Hi All
i have a 1973 DT3 250.  3200 miles.  I just cleaned the tank, installed new peacock, and rebuild the Mikuni VM-26 carburetor.  Bike ran well and idled ell before performing the work.
Here’s the issue.  The bike starts well and revs normally.  After about 15-20 seconds it will bog down and motor dies.  Plenty of fuel, and peacock has good flow.  Seems like I can rev the throttle and keep it running for a bit, but soon as I let off of throttle the motor bogs and dies like it’s just not getting enough fuel.  Motor will restart just fine.  I can restart it, it revs normally but will then bog and die.  Before performing the work and rebuilding the carburetor the motor ran fine and held a good idle.  All carburetor settings the same except I changed the needle clip one slot down from the number 4 slot to the number 3 slot on a 5 slot needle clip.
wondering what is causing the motor to bog and die 20-25 seconds After initial stat up?

Andrew B.
Fairbanks, Alaska

 from near Salisbury, UK.

Did you check/replace the rubber 'O' ring on the setter jet? If not, this is worth doing anyway - I buy the oem Yamaha part, as it is still available. 

Thank you for your quick reply.  Would this o-ring on the setter jet be the o-ring on the main jet (drain plug), or the washer on the needle valve seat.  Forgive my ignorance about which part is the setter jet.

Andrew

No problem.

Item number 7 on this parts list is the 'O' ring, and this sits in a groove on the setter jet, item number 5.

When it excellent condition, you should feel very slight resistance when you fit the float bowl to the main carb body.

Yamaha DT3 1972 1973 USA CARBURETOR - buy original CARBURETOR spares online

Thanks.  I’ll remove the carburetor and inspect.  I completed the rebuild job last summer and cannot clearly recall.  I did change out any part with new parts that came with the rebuild kit, but not sure if the kit contained this o-ring.  
So if I understand correctly, when this o-ring is worn or MISSING, it can create the condition where the engine starts but will big and cut out after 20 seconds.
Andrew
 

Just got done rebuilding same one and by chance did you put the floats in with the pin up and not down towards bottom of bowl?

I’m pretty sure I put the float pins in pin side down with the small plastic stops on the top side where they snap into place on top of the float pins.  I’ll also inspect this location.
Thanks

Mentioning "rebuild kit" makes me cringe. 

Do yourself a favor and put the original Mikuni jets and slide needle back into the carb if you used new components from a kit. 

Greetings from Tennessee 
 As MarkT says change your jets etc back to the original  and save yourself a headache 

I do have  the original Mikuni jets and slide needle.  They compare as exact replacements as the ones in the dreaded, “rebuild kit”.  I’ll go through the carburetor and inspect part #7 in the diagram, the o-ring for the setter jet.  I’ll also inspect the float pins for correct installation.
what is the issue with rebuild kit jets and slide needle?  Do they tend to be off in their dimensions compared to original Mikuni parts?