Jetting issues on a 1981 Cb750 DOHC

[RuppRanger]

Hello, I am new to the forum so hopefully i follow the desired protocol, but I have some questions and so far this group seems super helpful (properly diagnosed a loose starter flywheel). I recently bought a 1981 Cb 750 DOHC that has some fuel system struggles. The fellow I bought it off claimed it had been synched less than a year ago but I am not so sure. Anyway it idles terribly low until after about 20 min of driving it gets stuck up in the 2500-3000 range and takes forever to come back down. Also it cuts badly between 6500 and 7000 rpms when accelerating quickly. Lastly it backfires quite nicely around 3800 rpm. The previous own put a 4 in 1 exhaust kit on it but didn't mention anything about rejetting. It still has the stock airbox.

I know it is quite the chore list, but from what I am reading on this site it sounds like there are some potential easy fixes. The first that seems to sound nice is carb rebuilds, with new jets to match the modified exhaust and my altitude of 4500 ft. (almost every used bike I have ever owned has never had the sea level jets removed from it). Also upgrading to iridium plugs sounds like it could help with the backfire. And finally based off the condition the fuel lines and filters need to be replaced no matter what. Then when all of this is done a good synching to top it all off All of things I believe I have the prowess and know how to do except picking jet sizes. This is where I am all ears for suggestions. That and if you have any comments on other tuning ideas or things to check please share.

I've only had it for two weeks but I am loving the cb already. Thanks!

[Johnnylevi]

Have you done a valve height adjustment yet? Timing? checked for  vacuum leaks? adjusted the air fuel mixture screws? Just going over a list to start troubleshooting. I would take the carbs off  and check what jets are in there right now.

[FastCletus]

Do what johnny says, but I would first check for vaccum leaks.  simplest task and doesnt require turning a wrench to figure out if you got a leak

spray carb spray on boots.  if engine picks up or slows down, you got a leak

[RuppRanger]

Thanks for the quick reply, I couldn't see any major difference after doing the vacuum test, I did replace the plugs which has smoothed things out a lot, but there are still hiccups. I think I am about to the point where i am going to ride until winter then when the snow comes tear off the carbs and work from the ground up. Is there any order you would go about doing the other things you suggested? I've only done major work on single cylinder carbs and suddenly four it seems like it is very wise to plan your moves accordingly. I assume jetts, then air fuel mixture screws then carb sync? valves then timing? or does it matter?

Thanks again, you guys are great

[TOOLS1]

Sounds like you have a grasp on the situation. To determine what jets to use, first you need to know what jets it has now. Also the carb sync is the last thing you do when tuning a bike.
TOOLS