If it's a big enough plastic Y to accommodate the white piece with all the holes in it, your mechanic may have left out the important piece. If the black plastic Y is much too small diameter to accept the white piece with all the holes, it's probably not the flame trap. On my 81, I found a piece like you describe with no trap inside. I found out it was never meant to contain a trap, it was just a Y junction in the line. My actual flame trap was a brass cup sort of thing stuffed into the nipple of the breather box itself, which is the older design. Turned out it hadn't been changed much if at all, and the breather box was totally clogged. I replaced it, and installed a newer design flame trap with all new hoses in a convenient location. Breathes like a new car now. At www.foreigncarpartsonline.com you can see a picture of the weird old style flame trap that went in the nipple of the breather box. Good luck. Holler if you need help. I would not drive it without a flame trap. It's designed to prevent crankcase explosions in the event of a backfire.
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Thanks to everyone for the help, Doug C. 81 242 Brick Off Blocks, stock; 86 240, 129K