TOPIC:

Hey all. I have a 94 cop bike, KZ1000. It runs great, except for one thing. On the highway when cruising at 5000 RPM or greater, the engine runs fine for about 5-10 minutes, then starts running on 3 cylinders, then 2. At this point, of course, it starts seriously slowing down and I have to open the throttle waaay up to maintin any speed at all (and if I keep going like that, the engine will eventually splutter out). Normally I don't let it get to that point. When I notice it running on 3 cylinders, I get in the right lane and back off the throttle and slow down, and pretty soon it's running fine on all 4 cylinders again.

Now to me this sounds like fuel starvation at higher fuel flow rates. So I disconnected the line from the tank and turned the petcock to prime....fuel pours out fine. Next I tried riding on the highway with the gas cap open and petcock on prime. Still didn't fix the problem. Now I'm starting to wonder if this problem is perhaps caused by something other than fuel starvation.

Some additional info on the bike. It has a new fuel filter, but not the cone kind. I got another one from mikesxs.com, a squarish one. The one at the bottom of the page here:

www.mikesxs.com/mikesxs-fuel_system.php?category_id=4.7

Could that be the problem? Also, the bike has uni pod filters (the foam kind, soaked in filter oil), and I upped the main jets from 127 to 130 (I have Mikuni BS34 carbs). Could those things be the problem? The carbs have been cleaned, plugs and wires are new.

Any thoughts on this?

Any crud or other foreign material inside the fuel tank clogging the petcock filter? You probably already checked this, but just a thought. Was riding a new bike with freshly painted tank that would run fine for maybe a mile, then "run out of gas" -- crank back up, run fine again for maybe a mile, then "run out of gas" again. Kept doing this until discovered some paint residuals floating around inside the tank which would gradually make their way toward and gradually clog the inner filter, then when the motor was shut off, the crud would float away from the filter until again drawn back to the filter when gas started flowing. Maybe it might help to remove the petcock and flush out the tank with fresh gas. Shot in the dark, okay.

make sure the vacuum line to the fuel cock is not cracked or squashed as well......

make sure the vacuum line to the fuel cock is not cracked or squashed as well......

When's the last time you checked your valve clearances?

How do you know it drops to 3, then 2 cylinders?

Which cylinders actually cut out?

It may be a fuel cap venting problem?

Check all the simple things first such as,

fuel vents, airbox, air filter, pinched hoses.

Thanks for all the responses, folks!

Patton wrote:

Any crud or other foreign material inside the fuel tank clogging the petcock filter? You probably already checked this, but just a thought. Was riding a new bike with freshly painted tank that would run fine for maybe a mile, then "run out of gas" -- crank back up, run fine again for maybe a mile, then "run out of gas" again.

Could be. Perhaps I have some deposits, but not enough to be affecting it at city speeds. I'll open that and check it out.

Popeye wrote:

make sure the vacuum line to the fuel cock is not cracked or squashed as well......

Checked that, it's fine. Also I ran it with the petcock on prime, so the vacuum line doesn't affect the fuel flow.

ltdrider wrote:

When's the last time you checked your valve clearances?

Probably 15k miles ago, when it was still in cop service. I haven't done this myself since then. Could this cause the problem I describe?

larrycavan wrote:

How do you know it drops to 3, then 2 cylinders?

Good question. I don't. It just feels that way. It starts sounding different, more ragged, and I have to open the throttle much more to get the same speed. Then I slow down for a while; and after a bit, the engine operates normally again, and I can get back to normal high speed behavior for another 5 minutes.

So it seems to me that one of the floats goes dry first, quickly followed by another, then another after that....unless I let off the throttle for a bit and let the floats fill back up.

Are there other possible explanations?

larrycavan wrote:

It may be a fuel cap venting problem?

That's what I thought too. But as I said above, I tried riding the freeway with the cap propped open. Still did the same thing.

larrycavan wrote:

fuel vents, airbox, air filter, pinched hoses.

Checked all that, looks fine. I have pod filters, not an airbox--the pods are only 2 months old.

You might also have a timing problem. Try running it at these rpms in neutral and see if it does the same.

kzwolfsr wrote:

You might also have a timing problem. Try running it at these rpms in neutral and see if it does the same.

Hi kzwolf-- I have no problem revving it to 5k and beyond, on surface streets as I'm accelerating. No power loss or anything like that. It is only after extended cruising at 5k+ that it starts to miss. Could timing be causing such a problem? Also, I checked the timing with a timing light (both at idle and at full advance) and it seems to be fine.

has someone plugged the vent caps to the carb bowls?

nads.com wrote:

has someone plugged the vent caps to the carb bowls?

On these carbs you can't plug the vents, I think. My bowl vents are the type that vent into the airbox (or in my case, pods). They are molded into the casting of the carb body. One possibility though, I could check if the pod clamps are somehow causing the pod sleeves to block the bowl vents. I doubt it, but I can check.

Did this start just after you added pods and rejetted?maybee too much oil on pods?try useing the choke when it happens again to see if it wants more fuel.also check to see if you have 12volts to the coils.It could be low.