Ossa Mar Porting

[woody]

Here's a question for those who understand port timing - which I don't

Having a few problems with gear selection set up on my '75 310 MAR. Under the bench is another 310 engine I picked up about 25 years ago. It's a later engine from a green bike. I stripped it to compare selector mechanism and set up but whilst apart I thought I'd compare porting as I'd heard that the later bikes were different.

On the 75 bike, the inlet port measures 25mm top to bottom and is 25mm high in the barrel (from bottom of liner to bottom of port)

On the 79 bike, the inlet port measures 20mm top to bottom and is 30mm high in the barrel

On the 79 bike the port is also narrower in width but I couldn't measure by how much. So basically, smaller size all round and higher in the barrel meaning it opens later.

On both bikes, the distance from the top of the port to the top of the liner is the same. Both use the same piston.

Exhaust port and transfers all appear the same.

So the question is, what does this mean and how does it translate to engine performance? Just curious as I'd heard that the later bikes were sharper than the earlier lazier bikes. The later engine has been fitted with a huge counter weight on the clutch in place of the cush drive. It's a home done job but very neat but it weighs nearly as much as an ignition flywheel which suggests someone was trying to calm it down - why I've no idea as they were hardly 350 Bultaco / Montesa power.

Finally, anyone any idea of sources for gear selector shafts and return springs - I'm running out of useable items trying to sort this gearbox.

[mick annick]

Woody

Can't help with the porting question, I'm a bit technically inept; have you tried Steve Sell for the parts?

Mick

[woody]

No, haven't got a contact number or email for him - do you have one?

Just managed to source a decent selector shaft from ebay USA though.

[jay_lael]

The top edge of the exhaust port controls when the port opens. If it had been raised higher the "power band" would be higher in the rpm range with a slight increase in higher end power. By lowering the bottom edge of the port, and narrowing it slightly, they have increased the duration of the exhaust exit time, and narrowing the port incdreases the velocity of the gases as they escape. I suspect this mod would provied quicker low end response. A true tuning expert, such as Jon Stoodley could explain this more clearly. I'd be curious to hear how the two barrels compared in actual real world testing.

[woody]

Jay - it's the inlet that differs, exhaust and transfers are the same on both

[jay_lael]

Jay - it's the inlet that differs, exhaust and transfers are the same on both

Ooops. My bad.

[jc2]

Woody, I'm fairly sure the original MAR porting was the same as the Pioneer, which I always tho't was a little radical for trials, but I never rode an MAR. It was probably tamed by the heavy flywheel.

As I understand it the smaller inlet port/tract does increase the inlet charge velocity & may well give better crankcase filling at low RPM which probably would make it a little sharper response.

5mm higher on the bottom of the inlet (which controls inlet timing) is quite a bit. It amounts to about 20deg less inlet duration which you would think would knock a bit off the top end (both rpm & HP) but it also means less blow-back at very low RPM (the port closes 10deg earlier), making it smoother down low & (again) a crisper response.

Edited October 26, 2013 by jc2

[twinshock giles]

I had a very early 250 MAR with the short swinging arm which had a completely different power delivery to the later bikes - it was very very snappy (and also surprisingly quick on the road), I remember checking it at the time to see if the crank or flywheel had been lightened but it all looked standard so I would say that porting definately changed quite significantly over the years. What return springs do you need as I may have some but its likely to take me a good while to find them out, a photo would probably be good as most of my stuff is mixed up at the moment.

Giles

[woody]

Hi Giles, it's the return spring on the selector shaft but there are two sizes, so I'll need to measure it when I next get back into the garage. All I know is it is the larger one for the shaft that has the larger diameter piece where the spring locates (rest of the shaft is the same)

Not sure about the porting on the MK1 barrel but they had a smaller crank assembly so, I think, different crank compression and should rev up quicker. They fitted a larger crank to the MK2 and the rest from '74 onwards.

I had a go on a green 250 recently and was surprised at how well that picked up off the throttle, a fair bit quicker than the MK2 version that I'm more used to

[trialsrfun]

I had a go on a green 250 recently and was surprised at how well that picked up off the throttle, a fair bit quicker than the MK2 version that I'm more used to

Am following this topic with interest as I am currently restoring a green TR77 250. How was the gearing on the one you rode Woody, does anyone know the best sprocket sizes to opt for?

[twinshock giles]

Hi Giles, it's the return spring on the selector shaft but there are two sizes, so I'll need to measure it when I next get back into the garage. All I know is it is the larger one for the shaft that has the larger diameter piece where the spring locates (rest of the shaft is the same)

I'm sure I should have one of those, will have a look at the shafts I have when I can get at the parts.

[woody]

Just come in from the shed after reassembling the engine. I managed to get a good shaft from ebay USA so that is now in the engine.

The shaft I have with the weak spring is the only one I have with this larger spring - typically. The ID where it sits on the shaft is 25mm. All of the others are 20mm.

Don't make a special effort to look for one yet Giles, as although I'll want one eventually, the immediate panic is gone due to getting the shaft from ebay. I'll also speak to Steve Sell this week as I'll be at the two Miller rounds next weekend and he usually rides one or both of them so if he has one he can take it with him. I'll let you know after that whether I still need one.

[woody]

Am following this topic with interest as I am currently restoring a green TR77 250. How was the gearing on the one you rode Woody, does anyone know the best sprocket sizes to opt for?

The gearing I would guess as being standard. The original MK1 MAR used 12 : 46 which was changed to 11: 42 from the MK2 onwards.

The two bikes I had back in the 70s were both MK2. They were pretty quick on the road, especially compared to the '71 Sherpa that I'd had before which must have struggled to get 50-odd mph (never had a speedo on it) The Ossa would see 75mph on the speedo flat out and in a drag race could beat my mates' (3 of them) Honda CB200 off the start up to about 50mph until I changed into 5th which was a big drop from 4th and then they pulled away.

The gearing as standard was always a bit tall and these days I tend to run 10 : 42 instead of 11 : 42 as it gives a lower 1st gear which, with the heavy flywheel affect, makes it very difficult to stall and the clutch virtually redundant. The only problem with the 10 tooth sprocket is the chain can catch the sprocket nut, you need the nut with the boss on the inside which moves the actual nut outwards and keeps the chain away from it.

FYI - I use Barnett friction plates in my clutch and these were a huge improvement over the suflex plates I'd used previously. Break drive immediately with no drag and hardly ever stick if the bike has been standing. Because the plates are Kevlar or suchlike they bite harder so can be run with less spring tension which makes for a lighter clutch action. I got them from Doug Elke in the USA but he seems to have disappeared now and I haven't yet found anyone else that does them.

[longlegscanbeuseful]

FYI - I use Barnett friction plates in my clutch and these were a huge improvement over the suflex plates I'd used previously. Break drive immediately with no drag and hardly ever stick if the bike has been standing. Because the plates are Kevlar or suchlike they bite harder so can be run with less spring tension which makes for a lighter clutch action. I got them from Doug Elke in the USA but he seems to have disappeared now and I haven't yet found anyone else that does them.

I got fitted some Barnett clutch plates too, (Thank you Woody for the recommendation) and can confirm they do make a big difference. I got mine from Speed and Sport last year.

http://www.speedandsport.com/index.php/parts/drivetrain-and-tires/clutch-plates-fiber-barnett/