Did you make the readings with the engine warm and the throttle wide open? A rule of thumb is no more than 10% change between cylinders, but that might be just for cars. For my k5 the specs are 185-242 is normal compression. Overhall the engine if any one cylinder falls below 142 or all are below 185, and no more than 28 psi diff between cylinders.
It seems like 40 psi diff between your cylinders is bad ... Warm up your engine real good and open the throttle all the way on testing. I hope there isn't a problem.
The oil check will tell you if its valves or rings. Do a test the normal way, and then drop a little oil in the spark plug hole and retest. Look at the numbers. If they go up, its rings. If they change some but not much, its the valves. Might be time to adjust the valves.
I would ride the bike around the block a few times. Get it up to operational temp, and then redo the measurements. I think you'll have a better idea of were you stand then.
I would say that if your valves are way out, it would cause your bike to run pretty bad. Did this happen all of a sudden or did it creap up on you? A leak down test will tell you were to look if your compression seems questionable. I would guess that all your numbers should at least be around 190, and then do percent change, and it should be low. Like (190-150/190)x100 = 21% between cylinders. That seems high to me. But you did it on a cold engine I guess. Specs for your bike would be nice. Maybe someone has them here.
If I use my k5 specs as a guide, I get 14% change max between cylinders. Just picking random numbers:
(200-(200-28)/200) x 100 = 14% is the max percent change based off 28 psi max diff pressure.
But remember this is free advice and I'm no mechanic. You get what you pay for
Would be cool to know what it ended up being. Sounds to me like it's pretty lean, Devil exh and no mapping I'd not be suprised to hear that. It's proably not backfiring, but poping instead. Real common to misread the sound, but major poping is common when your real lean expecially once the RPMS are up. Didn't notice how many miles you were up to, and when you started noticing the change. Was it as soon as the exh was put on, or before?
I'd look into a tuner so you can get some more fuel to that thing soon.
Just hope no major damage was done up to this point.
When going with Aftermarket exhaust / Slip on Exhaust, You really need to have your carbs done" jet kit" . You will see the difference then. The stage of jetkit really depends on the bike and fuel-2-air ratio that is required so your performance is tip top. The reason for Aftermarket Exhaust goes Beyond The Sound of Your Bike. 🏍 Ride Safe. Zoom -Zoom.