If you have the 145 main with straight pipe and performance filter, you're way too lean on that. Did you adjust the idle mixture? What you should be running for that is a 152.5 or 155 main with one or two washers for the white spacer mod. Not sure it's the exact cause of your problems, but it could give you trouble.
The two things I can think of are inadequate fuel delivery or extremely inadequate jetting. Straight pipes are trouble-makers, I gather, and they don't really perform better than a good muffler. In fact, to some ways, they perform worse due to exhaust reversion.
You can:
A. Loosen up the float bowl screw and drain the fuel (or run the engine till it dies) with the petcock off.
Then turn it on and catch some in a pint jar; count the seconds after it begins to pour to calculate your fuel rate.B. Change your jetting!
AFAIK, your combination probably requires some of the richest jetting of anyone here, apart from those with internal engine mods (The experts here are free to correct me.)
C. Do both. Easy peasy.
Technically, the California models have even leaner jetting than stock, though I'm not sure how their pollution control stuff affects their ability to handle that. Nonetheless, you're too lean, and straight pipes are trouble. Get it fixed!