Starts right up and idles fine. Throttle response is quick  and returns to idle. Revs up no problem. First and second gear accelerates good. Once I get to top end of 3-4-5th gear it just loses power. No bogging or backfiring. Acts like it’s not getting fuel almost. Doesn’t die though. Won’t go over 35mph. If I shift back down it’s fine. I’ve never rode this before so Im not sure what normal is for first and second gear.Back story, 1976 dt125 sat outside for a few years until I got it. Since then I’ve cleaned the carb numerous times.Taken the clutch apart and  cleaned 
replaced the crank seals
bored cylinder .050 over with new wiseco piston and rings
new top end gaskets
repair gas tank and sealed inside
new petcock 
fixed governor
set timing with dial indicator.
and multiple new spark plugs.
adjusted oil pump
at a loss now. Looking for any ideas. Thanks 

I'll let RT325 tell you about a common cause...

Old foam filters can look to be clean and in good condition but not flow enough air.  Modern "sticky" filter oils can also wreak havoc.  Replace with a new Yamaha filter, lightly oiled, if you haven't. 

Ignition timing could be off.

"I'll let RT325 tell you about a common cause..."

As he'll be in bed at the moment, I'll tell you of my experience!

My own DT125, many years ago, showed similar symptoms. My mechanic cut the silencer in half to remove tons of crap and then welded it back together - problem sorted.

If you live in an area with no fire risk or neighbours close by, then you could stick it on a bonfire, bbq etc in order to burn out any crap.

Such an easy thing to do. Couple hours in the fire pit followed by a fresh paint job and it may just rev freely again.

imgur.com/a/kmQLpw1

Really? That would be an easy fix since I work in a fab shop. the bike has 6,000 miles on it. I’ll double check my compression and timing tonight and if that all checks out I’ll hack it apart in the morning. Lol 
I’ll get back to you once done. Thanks
 

Yep was gonna say--Exhaust. If you can stand the noise or live out of town you could try it minus the rear silencer the complete thing not just the end cap!! Take it out of the exhaust body. Then look in there also for a ring of carbon where that pipe ends. Garden hose size should slide right in. Then burn out the tail muffler regardless. If it's really bad you could benefit from burning the complete system front to back.

Well, you guys nailed it. The silencer was plugged up. Damn thing couldn’t breath. I’ll post some pics later today. Thanks again

Hopefully good news. 

To post photos, you'll need to use a third party host such as Imgur.

Yay we have a winner. Winners are grinners. That's something you'll never forget. Look forward to the pics. Now all you need is a 175 top end & you'll be away. Easy swap to do. Twostrokes eh!!, sent to try us.

I am not fond of 2 strokes. Everyone I’ve messed with has been a pain