pmk

New to trials stuff, but longtime rider. Silly question, can the drum brakes be too powerful on a vintage machine?

The modern Ohlins like many modern rear shocks often does require special tools to work on. I may be wrong about your specific shock, but most modern shocks operate best and last longer between rebuilds when vacuum bled. As a person with 40 years experience rebuilding suspension, yes almost any rear shock can be rebuilt without vacuum, but vacuum bled if the design supports it, is far superior. The KTM WP, PDS series is a good example of a rear shock that does best being vacuum bled. I designed and built my vacuum bleed equipment about two years prior to the trend of folks selling DIY vacuum bleed setups. Having done vacuum bleeds for about 12 years now, trust, it does do a better service if accomplished correctly. If the damper is not provisioned with a bleed port, a good shop can hand bleed the damper.

Yes there were bug nests inside the silencer, but none in the pipe. Those did fly out once it fired up and I slid the silencer back on.

Ohlins USA is in North Carolina, no shame in a factory rebuild

Schoolboy mistake I made on mine. While cleaning the entire machine of the previous owners last ride dirt in the 80s, I removed the carb and exhaust. I stuffed several paper towels into the intake spigot and exhaust port, then sealed them with duct tape. As i reinstalled the carb, I removed duct tape and paper towel. Installing the exhaust, not all of the paper towel came out. My prestart process had me visually inspect the cylinder wall via the exhaust port, and add two stroke oil, plus many turns of the engine by hand with the kick starter. When I tried to get it fired up, it always acted as if it wanted to run, but refused. Finally, i removed the silencer / spark arrester and gave it a kick. A paper towel flew out 10 feet. Next kick it came to life. No assurance this is the problem, simply a suggestion and means to laugh at myself.

Thank you Woody. That is what I was thinking, but not certain about it.

OK guys, I have read the Jubilee clamp comments a few days ago. Without sounding totally dumb, where is the Jubilee clamp actually located OR is this not an oem part inside the engine? As best I can tell, my own 73 MAR, does not have a Jubilee clamp installed, unless it is between the right side main engine case and the shifter claw that actuates the shift drum. I do recall, when I had the clutch side cover off, placed into a groove in the shift shaft was a snap ring / circlip external type, followed by a flat washer (which could be a hardened thrust washer), followed next by a spring washer, then the side cover. Does this sound correct? If so, where does the jubilee clamp reside? Thanks. I am making progress but have had some other requirements take time away from the OSSA.

Woody and others offered good info. I suspect that you may have forgotten to install the snap ring (I guess Jubilee clip is another name for it) and the thrust washer on the clutch end of the shaft. With those parts missing, the shaft is able to move left, and will see the same problem again. The others posting are well versed in these OSSA machines. I learned about the shift shaft end play when I removed the clutch cover. You mentioned. 1/2" of movement on the shaft. With my clutch side cover removed there is that much end play. With the snap ring and thrust washers fitted correcty, the shaft end play is zero. Try moving the shift shaft outboard, if it can be drawn out of the side case, you may want to remove the side cover and ensure all parts are installed.

Want to add, if you start plotting the leverage ratio curves at various points, you may find many twinshock machines have a very slight almost microscopic rising rate in regards the suspension geometry. If the machine utilized a dual spring or progressive wound spring, it artificially has a rising rate in regards to the spring force. As for gaining a progressive damping curve to artificially accomplish a rising rate of damping, you must look inside certain dampers. Researching the KTM PDS modern off road machine you will find internally they utilize a needle that alters the damping curve based on the dampers position of the shaft. This design not only adds progression but is position sensitive as well. This trials thing is new to me, however dirt bikes are not. My own twinshock I am working to finish as a rider is a 73 OSSA MAR. The original rear coilover shocks were Betors and on the machine when I got it. They had progressive wound springs. On account of corrosion on the shafts, the units have been replaced. The replacements are 10mm longer. Not concerned about that. The shocks are vintage Curnutts. These were a popular upgrade on dirt bikes back in the 70's. Internally these Curnutts use metering needles in some ways similar to the KTM WP PDS. So, my 73 vintage OSSA, with the slightest of slightest mechanical rising rate based on suspension geometry, will artificially produce rising rate on account of progressive wound springs and position sensitive damping within the damper itself. Will it be better or worse, time will tell. FWIW, trying to compare a race car chassis and suspension design to a motorcycle is often not a wise idea. The full chassis dynamics is considerably different in how the wheel movement arcs, the effects of drivetrain squat / anti-squat, and more. The RaceTech text book is a good read and reference book. While I do not agree with everything Paul Thede states, the book is well written and does a good job explainig moto suspension.

Not sure if your reference regarding shock mounting and the angles involves the upper or lower mount? If the lower mount is moved forward on the swingarm that was / is called forwarded mounted. When the upper mount is moved forward, typically that is called laydown. With the travel being not very long, the swingarm arcs are small and it takes a pretty serious excess forward mount to have bad falling rate. Linkage designs are more intricate, based on the location of the pull rod mount points and how much they position and rotate the relay link. The comments about spring rate and proper sag is true. Those factors can take any motorcycle, whether road or off road and make it stellar or a turd. Unless well versed and experienced in suspenion design, leave it stock or with proven link mods and focus more on proper springs and quality damping.

Woody, thanks so much for checking that. No doubt OSSA has a simple low cost method with the O ring, but it does become more complex if leaking. Back in the day, I guess the idea of pulling the engine, and splitting cases for a small leak or replacing a bent shift shaft was no big deal. Possibly I became spoiled by Japanese engines allowing an entire shift shaft to be removed by pulling the side cover and clutch. Regardless,, and with no leak on the right side shift shaft, this MAR is moving forward. Thanks again very much for checking that.

Good info guys. Thanks. The countershaft side seems to not be weeping, but clutch side is weeping slightly. For future planning, if the right side shift spline was cut off and deburred, would the shorter shaft then allow Oring replacement without splitting the cases?

I have a 73 MAR, but it appears many OSSA models are similar regarding the shifter shaft and the O rings that seal the shaft. My MAR has a splined stub on both sides of the engine, and could be run as left or right side shift. The O ring in the clutch cover is easily replaced by removing the clutch cover. With the clutch cover removed, can the shifter shaft be removed to replace the right side O ring or must the cases be split to replace the O ring. When I had the clutch cover removed previously, I noticed there was quite a bit of movement where possibly the shaft could be removed, but I did not attempt it.

In general, gas shock with a seperation device, either bladder or floating piston can be positioned at any angle if serviced correctly or has not lost any fluid. Emulsion gas shocks, that do not isolate the the fluid from gas need to be positioned in accordance with what the manufacturers recommends. As for inverting a shock after it has lost fluid can make the bike feel good during the initial part of suspension movement, but as the suspension compresses further, it is likely there will be no damping to slow the bottoming action, and the shocks themselves will be hammered into useless condition.

For whatever reason, the InMotion website no longer loads or directs to the site from my laptop or IPad. Anyone know what is going on with them?

As for the Curnutts being overdamped, the recommended oil was Type A transmission fluid. Seems Curnutt even recommended a mixture of Type A tranmission fluid and Diesel fuel. With so many fluids and weights available today, and the pretty basic design, I think they will respond well to different viscosities. The metering rods appear to engage well into the shock stroke, basically the great grandfather of the KTM White Power No linkage PDS. Regardless it will be fun.

Do you happen to know who has seals? The one pair of my MAR have badly pitted shafts, but good everything else. The pair my friend has were hammered and the pot metal sealheads are shattered. His shafts are not perfect, but should polish up ok I hope since that will be a display bike. My friends second MAR will be his rider. Like mine it will not have stock rear shocks or a stock carb. Ultimately, we should have two fun MAR riders.

The budget on cost was perfect, so no loss in trying the Curnutts. No doubt, The spring rates may be a big factor unless the oem Betor springs could be salvaged and made to work. As for Curnutt damping, I have rebuilt many shocks but these will be my first Curnutts. I also got a set of probably first generation Works Performance shocks. Those would need springs in order to work and the cooling fins will probably be a concern unless I cut a good portion of them off. Time will tell how it comes together.

Still moving forward. Guess I am doing a restomod as they call it with cars. Got a Clarke plastic tank in white. Broke the budget and ordered a Smartcarb. Got a box today with vintage shocks, probably going to install Curnutts. The rest will hopefully be ordered soon and seems available. Any thoughts on the Curnutt shocks? I also got a pair of very early Works Performance shocks, those would need softer springs and not so sure the finned bodies would fit well. Need to add, some of the items off my bike are headed towards my friends MAR he is making original. IZD single needle carb, 4 shocks hoping to make two originals, original headlight and wire harness, etc.

Local ACE Hardware had 7mm hex head bolts in stock. Not ideal, but fine for getting the skidplate back on and being able to safely keep it on the stand to work on it. Thanks everyone.

Good info on Menards. Those stores are not in this area, but we do have one local hardware chain that may carry them. I have been checking Fastenal also. These skid plate bolts are oddballs. Before I get too crazy, I think checking with some of the OSSA parts guys here in the USA may be best. I will post how it is sorted out.

Thanks Woody and others for the reply. I have been looking at EBay, Fastenal and several other online sources with minimal luck. Since I am doing this bike kind of restomod, rather than restoring it, it may be easiest to do as you suggest and convert to 8mm at all mount locations.

Here in the US, 7mm fasteners are not common. Not that I need many, but is there a reasonably priced source for this size fastener. The real oddballs I need, since I had to drill them to remove the skidplate, are the 7mm flush head slotted screws.

In regards to the Pirelli MT43, no doubt there are mixed comments about the history and different variations over the years. So, I recently acqired a 73 OSSA MAR. It still has the square block MT43 tires on it but they need replacing. Seeing as this machine is kind of porky with a claimed weight about 50 pounds more than a modern bike, anyone have opinions on if these older machines should use the older style tires with a more rigid casing or go with the more modern lighter tires? At the moment, if the bike is used for any competition, it will be fun based, primary plan is fun riding with a friend who also has a 72 MAR.

Guys, thanks for the replies. The machine I have is a vintage OSSA MAR, so the rear travel is pretty short travel. As original it has a springloaded tensioner with rubber type material slide block. Seems the slider style is still viable as they are used on modern machines also.