Newly acquired DT3    Ran fairly well with the aftermarket Torque Co. header.   My plan is to get the bike back to original so the first thing I purchased was an unknown but nice  stock exhaust and now the bike doesn't rev up past 3K or so and very weak with no power. The exhaust is very quiet , even when  compared to my other good DT3 .  This is making me think the newly acquired stock exhaust is possibly clogged .   Is there a procedure to clean these out ?  Do I take it to have it boiled out by a radiator shop ?   I'm tempted to put the header back on and see if it runs good again ...  I looked at the jets and the numbers were confusing    Is the starter jet the one that's horizontal down in the float bowl ?  I see its marked 160 ,  books calls for 60     couldn't see any numbers on the pilot jet but it should be 30 .  Thanks for any ideas you may have .    
 Tom   ( new member , first posting ) 

Most popular way, providing you have the space and it isn't against the law, is to stick it on a bonfire or bbq. It can be repainted as required.

Welcome. Yep it'll be blocked in the spark arrestor & sounds like its worse than most. There's a pic somewhere on the site of looking down the tail into the arrestor, it's non removable unless someones knocked it in then out breaking spot welds. Needs a bonfire for the night as be needing a burn right through i'm guessing.

Here's a random youtube video. I wasn't sure where it was heading without sound on but the last few seconds shows exactly what i'm on about down in there. Ok i see it came up blank but hit the youtube link on it & works. Ok different bike & different pipe but same problem & same looking arrestor down in.

The jet you have horozontally mounted in the float bowl is your main jet ,
The starter jet is not removeable . If your pilot jet isn't numbered , then it might be an aftermarket jet ,
which can be hit and miss regarding sizing accuracy . Stock Mikuni jets aren't expensive .

Starter jet is down the cavity in the bowl & is vertical but is fixed & controlled off & on by the choke lever. Choke jet in other words. 160 sounds about right for main jet. But something i totaly forgot was the importance of Oring7 that seals the bowl onto the needle jet so the main jet isn't compromised by a leaking Oring giving a huge hole to suck fuel through straight up the spout to the needle. Pic coming "7". Still thinking exhaust though.

Ok wouldn't load the pic but follow the link if it shows up.
www.partzilla.com/catalog/yamaha/motorcycle/1973/dt3/carburetor

Thanks for the ideas    I replaced the O ring that was mentioned when I did the carb ..   Bike started with no pipe attached on the first kick    I've been heating the pipe with MAP gas and moving the flame around   I don't think its enough heat .    I attached the output of my big shop vac ( blowing air) and not much flow out the pipe    Oil did runs out as it was melted.   Have to find a bon fire I guess   I hope it doesn't distort the shape  

Radiator shop idea will work too.  Someone here used to do it that way.

Get the gas heating directly on the arrestor & get it red hot but not melting then get the air blowing in there. Next one i do i want to try my leaf blower. But if you had a bonfire to leave it in it'll all shake out & wash out in the morning. Well, that's the plan but plans can go wrong.