pmk

Kind of as the title mentions, newbie question. I notice it seems the chain tensioners, whether on a modern bike or vintage machine use a non roller style wear block. Is there a valid reason for the slide to be preferred over rollers?

Good option.

My 73 MAR 250 has an IZD carb installed on it. This was what came stock on these machines. I am compiling a list of items needed, all cables are needed. Problem is, no where do I see the cable to go from the Amal throttle to an IZD carb. Would it be the same cable ad for an Amal carb or Bing?

After getting it running today, it is appaarent that the exhaust note is a bit harsh. Without doubt the spark arrester / silencer needs repacking. As for the mid pipe, I plan to rework that and repack it also.

Spent some time today doing the common clean the carb, lube cables and assemble it enough to see if it runs. After a bit of verifing spark, ensuring fuel she would give that poof like it wanted to run. Accidentally, I overlooked a paper towel in the head pipe, and that poof just sound like a plugged exhaust, some since the slincer had not been cleaned or verified clear, I pulled it thinking it was plugged up. So my care while washing and that paper towel had moved into the mid pipe stinger. Next kick it blew the paper towel out and fired up. Ran it several times on the stand. Engine sounded good with no bad sounds. Next, I will give it a couple easy runs to ensure the gearbox feels good under a load. After that, it comes appart for some cosmetic refinishing, and I plan to split the cases, replacing seals and bearings, with hopefully no issues on internal stuff. So at this point, it looks like she will be a nice machine, not restored but for riding while keeping the vintage theme.

The Motoplat, even after the abuse of sitting and corroding, cleaned up and fired the spark plug allowing it to run.

Found it. As mentioned, the case breathes via a small passageway at the center case screwhead above the countershaft.

Mine has the single needle IZD carb. Original owner mentioned it ran well with it for many years.

Mark, I had plans to do a case leakage test, but held off for the moment. When I removed the flywheel and stator, I noticed the crank seal sitting a bit proud. I did not mess with it at the time, simply went about cleaning the yuck from that area before placing the stator and flywheel back on. Curious, beyond the simply mark the stator to reinstall it, and wanting to read how the timing is set I browsed through the manuals, both my Clymer and the online version. As I read, I payed close attention to the photos that showed the seal and the text read the seal should be flush with the case. Not sure if the original owner installed a thicker seal or what, I did not want to set it flush. Rather I grabbed the seal with my super short finger nails and it pulled right out. Basically, the stator backing plate retained the seal from completely coming out. I cleaned the seal and for the initial test run, reinstalled it flush. I am not sure why the seal was sitting proud. Possibly the bike was sunk and water filled the bottom end, popping the seal out. Maybe, since the flywheel and stator had been removed previously, the seal was replaced and never seated. I honestly don't know. My friend I got the bike from told me as we loaded it up, she probably needs rings or a top end since it was down on power last time he rode it. Maybe the seal was leaking and it was one of those lean enough to need jetting but not lean enough to quit running. Again I don't know. At this point, once I know the ignition is good, carb is ok, and it at least fires up, It will come back apart, changing those seals and others at a minimum but probably split the cases and do more. Simply time will tell, it all adds up. Tires, cables, shocks, fork seals, paint work, fuel tank(?), chain tensioner stuff and maybe more. Best part is the parts are out there. Still have not gotten my 81 KTM 495 back together yet. Some of it on parts availability, some on simply refusing to pay far more than fair market value from dealers and private sellers.

Yes, it does have matching numbers and I agree parts are available. The possible wrench in the works is how much it will cost to make good vs what it is worth. I am not really into making it a perfect restoration, rather a nice rider that retains the look and ambiance of a vintage trials bike, I think...

I first will say I appreciate the experience and knowledge here. I grew up riding when these machines were being made, but never considered them as I was a kid growing up racing MX when the Japanese bikes arrived and became dominant. Fast forward to 2016, I wanted a trials bike. Many reasons why. My good friend and riding buddy purchased a 72 MAR about a year ago that was supposedly very nice. He learned it was good but lacking in the fine details and he does not have or own junk. A few weeks ago, knowing he needed some original parts, I noticed another MAR on a Facebook group that was for sale reasonably inexpensive. He acquired it. So he had plans to build a very original machine with original parts, and a second rider. His words were that of we could ride together. During the time while his second bike was in transit, the wife and I travelled for both family stuff and vacation. The wife and I ride tandem bicycles, both on road and tandem mountain bike. During an event, I spent time with a friend and his wife. With a combined age of my friends at 160 years, it is always fun to see them and ride with them. They still to this day ride a tandem mountain bike off road. During one of the meal gathering, I was talking with my tandem mtb riding friend, and long time dirt bike rider, about trials bikes. He knew I wanted one, and we discussed my local friends two MARs. He informed me he had one that he had purchased new in 1972. He had ridden it a bunch, but his wife also told me it was her favorite dirt bike of all those she rode. The following morning, they insisted I pick up the MAR and if possible bring new life into it, or if beyond reasonable effort and expense to get going, use parts as needed to build one of the other two into a more original machine. So we drove to their home on our return trip to Florida, and picked up the MAR. It had been sitting for a while, stored outside, but had been run randomly but not in a while. The owner said the last time he rode it the clutch was stuck and would not release. She is rough cosmetically, but viable still at the moment. Ironically the engine is free and smooth.

I have studied the parts diagram and notice there is a plastic tube inside the gearbox with slotted ends that serves as a gearbox breather. In my cleaning up of this old machine, removing concrete like mud, I have yet to find where this breather vents to ambient outside air. Could someone please explain where the gearbox breather vents externally? Thanks

Agree, the taper must not be abrasive blasted or cad plated. I considered this before removing corrosion. It will be possible to install a bolt through a washer, then through a rubber washer, through the flywheel taper, then a rubber washer followed by a washer and nut. This once tightened will seal the taper from abrasive grit blasting and the liquid cadmium bath. Previously I worked in an aircraft propeller repair shop. I will use them to replate the items needed and should be minimal cost if any. So I think plating it should be done, especially since where I live, here in Florida the humidity is high any everything rusts. As for the added vent, I had read about people doing this. Since the previous owner had some type of failure that cracked / broke the side cover top to bottom just aft of the clutch actuator, the case will be repaired and is a viable candidate to be modified after welding.

Photo before corrosion was removed and after removed.

When I got this machine, It had been sitting outdoors for a while. The tires were flat and the thing looks rough. The engine would not spin via turning by hand using the kickstarter initially. After a couple light attempts to just have it turn a little, it did come loose. My first thought was a stuck piston, but later turning the flywheel by hand, the engine is smooth. So I suspected and confirmed after removing the ignition side cover there had been some water inside it from sitting outside. Enough water to partially submerge the lower magnets and stator poles. These rusted and made the motor stuck. Fast forward, bike is home, pulled the flywheel and it certainly is not bare steel. There was very little reddish rust, mostly the white powder almost as if it was zinc or silver cadmium plated. Removing the corrosion, removed the coating. Currently, the flywheel is bare steel and coated with an oil to keep it from rusting. I had a look at various photos on the net after my initial post and most flywheels had a grayish color. Assuming the Motoplat fires the plug and the ignition is usable, I will probably have the flywheel and other various bits of hardware cad plated. Thanks for the replies. Still not disappointed, just some effort so far. Want to get it fired and ensure it runs compiling a list of needed items. Then pull the engine and chassis apart assuming it is viable to fix up vs part out.

My 1973 MAR has some corrosion on the flywheel and stator. With care, I have cleaned up the poles of the stator. To clean the corrosion from the flywheel I used abrasive cloth and cleaned the magnets and interior. I also used the abrasive cloth to clean the flywheel exterior. Removing the corrosion also removed whatever protective coating was there, so I ask, does anyone know what the oem finish was and what color a new flywheel would be.

Side tracking my own topic, I had no plans to buy an OSSA. My riding buddy always wanted one and picked one up last year. He has spent some time and dollars on what was supposedly a very good original bike. I recently noticed one and informed him of it as it had some original parts he needed, that one was reasonably inexpensive. So now he has two to build one creampuff and one rider. He thought it would be cool so we could ride together also. Still swayed more towards a semi modern trials bike, or more common TY250, I kept looking as the wife asked me to hold off a bit. We recently did a bicycle event that had us travel quite a distance but allowed us to see friends. While there, I was talking dirt bikes with a friend and told him about the MARs. He informed me he had one that he had bought new in 1972. The next morning at breakfast he insisted I drive to his house and take it home with me. So it made a 1000 mile trip. His wife told my wife that if our Tacoma pickup was not full with a bicycle and now an MAR she would have me take home more dirt bikes... He knew it was going to a good home and it would either be made into a rider or have parts pulled for the creampuff. Trying to get it where I feel comfortable to start it and cause no damage. Just takes time.

Wanted to that if needed, I will repack the midpipe, maybe mod it so it can be repacked easily as needed. No plans to build a restored MAR, I want a reliable rider. Would like quiet also as I hope to get it tagged for trail hopping and riding close to homes.

This machine I got from the original owner. I learned from my friend that is building his 72 MAR, that we do not have the oem pickle silencer. It seems the setup, maybe just here in the USA was to replace the oem silencer with an aftermarket silencer / spark arrester. My friends 72 and this 73 both have identical silencer / spark arresters that can be easily repacked. Inspecting parts and learning OSSA quirkiness as I pull it apart to clean and evaluate. This old machine was last ridden years ago and has been stored outside for a while. Looks rough but is mostly there, but does need some TLC and someb parts. So far everything I had concerns about is coming apart pretty good after some penetrating oil. This machine has what is possibly a later model aliminim skidplate. Those slotted screws were not moving and the mud had cured like concrete so I did drill the heads off and all is apart no worse except needing screws. Next is removing the flywheel and ignition so I can get that area cleaned. Need to verify spark, carb looks good somaybe during the weekend I can get it running. Then go from there. Nice to work on a simpler bike, though I don't envy splitting the cases but can if needed, read about the shimming, seems tedious, but not difficulult.

Woody, it does appear the midpipe was possibly packed with sound absorbing material when new. I questioned it being baffled as no light would pass through. After reading these replies, I found a nylon hose and fed it through the midpipe until it appeared at the opposite end. Apparently light would not pass on account of some kind of insect nest. The nylon hose cleared it, and now light can be seen through. If this possible runner does not become cost prohibitive to make into a daily rider, I will repack the pipe at that point. Currently working my way through the details to get it appart enough to ensure no damage is done if and when it starts. Thanks for the info on this topic and many others prior to it.

Bondy, when you say just did yours, what was accomplished?

I have taken the time to search and read about these bikes. I recently acquired a 1973 MAR and was wondering if the center exhaust section contains or contained at one point packing material, or is it baffled, or simply a small expansion chamber? I ask on account of wanting to clean it off carbon deposits internally and externally prep it for paint.

Recently acquired a 1973 OSSA MAR from the original owner. Rough but mostly there. Not running and needs some attention. The MAR was not a planned machine to own, was looking more at a modern trials bike but this was offered to me knowing it was going to a good home. Time will tell if it is viable to fix up or is cost prohibitive. Always wanted a trials bike, hope to attend some of the Florida events, even if as a spectator.