High Speed Power Loss

Hey Guys.

I took a 45 min ride south on the highway yesterday and the day before and noticed a problem when at high speeds & in 5th gear.

The RPMs would be around 6K and I'd be at about 80mph (75 speed limit). I'd notice the speedometer fluctuate about five mph up/down even though the speed was staying constant. Shortly thereafter the bike would loose power even though the throttle was pulled all the way back.

The speed would just keep going down until it hit about 60 mph. The only way to increase the speed is for me to drop it down to 4th gear. Then I could get it back up to about 75 mph but the RPMs would be between 7 & 8K. One time it started to do the speedo fluctuation like when in 5th gear and started dropping power slightly but then self corrected.

If I put it back in 5th gear while in 4th at 7-8K RPM, the bike immediately drops to 6K RPM and starts loosing power again.

It only happens on one particular stretch of highway & I believe that the altitude is much higher at that point than anywhere else on 25 in this area.

The other thing I notice is that my low-fuel/brake light failure indicator light is flickering on and off erratically for the whole ride (not just on that stretch). I know that I have an electrical short or open on that circuit somewhere but haven't fixed it yet. Could that cause the problem? If so, why only at high speeds/gears?

Even if it is altitude related, it isn't safe to just loose power like that so I'm hoping that there is something I can fix to make it so it stops doing that.

Is this just a quirk of KZs or is something not working properly? Any ideas?

If I had to guess, I'd say you're looking at several different problems.

The fuel light flickering is probably caused by corrosion in your connectors.

The power loss sounds like it's probably altitude related, as the bike will run rich at those altitudes because the air is thin. Not much you can do about that because if you rejet for the peaks, it'll run lean in the valleys and you risk burning a valve.

Your flickering low fuel/brake light warning maybe because your brake light bulb is blown, or your brake light switch on the pedal is set too tight and bumps in the road is causing the pedal to move enough to trip the switch. That's easy enough to test, just turn you key on and see how much movement of the pedal turns the light on unless it's blown and doesn't light at all.

I don't think it's altitude related. My 650 is jetted good . I live at 1,200ft and can go to sea level with no problem and last year went to Big Bear (8,000ft) with just a slight loss of power but still pulled redline in every gear. Like GG says, it could be many things, but a bad gas cap comes to mind. If the vent is not working, a vacuum will form in tank and engine will starve for fuel. Next time it does that open the fuel cap and if that does nothing try raising choke and see what that does. Overall, how is bike jetted? If neither one does nothing I would think maybe electrical would be your next potential problem. Let us know...

Thanks for the replies guys. I'll try the gas cap/choke suggestion next time. I know it isn't the pedal brake switch causing the flickering because I adjusted it so that I have to push down a good amount in order for it to work.

It could be corrosion causing the flickering but previous troubleshooting is pointing me to a short somewhere in the wiring because it happens when both the fuel level & outage indicator switch are disconnected from the bike.

If the issue is only altitude related, why would other bikes not have the problem?

Does application of choke worsen the problem (by increasing over-richness of mixture in thin air condition)?

Are valve clearances set to specs (might be too tight)?

At idle speed, do plugs have a good strong fat blue spark?

Is automatic ignition timing advancer unit operating correctly (fully advancing)? If not already done, would use strobe-type timing light to assure correct ignition timing and function of advancer unit.

Good luck with the diagnosis!

For me and my bike, this problem was caused by poor fuel flow eventually emptying out the float bowls. But the fact that he gets power back when downshifting kinda negates that possibility huh?

Qdude wrote:

For me and my bike, this problem was caused by poor fuel flow eventually emptying out the float bowls. But the fact that he gets power back when downshifting kinda negates that possibility huh?

My thinking also.

After power drops off in top gear, and before downshifting, am wondering about a spark plug reading if the engine is intentionally shut down at that point. This could help better determine what is going on inside the combustion chamber when the power drops off.

As bluemeanie earlier suggested -- what about the jetting? What exhaust system is in place? Baffles? Air box or pods? Type air filter? Existing main jet size?

EDIT: Remember to carry a short length of hose that fits snugly over a spark plug to remove and replace a hot plug without burning your fingers.

Good Luck with the diagnosis!

Post edited by: Patton, at: 2008/02/25 10:03

i had similiar problems. turns out my carbs were flooding in 5th and when i kicked down into 4th this would clean out the extra gas. then could upshift into 5th for some time but not long.

Thanks for all the additional replies guys. A lot of good ideas there. I haven't had time to do any of the recommendations yet and may not till the weekend. I'll surely report back on the issue though.

My bike has the stock muffler/exhaust/airbox on it. As far as I know, the carbs are jetted as they came from the factory but I have no way of knowing that for sure. When I had everything apart earlier, I just put the carbs back on as before without even opening them up except to replace the carb covers with poly tops.

@bubba_d how did you find out it was the carbs doing that and why did they only do it in 5th (if you know)?

@ Patton what do you mean by doing a plug reading after killing the engine at that speed? Seems like it would be pretty dangerous to do on the highway...

EDIT: The valve clearances are in speck. This is one of the things I did while everything was apart.

Post edited by: Little B, at: 2008/02/25 18:56

i was also getting poor fuel mileage. and when i say poor i mean bad. 12 mpg. fuel was leaking past the needle valve due to worn out needle and seat and the fact that i had never set float levels before and obviously didnt have a clue when i tried the first time. i have no idea why it only did it in 5th. someone else pointed me in this direction.

Little B wrote:

...Patton what do you mean by doing a plug reading after killing the engine at that speed? Seems like it would be pretty dangerous to do on the highway....

Would only do this in a light or no-traffic area permitting safely coasting to a protected site to pull off and stop, and without exposing youself to any traffic risk whatsoever.

Exercise good judgment of course, use common sense, and not do anything feeling uncomfortable with, while remaining always acutely aware of the surrounding traffic situation.

Good Luck and Safety First!