Power Loss Mid-Range

I wasn't sure which category to post this in, but figured my problem is with carburetors? This actually deals with a 1995 Kawasaki sport bike; EX500-D2, so I apologize beforehand if I annoy anyone with a non-KZ issue. I'm trying to help a youngster with his bike, and could use some help and insight from you experienced folks.

The problem is he's experiencing power loss in the mid-range - low band and high rpm is fine. His comment is that the bike seems to "bog down" in the mid rpm range, then picks back up again when he passes 5-6K.

Do any of you have any suggestions where to start looking? He hasn't dropped the bike off yet, but I believe it's all stock. An acqaintance mentioned jetting as a problem source, but I don't see why anyone would have re-jetted if the bike is stock? I believe the bike has Keihin CVK34 carbs, at least the 1996 model does.

Where should I start? Appreciate any insight and guidance.

I know zip about that bike but I would look to see if the vacuum hose from the carbs is connected to the TPS assembly on the IC Igniter. Most of those mid-90s style bikes have a Throttle Position Sensor (TPS) switch. The info from this switch is relayed to the IC Igniter to vary the ignition advance. The vacuum hose is real skinny and will generally be on its own fitting (not the one used for sync) on CVK series.

You might also look for vacuum lock... ride the bike and if it bogs or feels like it is running out of gas, pop open the gas cap (wear flameproof underwear I guess) and see if the "bog" goes away. Sometimes, the venting on gas caps is poor and a vacuum can set up in the tank and not allow gas to flow.

If the bike has a fuel pump, change the fuel filter. It is probably the original and probably filthy anyway. Also, there will be small screen filters inserted into the gas line ends. Check to ensure they are not clogged... remove them and blow them out. Make sure the gas line is routed without kinks or sharp bends.

If this external stuff doesn't fix the problem, check for rejet. The most likely problem will be someone has installed a "JET KIT"... a Dynajet kit will have what look like stainless steel jet needles with six slots. If you find those things in the slides, remove them, and find some stock jet needles and put those in. That will likely clear up the problem which is that the Dynajet jet needles are tooooooo rich across their entire range... you can try and reclip those things up towards the 2nd or 1st slot but a lot of times, it doesn't work...

To get to the jet needles, you have to remove the carb tops, springs and retainers. When you pull out the jet needles, also be looking for extra washers. The nice thing about CV carbs is that you don't have to resync if you pull the jet needles. There is also a possibility that the main jets are way too big as well. If the bike has stock airbox and pipes, or even aftermarket pipes, you might want to find some stock size of perhaps only slightly larger Keihin mains and install them if you find Dynajet main jets as the bike could be bogging from wayyyyyy too big main jets.

Last, if the rider isn't that familiar with bikes, a 96 model COULD have a slipping clutch but the ride characteristics don't much sound like it (lower rpms).

Wow, WiredGeorge, you just gave me an entire class there! Thank you for taking time to provide this excellent trouble-shooting advice, and it's exactly what I needed.

I'll go to work on it, and I will post the results. Thanks again!