rick, you said you "don't" have a problem, the 70' came with a cap like that???. so why not run it untill you do. "if it ain't broke don't fix it"...frank...
Frank I don't think a 70 model came with a cap like that, if it did it had to be vented, this one wasn't. I went ahead and cut some slots in the gasket and it works fine now. As much vacuum that was on that cap after some short runs, something would have to give on a long haul.
Actually I believe I did see a NOS cap like that at Carlisle a few years ago but it is a different part number because of the venting qualities.
My 1970 Grabber had that cap on it from the factory (I think). I never looked at the cap to see if it was vented or not.
An unvented tank could come to a point that the fuel pump can't pull fuel any longer against the vacuum. Engine sputters and quits. Use proper cap that doesn't allow a full tank to let gas slop out.