Since I've gotten my CT-1 running  I've put over 2,000 miles on it. I try to keep the gas tank topped off, and the only time I've gone to reserve, it was at a stoplight. The idle started speeding up, and that told me it was getting lean, so I switched to reserve before the light changed . Back in the day when I was riding at speed, if it started sputtering and cut out, I'd quickly reach down and switch to reserve, so I never had a overheating issue.

However, if you're already on reserve and that's getting low, the remaining gas is splashing around, possibly allowing air into the line. If you're at WOT with those conditions, you could go lean with overheating, and having a CHT meter would really be useful. Maybe the lesson learned is if you're on reserve, just ride at a moderate speed to the nearest gas station, or back to camp.

Great advice Tom!

An interesting thing I got told by an old school owner of a US bike shop several years ago, was that when they were involved with working on air-cooled 2T go-kart motors, was that using Kal Gard Gunkote, on the top end parts, reduced the CHT a good amount and was well worth doing.

A little common sense about CHT and EGT gauges:
Rule number one, There is no magic temperature number. You do not tune the bike to hit a certain temperature.
Rule #2, After tuning the jets to where the engine just quits blubbering rich, and runs clean, note the maximum temperature, this number only applies to your bike on that day
Rule # 3 If the temperature suddenly exceeds your normal maximum, let off the gas and let the engine cool off,

At the Go Kart track, we have high and low speed adjustable fuel mixture needles, and we tune to the way the engine runs: Open the high speed needle until the engine starts to " 4-stoke" about 1/2 way down the longest straight. Themn slowly, about 1/8 th turn at a time, start leaning the high speed needle until the engine just cleans up, or just breaks into t 4 stroke right at the point you would be lifting off the throttle for the turn at the end of the straight.

Plug color, and CHT/ EGT temperature are only indicators of a proper mixture, but you need to set the jetting so you are on the rich side of going lean at WFO on a straight. Only then do the numbers you get make sense, for your bike on that day, as the air density changes and then you have to re-tune. This is why you set the jetting up a little rich so on cold dry days( high air density) you do not go lean and pack it in. It should be obviouse that a bike ridden in Death Valley will read higher CHT than one ridden in Alaska during the winter,

Lastly, the best way to seize a bike is to hold it wfo for long periods of time (more than 30 seconds) without letting off.

It would be very useful if the OP could provide an update, as I would have thought had the jetting been far enough out to hole a piston, that the bike wouldn't have been that nice to ride, and probably noticeable that there was something wrong, before any damage occurred?

Nothing really to update.

Piston is holed and the bike will not get any attention till the little one is finshed.  It is just languishing in the conservatory.
I have not even bothered removing the tank or seat.

I did allude to a transitory "issue" earlier.  I was going through a small country town earlier in the ride.  Very narrow streets.  Two cars cannot pass each other narrow.
I came to a T junction and the rpms hung for a very small moment.  Not hung as in revving its arse off, but not falling back to idle as crisply as it did, it took a second or two to drop from 3000rpm to 1500.  Also the exhaust seemed to be louder at the tailpipe than usual.  Less muffled and more crackle.

After that one time, it never did it again.  Everything seemed normal for another 10 miles. One of those things you shrug your shoulders at.  Perhaps it was just fuel getting heated up more in the pipes and carb as very low speeds, low airflow and low fuel flow made everything run hotter.


 

Ok. Must be really depressing after all that work! If it were mine, probably be under a dust sheet by now, and not going to see the light of day for quite a while.

Nah. I am not really down in the mouth about it. For the first time I felt that it was getting really close to completion and was enjoying riding it and I have learned a lot from this episide.

I perhaps went too far in going down two jet sizes from my spurious 130 to the spurious 120 that came with the bike.
130 was obviously too rich, it was not producing the power.
Now I know that 120 is too lean. The issue was compounded by dropping the needle a notch to get the midrange right, compensating for that likely overgenerous, spurious chinese 130.

I have resolved to find and fit a genuine Mikuni #130 main in both the DT100 and the 175, as per official Yamaha spec. DT100 main jet having being tampered with sometime in the past.

Total cost maybe £80 for a new genuine std piston. I am reusing the old rings as they are bedded in to the bore, provided they are not blued, nor have damage. If they feel like toffee, of course I will have to fit new and hone the bore.
£15 for a jet.
£10 for a gasket set and odds and ends.
Plus maybe 5 hours work dropping the motor, stripping it down, checking crank alignment, cleaning the bits of melted alloy out, rebuilding it and slotting it back in.

A small price to pay to finally rid the demon hand from this bike.

Not too bad then. I generally start off with around 120 for 175 trials bikes. Had TY175 on a dyno, and going up to 125 did make it run noticeably better.

Customer had insisted on very long front pipe though, which seemed to reduce power at all engine speeds?

120 is jet dimensions though, and not sure if your Mikuni is CC flow rated jet?

I looked at the Parts list for the 175 carb and it shows two "U.R." main jet sizes: 150, 140. This means for different conditions either jet "may be" appropriate.

When you have midrange fatness/rich condition, and the pipe and airbox are stock, head and porting stock, it means the pilot jet, or needle jet is too big. The needle jet (not the needle) is often called the emulsion tube. If the bike sat with fuel in it for long periods of time, it can get corroded, making the orifice bigger and causing a rich condition in the midrange, The same is true for the pilot jet, it can get corroded and allow too much fuel in the midrange.

I have run into this many times, and the number one symptom is you can partially cure the condition by dropping the needle, but that is not a fix. The "floating" idle you experienced is a lean idle circuit condition. Normally turning in the pilot air screw a 1/4 to 1/2 turn should cure that.

Double check the float height and make sure it is not too low.

I would change all the jets and closely inspect the narrowest diameter of the Needle Jet using magnificationto be sure the critical orifice diameter has not been damaged by corrosion. I would also replace the pilot jet, as they are cheap.

HTH,

Jay