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black111r

Ossa 250 Mar Miuni jetting question

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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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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!

Thanks in advance

Chris

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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.

Edited by bullylover

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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.

Theoretically the bike should run cooler.

Chris

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if your running synthetic 2 stroke, then leave at 33/1 or better go to 50/1. you may need to go up on the pilot to 35 as tony suggested. afer you try that, then maybe up the main slightly, and try lifting the needle.

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Okay what should I try next?

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?

Any thoughts

Chris

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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.

Any thoughts

Chris

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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.

Edited by feetupfun

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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

Thanks

Chris

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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

Thanks

Chris

Chris - I think as David has pointed out your Ossa has a CDI so there are no points or condensor - obviously much more advanced than the other spaniards of the day.

Edited by keychange

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