I don't believe so. I used one of those crush gaskets when I mounted it and it's also not super loud at the cylinder like it would be if I had an exhaust leak. I'm thinking it's just tuning at this point. Ive ridden a bit today and it seems to be hovering around 350 now and hasn't gotten above 375 since yesterday.
Also I've been reading a bit today and it sounds like more strain on the engine can also cause higher temps. When it was staying as cool as it was before I still had 16" wheels and 27x22 gearing. Now I've got 17" wheels and 30x24 gearing. So I'd say there's definitely more of a load. That may be part of why it seems like it's getting so much hotter.
Sounds like from what people are saying I should be good as long as I'm not breaking 400 anymore which I'm not. It just doesn't sound right to me that an aluminum cylinder would be fine with sustained temps of 350+ but I'm not a scientist. so what do I know?
I just need to keep getting it dialed in and I'm sure it'll be fine once I have my carb tuned all the way. It's already way better than it was yesterday and the old lady is working all night so I've got ample time this evening to tune and rip around downtown Louisville.