1980 kz550 ltd. Motorcyle lags after few minute of riding.

Hope someone can help me here. Been trouble-shooting this problem for weeks now, and can't figure out what's the problem.
Bike starts up easy, runs well with choke on, though it does belch some smoke. Hey it's an old bike, I get that. On my daily commute she runs well for the first few minutes, then at about the same point each daily trip, performance falls, the bike starts lagging, sounding/running rough, and I have to keep on the gas a little during idling so it doesn't stall. This will last for a couple more minutes, then the bike surges, and returns to normal performance.
Over the last few weeks, I've done the following: changed spark plugs, air filter, and fuel lines; rebuilt the fuel tank petcock; drained out the tank and carbs; sprayed carb cleaner in to carbs while bike is running; and finally put a can of Berryman's B-12 fuel treatment in the tank.
While I've noticed some improvement, and many times the bike runs without fault, other times I'll notice the same loss of power and rough running for a short spell.

Any ideas?

Did move the ignition Timing advancer freely, (cleaning and some lube)?
Valve clearence checked?

Had very similar problems to this on my z500, turned out to be worn needle jet tubes, check out mine, replaced them with new and all those issues went away. Initially I thought it was ignition timing, then air leak, then went down the whole list but problem kept returning. Photos of old and new tubes below. Ps new tubes are almost impossible to find for TK carbs

Are you riding it with the choke on?

I use the choke when cold, but close as soon as I think the engine's warm.

Any chance the vent in the gas cap is plugged? Next time it happens pop the gas cap and see if it doesn't go away.
Bill

KZQ wrote: Any chance the vent in the gas cap is plugged? Next time it happens pop the gas cap and see if it doesn't go away.
Bill

Good call, I forgot about the gas cap venting. Had the same issue on my son's bike , lucky I caught on quickly

I had never considered that. Thanks. How does one clean a gas cap vent?

Here's a couple clips on it.
Steve


Start at 14:51

geneaskew wrote: sprayed carb cleaner in to carbs while bike is running

Any ideas?

Spraying carb cleaner while bike is running is not the same as taking the carburetors apart and cleaning them.