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!
I have been test running it with clear hose as a fuel line. Petrol flow is constant, though the misfire, when warm, feels and sounds like it is running out of fuel. I do not think this is the case though as I have tested it with two carbs, one of them being brand new.
I am thinking that the issue lies with the LT/HT wiring. ie something is getting warm/hot then expanding and contact being lost.
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?
Looking at the intake spigot is no good...you have to test it.
You can spray water on it while the bike is running to see if the idle changes.
You can also point a propane torch at it (don't light it!!) and the idle will change if it's sucking propane.
It's probably electrical, though.
Points and condenser are cheap and easy to replace.
Often the first sign of a coil going belly up is it works for a few minutes, then starts to miss.
Thanks to all
As part of yesterdays work I took the carb off along with the rubber intake spigot. It is one that I have recently made from new 35mm internal dia automotive hose. No splits but once the bike is running again I will try the tests you suggest to see if air is leaking in
Chris unless someone has modified your bike it should not have a condenser or points. It should have a Motoplat CDI. Something to check that is a weakness in the MAR design is that the LT ignition cables come out of the magneto housing underneath and can easily be damaged by the bashplate pinching them against the underside of the motor. Mine had been damaged in that way, but despite the wires being squashed and having conductors exposed, the motor still ran fine.
If you search for OSSA on the Trials Central forums or the trials.com.au forums you should find a photo sequence of how I repaired and re-routed the ignition cables on mine out the front of the magneto housing, after a suggestion from Woody.
Your ignition may be getting affected by this problem.
Other things to check for are loose motor mounts,loose stator screws and a killswitch cable possibly shorting to earth.
Hi David
I will investigate this tomorrow
Interestedly enough In-motion list new condensers on their website but I cannot find references to them on any Ossa websites/info!
Looks as if I will have to pull the bike apart, check the cabling.
I will do a search for your photos and will let you know about the puller
Definitely the condensor first - they break down much more often, especially under heat and they are much cheaper than a coil to replace. But also check the points although they normally would be bad from scratch - you might have short in the low tension wire that only shows when hot ie: metals expand when hot and so can short out hot and not when cold.
However condensor is a very common cause of misfire when hot - they only cost a few dollars
Any ideas on a local source for a new condenser
I can order one in from the UK (In motion 8.00GBP) but this will mean another week of waiting.
Following advice i took a deep breath and have put the old Amal back onto the bike to see if I can isolate the problem.
Just to spite me the bike started first kick! However after running for 3-5 mins (as before) it started to misfire in exactly the same manner as before.
I took the chance to inspect the rubber inlet spigot hose- it was perfect with no splits or deformity in the spigots.
Pulling the plug this time, however, did show a lot more oil so it would suggest that the new Mikuni carb is helping the fuel to be burnt more efficiently.
But back to the misfire.
Should I replace the Motoplat with a new coil or just replace the condenser. Which is the most likely to be causing the problem?
The HT lead does show signs of being burnt against the exhaust however I have shielded these areas with hose so I do not think that it is shorting. in fact the lead has always been like this so i do not think that this is the cause of this misfire.
It is also sealed into the Motoplat so replacing it will mean a new Motoplat coil.
Followings Andrews advice i took a deep breath and have put the old Amal back onto the bike.
Just to spite me the bike started first kick! However after running for 3-5 mins (as before) it started to misfire in exactly the same manner as before.
I took the chance to inspect the rubber inlet spigot hose- it was perfect with no splits or deformity in the spigots.
Pulling the plug this time, however, did show a lot more oil so it would suggest that the new Mikuni carb is helping the fuel to be burnt more efficiently.
But back to the misfire.
Should I replace the Motoplat with a new coil or just replace the condenser. Which is the most likely to be causing the problem?
The HT lead does show signs of being burnt against the exhaust however I have shielded these areas with hose so I do not think that it is shorting. in fact the lead has always been like this so i do not think that this is the cause of this misfire.
It is also sealed into the Motoplat so replacing it will mean a new Motoplat coil.
Mixture back to 25-1, Correct plug- NGK-BP7E gapped to 0.20 (0.51mm). Inlet hose perfect with no air leaks, clean filter. New carb (Mikuni VM26, 150 main 30 pilot)
Starts easily with the choke. Choke off, then bike runs cleanly. After say 3-5mins of just riding around bike starts to misfire v badly and will not run with an open throttle, will still idle though.
The plug comes out clean, copper colour with no hint of soot or white powder. Same colour in fact as with the mix at 33-1
This type of problematic running is new. With the amal is was starting that was hard but it is hard to say whether this new problem was developing.
Is it time to try a new Motoplat coil, the existing looks original so is the best part of 35 yrs old. The coil does not feel hot to the touch when the misfiring starts and I do know that the coil is well grounded to the frame. Any better alternatives?
Mixture back to 25-1, Correct plug- NGK-BP7E gapped to 0.20 (0.51mm). Inlet hose perfect with no air leaks, clean filter. New carb (Mikuni VM26, 150 main 30 pilot)
Starts easily with the choke. Choke off, then bike runs cleanly. After say 3-5mins of just riding around bike starts to misfire v badly and will not run with an open throttle, will still idle though.
The plug comes out clean, copper colour with no hint of soot or white powder.
This type of problematic running is new. With the amal is was starting that was hard but it is hard to say whether this new problem was developing.
Is it time to try a new Motoplat coil, the existing looks original so is the best part of 35 yrs old. The coil does not feel hot to the touch when the misfiring starts and I do know that the coil is well grounded to the frame. Any better alternatives?
I'd stick to the recommended plug and try and fix the problem rather than treat the symptoms.
What can cause the symptoms: timing, jetting and mixture setting, air leaking around inlet or in carbie, crank seals... it could be a combination and so you just have to start (with easy first) and work your way through. By the time it's over you will really know your engine much better
Firstly I will try the mix back at the recommended 25-1, using the recommended NGK BP-7E.
Hello black111r, we met at the last WDTC club trial. What type of oil are you using mineral or synthetic. I run my Sherpa on Castrol TTS synthetic oil at 50 to 1 and it runs very cleanly and whenever I pull the head off it there is no carbon to clean off unlike mineral oil. Any of the modern synthetic oils are pretty good. I am not sure on the spark plug though I think the Ossa`s should run a BP5 or 6es. One thing you should check is the plug gap as I know the Bully`s are very particular about there plug gaps. The Ossa`s have an electronic ignition so should`nt be as fussy. Did it miss with the Amal on it. It sounds more like ignition than carburation. The air screw won`t affect the top end to much. To set the air screw have it idleling and adjust the air screw to give the bike its highest idle and that is usually it. Just make sure it revs cleanly. Sorry I can`t be of more help Chris. Graham.
I am also running the same oil but at 33-1.
I ran it at 25-1 in the original Amal carb but the plug was always very sooty with a lot of exhaust smoke. Many people suggested a change in the petrol/oil mix so that is what I have started with now that I am using the new Mikuni.
The bike never misfired when using the Amal at 25-1, it was just v hard to start. hot and cold. The slide looked very worn and moved around inside the carb body so a change was in order.
With the new Mikuni starting is way easier and it had been running very cleanly with the jets at 150 main, 30 pilot until the bike got hot, than the misfire started. The plug has no hint of soot so i will try it back at 25-1 to see how that changes things.
Many thanks, what a great series of thoughts on the link between oil/petrol mix, jet sizes, plugs and the the temp that engines run at.
Next step is back to the manufacturers recommended 25-1 and see how that helps.
Interestingly enough I have only recently tried leaning out the mixture due to every other person that I talk to at trials recommending that I do so. i.e "with modern oils you can go to 50-1, 100-1" etc etc.
One question though. If an engine is running hot and misfiring because of it what would be the correct plug type to change to. It currently has the manufactures recommended NGK BP-7E.
Well I have got the new Mikuni VM26 set up with a 150 main and 30 pilot.
Starts pretty good, and runs cleanly but it is now missing when it gets hot.
I am running it on 33-1 and with the air screw by 1.5 turns out. Rubber inlet hose in good condition with no leaks.
Looking at the plug I have noted that the NGK BP-7ES look pretty hot (ie no hint of soot, and a copper hue). Could this be the problem.
Any thoughts of the next step to get the bike running cleanly.
Is it worth changing the plug (to what type?), revert back to 25-1, or backing out the airscrew. Or all of the above!
Thanks in advance
Chris
Posted 13 September 2011 - 06:31 PM
Hi
I am wondering if anybody could help me.
I have just finished restoring an OSSA 250mar Mk 2(1975) and have bought a new Mikuni VM26 for it as the bike was running badly with an old worn Amal. It was hard to start and lost its tune very easily and constantly oiled the plugs
However after fitting the new Mikuni VM26 it starts easy but hiccups and wont rev out when even slightly warm.
The new carb has been set up with a 120 main and a 2.5 pilot but when running it appears starved of fuel.
Any thoughts on what the correct jetting and settings should be?
Any thoughts most welcomed
Chris White
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#2 tony283
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Bike:Yamaha/Triumph/RE
Club:AHRMA/ITSA/BARHAM
Posted 13 September 2011 - 08:43 PM
Use a 150-160 Main and a 35 pilot with airscrew at around 1.5 - 1.75 turns out.
Tony
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#3 black111r
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Joined:13-September 11
Location:Brisbane, Australia
Bike:Ossa 250mar 1975
Club:WDTC
Posted Today, 05:31 PM
Hi
Well I have got the new Mikuni VM26 set up with a 150 main and 30 pilot.
Starts pretty good, and runs cleanly but it is now missing when it gets hot.
I am running it on 33-1 and with the air screw by 1.5 turns out. Rubber inlet hose in good condition with no leaks.
Looking at the plug I have noted that the NGK BP-7ES look pretty hot (ie no hint of soot, and a copper hue). Could this be the problem.
Any thoughts of the next step to get the bike running cleanly.
Is it worth changing the plug (to what type?), revert back to 25-1, or backing out the airscrew. Or all of the above!
I have just finished restoring an OSSA 250mar Mk 2(1975) and have bought a new Mikuni VM26 for it as the bike was running badly with an old worn Amal. It was hard to start and lost its tune very easily and constantly oiled the plugs
However after fitting the new Mikuni VM26 it starts easy but hiccups and wont rev out when even slightly warm.
The new carb has been set up with a 120 main and a 2.5 pilot but when running it appears starved of fuel.
Any thoughts on what the correct jetting and settings should be?
Ossa 250mar 1975 misfiring
in Twinshock
Posted
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