Hello from new owner of 1960 125 Sport

[BDang]

Hi everyone,

I’m Brian Dang, proud owner of the little nice bike. Bought it some months ago and the seller was not sure it was 125 ST or Sport. It is complete but mixed parts. Based on the engine and chassis numbers, I think it is a Sport. It has the UB20BS carb even though the seller said UA20BS. I don’t know the differences between them after study the diagram from carb dealer websites.

I recently joint Ducati.MS but I think this forum is the right one for me and my bike.

Anyway over the weekend I checked and set the cam valve clearance. It was too wide by feel, and I set them to 0.0025”. I was a bit surprised when I read the valve spec this small from Manual. In a rush, I tried to set the ignition using the cam gear (next to the top bevel gear). It had 56 counts (one tooth to the next is counted as 2), and so 20* static is like 1.5 counts. I eyed ball the cam gear and set the point timing. It fired up, running ok at high idle, could stall at idle. I tried to adjust the carb screws but making big mistake of not record down the initial turns of slide and air screws. I failed to get it run again. During kick,kick,kick sometime the kick pedal locked up solid. I was worried something jammed inside, but after removed the plug, it turned fine. It was weird. But now I think the lock-up might be caused by too advance ignition and partial ignited mixture stopped the piston going over the TDC. I had found and printed out the degree chart to be mounted on the crank for a better ignition timing. Anyone know how to set the carb screws to their initial setting to start with?

Thanks and looking forward to knowing all of you.

Brian
SoCal, USA

[Snakeoil]

Pretty little bike.

Rob

[Rick]

Brian,
I'd say it's a Sport, not a TS. It should probably have clipon handlebars and the 'Sport' profile steel rims, but the rims are often rusted beyond restoration and it's possible the US spec 125S model came with the top clamp mounted bars. The seat may be from the European spec TS- I've seen it in a printed brochure, but don't recall seeing one on a Sport.
The UB carburetor is a very basic instrument- the 'idle speed' screw just stops the slide, and the 'air screw'(idle/pilot) is the only way to adjust the idle mixture- no pilot fuel adjuster or jet that can be replaced. You could start by bottoming the air screw and then backing it out a little- look at the sparkplug- you should be able to tell if it needs more or less air in the mixture. The engine should run from the idle circuit unless it's plugged, and it's common for the idle circuit outlet on the engine side to be plugged- a tiny hole- if the outlet is plugged than you don't have an idle circuit. It's not hard to damage the kickstart mechanism- if you don't hear any sign of the engine firing it's useless to flail away on the kickstart lever.
I didn't follow how you were setting the ignition timing, but the traditional method of going to TDC on the compression stroke and then using a degree wheel to set the point opening never fails.
The 160 engine was based on the 125- it's easier to find manuals, specs, parts, etc, for the 160. There is a really good shop manual for the 125, but it's hard to find- it's the manual with the blue cover. I'll attach the page that shows timing information- valve/cam timing at the top.
That's a nice motorcycle- it shouldn't be hard to get it going.
Rick

125 timing.jpg

[BDang]

Hi Rick,

Thank you for the detail information of the bike, and for posting the timing page. This is the first time I have seen it in English. I was guessing at the Italian version and came up close – technical information is easier to understand from foreign languish than literature.

For the posted valve timing chart, it says 44* open for 125S till m. 200967, and 24* starts from m. 204787. So what to use for bikes between 200967 and 204787, and are these engine or chassis number? My bike’s frame is #203731, but I don’t remember the engine #.

As about my confusing ignition timing process description, I was trying to use the cam gear tooth counts as a rough version of the crank mounted degree wheel (I couldn't find the 14mm Allen key for the crank plug, but yesterday I used a thick 14mm cross nut and socket, and it worked). I think the cam has 28 teeth total, and I counted the top + base of one tooth as 2 counts, so one cam revolution has 56 counts encoder wheel. So 18* crank = 56 * 18 / (360 *2) = 1.4 counts on cam gear. Assuming compression TDC is when the cam keyway is at its top position (relatively to the 10* tilt of the cylinder), 1.4 counts before the keyway top position, CW rotation, is 18* advance ignition timing. The cam gear OD is too small for this timing purpose, and I will soon find out how much off with the degree wheel installed.

As about the carb air screw, why its pointed end needs a drilled hole across the shaft?

To check the engine oil level, the 160 Monza Jr says that when resting the dipstick plug on the filler, the oil should be between the two marks. I followed this and with oil level at MIN mark, it was about the same level at the top of the oil fill hole without the filler installed, and thought it was too high (MAX would be too high if the filler extension is not installed). Was the book correct or should the plug be screwed in for checking the level?

Regards,
Brian

[BDang]

While with the top bevel gear cover off, I noticed 2 degrees or so backlash between cam gear and bevel, and I thought this was too much. It seems the only way to adjust is to shim out the cam gear. It also seems to me that the upper bevel shaft couples to the lower one with tongue and groove, but the photo I had seen looked like tongue to tongue, but this makes no sense since they will push themselves out? What keep the upper shaft from sliding vertically, the bearing inside the tube housing, two bearings inside?

Regards,
Brian

[Rick]

Brian,
I can't explain the gap in the engine numbers- I have an engine 202xxx that falls in that range also.
The cross drilled hole in the needle is probably to ensure there's always some air available in the idle mixture- even with the screw bottomed out in the body.
I think the dipstick is meant to just sit on the extension to check the level.
Spiral bevel gears should keep in contact. The upper bevel shaft is constrained in the bearing. There's a steel cup pressed into the head that the bearing fits into, and the combination of the shoulder on the gear, shims, and snap ring should hold the shaft in the bearing. The only variable is the gasket below the tower tube- the protruding shoulder on the tube should bear against the bearing, but if the gasket is too thick it keeps the shoulder from contact and the whole assembly can move axially.
The two bevel shafts have semi-circular ends that nest into each other and are held in contact by a sleeve that slides over them.
If you haven't seen this drawing from the manual it should help you see how everything fits.
Rick

cam drive cross section.jpg

[BDang]

Hi Rick,

Thank you for the drawing showing the bevel gear housing. I start reading the 4-5-Speed Shop manual today and understand your explanation. So there should be no play between those bevel gear mesh. Mine has too much backlash and I have to find out if the cam or the drive needs more shims. On the same drawing, how do they keep the left cam bearing from sliding to the right, or the bearing was interference fitted to the housing with heated housing?

Today I made the TDC finder using an old Bosch spark plug. After break off the ground electrode, I tapped on the center electrode straight head-on with small hammer to shatter the ceramic insulator, then with a small flat screwdriver pushing the copper core electrode back and forth to break it off at its base deep in the hole. A friend machined a 1” long aluminum plug with a drilled center hole for venting. He also drilled though the remain soft copper electrode, and pressed in the aluminum plug to the Bosch.

Regards,
Brian

[BDang]

With the top bevel gear has too much backlash, I was thinking what could cause this. The previous repairer could mistakenly only installed the seeger ring below the top bearing and had the shim on top of the bearing, instead of shim placed above the seeger ring. This would lower the drive shaft and cause backlash. I worry what else going wrong inside the engine case. I want to open up the whole engine and check over everything. Can Anyone recommend a good place to buy a set of new engine gasket and seal? I'm in Southern California, USA.

Regards,
Brian

[BDang]

So I saw Rob's 250 had the crankcase vent next to the alternator wire outport, and my 125 has no similar vent set up. Where does the 125 vent the bypass gas?

Regards,
Brian

[Rick]

Brian,
If you look in the back of most bearing catalogs there's usually an 'arrangements' section- you'll see it's common practice to mount bearings on a rotating shaft in a 'fixed and floating' combination. In this case, the right side(bevel gear side) bearing is used to locate the camshaft axially and radially, and the left side is radial only- as the camshaft or head casting expands from heat the camshaft can slide a little through the left bearing. Without the 'floating' bearing, axial loads are transmitted to the bearing, and ball bearings aren't designed for high axial loads.
I've always found the shims for the upper bevel gear on the bottom- between the snap ring and the gear shoulder- if the shims were on the top it would drop the shaft. But- lots of old motorcycles are taken apart with good intentions and end up sold as parts on Ebay, so I usually discourage guys from ripping into a bike they're not familiar with until they've done some research- it's good that you're studying before you start work on the bike- lot's of info on this forum.
Be sure you understand the camshaft timing ritual before you move any of the gears- once the timing is lost there's no option but to pull the timing side cover off and start over.
Rick
Edit:
My description wasn't very good- the 'fixed/floating' bearing arrangement is usually used when both bearings are pulled up against a shoulder on the shaft, so one bearing needs to float- in the Ducati head the left bearing is not really floating, it's a shrink/press fit into the head casting, so it should stay in place, but the camshaft can slide in the left bearing to allow for thermal expansion. I've never seen a bearing come loose in the left side carrier, but I guess they could have sunk the bearing further into the carrier and used another snap ring to hold it- doesn't seem to be a weak point without the snap ring. And even a floating bearing doesn't move much- would be hard to measure.