Sounds like you've actually developed an intake leak. On the 77-79 750s, behind the rubber intake boots/manifolds there are viton rubber orings..after time these get hard, brittle crack and break, thusly allowing air between the head and the boot. This will lean your mixture out, and cause the popping on deceleration that you are experiencing, as well as a host of other problems...failure to correct a lean condition will eventually result in expensive and extended mechanical failures, mostly in the combustion chamber. Valve heads, rings, pistons...the cost will rack up...
www.cycleorings.com sells the needed rings, as well as the orings for you to rebuild/refurbish your VM carburetors (I would strongly recommend doing this if you havent already. Even if they're fairly clean carburetors, the orings break down over the years, and allow air and fuel in uncontroled volume which will further compound your running issues) Also, valve adjustment SHOULD be checked and/or performed every 5K miles at the latest. Its not difficult once you get the hang of it, and Steve here on the board offers a very helpfull spreadsheet that will help you to keep track of what shim sizes youre currently employing, and likely which sizes you will need to order or source next. Shims are available new thru
www.z1enterprises.com (although your local dealer can probably get them too, but for at least a few bucks more each) Some guys on the board, such as Steve and myself will even participate in a shim swap where we supply you with a needed size, you send us your old shim(s) back plus a dollar for each shim you required...Works pretty good, and if you only need one or two, its cake.
I have a pretty soft spot for those old late 70s 8v 750s and 1000s. they are bulletproof machines, alot of fun and will take to tuning well. If you have any more questions, please dont hesitate to ask, I will assist the best i can. Also, on your "vent" lines, Only ONE of the carbs (number 3) is the petcock vaccum hose to be attached to. The other rubber lines, between one and two, and three and four, are vent to air. If you are running pods and a pipe, it has been suggested, and I have followed and found it helpfull, to run NO RUBBER LINE from those vents. It has something to with barometric pressure and some scientific stuff that i cant even begin to understand
Also, I see that you have a 750 "L" model i think? If you need, i have a tank in decent shape, actually off of an 1000L but its slightly bigger (a plus no doubt) and will need a new petcock. The tank had recently been cleaned at a radiator shop, and has little if no rust inside (been a while since ive looked) no rot holes forming either. Its just sitting around, will need paint, but I cant recall it having any dents to speak of..maybe a ding or two. If you're interested, I would be happy to let it go for a reasonable price.
