High Speed Power Loss

I had a problem like that also, but not specifically with which gear it was in. The 650 just started dying on me on my way to Fort Collins and I found out that the vacuum operated petcock was doing, if not less than what it was supposed to do. I had to switch it to prime to continue the ride.

Test results:

Ok guys, I went back out tonight and tried popping the gas cap when I started loosing power. There was no immediate change so I shut it again.

I also tried the choke idea and turned it on when loosing power. The engine started bogging down and would have died on me if I had kept it in. So, I think it is getting enough air even at the high altitude. If that assumption is wrong, please correct me.

I noticed the tach gauge fluttering or bobbing. (It has also happened with the speedo gauge.) Does that just happen on older bikes or does it show something is wrong? When it does this, I can't tell any difference in how the engine sounds/performs.

As far as 'redlining' the engine, I can't even get it close to redline on any gear. I believe that the RPMs max out at about 7-8K or so. I don't know if that is normal or not but since some of ya'll said that you can redline in each gear, that tells me something is up.

I rode that section several times just to make sure the issue wasn't intermittent. Well, I noticed that while I didn't get full power I was able to keep it around 65-70 after I had popped the gas cap earlier. This confused me so I decided to go further down the road and turn around again.

Again, there was hardly any 'problem' (no redline ability ever) on the way back at what I assume was the higher altitude. I started having problems again, only further down the road from where I was having the problem before. (I assume at a lower altitude.)

Again, I tried popping the cap and after that failed to yeild results, tried the choke idea. Same thing as before, no change other than the engine bogging down when the additional air was added to the mix.

I had the issues for quite a while thereafter but it eventually worked itself out and returned to 'normal'.

Also, the light I mentioned earlier in the thread wasn't blinking the whole time this go around either. When it did blink, it caused no change to the engine's performance. So, I'm assuming that these are separate problems.

I heard a loud hissing after removing my gear. It was my gas tank cap. So I popped the top and it sucked air in. Closed it and it started sucking again. The last time something like this (the gas cap hissing after a ride) happened I took the gas cap apart returned everything back to how the factory made it and cleaned out a bunch of gunk. Then it stopped doing so.

I'm wondering if its hissing due to there being more pressure at a lower altitude which then causes the gas to compress inside the tank and thus need more air to replace the compressed gas? Since I equalized the pressure at the higher altitude (where the opposite was happening) by popping the cap, it may point to a gas cap venting problem...

But, if this loss of power is just the result of a cap venting issue, why wouldn't it have corrected itself virtually immediately when I popped it while testing?

Any and all insights are welcome.

Do not use the choke! Who told you to use choke? Don't say me. I said put it on prime. On your petcock it has a prime and afuel operated selection. Put it on prime and try it again. You just might see the difference. When you put it on prim you are gonna smog your plugs, be toolless and strand yourself.

Applying choke while riding is a test to help determine whether fuel mixture is already too rich to begin with, as the engine may run even worse with choke applied. Which is more likely in the higher altitude (thinner air) enviornment. And choke does not add air.

If not already done, would check for clogged air filter or obstruction to air intake (such as something stowed under the seat).

If spark plugs are black and sooty, the fuel mixture might be too rich (which would worsen at higher altitude where the air is thinner).

Where spark plugs are black and sooty, if not already done, would check for correct float bowl fuel level by doing the quick easy clear plastic tube test.

And would keep in mind that black and sooty spark plugs may (and often do) result from other reasons besides over-rich fuel mixture (such as ignition and/or compression issues).

Even perfect carbs cannot compensate for weak ignition, poor compression, bad fuel, inadequate fuel supply, or obstructed air supply.

Good Luck!

Post edited by: Patton, at: 2008/03/02 06:34

Sounds like a bad cap to me, you would not get "immediate" results as gas now has to flow into all carbs. Will take a few seconds. May I suggest leaving the cap loose and going on that ride and see if problem even starts?

Thanks for the ideas. I'll try leaving the cap loose for the whole ride next time and see what happens.

If that doesn't make things better, I'll try putting it on prime and see if that helps anything.

I guess I don't really understand how a choke works since I thought it added air.:blush: When I pull the lever up, big round metal discs move from vertical to horizontal on the back of the carbs which is where I got the idea that they must add air since the intake to the carbs appeared less obstructed at that point... So I guess it adds additional fuel instead???

Once the bike is warmed up, the choke always makes it run worse. I had thought that that was the way it was supposed to work...

The plugs are not sooty or black. From what I can tell they look 'good' & the gap is correct. Air supply is un-obstructed and there is a brand new K&N air filter in the airbox. So I'm going to take another wild stab at it and say that the fuel/air mixture is right or close to right....right? :blink:



I have to wait until the snowstorm we got today is all melted before trying again but will report back with the results.

Thanks again.

Not familier with your carbs but if it has choke plates that open and close you would be cutting OFF the air flow to carbs which results in a rich mixture when choke applied. Remember carbs on cars before fuel injection? Top choke plate, same thing.

Post edited by: Bluemeanie, at: 2008/03/02 22:21

I am too lazy to look up your bike on parts fiche. List the type of carburetors on your bike. They will be either CV34 or BS34. The former use a choke system. A butterfly shuts airflow when the choke arm is actuated and enrichens the mixture. The BS34 doesn't have a choke. It has an enrichener circuit. This circuit adds both air and fuel to the bike and essentially enrichens the mixture. While both a choke and enrichener richen the mixture, they do it different ways.

In any case, your bike is running lean. That is why you can't pull redline. The reason may not be jetting. It can also be fuel obstruction or other problems. This can be cause by a variety of reasons:
1. dirt in tank
2. vacuum in tank caused by lack of venting in the cap
3. petcock not working properly
4. carbs dirty and needing rebuilt
5. dirty air filter

To fix first issue, you would flush the tank, examine it for excessive rust and treat, if necessary and definitely use an inline gas filter. If you already have a filter, the symptoms you describe are consistent with a clogged filter.

When folks asked you to pop your gas cap, the purpose of this exercise was to relieve a vacuum lock in the tank. If the vent in the cap is clogged, your engine will draw gas for awhile and then it won't pull gas in anymore because of the vacuum lock. If you pop your cap and the problem goes away immediately (REPEAT IMMEDIATELY) then the problem is a vacuum lock.

Your petcock may be a problem. Engine vacuum actuates a diaphragm in the petcock which allows gas to flow. If the diaphragm is compromised, gas flow will be as well. The suggestion was made to operate the bike in PRI mode. This mode (PRIME) allows gas to just flow by bypassing control by the diaphragm. Try this to determine if the petcock is the issue. If the bike runs fine with the petcock in PRI, the petcock or hoses leading to the petcock are the issue.

If the carbs are dirty internally, they need to be cleaned and rebuilt. This isn't unusual on a 30 year old machine.

You are operating the bike at altitude. This means less air relative to fuel. If your air filter is dirty, this makes the problem even worse.

Any of the "rich" symptoms will foul plugs and you should likely clean or replace your spark plugs. See the "coil repowering" mod on our website under wg's Tech Stuff Index. While this won't fix the problem, it will help you avoid sooty plugs from weak spark.

Thanks for the help WG! I'll try your suggestions as well and let ya'll know what I find.

Hey Guys,

Went out last night with the gas cap loose and had no problems at all. So, as several of you suggested, it looks like a fuel delivery problem due to a vacuum lock from a bad gas cap.

I'm going to replace the inline fuel filter as WG recommended. They are fairly inexpensive so I figure that it is worth the little extra cost to know that that won't cause an issue in the future.

BTW, if anyone else needs a new rectangular gas cap for their KZ, check out oldbikebarn. They have them for $30. I thought it was a pretty good deal. They also were the only place I found them at.

Hey Guys.

Just wanted to share a quick update. I got the new gas cap in and tried out that stretch of road again. She works great!

Thanks again for the help!

PS. If any of ya'll are gamers or have a son/daughter who is a gamer, I have a copy of Half Life 2 I can give you/them. If interested, just PM me.

Glad everything worked out for you. Sometimes the biggest problem has the simplist solution. Enjoy the ride...