Bike super sluggish after starting until revved a bit under load

I've long considered my KZ650 ('78 B2A) a cold-blooded bike that doesn't like starting and has to be tended to until it gets warm. However, for several years now, the bike acts like it has no fuel in the carbs and chugs along with no power until I'm able to rev it up to 4-5k RPMs while under load (just revving it in the driveway seems to have no effect). After I hit those revs, the bike is fine and has its normal power.

- Put in a new fuel petcock a few years back. The problem existed before and after the petcock
- Exhibits the issue in any petcock setting (normal, prime, or reserve)
- No amount of time idling in the driveway seems to affect this
- While in the driveway, revving in neutral also does nothing. It apparently has to be under load
- Bike acts like it is fuel starved, wanting to die if I give it slightly too much throttle while under load until I can get it to rev a bit under load
- Went through the carbs a few years back and am waiting on an ultrasonic cleaner to go through them again this year to get them super clean

The only thing that comes to mind is that it needs a sufficient vacuum from the carbs to open up the petcock diaphragm and the bike needs to be under load to do so. However, the same things happen even when the petcock is on "Prime" with the fuel flowing freely.

When is the last time you changed and lubed the ignition points? Ed

I don't remember. I assume that means it is something I should do, though. Thanks for the suggestion.

Yes, that might solve the problem. Also, if you haven't performed all the other maintenance described in the maintenance schedule in the service manual it would be a good idea to do so. These bikes are extremely reliable, but they do require normal maintenance to be performed to stay that way. Ed

Have you tried checking the condition of the spark plugs to see if they reflect any fuel starvation/lean running conditions? As for the carbs, I go through mine about once a year and it always helps give some pep back to the bike.

If you store your bike over the winter, do you use a fuel stabilizer?

Does the ignition advancer have free and full movement?

Rick H.