ferret1964 Posted March 14, 2011 Report Share Posted March 14, 2011 Hi fairly new to this forum, i have just started trialing a 1974 Ossa 250 mar, and i have a few mods i would like to do so any advice regarding these mods would be much appreciated. Firstly, front suspension seems very soft,and and appear to creak a bit when riding, forks are mainly compressed when riding and dont seem to have much travel, i have put heavier weight oil in but it doesnt seem to have made much difference! i am 6'3" tall and a bit on the heavy side, at present the bars seem a bit low(renthal 3"rise) which puts all my weight over the front end, i have ordered some 6" rise renthals so this might help and then will posibly add some bar risers to get me more upright, but any suggestions which may help would be gratefully appreciated. Secondly kick start feels awkward, being on the left it appears to be too long, has anyone replaced the long kickstart with a shorter one, if so from what? , or has anyone cut and welded the origional to make it a little shorter (would it be strong enough, cut and welded or will it be too weak} thanks in advance for any help and suggestions stuart Quote Link to comment Share on other sites More sharing options...
old trials fanatic Posted March 14, 2011 Report Share Posted March 14, 2011 Hi fairly new to this forum, i have just started trialing a 1974 Ossa 250 mar, and i have a few mods i would like to do so any advice regarding these mods would be much appreciated. Firstly, front suspension seems very soft,and and appear to creak a bit when riding, forks are mainly compressed when riding and dont seem to have much travel, i have put heavier weight oil in but it doesnt seem to have made much difference! i am 6'3" tall and a bit on the heavy side, at present the bars seem a bit low(renthal 3"rise) which puts all my weight over the front end, i have ordered some 6" rise renthals so this might help and then will posibly add some bar risers to get me more upright, but any suggestions which may help would be gratefully appreciated. Secondly kick start feels awkward, being on the left it appears to be too long, has anyone replaced the long kickstart with a shorter one, if so from what? , or has anyone cut and welded the origional to make it a little shorter (would it be strong enough, cut and welded or will it be too weak} thanks in advance for any help and suggestions stuart Sounds like your fork springs are shot. Fit new ones. In Motion should be able to help. 6" Renthals should be about right. Drop the footrests, normally min 2" and back 1" - 2" and fit wider more modern ones. In my experience if you shorten the kickstart you will struggle to kick it over due to the primary gearing but thats up to you. Try it you can always buy a new one. They are long for a reason. Quote Link to comment Share on other sites More sharing options...
woody Posted March 14, 2011 Report Share Posted March 14, 2011 Front forks on the MAR work very well and were quite a soft action when new, although they never bottomed or topped out. Their action is actually very good for a fork of that age. The springs should have spacers fitted at the top. Usually one per leg and about 20mm long (guessing). You can try using longer but if you need to go longer than say a couple of inches, chances are the springs have sagged with age. Remember though, they are a softly sprung fork, not like a modern Beta or GasGas in their action (ie: very bouncy) Oil quantity between 180cc and 200cc per leg. Start at 180 and 200 is definitely the maximum for those forks. SAE 10 is probably best, but ultimately, quantity and thickness is personal preference. Footrests can be lowered by adding plates extending rearwards from the frame tube below the original footrest position. These will take new hangers for modern rests positioned just level/above the bashplate line. No lower (see photo) The added plates will need gusseting behind to stop them bending. That with 6" bars should give a comfortable riding position as well as positioning you lower in the bike. It's easier to start by using bar risers under the clamps to achieve the effect of having old style 9" high bars, which is roughly what we used back then, but you are still perched high on the rests in their original position, highering your centre of gravity compared to lowered rests. If you can lower the rests, it is a better solution. You'll get used to the kickstart. If you shorten it you may find that the kicksart gearing and high engine comnpression make it really difficult to start. Also, the Ossa has a habit of occassionally slipping on the kickstart ratchet resulting in all your leg energy kicking against no resistance when it slips. If you have a shorter kickstart, when this happens you will know about it as the results can be quite painful to calf and foot with the standard kicker when it lets go and you'll be kicking harder with a shorter lever. If your cylinder head has a secong plug hole, fit a decompressor. You can use this to help starting and also save the 'ratchet slip'. Pull the decompressor when you kick it and let it out after you hear the first couple of engine rotations and continue the kick through it's full swing. The lack of compression with the decompressor pulled in makes the swing of the kicker easy by removing the resistance of the engine compression. Then, as you let it out compression is restored and the engine should fire. Easier on the leg and easier on the ratchet gear. It's also a good idea to first spin the engine over twice using the decompressor and the throttle wide open when cold. This helps prime the engine and it will usually fire first kick. The MAR from that period is a good bike and very forgiving to ride, so hopefully you should enjoy trialing it. Quote Link to comment Share on other sites More sharing options...
johnnyboxer Posted March 14, 2011 Report Share Posted March 14, 2011 (edited) Hi fairly new to this forum, i have just started trialing a 1974 Ossa 250 MAR I am 6'3" tall and a bit on the heavy side stuart Keep off the pies Fit some new springs and oil, loads of good advice here..................much more than I know about Ossa's Edited March 14, 2011 by Johnnyboxer Quote Link to comment Share on other sites More sharing options...
ferret1964 Posted March 14, 2011 Author Report Share Posted March 14, 2011 Keep off the pies Fit some new springs and oil, loads of good advice here..................much more than I know about Ossa's ime not that new johnny ___k off Quote Link to comment Share on other sites More sharing options...
johnnyboxer Posted March 15, 2011 Report Share Posted March 15, 2011 ime not that new johnny ___k off Top of the day to you, sir........................and it's like the 'pot calling the kettle black' coming from my goodself Quote Link to comment Share on other sites More sharing options...
ferret1964 Posted March 15, 2011 Author Report Share Posted March 15, 2011 Top of the day to you, sir........................and it's like the 'pot calling the kettle black' coming from my goodself Thanks for the advice Woody, i will have to look into the footpeg conversion, do you know anybody locally that can carry that type of work out, Ime not far from Wolverhampton/Birmingham areas. thanks Stuart Quote Link to comment Share on other sites More sharing options...
woody Posted March 15, 2011 Report Share Posted March 15, 2011 Try Colin at TY Offroad http://www.tyoffroad.webeden.co.uk/ Quote Link to comment Share on other sites More sharing options...
ferret1964 Posted March 15, 2011 Author Report Share Posted March 15, 2011 Try Colin at TY Offroad http://www.tyoffroad.webeden.co.uk/ Cheers WOODY, just spoke with Colin, hopefuly going across on Monday to have a chat and see. Quote Link to comment Share on other sites More sharing options...
amo Posted March 15, 2011 Report Share Posted March 15, 2011 I've done the kickstart mod to a couple of bikes, seems to work ok, not used the bikes regularly though I took approx 2" out of the vertical section when viewed on bike Quote Link to comment Share on other sites More sharing options...
ferret1964 Posted March 20, 2011 Author Report Share Posted March 20, 2011 I've done the kickstart mod to a couple of bikes, seems to work ok, not used the bikes regularly though I took approx 2" out of the vertical section when viewed on bike Thanks for all the replys, ive fitted new fork springs today, obtained from inmotion bultaco, when i compared them to the old springs, the old ones were 3" shorter so dont know if previous owners had inserted shorter softer springs, I have also fitted the 6" rise renthals and a small 1" bar riser, have to say the bike has been transformed, ime far more upright on the bike and the front end of the bike is now sitting up properly and not sagging as it did before. Just got to get the wide foot pegs i have purchased fitted now (2" back and in line with the bottom of the bashplate), cant wait for the next trial now to see the real difference. Thanks again Ferret Quote Link to comment Share on other sites More sharing options...
trialsrfun Posted July 28, 2013 Report Share Posted July 28, 2013 Front forks on the MAR work very well and were quite a soft action when new, although they never bottomed or topped out. Their action is actually very good for a fork of that age. The springs should have spacers fitted at the top. Usually one per leg and about 20mm long (guessing). You can try using longer but if you need to go longer than say a couple of inches, chances are the springs have sagged with age. Remember though, they are a softly sprung fork, not like a modern Beta or GasGas in their action (ie: very bouncy) Oil quantity between 180cc and 200cc per leg. Start at 180 and 200 is definitely the maximum for those forks. SAE 10 is probably best, but ultimately, quantity and thickness is personal preference. Footrests can be lowered by adding plates extending rearwards from the frame tube below the original footrest position. These will take new hangers for modern rests positioned just level/above the bashplate line. No lower (see photo) The added plates will need gusseting behind to stop them bending. That with 6" bars should give a comfortable riding position as well as positioning you lower in the bike. It's easier to start by using bar risers under the clamps to achieve the effect of having old style 9" high bars, which is roughly what we used back then, but you are still perched high on the rests in their original position, highering your centre of gravity compared to lowered rests. If you can lower the rests, it is a better solution. You'll get used to the kickstart. If you shorten it you may find that the kicksart gearing and high engine comnpression make it really difficult to start. Also, the Ossa has a habit of occassionally slipping on the kickstart ratchet resulting in all your leg energy kicking against no resistance when it slips. If you have a shorter kickstart, when this happens you will know about it as the results can be quite painful to calf and foot with the standard kicker when it lets go and you'll be kicking harder with a shorter lever. If your cylinder head has a secong plug hole, fit a decompressor. You can use this to help starting and also save the 'ratchet slip'. Pull the decompressor when you kick it and let it out after you hear the first couple of engine rotations and continue the kick through it's full swing. The lack of compression with the decompressor pulled in makes the swing of the kicker easy by removing the resistance of the engine compression. Then, as you let it out compression is restored and the engine should fire. Easier on the leg and easier on the ratchet gear. It's also a good idea to first spin the engine over twice using the decompressor and the throttle wide open when cold. This helps prime the engine and it will usually fire first kick. The MAR from that period is a good bike and very forgiving to ride, so hopefully you should enjoy trialing it. I have just found this very interesting article, thank you Woody for the details. Have you changed the brake pedal or does it operate ok as it is with the new lower footrest position? Quote Link to comment Share on other sites More sharing options...
woody Posted July 29, 2013 Report Share Posted July 29, 2013 I left the brake pedal as it is. In reality, there is little you can do with it due to the design but I can still get to it ok where it is. Quote Link to comment Share on other sites More sharing options...
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