open and a spark occurs.
By the adjustment the spark is generated earlier whereas the voltage is lower.
Is the basic setting incorrect, during the adjustment the ignition point is displaced to a range where the spark does not have sufficient voltage or tension.
There will be a misfire.
The magneto Noris adjusts the
ignition points up to 15 °. Hereby the spark is weaker at higher revolution
speed. When ignition spark opens already when the voltage is not so high, so
that of an ignitable spark can occur, then there is misfiring. Cause of the slow
down of sparks is that the opening of the contact is shifted downward from the
highest voltage point by this adjustment.
As for the Bosch magneto the ignition spark has always the same strength, since the adjustment to advanced ignition does not change the position of the armature to open the contact.
How do I set the ground plate?
|The magneto is driven by an electric engine with at least
Set distance of the contact breaker to 0.25 mm.
When the fixation of the ground plate is made directly at
the border area, the adjustment of the ignition contact can already cause the
collapse of the spark.
Once the optimally set magneto is mounted on the engine, we check where the ignition points really are.
Best way to do it, is to take cigarette paper and stick it between the contacts. Then slowly turn the engine to OT. (OT is the top dead center at which the piston reaches.) At the same time watch through the inspection hole the mark on the crankshaft. When the contact opens, the marking should be centered in the inspection hole at the latest after 25 mm. Thus, the ignition may take place up to 25 mm before OT.
In my experience, an engine is best running at this advanced ignition.
The exact adjustment of the ignition point to the exact center of the inspection hole would have its price. Any adjustment of the ignition contact will result in a lower ignition voltage. This we had previously set optimally with the distance of 0.25 mm.
We must not forget that today we use a completely different fuel for combustion then in the year one.
I have to tell the people who try to adjust the ignition with their stroboscopic lights: Forget it!
Further causes of ignition failures.
Ignition failure is often caused by poorly manufactured coils. The coils move because of generated heat and create a short circuit, which weakens the ignition voltage so that the spark for ignition is no longer sufficient.
Also poorly manufactured ignition cams cause a rapid wear on the rotor Ignition Verschleiß des Läufers am Zündkontakt.
The runner from use and reduces the contact gap to zero, so that may arise, no spark more. Der Läufer nutzt ab und verkleinert den Kontaktabstand bis auf Null, sodass kein Zündfunke mehr entstehen kann.
These runners have a too rapid deployment and are on the tread just fine turned and not ground. Diese Läufer haben auch eine zu kurze Öffnungszeit und sind an der Lauffläche nur feingedreht und nicht geschliffen .
Check: Scratch across the contact surface with a fingernail. Do you feel grooves? Then this is an inferior copy. Good ignition cams are polished and smooth.
It looks easy to assemble a Noris magneto.
But there are a lot of ways to incorporate errors, to make incorrect settings or use fake spare parts which have mistakes that are not immediately recognizable and show their effect only later.
The magneto works with a high voltage. What is not really made of puncture-resistant material, will be penetrated with the times and create new problems which lead to the failure of the ignition system.
H-P Hommes Ersatzteil-Dienst
Ersatzteile + Instandsetzungen
repairing + restoring
Zurück zur Hauptseite