Ossa 250mar Misfire

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FM350
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Re: Ossa 250mar Misfire

Postby FM350 » Tue Sep 20, 2011 10:24 am

The MAR has Motoplat CDI ignition, and on these systems its common for insulation on the source coil to fail. In some cases this will mean the bike loses its spark when the motor gets hot, and it returns again when it cools down, and in others there will be no spark at all.

Common fault that is easily fixed though, is breaks in the wiring to the stator, just where it exits the motor. This requires removal of the flywheel and stator, and then simply replace the wiring, using same colour wires if available.

Motoplat systems are repairable, but this will be quite costly unless you have someone in Aus who can help. Other alternative is to try and find a used stator, but obviously no guarantee this will be any good.



Black111r
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Re: Ossa 250mar Misfire

Postby Black111r » Tue Sep 20, 2011 3:07 pm

The bike has been stripped down and the first thing I notice is that the HT lead is slightly loose where it enters the Motoplat HT coil. Quite possibly it could have been worked loose with the constant plug changing that I have done. It is also pretty burnt where it has been in contact with the exhaust. though I cannot see metal through the flattened plastic sheathing.

I has Araldiited it into place and will get the bike running tomorrow to see if that issue is the cause of the mis-fire when warm. the burnt areas can be shielded from the frame using a slipover hose and i can cable tie it all tight.


The second point that I have noticed is that the LT wiring loom exiting from under the Magneto is pretty flattened as suggested by David.

I have tried to check the internal condition of the four wires within the loom but will need to remove the flywheel to do so as it is all pretty tight.

I have take a hammer to the aluminum bash plate as you could see rubbing marks on where the wires exit from under the magneto. I will try and give the area for the loom a little space and get the bike running as noted above before attempting Davids rewiring tip.


Will keep you posted and thanks to all for their tips and advice.


Chris

PS If the coil proves to be defective does anyone know a source for an Motoplat HT coil (it has the letters FN on the metal bar), Or a spare knocking around?

Speed and Sport in the US has a universal HT, does anyone know whether this will do the job?

http://www.speedands.../ignition-coil/



David Lahey
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Re: Ossa 250mar Misfire

Postby David Lahey » Tue Sep 20, 2011 4:11 pm

For OSSA Motoplat HT coil info, have a talk with Keith Lynas (in USA). Contact details http://www.ossaplanet.com/
Another option is complete ignitions for the MAR are available from France Trial Classic
David


relax, nothing is under control

Black111r
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Re: Ossa 250mar Misfire

Postby Black111r » Tue Sep 20, 2011 5:15 pm

Thanks

France Trial is out of stock for the HT Coil, lets hope my araldite fix does it.

Will contact Keith as required


Chris



Guy53
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Re: Ossa 250mar Misfire

Postby Guy53 » Wed Sep 21, 2011 1:03 am

What quind of spark plug cap are you using? Is it plastic or ruber? With resistance or not?

If it's possible change it

Guy



Black111r
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Re: Ossa 250mar Misfire

Postby Black111r » Wed Sep 21, 2011 9:05 am

rubber cap, no resistor

Will be cutting back the HT lead today an ensuring good clean cap/plug contact


Thanks to All


Chris



FM350
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Re: Ossa 250mar Misfire

Postby FM350 » Wed Sep 21, 2011 6:10 pm

Unlikely that misfire is caused by coil/ht lead, as in most cases any problems related to these parts will be there all the time. Might be a good idea to take some readings from the various parts and contact Steve Hardaker in Spain, who is probably knows more than anyone about the Motoplat ignitions: http://www.uk-motoplat.com/testing.html



Black111r
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Re: Ossa 250mar Misfire

Postby Black111r » Wed Sep 21, 2011 7:11 pm

Got the bike back together again today.

I had glued the HT lead back into the HT coil (as it was v. loose - most likely due to hundreds of plug changes that I had made over the last couple of months).

All the earth contacts points were cleaned and the HT lead cut back, recapped and the cap cable tied tight to the HT lead. I also encased the HT lead , to sheath the flatten (burnt) areas, then cable tied all to the under tank frame. Air box cleaned and a good seal to the carb flange.

I did not undertake David rewiring trick but took a heavy hammer to the aluminum bash plate and made a bit of space for the LT wires where they exit from under the stator. The wires looked OK but one had some slight damage.(red).

Put it all back together, filled it with 25-1.................and the bloody thing started first time and proceeded to run flawlessly, with a beautiful idle for half and hour or so.

The bike now has a good clean idle with a good plug colour. (this must be the new Mikuni - Speed and Sport in the US, jet sizes as mentioned before - $US120)


So to summarise I think that it was the HT lead and previous to that it was the combination of a worn Amal carb that had me changing plugs that caused the damage to the HT cable. Or it might have been loose, who knows ?


Anyway thanks for all for the advice and encouragement to keep going.

I am new to trials and haven't touched a motorcycle since I was 15 (some 37 years ago) Its a big learning curve, but great fun (somethimes!).

See you at the next trial................have to learn to ride it now!


Chris



FM350
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Re: Ossa 250mar Misfire

Postby FM350 » Thu Sep 22, 2011 5:06 am

If your HT lead is burnt then cut it in the middle of the burnt area, and join together with an NGK HT lead splice. Reason for rewiring is due to problems with breaks in the wires themselves, where they exit the cases. Breaks here most often lead to burnt out stator windings, and a pretty costly repair, so always worth rewiring an old Ossa to prevent future problems.




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