Bung the ends and fill the exhaust with Castrol SuperClean (you can get 2.5 ga containers of it al Wally world).  Let it stew for a few days and flip/agitate once or twice a day.  Then dump it out into a bucket and flush the pipe with strong steam of water from your hose, until it runs clear.  You'll be amazed at what comes out.  Next best thing to burning it out.  SuperClean is very caustic so wear proper PPE when handling it and work outside in the fresh air.

Thanks so much for the suggestion. I will give this a try!

Mine had the same issues, and my reed block had a small crack in it and the reeds weren’t fully closing. I changed the reed block and reeds from a parts bike, and it fixed it. 

My reed block is okay, but reeds were bad so I replaced them. Didn't help my problem much.
Thanks for the suggestion.

Gotta do that exhaust somehow.

Just wanted to wrap up this thread. First, thanks to all who offered their ideas and suggestions. My DT175 is now running well. I can’t pinpoint any one thing alone that fixed the problem. After boring, new piston and rings to bring up the compression, rebuilding the carb twice, replacing the reeds, and going through the ignition system multiple times and replacing the condenser, plug, and plug cap, it improved from only being able to idle to revving to about 3500 rpm.So, then I focused on intake and exhaust. Exhaust didn’t look very plugged up. Not having a large torch, or a place to burn it out, I used a propane torch. Got a lot of smoke, but no “fireworks”. Then I tried filling it with caustic soda and let that sit a few days. It dissolved some more carbon but didn’t appear to be a lot. Now engine would rev to 4500 or so. Next, I took out the air cleaner box, as it had dirt/grease buildup inside where the air comes down the upright and into the box. The holes (drain?) in the bottom also had a lot of debris in them. This made a little difference. Lastly, I put in a new air filter. The one that came in the bike looked brand new-very clean. But replacing it finally did the trick. The engine will now rev to redline!Thanks again. Now I’m on to the next problem-battery quickly goes dead when running instead of charging. Already replaced the rectifier and checked wiring. Resistance on the coils checks out okay, but I suspect it has something to do with the stator.

After going through all the same steps it is clear the air intake system is very restrictive and anything but the most free flowing air filter will work.   Pull the air cleaner out and check out the performance difference as some have noted.    Anyone trying to increase performance will be limited by this intake flow restriction.    Bigger carbs, pipe mods, porting etc won't help if it can't breath freeIy.   I tried in a new Uni-Filter with a double wrapped cage - dry it works decent but added sticky blue air filter oil and the stumbling returns - good to maybe 4500 rpm max.   The original  air intake design was highly restrictive and baffled to control intake noise.  Hence the small opening, rubber snorkel and internal labyrinth.  I got an old box and am looking at ways to open it up and install a less restrictive filter.   Looks a bit messy to fab.    I may put auto damping sheet on some surfaces of the air box - the metal box is a great resonant speaker.   Just tap test the sides and listen to the tones (mini steel drum).    Something to be said for plastic air boxes! Highly damped.   I was considering 3D scanning the box, cleaning up the model and printing a new box in fiber reenforced plastic...maybe later.   

I'm sounding like a broken record i know. But if the exhaust isn't working as new & is even partially blocked--not talking build up along the way--but partial blockage by a lump--& it acts like it's too rich on the main. So you go looking at air filter for a better air flow than factory original, or blaming a restrictive airbox snorkal, but it was ok for 45 years & now i isn't. Just pushing the point that sometimes you can be sure it's one thing but can be caused by another, that's all.

Certainly the exhaust was part of the problem, but like I said, it didn't look like it was any single thing, but probably a combination of things. I burned out the exhaust as best I could with a propane torch, and soaked it for days in caustic soda. This noticeably improved performance, but didn't solve the problem completely. The air filter that was in the bike when I got it was Yamaha, and although it looked brand new, I don't know how many years it had sat not running. Maybe the foam had broken down, maybe it was over oiled. When I removed the filter, the bike ran great. And when I put in a brand new Yamaha air filter, it resolved the problem. Most everything I did to improve air flow made it run better. Hard to say what was causing the most trouble.

All good. I only mentioned exhaust again as you can never be 100% sure unless you cut it open or burn it red right through--well almost. I've seen some bad ones over the years by cutting them open. Sounds like ya all in control though. Enjoy the weekend ride if ya have one.