feetupfun

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

The reason that an ideal points gap, or an ideal range for the gap to fall within is specified by the manufacturer, is because the sizing of the capacitor is chosen to match that gap (and the area of the contact surfaces of the points). The actual gap is not particularly important in the scheme of things, but the position of the piston/crankshaft when the points start to open is very important. On some bikes you can rotate the backing plate or change the points gap to adjust the timing and on other bikes you can only change the points gap to adjust the timing. As you move the backing plate away from the mid position in either direction, the voltage being generated in the stator coil (the primary circuit) at the time the points open will be (slightly) lessened, compared to if the backing plate is in the mid position. This means that if you want the strongest spark, keep the stator near the mid position and get the timing right by adjusting the points.

If you want an easy way to ride your Cota at night and don't need to get road registration, you can use a cheap LED light bar that will fit across the front of the fork tops, do a great job of lighting the way and not be sensitive to voltage. Bikes like yours can be started with the de-compressor engaged, if you use the type that are fitted to some chain saws. This type are "cocked" by hand prior to starting the motor, and as soon as the motor fires, they snap shut automatically. A friend has fitted one on his 325cc Italjet in an effort to prolong the life of the notoriously weak kickstart shaft, and it works beautifully. No good as a kill switch though!

Yuk

OK If you are familiar with the Bultaco then I would describe the 348 as compared to the Bultaco 325: Motor has faster pickup Motor feels a bit more powerful Motor not as smooth as late model 325. Similar to first model 325. Higher footpegs (riding position) Feels like you are closer to the font wheel Front wheel placement slightly less important Feels light until you get a bit out of shape Gear selection is sketchy Clutch has unpredictable and delayed engagement More susceptible to overheating (in my climate) Less ergonomic and heavier kickstart action

I find that the Cota 348 and 1977 Sherpa T are a joy to ride but both are hard work in tight sections. They are both suited to sections that are more open and have stable, relaxing handling. They feel different to each other in the handling and steering and which one feels better to someone depends on personal preference. If you are talking about the 325 Bultaco, both the Cota 348 and 325 Bultaco are difficult to ride with precision unless the motor is running perfectly, because the behaviour of the motor dominates the behaviour of the whole bike. If you were wanting the best chance of a low score in modern twinshock trials with tight turns and stop-allowed riding techniques, you would probably not ride a standard version of either of them. If you want a twinshock that feels amazing to freeride and has awesome power, then they are a great choice.

The wearing points that make it run rich are the slide needle and needle jet. A worn slide can also cause a few problems. In your case the plating is coming off the slide, which can sometimes cause the slide to jam (open), which is very dangerous. As far as the sloppiness of the fit of the slide in its hole is concerned, in the case of your TK carby, I suspect that the hole that the slide fits into would be where most of the wear would have occurred, and this wear would have been hastened by the way the plating has been coming off the slide. The plating is chrome which is very hard and the pieces that come off the slide will damage the piston, rings, cylinder bore and engine bearings as they are digested by the engine. If that bike was mine, I would fit a new carby to avoid further damage to the engine. If you fit a new slide, it will probably not change the sloppiness of the fit because the carby body will be worn. Carbies can be resleeved but it is not cheap. If you wanted it to look totally original then it might be worth the cost of resleeving and fitting a new slide. Yes a 3.0 slide will give different jetting compared with a 2.5 While I said that the original TK carby on the A model TY250 worked very well when in good condition, my own TK carby eventually developed wear on the slide needle and needle jet and a few weeks ago, I fitted a new OKO carby and it was back to running sweet as a nut.

My Bultacos and OSSAs have Betor forks. The brand is stamped into them. I can't remember what is marked on Montesa forks apart from the big M. They are similar to but different to Betors on the inside. Montesas had Telesco shocks while OSSA and Bultaco had Betor shocks. What prompted you to wonder about this? The Spanish bike brands had lots of parts made by external contractors. I learned today that a Bulto company made the Mahle branded pistons used in Bultacos under licence in Spain (and a very nice job they made of them too). The handlebars for Bultacos were made by another company, as were the frames and probably lots of other parts (a lot like nowadays where the parts for a new trials bike are made by a myriad of suppliers). In the case of the Spanish brands in the 60s, 70s and 80s, there were import limitations imposed by the government so most (all?) of the components had to be made in Spain. Today's bikes come together from all over the world.

Definitely bent shaft. Will need to split the cases to change it if you can't straighten it enough in-situ. Normal practice when using a strobe on a Bultaco is to start the engine with the cover on then take the cover off. Same for Montesas. Fully equipped workshop would start the motor on starter rollers driving the rear wheel. I have no idea how you got away with starting it that way before, even with low compression.

Fantastic. Thanks for allowing us to enjoy those films. While watching, I kept thinking about how much improvement there has been in tyre and rear shock technology since then.