Motoplat System on Air Cooled Gas Gas

[nzpeterb]

I have just purchased a 1989 Gas Gas 327 Air Cooled Mono.

It is fitted with Motoplat Electrics and has no spark.

I have been unable to find a wiring diagram that looks like mine.

The stator has a black, grey and 2 blue wires plus the yellow which is not connected. I understand the yellow is for lights but the bike has none fitted.

The resistance of the spark plug coil tests OK but, from reading what I can find, should the bike have a different coil with both the green and white wires connected?

Does anyone have an original wiring diagram and measurements for testing the components.

Regards,

Peter

(in New Zealand)

Motoplat System.pdf

[pschrauber]

The ignition is of the incast stator type, no sparks means that some of the incast electronic parts are toast or shot.

The casting around the electronic devices build up pressure when the stator gets hot this will lead to damage and malfunction of the thyristors inside.

What you have to do is cut the stator open and replace them, that is a delicate job, you might sent it to a company that does repairs to old electronic
ignitions. I know a guy in Austria that can repair these ignitions and generators and he did so to the same ignition to my Beta TR 34.

But you live in New Zealand ... to far away ...

 

 

[nzpeterb]

Thanks for the reply.

I have found names for people in UK, America, and Australia who can repair the units.

Rather than send the whole system, (stator, control box, and coil) I was hoping to be able to identify which component is faulty, then replace or have repaired.

At present I believe the coil fitted may not be compatible with the Motoplat control box so would first like to resolve if that is right.

Regards,

Peter. 

[pschrauber]

There are components in the Stator and in the coil unit so you have to repair / replace both if you want to use something else.

The repair is deilcate as you have to know where to cut the plastic open and not to destroy the platina or other units,
after the "Treatment" the components should be casted in with 2 compond glue which is btw. softer and will not harm
the units in my experience. 

To control the thyristor in the ignition switching device, only a few milliamps of current are sufficient, which faulty diodes or resistors that are too high can somehow create under limit conditions. That's why you usually only notice such problems when the engine stops or it ignites three times per revolution! If you have a multimeter with a diode test, you can quickly check the forward voltage of the diodes in both directions

In the damped current direction, the voltage drop with a milliamp test current is, for example, 527 mV and if you swap the two test cables (red and black), the forward voltage of the other diode (without 30 Ohm resistor) is measured, the voltage drop (or the forward voltage the diode) should then be about 500 millivolts! However, the flux voltages given are only guidelines because they are also strongly temperature-dependent. If, however, one of the two flow directions does not give a reasonable measured value or an infinitely high flow voltage, the fault is clearly localized.

 

[paulc76]

I am just about to look at one with no spark and i have a running one to compare so will let you know the reading the ignition you have is the type with the separate ignition unit usually hiding between the headstock gussets the coil should be a single wire type 1.5ohm connected to the green wire the white wire  should go to the kill switch you should also see a voltage between the two blue wires when you kick it over the black and grey are the trigger wires i'll let you know the resistance once I've measured mine

[paulc76]

Ok I'm sparking again now stator was bad but had a spare the readings are as follows

coil primary 1.5 ohms

coil secondary 18.2 Kohms

Blue wire to blue wire 281 ohms

Black to grey 25.2 ohms

grey to ground 107 ohms

black to ground 82 ohms

blue wire to earth several Mohms so no connection really

yellow wire is lighting

yellow/green from ignition unit to earth

white from ignition unit to kill switch

other side of kill switch to ground

hope this helps Paul

 

[paulc76]

Also most of the info you find online is for the more common type of motoplat where the switching electronics are within the coil yours is not this type so the info does not apply

[peterb]

Hi Peter, I used to own an air cooled 1989 GG 250 mono, it now belongs to another rider in Te Awamutu. That has the same Motoplat system. I will be catching up with him at this weekends Auckland National event. If you need to know any further info over what was posted above I can try to find out for you. Jim Snell may have a wiring diagram for these on the net, I have a parts list but not much else. Bye, Peter B.

[faussy]

Imagine having a rare bike with a rare ignition system and then PaulC posting all that info. What a God lol

[nzpeterb]

Thanks for the replies and especially to Paul who provided a brilliant reply.

I also sent an enquiry to America and got a very helpful reply there too.

Spent this evening using the information provided and was able to start the engine. What I did is summarised below. 

I am advised the original coil was a single wire red Motoplat, and the Denso CDI coil I have fitted is a compatible replacement.

It was also confirmed that the wiring diagram I posted is also correct.

All measurements I recorded for the coil and stator were within 1 or 2 ohms of the values Paul provided with the exception of one.

When measuring blue to blue Paul got 281 ohms. One mine each time I connected the meter I got around 100 ohms briefly then it went to no circuit.

I had read on my trawling through the web that some Motoplat coils had this characteristic, so unsure whether my stator was good or bad I tried kicking the bike over with no plug.

With each kick an AC volt meter connected across the two blue wires briefly showed a voltage between 10 and 17 volts.

Next cleaned all the connection plug contact surfaces, then plugged the control box back in but left it hanging so no part of the case was touching the bike, connected the earth wire from the control box direct to the coil, sat the coil on top of the head, and disconnected the kill switch. Kicked the bike over and there was a good spark!

Then bolted the coil into place in the frame and retried but no spark. Scraped paint off the coil mounts to get bare metal, put the coil in place (still with the control box hanging) and the bike started, so this problem is solved.

Found the 1991 parts book on the web and this gives the plug as an NGK BP5ES. Is this still the recommendation?

Also what is the standard idle mixture screw setting? Mine was 3 1/2 turns out but seems happier at 2.5 turns.

So thanks to the help from this forum the bike is running and motor seems good, though bike needs the following new parts: tyres, chain and sprockets, rear suspension bushes/bearings, and a rear guard, plus sticking rear brake, and a number of small parts, but feels like it will be nice to ride.

Thanks again,

Peter. 

Edited March 16, 2020 by nzpeterb