I am having (had?) a similar problem with an atomic4 on my Cal33.

Check the gas tank vent tube, it was the last thing I checked and I found a whole family of spiders in residence, with what looked to be several generations of ancestors. I sucked the whole civilization out with my shopvac. I had to leave before I did a thorough follow up test, but removing a tribe of spiders from the tube cannot have hurt.

Good Luck

alistair