feetupfun

Yes the earlier one has a narrower brake drum contact surface, the smaller spoke flange is part of the hub casting and the fins on the drum are machined not cast

DT175 gear ratios are better for trail riding and worse for trials

If you post your question in here http://www.trials.com.au/forum/viewforum.php?f=16 a bloke called Greg Harding will probably be able to answer any question about the conversion. Here is one of his Suzukis

Here is one type of M49 rear hub. I just measured the liner of this one as 144mm OD. Nominal ID is 140mm. I am planning plan to machine this one out to either no grooves or a maximum of 142mm ID with some grooves remaining. There is another type of M49 rear hub fitted to early build M49s that I will post up a photo of later if you think it is helpful. This earlier hub has a liner with the same nominal ID (140mm) but has a much bigger OD (147mm)

Piston manufacturers recommend a certain piston/cylinder clearance be achieved at workshop temperature during the rebore for their piston, so that when someone uses the bike to plough through deep sand in an Australian summer, there will still be enough piston/cylinder clearance to avoid seizure. For those of us who have mechanical sympathy for their motorbike engine and not get it extremely hot, we can get away with less piston/cylinder clearance (at room temperature), which means less piston rattle when we ride trials. What I was referring to about different pistons is that some pistons are made from an alloy that has a higher coefficient of expansion than others. This means that the difference in piston/cylinder clearance from cold to hot is greater in some pistons than others. In the case of those high coefficient pistons, there needs to be more clearance at workshop temperature and at trials temperature to be able to have enough clearance if the bike is ridden in a way that gets the motor very hot. Because of this requirement, the standard clearance recommended for high coefficient pistons is greater than for low coefficient pistons. If the person doing the rebore uses the standard clearance as recommended for the piston, a high coefficient piston will rattle more than a low coefficient piston at trials temperature. I was pleased to read the instructions that came with a piston I bought recently because it had three different recommended clearances for the piston, depending on the intended use of the bike it was going into.

They are a notoriously rattly motor and can function perfectly for many years despite obvious piston rattle. Because you have the motor apart, I would recommend that you decide if it needs new rings or a rebore by measuring the piston ring end gap using the standard method. Also be aware that some brands of pistons are more rattly than others.

Guy the 0.6mm is not the timing. It is the amount further advanced I set the timing compared with standard setting. As an example, if standard for the bike is 3 mm, I use 3mm + 0.6mm = 3.6mm I don't have a hyperlink to info about combustion chamber design. I spoke to the tuning expert on the phone.

Unless you are going to road register it, you can machine the drum out as far as you dare. The lining material is available up to 6mm thick so you have quite a lot of lattitude to play with. Bear in mind you don't have to machine out every last groove from the drum to get good brakes. Even something that is flat for 90% of the surface is way better than a drum with a surface that is uneven all the way across. The standard rear brake on them is quite strong by the way. Nothing like the little, lightweight Sherpa T rear hubs that came out in the mid 1970s

The rings you need to buy are determined by the diameter of the cylinder bore, the cross-sectional shape of the rings and the shape of the ends of the rings. A model TY250 came out in 1974 and has two rings of the same type with the top ring set down from the top edge. Later model TY250 twinshocks came with an L shaped top ring and a lower ring shaped like the TY250 A rings. Those numbers you posted are not the size of the piston or the rings. They look like something the manufacturer put there to keep a track of what type of blank the piston is and are not helpful for you. Sometimes but not always the oversize of a piston is stamped on the crown. Oversize numbers are nothing like 6-33. They are usually a number that represents the number of hundredths of a mm the piston is bigger in diameter than standard. They can also represent thousandths of an inch. Typical Japanese piston oversize stampings would be 25, 50, 75, 100. Imperial oversize stampings would be something like 010, 020, 030, 040 First work out what the piston is. A model, BCDE model or aftermarket supply, then measure the bore diameter. If it is a Yamaha or Wiseco piston you should be able to get rings. It is rare nowadays for people to replace just the rings on an old trials bike. A piston kit and re-bore is more commonly chosen due to the similar labour component for both jobs. A decent photo may speed up identification of the piston.

I run 0.4 mm gap with standard TY twinshock ignitions and 1.0 mm with John Cane Ty twinshock electronic ignitions. Never had a problem with either setup. NGK B6ES for trials. B7ES for trail riding.

The OKO is certainly good. I just fitted a new one to my TY250A to replace the original TK and it works very well. Not a fair comparison though because the TK had started to wear on the needle and needle jet. It is just as good as a standard Yamaha TY250D carby that is in very good condition that I have on a TY250D motor. The problem you describe is one I have lived through on a couple of bikes, but not any Yamahas. I had exactly the same issue with my Cota 348. The original AMAL was terrible so I soon replaced that with a KT250 carby that ran perfectly except for when the motor was fully warmed up and it would then act very hot off-idle. I also had the same issue with a Bultaco Alpina 350. The original AMAL MK1 was OK but suffered from an attitude problem in that when the front of the bike was low and wouldn't idle. First off I fitted a new 26mm VM Mikuni sold to me as jetted correctly for the bike, but I spent a few hundred dollars and about 25 hours of test and tune time trying to get it to go as well as it did on the AMAL, with symptoms that sound just like yours. As well as trying different needles, needle jets, pilot jets, idle air jets and different slides, I also modified the downstream edge of the slide with a groove intended to increase the fuel just off idle. Trouble was that even with that serious sort of modification, it would run rich off idle until it was warm, run well for a brief period then run hot and what seemed like too lean just off idle as it reached full operating temperature. On this bike and the Cota 348, I then fitted new OKO carbies and was impressed with the improvement, but the problem was still there to a lesser degree. I then consulted a two stroke engine builder/tuning expert who told me that it was a common problem that is due to the combustion properties of modern fuel being different to the 1970s fuel. He also told me that it was a common practice in vintage MX circles to modify the combustion chamber shape to get rid of the problem. The idea is to replicate a modern design shape and squish zone in the old engines and that it cures the problem very well. He also suggested that running a slightly advanced spark would reduce the problem and I have done this in the two bikes I mentioned and it did provide a benefit. I went about 0.6mm more before TDC on both motors.

TY250 forks are about 20mm shorter overall than TY175 forks. Standard setting with TY175 forks is about 20mm protruding through the top clamp, so if you run TY250 forks flush with the top clamp, the height of the front end will end up close to standard. A TY250 front end will fit without modification. The fork tubes, being bigger diameter, end up a bit closer to the fuel tank. Some people use 340mm shocks and some use 360mm shocks. If 360mm shocks are used, the bike usually has the TY175 forks set flush with the top clamp. With TY250 forks you won't be able to lift the front any higher than standard. In my opinion, lifting the bike makes it handle worse and with such a short wheelbase is unnecessary. If you are using TY250 forks, then I recommend using 340mm shocks. If you are buying Falcon shocks, choose 40 pounds/inch springs. I run TY250 forks in a TY175 with standard springs, standard damper rods and standard preload and 5WT oil with the oil height set to provide just the right overall spring rate.

If you want a close look at some to compare them, Robert Gough's 199B has the taper top tubes

At the time they developed the bigfoot, there was a big push to make dirt bikes quieter and it wasn't just Bultaco who developed quiet exhaust systems.

Yes some 199Bs have taper top fork tubes and triple clamps. I don't know why

Graham, George did mine by using a milling machine to enlarge the hole and then welded in the aluminium insert that I had bought from Keith Lynas. He said next time I needed one done, I should consider cutting a bigger diameter thread in the cylinder and make a new steel nut (male piece) to suit the bigger diameter thread.

Definitely is louder than the clubfoot

The manufacturers choose the location of the rear axle to give the best handling overall. If you only ever jumped up things, the ideal location would be different. So would lots of other things on the bike be different. As far as the slot goes, it is there so you can adjust the chain tension. Yes you can change the handling by having it at the back compared with at the front, but it is only a very small difference and 99.9% of riders would not be able to tell the difference just by riding the bike.

It sounds like you are trying to turn like you were still on a modern bike, and it isn't a modern bike. The techniques required are quite different.

Ty175 can suffer from a pitted/worn surface on the camshaft which could cause it to engage more suddenly than normal. The pit forms where the pushrod rubs on it. I agree with Carl Ekblom that the condition of the plate driving surfaces on the basket and hub are very important for progressive engagement and is probably the most common cause of poor engagement and disengagement on TY175 clutches

I would include lower shockie mounts on that list

More is not always better (280 vs 325) If a 250 needs a rebore, it would not cost anything extra to make it 280 if you do it at the time

I don't think I've ever heard of a 348 that hasn't developed a frame problem with use. I've got two bent 348 frames (I think from frontal impact) and one really rusty 348 frame that might be bent or cracked but I haven't looked. A friend originally from South Africa rode a 348 in trials over there that someone had modified the steering head angle to INCREASE the rake to make it a better enduro bike (in their opinion). Seen plenty with cracks near the headstock too. The lower shock mounts on my ride 348 are 1/2" UNC bolts that someone welded on in place of the elegant (but obviously not strong enough) original shock mounts.

There is a law of diminishing returns when painting something black because all paint acts as an insulator, so while a single thin coat of matt black may well increase the efficiency of heat transfer compared with bare metal, above a certain thickness of paint, the overall heat transfer efficiency will start reducing

Just thinking, a 2mm liner makes for a nicely lightweight cylinder assembly although I see the latest thing in old bikes is to use a coated aluminium liner