Needle jets are also subject to mechanical wear, sometimes so bad, its possible to see the jet is oval.

Mikuni carb bodies dont tend to wear noticeably, on something like an Amal carb, the carb body being badly worn is very common.

Here in the UK it's often difficult/impossible to get wear parts for very old OE carbs, and in some cases this leads to bikes that don't run that great.

>>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<<

My take is that the temporary floating idle was a lean condition brought on by running on reserve, with air pockets getting into the line from the low gas level splashing in the tank, which cured itself for a short while after he stopped at the T intersection. Then, while running at WOT he finally ran out of gas, even though some remained in the filter.

One other thing I forgot to mention,
Are you running 91-92 octane? 
Yamaha, for many of their bikes recommends  running one stage colder plug for high speed riding.
For example my RZ 350  uses BR8ES around town,,but requires BR9ES to avoid problems at freeway speeds.
The other issue to look at is the combustion chamber volume; did some previous owner mill the head? 
We had a bike with piston failure problems and found the head was milled, so we added a second head gasket, problem solved!

I am still suspicious of your needle jet because I run a stock 150 main jet and my DT175 runs crisp, not too fat, on the top end.
The fact that you are down to a 130 main and a 140 is too rich  means the bike is getting fuel from one of the other jets, like a corroded Needle jet or pilot jet.

UK Parts book for my model deffo states 1371414326 # 130 main is std with 125 and 135 optional.
needle jet and needle are correct.  IIRC 02 and 4j13-3.  Slide 2.0mm cutaway.

Running premium 97 octane fuel in all my bikes.

As for wear.  Bear in mind the bike has just rolled over 2000 miles from new.

Did you use to own a DT175MX when they were new? I had a new one, probably about 78.......

Yep. That's exactly what mine looked like. Some of those bits made it all the way to the end cap. Hole was about the size of my fingernail. Could not believe al of the bits I flushed out of the case, but the hole was in the thickest part of the top. Man, don't ya feel lucky when that happens?

I was going to buy a $60 Trail Tech CHT meter, but saw that the battery is sealed in. Some folks say they last a year or two, then you have to replace the entire unit, or try to cut it open. So I decided to try the cheaper Runleader version, that has a replaceable CR2032 battery. It has mixed reviews, many saying it arrived DOA, or took three exchanges until they got one that worked, while others said it worked fine. So for $27 I ordered one. Well... none of the buttons work, but it does measure temps fairly accurately, so far. I contacted the seller, and they responded within 10 minutes offering a replacement, or refund, and I can keep the defective unit. Since it does read temps, I decided to keep it, and try it out, and had the refund credited back this morning.

Since the buttons don't work, I can only use use Celsius, and the backlight doesn't function. It's also preset to blink when the temps are above 100 C, and I can't change that. I put the sensor on the spark plug I'm not using, after spinning off the crush washer. The sensor ring is about .4 mm thinner than a flattened crush washer, but should be fine IMO. BTW, I bought a mounting plate with the first cheap meter I ordered (another brand) , but sent that meter back because it only read up to 120 C. I put the bracket on the left mirror mount, so it's easy to see with head up.

Just took a long ride, and the temps match my IR gun very closely, when measuring on the head near the plug hole. It runs ~145 C to ~155 C at speed, with the highest reading around 160 C at 60 MPH. So heck, I'll just run with this one (for free) and see how long it lasts. 

youtube.com/shorts/gFPvNZPEJXQ?si=t4E6G4HeXY66ZX-f

This Runleader temp thing is still working great, except for the buttons not working. Did another ride today, and it stays right between 145 C and 155 C. As it cools down after coming home, the meter matches my IR gun almost perfectly. The seller has also been back in touch, and their techs are trying to figure out why they're selling so many duds. If they could get it sorted, it would be a big seller IMO. I let them know I'm using it, and other than the buttons, it's working fine.

Just an update on this Runleader temp gauge. I bought another one, and this one is working, so far. It's a bit fiddly to set, but I figured out how to set the alarm temp (where it blinks) and changed the temp unit to Fahrenheit (just because that's the scale I'm used to). The backlight works as well, after a button press. Seems to read the same as the other one, and I'm using the sensor from the old one. I'll keep the new ring sensor as a spare. Just went for a ride and after cruising at 55 to 60 mph for a few miles it was up to 315 F, then sitting at a stop light with a 70 F breeze it began cooling down to around 300 F.

I think a button on the other one was stuck because the battery was only showing 1/2 reserve, and was new just a few weeks ago.

www.amazon.com/dp/B01G53N7C8?psc=1&ref=p...dt_b_product_details

Holy thread ressurection Batman.

DT100 finally put to one side, time to pull the head and check the bore for damage before ordering a piston.

Thankfully the bore has escaped with no apparent damage, certainly above the exhaust port.  I will pull the jug tomorrow.  I only stay in the conservatory for a half hour at a time in the winter.  With it being cold in there, I do not want my breath causing a problem with humidity/ condensation.  

I can now order a new OEM std piston.  I have been watching one for a few weeks.  Since the rings are new and are just bedded into the bore, I will reuse those.

Parts already arrived:
Genuine mikuni main jet #130 as fitted as std.
New mains.  I could clean the old ones out, I doubt they are damaged, but while I have the motor apart... For the sake of a tenner for new ones.
Gasket set.
175 wide small end bearing.
Piston circlips.

I will not bother disturbing the crank seals, just make sure they are squeaky clean.

Holy piston.  Thanks for the five mile / 3 hour push home.


Undamaged bore.  PJME did a good job and rest assured, the sleeve has not come loose as someone would have liked to have happened 

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