Ok, we're getting there. I sure hope its only exhaust issues. The spark arrestor isn't removable although i've seen them knocked out--or knocked in to break the spot welds then get it to drop out the back. If its still in there it's best left in nowdays with all the modern legal stuff. Just burn the pipe out & especially that bit. If it's been removed then could just make a non restrictive straight through decent length tail muffler with lots of packing around the perforated tube. There's folk on here that used to modify pipes by going in from the side near the back. Anything i say is just thinking back 50 years from working in the Yam shop. Happy to be wrong & corrected.
Here's my relatively quiet DT1.
I know pics don't load but here's a video from a couple of years ago starting my old DT1 after a long hibernation & stale fuel--too lazy to clean the carb but probably had to in the end lol. But point of this is you can see a long homemade tail muffler which works good & non restrictive. There's more inlet noise than exhaust as have no airbox & just a big K&N sitting in the open. I might look old & #$% but i'm worse now a couple of years later haha. Mind you i think i'd been kicking it for 10 minutes before that video as i saw a 10 minute one i'd made but didn't want to bore you. Shoulda cleaned the carb from the start. In fact i think it was that, that i found stale fuel powder stuff had grown up on the inside of the emulsion tube until it fitted around the slide needle stopping much fuel getting past. Took me a couple of goes to find it as very unusual but something i'll never forget. Had sat for a couple of years & our fuel is terrible. Had a car fuel pump in the tank totally blocked from spinning after 4 years.

Video i see says may 2018 so 4 years ago. Using it regularly now so no fuel problems.

I like the way you circled around the bike at about 1:10 to get a whiff of exhaust smoke! (:
Sneezles61

Ha, yeah did look like that but doubt it as would've smelt horrible with old fuel & who knows what oil i had in the mix haha.

Liking the number board ,and i much prefer the earlier rounder tanks,im after an sc 500 tank,do you know if they fit.anyway after looking down the pipe once more i realised the blanking off that i saw is actually the centre of the spark arrester,that looks like its been bashed forward,ive drilled out the centre sectionan got a little more power from the bike,im hoping to get the whole unit out , then rig up an aftermarket silencer with some wadding.What does the spark arrester do anyway?

I just went looking for this post, figuring it would be way down the list and it got bumped to the top!

Anyway, I got the test article of the RT3 from 1973.  Really enjoyed reading it...  it had some insights from the times like how they praised Yamaha for making a 360 that had a lot of low end grunt rather than high rpm horsepower.  They praised the bike for having great power perfect for a low to medium speed "playbike" for off road use. 

Anyway, dyno test had the RT3 at only 18.65 HP @ 6000 RPM (corrected rear wheel horsepower).  It had close to 8 HP at 2500 RPM if I remember correctly.  Standing quarter mile measured at 16.52 seconds at 75.44 MPH. 

The same magazine issue has a test of the 1973 Yamaha RD250 which had 26.2 HP @ 7,500 RPM (corrected rear wheel horsepower).  Standing quarter mile for the RD250 was measured at 15.15 seconds at 84.98 MPH...  significantly quicker.   

The RT3 is GREAT at what it was designed for...a bike that can be ridden on the street and is a BLAST off road.  

P.S.  As far as "noise" as that has been brought up on the forum quite a bit lately regarding these old bikes...  one of the things they spoke about in the article was how the RT3 measured only 85 dB(a) but "the bark of the engine is irritating to the rider, his neighbors, people in nearby cars; the noise is especially abusive to the serenity of wilderness settings."

 

These bikes are quiet compared to all the cars running straight pipes around Shelby county almost every muscle car has straight pipes driving like they stole it and probably did!!

I agree Ht1kid...  I don't find these bike "loud" at all...  but several (perhaps younger?) members have complained that even their stock DT125 is "too loud".  That's just the way they were and are...  the article supports that some people agreed when the bikes were new.  Might be something to do with the tone or type of sound that comes from a two-stroke single more than the actual sound level?  

The article went on to say "this isn't an XL250" which makes me think the author might have been more of a four stroke guy. 

I like the sound these bikes make but I grew up with it.  It sure turns heads to ride down the street these days.  Nothing today sounds like one of these old two stroke singles. 

MarkT they sound great to me too I’m sure you put open expansion chambers on yours back in the day. We were lucky to have a local guy who built custom ones for us any style we wanted. Fun Times

A while back I was on here wondering if there was a way I could make my CT1 a bit quieter, but the more I ride it, the more it seems fine. I'm certainly not a youngster, having ridden one when I was in high school in 1971, but I guess I forgot what they sounded like. Also, most of my neighbors have stock four strokes, which are very quiet. I usually short shift it when I'm riding inside the neighborhood, so people don't start complaining

That said, when I'm riding along Pacific Coast Highway, occasionally some custom road bike comes roaring past and about blows me out of my seat, they're so loud.

Heck 2 stroke weed wackers are louder than our enduros