pictures from down under

I recieved some more pictures from Stew's RSC90 project.


He found an original honda stand for only 10 dollars at a swap meat. Think it's from an MT125 or RS125 2stroke racer, but I'm not sure, my stands were homemade from pictures.
Note the details, MT125R top yoke, lightened rear sprocket and everything else.

Long stroke experiment

Sadly the experiment didn't turn out to have a big effect.
After a lot of fine tuning it might give a little bit more then the current short stroke motor, but not enough to make a real impact in the 2stroke dominated 50cc class. So I'm thinking of quitting the 50cc altogether to focus on my RSC90 and maybe a CB72 that I've been trying for the last two events.

For now the 50cc has been converted back into a short stroke, with some minor adjustments to the head, valves and camtiming to get power at higher revs.
Next weekend is the Honda-day with a dyno, lets see what the little 50 has to give.

Long stroke experiment

A new experiment to get some more torque out of the 50cc engine, as it is originally a short stroke, at 35,6mm with a 42mm bore.
Now a test using an XR75 cranck wich has a 41,4mm stroke, just like an SS50.

Cylinder is sleeved to get to a 39mm bore, combined with a GB2 piston, originally of an engine with horizontal cylinder.

Tomorrow its first run....

R90/6 café transformation

The transformation into a caferacer has begun.
Clip-on handlebars, found in the spareparts bin of my dad's racing history, originally 35mm but adapted for the 36mm BMW forks.
Under the black paint, it turned out I had a chromed top fork plate in good condition.

Real magura's, no modern stuff.

And the Laverda SFC tophalf fairing I want to put on. I started with a different style tophalf, but that didn't have enough height to fit the headlight.

5speed first gear

Since I've been riding with the SS50 motorcycle it has had an original unmodified 5speed engine. Now only the ports and valves have been lightly tuned, but wat has been buggin me from the start; the short first gear compared to the closeness of the rest of the gears.
My other engines have 4speed gearboxes, that have a problem with a big step between third and fourth, but that is not as annoying as the 5speed first gear.
Also this is dangerous, as the 5speed gearboxes are known to break when shifting from second to first while riding, or they break when not properly kickstarted, as the gears are to narrow and the kick-gear only engages for a few millimeters.
(Three out of four of my 5speed gearboxes had a broken first gear)

To avoid breaking and to get a closer gearbox, I decided that the "oldschool-tuning-engine" although newly built and never run, was going to be opened up to change out the first gear for a modified first from a 4speed gearbox.
Thanx to my dad for the modifications of the shaft and sprocket.

Top: modified 4speed first gear (13:35)
bottom: original, broken, 5speed first gear (12:36)


And some other tricks that have been applied to this engine:
Old copperplated rockers, lightened.

Thin homemade cylinderbase gasket.

2-plate clutch to prevent slipping, please forgive me for it is from a chinese engine.

But to make up for the chinese clutch, an S65 clutchcover with homemade finned adjustment cover. Note that there is no indentation under the adjustment cover as in the later types, this means 0,73L of oil instead of 0,7L.
On the front is the oil line for the carb-heating that is blocked of now, I want to use this for an oil-cooler in the future. And mounted onto the filler opening, a nice breather unit like the one on my RSC90.

And a lightened cranckshaft, turned on the lathe to remove weight, but only where it could be removed over the full 360 degrees so I didn't have to balance the cranck again. And onto that a new high compression piston, not the old one from the SS50 5speed models, but from some new dax, it has the same compression ratio, but it weighs 20% less.

engine overhaul

The 5speed SS50 engine that was under the motorcycle was leaking some oil from the cylinder base and behind the generator cover, so it needed to be taken apart, even though it has only done about 500km.
The problem at the cylinder base turned out to be a cylinder that wasn't flat, so it was turned on the lathe to get a flat surface for the gasket to sit against.
I thought it would also have a leaky cranckshaft oil-seal, but it was dry in the center of the generator. Next guess was the O-ring around the stator plate, when I took it off the plate, it was so dried out, that it broke instead of stretching.
So hardened it doesn't even lose its shape when broken:

when the head was off, I took some pictures of it, next to my "old-school tuned engine" this has an older type head with even bigger fins:

and more importantly, nicer ports:

Both are cylinderheads from the 5speed models, with extra and bigger fins then the 3 and 4speed models, but even then there are differences.

The other head has since been flowed to look more like the "old-school" one

breakdown

Tuesday I took the moped to work, but when driving home, it began to vibrate at high revs and it made some funny noises. 5KM later I was at the workshop, had a quick look if the flywheel nut might have come loose, but it didn't, so I thought it should be a clutch problem. After all these years of abuse, I finaly managed to break the engine.

Today I started disassembly by taking of the exhaust to have a look at the clutch, but to my surprise, it turned out to be as simple as a broken motormount bolt.

I had a new bolt, put it in, put the exhaust back on, started the bike, and the vibrations were gone, engine still in working order, dispite years of abuse.
This motor has almost no compression, you can hear the piston ratteling in the cylinderwall, it never gets new oil, at best the old stuff out of a race-engine, it gets overrevved, overheated with burnouts..... but I still can't get it to really break. Real Honda quality, no chinese parts.

Jet problems

The SS50 motorcycle engine is now pretty much run in, but the carb settings are a litlle of. So I thought I would try a different main jet but to my surprise the keihin SS50A carb is so old that it doesn't have the standardized jets still used today. Now available are jets with 4mm and 5mm treads, as far as I know M4 and M5, but mine has a coarser thread M5x1 ???
In any case it has the same threads as the jets used in the CB72 PW22hov carbs I have, the powerjet type.

some measurements:
5mm threads


6mm head


8mm long


and a view of the coarse threads


If anyone knows where to get these old jets, please let me know, I need something like a 75 or 78

BMW test

I've been riding my R90/6 for a little while now, and I am starting to get used to it, but at first I thought it was a real heavy beast that didn't want to go round corners. Making it into a caferacer would then only make that worse, with narrow clip-on bars and such.

When I was young (about 15 years ago?) my dad built a BMW caferacer together with Jan Hoogstraten who stil ownes the bike. We asked him if we could borrow it for me to experience what a BMW caferacer would feel like. Here it is next to my R90/6


I've only ridden it for a few kilometers now, but it isn't disappointing, I want to ride it some more, to get a good feel of the bike, but I think I'll keep my R90/6 to turn it into the JBT-interceptor styled caferacer I had planned.

The M10s on the back is the name of the bike, being compiled of the enginetype; R90s and my dad's name; Martien ("tien" means "10" in dutch)
Note the single sided paralever swingarm from a later type.

slipperclutch

The SS50 motorcycle had some problems with the clutch slipping through at high speeds when asking full power. As this weekend I want to ride it in a night time honda-run, I wanted this fixed, so I opened up the engine, even though it has not even run more than 500km yet.

I found that the clutchsprings I had put in, where probably new as there was still yellow paint on them, but they were weak compared to the new springs I had with partnumber: 22401-036-000.
when measured at compression to 15mm the yellow springs had 5kg while the 036-springs had 8kg.
So new springs were mounted with new plates, as you don't know if they might have been warped from slipping, although I saw no blue spots on them.
First testrun it seems okay, lets see if it will hold.