feetupfun

My model 138 Alpina (engine very similar or identical to model 116) has what I think is a 360 Pursang piston which is not exactly right for the model 138 Alpina but still works great. The Pursang cylinder has a bridged exhaust port so this piston I'm using has holes through the front to lubricated the bridge. The 350 Alpina has an unbridged exhaust port so shouldn't have holes there. The other difference is that Pursangs have additional transfer ports that work in with other holes in the piston and so those piston holes are superfluous in the Alpina engine. Some Bultaco 360/370 pistons might have a shorter rear skirt than the 350 Alpina but from memory the Pursang piston in my Alpina had the same skirt length as the piston that came out. My memory is not that certain though as it was over 20 years ago that the model 138 motor was rebored and I haven't had it apart since then. I don't know of any other brand bikes that use a piston that can be used in a 350 Alpina.

Yes that's the normal arrangement

Engine number prefix will tell you what motor it is. It's not a 350 because it is a round barrel. Lobitos are 100, 125 and 175.

It's not a Sherpa T. It's a Lobito. The frame number prefix will tell you what model Lobito it is.

Some people want to run modern lights (LEDs) which need 12V DC to work so they make that available. Bultacos only came standard with 6V lighting systems. Some Bultaco models had had a rectifier and a battery (so the lights would work with the motor stopped) but Sherpa Ts had no battery and the lights ran on 6V AC

I just tried to post up some contact info for the seller in Australia but suspect I broke a rule for this forum so all I'll say is yes he is still selling them

Jetting a bike using internet forum messages is a bridge too far for me

Over the past few years I've fitted five 26mm OKOs, all from Roger. Some didn't require any change. Some needed the pilot jet swapped and one needed a different slide needle. For that one I bought a range of Keihin needles to try.

Yes those are forwards and down. They are from a Yamaha kit that had those peg mounts, a longer brake pedal, a big seat, a longer swingarm pivot bolt and there could have been a different shift lever too

Yes most of my riding is near sea level and the highest riding in my state is about 4000 feet. When I said it didn't need adjustment when I fitted the B model motor, I meant that changing from one motor to a different motor did not require any adjustments to the carby. Both of the OKOs I fitted to TY250 motors did need adjustment when originally fitted.

Yamaha TY250 trailbike footpeg kit photo

Here's what one of mine looks like. 26mm OKO from Roger. Stretched Yamaha intake hose. This carby was fitted a couple of years ago when this bike had an A model motor. Fitted a B model motor in the bike a few weeks ago and the carby didn't need any adjustments.

My TY250s with the standard points and standard timing sometimes kick back if I give them a half-hearted kick. If you get an ignition with built-in timing curve and set it correctly it will not kick back. This is because at kickstart RPM, the spark timing is closer to TDC than it is with a fixed timing ignition

first gear is at the bottom

I have no idea. You could try phoning them to find out. Just because I bought something many years ago doesn't mean I would buy the same thing again. Things change. If I wanted to buy a TY ignition today it would be from Rex's Speed Shop. Those ignitions from John Cane many years ago are still going perfectly, as are my TYs that have standard Yamaha ignition systems.

That's what I was thinking too

There should be a washer under the clamp screw. Here are some tricks to help you: When changing the points setting, clamp the points just firmly enough so they don't move, but loose enough that you can just move them by twisting a flat, mid-width screwdriver blade. It helps to use a screwdriver that is not attracted to magnets because those flywheel magnets are pretty strong. Make a timing mark on your flywheel rim at the spot where the points should open. You should not have had to take the head or the tank off to set the timing. To find where to put this timing mark, make a mark on the flywheel rim (with the piston at top dead centre) beside one of the holes for the cover screws. From the TDC mark, measure 24mm counter-clockwise around the steel band on the flywheel and make another mark. That's the timing mark. You can find top dead centre on a TY250 with the tank and head in place by putting something down through the plug hole and using it to feel what the piston is doing. Move the flywheel until you can feel that the piston is at the top and that's top dead centre. The 24 mm measured on the flywheel reproduces the piston being whatever the standard timing distance is in mm BTDC. Yes electronic ignition is a popular way to get a bike running well if you are not confident setting points.

Are you saying you won't ride it until the frame is painted the original colour?

Common issues are: Bent or cracked clutch actuator arm. Split mid section of the exhaust.

Using the number is not that easy. If you post a photo showing the head and cylinder finning and the type of connection for the carby you will probably get an ID on the cylinder and head. The ID number for the bottom end of the motor is usually on one of the front engine mounting lugs.

Excellent. I see they do them in 50, 55 and 58 teeth.

I recently bought one from PBI sprockets in the USA. You can get any sort of rear wheel sprocket from them. When you order you nominate the chain type, number of teeth and the diameter of the hole in the middle. They will even drill the mounting holes for you if you want but I usually do that bit. They are flat sprockets so you need a spacer ring under them to get the chain alignment right.

No I don't. They are made for it. I've been trail riding with trials pressures in competition trials tyres for about 40 years now and the only problem comes if you ride at sustained high speed which can cause them to overheat. If it is trail riding, no issues whatsoever. What's the point of fitting a rear trials tyre and running high pressure anyway? They are terrible to ride with unless the pressure is down near 4 psi on the rear. Might as well stick with a knobby. It would be better to ride with than a trials tyre with too much pressure in it.

It's true that it does nothing for the internal gear ratios but the lower the overall gearing, the closer the bike speed is at the same RPM in successive gears. Imagine that a bike will do 100 km/h at 5000 RPM in 5th gear with standard gearing and it can do 80 km/h in 4th gear at 5000 RPM. That is a speed difference of 100 - 80 = 20 km/h between those two gears. Then you gear it down by 10 %. With the lower gearing it now only does 0.9 x 100 = 90 km/h in 5th gear at 5000 RPM and it does 0.9 x 80 = 72 km/h in 4th gear at 5000 RPM. That is a speed difference of 90 - 72 = 18 km/h difference between those two gears.

Tilllerman I think I need to clarify: 12/55 is even better than 13/53 for trail riding, because it brings 3rd 4th and 5th gear even closer together than 13/53. For some reason I thought you were thinking about getting a 14T front. The reason I said to take off in 3rd gear is because you said you were not liking taking off in 1st gear. A TY250 with 12/55 gearing can start off easily in any gear except 5th. When I said that you should use the TY250 broad power to your advantage I meant that when riding single track you should avoid rowing up and down through the gears. I've just seen the photos of your bike and that rear tyre will be making it much harder to ride rocky and loose trails than if you had a competition trials tyre on it with 4 psi.