Oh noes! Carbs again?

So I'm out with the old KZ400 the other day and she starts and idles fine. Winds out the first few gears nicely. Great, right?

Not so fast. Above 40-45 mph in top gear she loses power. If I roll the throttle on, she picks up but runs rough. Come to a stoplight, and she's fine. Take off, revs through first, second, third, perfectly. Same issue, at cruise in fourth and fifth.

Seems like she's running out of fuel, Tank is full. Really don't wanna pull the carbs off again. Sigh.

Anybody got any advice?

No crap in the tank and feed line? New plug/wire/coil?

Plugs look good. They aren't old and have very few miles. Neither exhibits any evidence of fouling or running hot.

Make sure there's no water in the tank. Some fuel valves have a filter on them that can get stopped up. Is there an external fuel filter?

I would make sure that your carbs are getting a good supply of fuel before doing anything else. If you don't have a filter in the line, now would be a good time to install one, right after you pull the tank and make sure that the petcock is not clogged up.

One other thing just came to mind, does this bike have points & condenser ignition? I've seen a bad condenser run fine at idle, and cause poor performance at running speed. That can be tricky to diagnose.

I do have some inline filters, and the petcock is fishy, but quite new. I just replaced the condenser last year. (Try to find a condenser nowadays!)

I will check the fuel supply to the carbs next time out to the garage.

Rubber tipped air nozzles.....

Get it up to speed where it noses over and crack your gas cap. if it runs better after a few yards your cap vent has corroded closed and your pulling a vacuum in the tank. The new fuel is hell on the check balls in the caps. Spray with penetrant oil and then a quick spray twice a year with chain lube will keep it working.

Had the KZ out again today. I'm thinking that maybe I'm just not riding it enough. I checked it out before my ride. Gas cap looks nice. I just had it apart last year and put a new gasket on it. No corrosion to speak of. Fuel system looks clean, flow is good.

On my ride today, I encountered the problem less frequently. When the engine stopped pulling, I rolled on a bit more throttle, and it picked right up and pulled as normal. So maybe it's more of a flat spot. Maybe one or both carbs gummed up a bit? I'm going to try riding it more, putting in some fresh gas, and see if that does the trick.

On a side note, my brand new battery is dead. Can't keep a battery in this bike! I had this battery on the charger just a couple days ago. Not enough in it to turn the starter this morning. Kick start saves the day! Guess I need to find a wiring fault or just disconnect the battery after every ride.

Check out Oregon cycle parts for charging circuit help http://www.oregonmotorcycleparts.com/index.html

It would not surprise me if there a charging problem that causes the hick up.

Also you have points if you have a kick starter so check the advance mechanism as there prone to getting sticky. If you take it apart use Kytox grease or oil to lube it. yes tinny bottle is $30 but it works and never leaves the part. http://www.amazon.com/DuPont-Krytox-Lubricant-Perfluoropolyether-Industrial/dp/B0096VV9ZE

Oh you may wish to try on a near empty tank add a gallon or two of a caned fuel like Sunoco "Standard" or VP "C12" both will help clean as they go through and will also let you know if its a jetting overlap problem due to higher Alky content since it was likely jeted before the E10 mandate.

You have a better chance of seeing Jesus than of getting a KZ400 to run without an airbox.

In reply to 4g63t:

Bike is as near stock as it can be, and will stay that way as long as I own it. Stock airbox and a OEM-type filter.

had a 75 kz400d, I miss it, adjust the valves???