rpms won't go above 3500 when in gear

I have a 79 kz400 that is not running quite right. With the clutch in or in neutral, the engine will rev to approximately 6000 rpm but no more. When it is in gear, it revs to 3500 rpms without a problem but then sputters a little and will not go anymore. I have normal power up to 3500 rpms which gets me to about 40 mph

This morning when I started it up, it would not rev past 3500 even in neutral. I also had no power (could not get above 20 mph and that took a while). After riding for about 4 miles, it started sputtering and sounded like something blew clear and ran as described above for the rest of the day.

I have: adjusted cam-tensioner, checked valve clearances, check ingition timing. Is it possible that any of these were done incorrectly (this is my first bike). The plugs also look like they are getting carbon fouled. Any suggestions on where to go from here would be helpful.

Post edited by: mpfgrs, at: 2006/09/15 02:09

When you start it up check the exhaust pipes to see if they are both getting hot(be careful! they get hot quick ). I one is not getting hot you have one cyl that is not running. I would check for good compression, replace the spark plugs and clean the carbs. Also could check the air filter....could be a home for a mouse!

Sounds like coils/ignition could be one possible culprit. One of the coils on my 78 KZ650 basically stops working once it warms up, about 30 minutes of engine running. When that happens, I can't get many RPM's from the engine.
I found mine by using a timing light.
It could also be caused by points or condensor or other ignition piece besides coils. However, it will be noticable with a timing light that the spark plug is not getting spark and you can troubleshoot from there.
The main idea would be to try to isolate the major system, fuel, ignition or compression. Then work from there. That's how I do it. Simple Physics, if an engine has fuel, ignition and compression it has to run. Physics doesn't give it any other options!!! So, if it ain't running, or running correctly, It has to be in one of those things (or a combination (which is extrememly rare (almost as rare as me answering a question with less than 500 words(possiblily even rarer)))).
That's one idea anyway. Good Luck.

Start with a tune up , Replace the Points and Condenser, new spark plugs , set the timing .. You may need a service manual to do this ,, Get the ignition Right first then tune the carbs if need be .. My guess is the points have worn closing the gap up a bit ,, But replace them not just re-set them .. Happy Tuning

I started my bike, and after running for about 3-5 minutes, the right hand pipe was still cool. the left pipe was hot. with this information what is the next step i should take?

on my Honda 550 when it had the same problem my carbs were dirty. I pulled them apart, cleaned everything, and put on new filters. when i put it back together it ran perfect.

Another thing that was affecting my honda was not enough power (under 11 volts charging).

mpfgrs wrote:

I started my bike, and after running for about 3-5 minutes, the right hand pipe was still cool. the left pipe was hot. with this information what is the next step i should take?

Do you have spark on the right? When you pull the plug is it wet or dry?

Post edited by: OKC_Kent, at: 2006/09/19 11:00

OKC_Kent wrote:

...Do you have spark on the right?

One super-easy way to tell is by switching the pspark plug wires and see if the other side is cool.

I switched the wires, and at first i could not get the bike to start. I pulled the plugs and they were black. I will post a picture of them later today. The bike had less than an hour on it with these plugs in. I put new plugs in and the bike fired right up. I rode it for about 5 minutes, and both pipes were hot. In neutral the bike would rev to 8000 rpms with a very slight hesitation around 4000. With it in gear it would still hesitate around 3500 and not really go higher than 4500.

It would help by starting with a tuneup first, as Shoe48 said. Once that is done see how it runs.

I can think of three things that would keep an engine from reving high. It's fuel, air and spark...as always

1. Low fuel level in the carb bowls. Maybe it's this and starving the engine.
2. Clogged up air intake system. Make sure your air filter is clean, no rags are blocking the air supply, mouse nest , etc.
3. Rusted up ignition advancer

Can you drop the carb bowls and see how clean the carbs look? Could be old gas plugging things up and it's time for a good cleaning...

Another thing I just thought of, could you have bought a tank of bad gas? Maybe some water in there?

Post edited by: OKC_Kent, at: 2006/09/22 16:24

What all do you mean by a tuneup: as mentioned in my earlier post, i have already done: valve adjustment, ignition timing, cam chain adjustment. Are there any of these that perhaps i did not do correctly that i might need to recheck?

I have also taken the carbs apart, disassembled, and cleaned. I replaced the front carb boots when i did this. I also just put a brand new air filter and element on. I have also taken off the tank and drained it to see if there was gunk in it and it did not appear as though there was.

any ideas of what would be causing the plugs to carbon foul?

I'm sorry I didn't notice you did all that.

Fouled plugs can result from a weak ignition system. Go to the Filebase and look for the coil repowering mod.
If coils do not get a full 12 volts, then the spark plugs tend to foul. The path the 12volts from the battery has to follow takes it on a long, convoluted route, with many connections. By the time the voltage gets to the coils it has dropped due to resistance from old, corroded connections. The coil mod restores the 12 volts to the coils.

You prior work would seem to have ruled out the other causes of fouled plugs, so check the voltage at the coils and tell us what you have there.

Link to Coil Mod

When you cleaned your carbs how did you set/adjust the fuel level in the bowls?

Post edited by: OKC_Kent, at: 2006/09/22 16:44