feetupfun

Standard was 70mm which was 238cc. The earlier "250" Sherpa T engine was 72mm bore standard. Pistons are generally only available in 72+mm sizes now. The 198 and later cylinders can be bored to suit 72+mm pistons without needing a bigger sleeve. One thing that is visually distinctive about the 198 and later "250" Sherpa Ts (198A and 198B) is that they use a metal head gasket instead of the earlier design that had the head seal directly against the cylinder sleeve (so the sleeve on a 198 cylinder does not protrude). The gasket type head design may have first come out on the model preceding the 198 but I don't know for sure. I can do photos of a 198 cylinder and head disassembled if you need it. 198A is the same as 198.

Its based on a 125 Ducati motor

There's an Italjet forum on here with people who know everything about them. In about 10 seconds searching I found someone in the US wrecking an Italjet trials bike like yours and advertising on eBay. I searched ebay using "Italjet Trials" Here is one of the listings from people wrecking one https://www.ebay.com.au/itm/1983-Italjet-T3-350T-Trials-350-Transmission-Lay-Shaft/120688282264?hash=item1c19950a98:g:f~gAAOxybi9Rcr0e

There's a thing called a Clake that uses the left bar lever as a clutch and rear brake that might interest you. I've also often thought that mountain bikes and electric trials bikes with the rear brake on the left handlebar lever had a slight advantage for bike control compared with using one of your feet for the rear brake. As for the having the brakes both work off one lever, it would be a significant handicap when riding trials because the reason they are independent is to provide better bike control. From the sound of your question I'd say that when you have had some more trials riding experience you will appreciate why the brakes are independent.

I avoid needing to use a dial indicator by measuring the advance around the rim of the flywheel. Initially you need to establish where top dead centre is, and you don't need a dial indicator for this. I use a TDC tool which is really just a rod in a guide that screws into the plug hole. Anything that will fit will do the job. Then make a mark 21mm anticlockwise around the rim from top dead centre and use that position for checking the position of the points opening. The TDC tool is marked in mm and some people use this scale to measure piston position BTDC but I don't think this is accurate enough for the job. https://www.inmotiontrials.com/product/tdc-timing-tool-14mm-plug-hole/ If you still feel the need to use a dial indicator but don't want to take the head off, you could take off the exhaust, loosen the carburettor and remove two of the engine mount bolts so you can tilt the engine down at the front and fit the dial indicator.

Maybe it's just coolant that was still sitting in the crankcase after the o rings failed and when you tried starting it after fitting the new o rings, the coolant was blown up out of the crankcase and onto the piston and spark plug

It's time based so the cost depends on how much preparation the operator has to do. If you de-grease and remove paint, silicone, gaskets etc it will save paying someone else to do it. The bloke who does mine cleans external surfaces with bead blast to remove corrosion and ingrained dirt before the vapour blast, which also saves time. I use bead blast if I am going to paint something. The paint sticks better and it costs less. I use vapour blast for things that are going to be left bare like wheel rims, fork yokes and fork sliders

I'm size 45 and "ample" calves. Not Forma boulder. They only just overlap on the calf for me. The old Alpine Stars were OK on my calves but I haven't tried the current stitched-on-sole Alpine Stars

Those two model Sherpa T you mention use a circumferential clamp at the top ends of the fork tubes that is fastened with 6mm bolts. Your Mk4 Lobito was made around the time where some Bultaco models had circumferential top clamps and some models had a cone (tapered) shape to the top end of the fork tubes that fitted into a matching conical hole in the top clamp. For you to be able to interchange fork tubes the fastening method for the top end of both needs to be the same

If you have both bikes why not measure them yourself?

Adding to this topic because of recent interest shown in aftermarket seats for TY250 twinshocks

I agree with nigel and can add that to get close to that ideal setting while you are still in the workshop, set the clutch spring preload so that you can just get it to go over compression with the kickstart. With 5 Barnett plates and the ideal preload setting, you may find the need to shorten the nuts to provide clearance to the casing.

The dots on the nose do look odd. I'm not certain but those dots might be what I was talking about earlier when a change in fuel type can loosen existing carbon deposits. Also you flogging the Alpina hard may also have loosened existing carbon deposits.

It may not be lead fouling. It may be the change in fuel type causing existing carbon deposits to be released and which then stick to the plug. When we (in Australia) changed to unleaded "pump" petrol in 1986 from leaded petrol, fouled plugs on two strokes were suddenly very common where they hadn't been a problem previously. After a short time of running on the new fuel, the frequency of plug fouling went back to normal. You could see carbon deposits on the plug nose that looked nothing like a deposit that had formed gradually. Some but not all were like whiskers. I will be interested to see your photos.

I tried Acerbis white plastic barkbusters on a TY250A twinshock trials bike while setting out an enduro course and decided that for me, the increased inertia in the steering when I did some trials-type riding outweighed the potential benefit of avoiding lever damage. If I was only using the bike for trail riding I would have left them on.

Sounds like normal symptoms of clutch drag caused by the clutch plates touching when they should not be touching. Usual causes are: Plates might not be flat. Plates might not be moving apart parallel. Less common causes are: May be side float in the gearbox shaft. May be grooves worn into the clutch basket fingers. Maybe the bearing that the basket spins on has started seizing.

why do you mix it with E85?

If you use a pre-65 electric motor instead of a pre-65 petrol motor it would be hard to argue against riding in pre-65 class but you didn't ask that. If you could hide a modern EM electric motor inside a pre-65 petrol motor and hide the cells inside a pre-65 fuel tank, then it would meet the eligibility criteria for pre-65 where I live.

I used neutral cure silicone rubber sealant which has worked pretty well for about 15 years so far

An angle grinder with a 100mm disc will work - I know because that's what I did, and didn't have to take that chain guard off either. The sprocket will pop loose when the disc gets close to the shaft

Replacement sprockets don't have a keyway and work fine so yes, it was overkill to have a key. The swingarm bushes may be rubber isolastic bushings. I have found them in a couple of bikes and got them out by burning the rubber away. This allows you to get the centre bushing out then carefully cutting through the outer steel casing of the bushing with a hacksaw blade. You can replace them with either new isolastic bushings or more conventional steel inner/brass outer bushings

The TY250A when standard has a different rear rim to other TY250 twinshocks. X11 tubeless tyres usually stay on TY250A bead seats. The bead seats on the A model are wide and it is also a wider rim than the later TY250s. The A model is the 434 model. Many other 1970s era tube type rear rims can give problems with X11 tubeless tyres though. The standard KT rear rim is the worst I've had for fitting an X11 tubeless, but if you do what "still trying" says, the X11 will stay on the KT rim bead seats nicely. The IRC tube type rear is fine on any of the old rear rims but where I live it is very hard to buy them, so I buy Michelin X11.

I'm thinking the 247 engine covers changed to being the same as the Cota 348 when the Cota 247 UKR came out but I'm not certain. For the sprocket, if it's on a taper like a 348 sprocket is, use a 100mm disc on an angle grinder. It will fit in there without the disc having to touch anything else. A bigger disc won't. I found that when I got to about 1 mm from the gearbox shaft the remainder of the sprocket sprung apart when a crack formed. For the cylinder nuts, heat them up with an oxy-acetylene welding flame. If you have a steady enough hand you should be able to get each nut very hot without melting any aluminium. You can buy new nuts easily so don't worry about damaging them

Mitas has stiff carcass and low aspect ratio. When they first came out, I bought one, tried it on a modern trials bike for a few minutes and then on a twinshock trials bike for a few minutes and then gave it away. The only place they grip is on a smooth, hard surface

Well done Wufdog. What you've done may be more reliable than the Scorpa version.