KZ400 throttle issues

I have a '79 KZ400ltd that was running great until a few days ago when it started bogging down around 40mph. I pulled over and my throttle/engine problems continued to where it would bog down with the slightest throttle turn and then wouldn't start. I pushed the bike back home and proceeded to buy new spark plugs, and cleaned the carbs twice. I am now to the point where I can start the bike, but it has no acceleration and bogs down around 3500rpm. I searched the forum for similar problems and found some people has issues with their jets. I guess this could be a possibility, but I'm not sure why I would suddenly have jet problems or need to get larger jets... anyway I attached a video of the issue, any help would be appreciated.



Thanks!

Could be fuel starvation, which may result from several conditions that can cause the float bowls to refill too slowly.

Would first add fuel and then try the same test with the fuel tank CAP open.

Next, re-try the same test with the petcock set on PRI (prime).

With engine not running, assure ample fuel is flowing thru the petcock when set on PRI.
(Can loosen drain plugs and watch for fuel flowing out of the float bowls.)

Let us know if the bogging issue persists after doing the above.

Good Fortune!

Been a while since I had my kz440 but about 37 seconds into the video, you are looking at the carbs and they appear to me to have slipped back out of the carb holders? At least there is it appears about 3/8" gap between the carb and the face of the carb mount.

Going along with what Patton said - fuel starvation - maybe it's just sucking air at this point and thus starving for fuel when you crack open the throttle?

Patton: I have plenty of relatively new fuel in the tank, this was my first thought as to why the bike was dying, but there's well over 1/2 a tank in there.
I tried running it with the fuel cap open, no change, same with PRI setting.
In my previous attempts to fix the problem I ensured there was flow through the the tank and fuel lines.

Wrenchmonkey: good call, I guess I've been running my carbs set back from the intake for awhile. I was able to advance the carbs which are now flush with the engine...still no change.

Any thoughts?

Are both cylinders running the same? You can tell by exhaust temp at the heads or spark plug color. making sure your advancer is lubed and set correctly might not be a bad idea. Vacume leaks can appear suddenly so I would grab a can of WD40 or starting fluid and start spraying the boots to see if you can find a leak. So suddenly and RPM specific sounds like a vacume leak to me, Could be a restricted fuel flow. If it is, both cylinders will be running identically poorly . vacume leak on a carb holder should show up as one cylinder running lean or hot comparatively.

Just for fun I pulled my spark plugs (installed yesterday) to see if I was running lean or rich. I'm guessing I'm running rich instead of lean?

Ahh, but they do not look very similar . one looks like over roasted penutbutter the other looks wet and black. They should be very similar. That one looking wet makes me wonder if you are getting intermittent spark on that cylinder or something limiting combustion. How does the exhaust look by comparison? Are you noticing a lot of unburnt fuel as you rev the engine coming from the black spark plugs cylinder?

I really think it's just the lighting. Neither one is wet.
The exhaust appears even on both sides.
I haven't really noticed a difference in unburnt fuel, but I'm also not sure what exactly I'm looking for from that standpoint

Could be lighting, that's why I was wondering if your getting grey puffs of unburnt fuel from your exhaust pipe when it starts boging down. More just throwing out that comparing cylinders can help elimante a lot of possibilities. Maybe its from running with the choke on but it looks pretty rich.

Ya, that was my thought too. I've been running it with the choke on because it idles the best and I can run it for a minute without worrying about it dying. I'm just not sure where to go from here...

If your inline fuel filter is the same as mine you may have it backwards from what I could see in the video. It was hard to make out but you might want to double check the flow direction. It could have got clogged. Just a thought.

It is backwards, but it's been running that way for the last several years. I think tomorrow I'm going to empty and refill the tank and flip the filter.

I'm not sure how much of a difference that will make though because every time I've cleaned the bowls they've been completely full of gas (though it would be nice it the filter was clogged and that solved my problem).