Sometimes, only sometimes

[reg]

I have a 1980 DR 400 that i use for road use and sometimes when i start it, warm it up and
bring the revs up to 2-3 grand it runs lean, header pipe begins to glow and backfires when i throttle off.

The next time i start it its fine 

It has a new air filter, plug and always fresh 98 fuel.

Any help appreciated
Reg

[mx250]

Sticking float?? Crud in bowl??

[David Lahey]

If you are doing the test stationary in neutral I would say it is crap part-blocking the pilot jet.
Maybe connected with this is that modern petrol seems to make a coating on carby wet parts (including in the orifice in the pilot jet). In my case it seems to be temporary ie once enough fuel goes through the pilot jet the problem goes away. I suspect it happens if I shut the fuel tap off with a hot engine causing much of the petrol in the bowl to evaporate before I ride next. Not saying this is what is happening to your DR400, just what I see with some of my bikes.

[vmxrider]

Crud in the fuel tap?