'06 yz450f - intermittent blubbering

As always, I checked multiple posts and I hope this one doesn't become a mind-bender. I recently did a valve job. Rode a Hare Scramble, MX practiced a few times, then yesterday, the bike started intermittently blubbering. I rode it back to the pits and it seemed fine. Checked coolant (I am getting some weeping from the impeller seal that I've yet to service) but it was at an acceptable level. Air filter is dirty having done a double-header Sat and Sun (an hour of riding in butter conditions w/out a filter service -nothing I haven't done before). After it cooled, it fired first kick, raced it around a bit before loading up and it seemed fine. I just don't want to pay a big entry fee next race and have it fail, there's got to be somthing wrong. Any ideas?

If you are using "blubbering" the way I do, it's rich. Note that you said it ran fine when you started it cold it was OK. So, why would it be intermittently rich? One possibility is that the float needle may be leaky.

But clean the filter to start with.

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