Ok. I've had a wicked backfiring problem, probably from 1978, but I never really noticed it until I installed the shorty, glass pack reversed megaphones in 2014. Anyway, at the rally, Maxpete said, "hey let's change the low speed adjustment." So we did, turned it out to 1-1/4 instead of 3/4, and guess what, It didn't backfire nearly as much, but there was an initial hesitation on take off. Screwed around with the low speed to no avail. Then I read the excellent carb thesis, (several times to allow it to sink in), by Grisld1 and 5twins. And I mean excellent. Great job guys! http://www.amckayltd.com/carbguide.pdf I have a 75 and from their charts, I pulled the larger #45 pilot jet, bought 2 , #132.5 main jets and moved my needle to the #4 slot. Yes, it's supposed to be there, but I moved it to the #3 spot back in 1978. Why, I don't know. Nothing changed back then so I left it that way. Long story short, The slide rubbers on my 72 were showing signs of cracking, so in June when I did the 750 thing, I used the 75 slides. They worked great, like the old ones and still with the same backfiring. Now I'm ready to change all the stuff in the carbs and looking thru from the back, I notice that the slides seem to be slightly askew. Was it my eyes? Did I install them wrong and crush the little tab in the rubber. No, and only a complete idiot could do that. (No offense to those who have screwed that up). Look at these Picts. You tell me if you've ever seen this before. The 1st 2 Picts are of the 72 slides. You can see light coming thru the holes on one of them. The 3rd and 4th are of the 75 slides. Use the horizontal line on the slides as a reference and in your mind, draw a perpendicular line straight up thru the 2 holes to the tab on the rubber bellows. What do you see? Yes, it's crooked on the 75. It still runs fine. But what gives from Yamaha? Has anyone ever noticed this? But the biggest question is, (dramatic pause), can the bellows be turned? Has anyone ever tried to? At $185 a pop, I wouldn't want to break them. Well?