1974 dt360a.  Very confused now on proper jetting.  According to Clymer's (1981 Ed. p.97) exploded diagram of the independent float Mikuni carburetor, there are TWO "main jets".  One on the end of the float bowl banjo bolt that is depicted with a round head.  The other is at the end of the needle jet assembly.  Right now, there is a hex head jet with an "O" ring in a groove.  Clymer's depicts this one also with a round head and washer but no "O" ring.  I have a ton of jets of all sizes with hex, large round, and small round heads.  Engine starts and idles fine but will only get a max of 3,800 rpm in neutral at any throttle setting and doesn't put out enough power to ride even in 1st gear.  I live at 5,000'.  I bought this bike new and I can't give up on the old 's part of the family!  Any guidance or recommendations will be greatly appreciated.  Pat

Main set sits on the banjo bolt in the float. The part with the 'O' ring (which needs to be in good condition) is the Needle Jet Setter.

CMSNL has good images of the parts.

Yamaha DT360 1974 USA CARBURETOR - buy original CARBURETOR spares online

Not revving out could be one or more of blocked/over oiled air filter, blocked exhaust or a faulty cdi (if still running the original Yamaha set up). 

Thanks very much Gr8uncleal for the two posts.  I'll try a new "O" ring from the site you shared.  I had the CDI bench checked by a professional shop and it tested fine.  My son had this bike for a few years...brought it back to me in poor condition.  The trans was way overfilled with synthetic, smoked very badly, and ran badly.  Bike is all original.  I took the exhaust off and sprayed a whole can of Easy Off oven cleaner in it and let it soak overnight then flushed it from both ends with the hose.  Could still be blocked though.  Air filter is new.  Is there a good way to clean the exhaust?  Someone suggested using a gas torch with a rosebud on low to "cook out" the residue.  Thanks man, Pat

Yamaha still make them, so save yourself the postage and ask your local bike shop to order you one - just supply them with the part number.

Awesome...thanks!

If you've got plenty of space, no neighbours in close proximity and are not in drought conditions (ie it won't be a wildfire hazard), then chuck the exhaust on a bonfire/bbq. Obviously it will then need to be repainted!

The "at any throttle setting" comment says to me it's either restricted (intake or exhaust) or ignition.

Try removing the foam air filter.  More than once I've seen foam that "looks new" but is either over-oiled or the cells in the foam have closed off.  This can happen if the filter is just 2 or 3 years old.

You also have a removable muffler/spark arrestor.  That's the part most likely to clog up.  You can also run the bike without it to test if it changes anything.

Make sure you have a good and correct o-ring on the jet setter too.

After the above, I'd strongly suspect the ignition system.

 

Thank you Mark for the suggestions.  I had the CDI tested and it came back good.  Also replaced the ignition coil, plug wire, and plug.  Could a weak pulser coil cause these symptoms?  I put a new "O" ring in but don't think it was OEM.  I will try it without the foam air filter and spark arrestor.  Much appreciated, Pat

Ummm...  even Yamaha couldn't test the CDI in a way that proved it was good.  Yamaha troubleshooting did include testing which may or may not discover an issue.  After you tried all the steps, the last thing in the Yamaha troubleshooting process was "replace with known good". 

One of my friends on here loves the DT360 and had four complete original CDI's tested and reconditioned by a reputable source that specializes in Yamahas and has a factory Yamaha CDI tester for these bikes.  

All four CDI's "tested" good and had a good spark when he got them back.  Only ONE of the four ignitions actually worked correctly on a running bike!  

Perhaps you could send your complete ignition to a member who could test it on their bike... Or maybe someone has a spare "known good" (even points CDI from the 250) assembly they could loan to troubleshoot with.