TOPIC: GPz900R refuse WOT in 5th and 6th gear

Ok.. I need help.
Stock 1984 gpz900r, valves are gapped, new ignition coils and caps and plugs
Carburetor operation smooth, good idle, good response, she go to redline in any gear and no problem except when i am in 5 or 6 gear.
The bike hit only 7000 rpm and then wont go over this point.
Alternator is new and is working correctly according to the shop manual.
I have tried another CDI just to be sure but no luck.
Pickup coil resistance ok
New plug wires too
What can i try? Thanks!

I forgot to say i have the stock gearing ratio trasmission

Have you checked the battery & charged it with a good motorcycle charger? and what type of battery is it?
I use a Ctek US 0.8 smart charger & VC97 digital multimeter and occasionally do a load test.

This may not be the same thing you're experiencing. I failed to check the electrolyte level for a number of months one year which included a very hot summer & hot fall. The motorcycle started running poorly at high speeds then would quit. I was able to restart & limp home in low gears and found out that the electrolyte was down about half way. Another time the bike started doing the same thing: the electrolyte was fine, but the battery was shot. It failed a headlight high beam load test. After charging with a good smart charger >> 12.6 VDC. engine OFF >> ignition switch ON & headlight high beam ON >> the voltage dropped significantly below 11 VDC. a good battery should only drop to about 12.2 VDC.

Battery is new. Is a good quality battery, i think this is not the problem

I would still test it with a digital multimeter & do a load test. new batteries are known to be bad or go bad. I had a Scorpion AGM that quit holding a charge after 2 yrs, when they were purported to last 5-7 yrs. I agree that this might not be the problem, but you never know. I check the easy stuff first, just to make sure.



...

Just tested mine. 12.73 volts no load, key on headlight high beam on 12,10 volts

Do you have CV carbs on it (Like Mikuni BS34s) with pods instead of the stock airbox?

No, stock cv carburetors with airbox, new manifold at the intake and between carb and airbox. Filter ok
I think this is not fuel related because if when underload i use the choke lever nothing change in high rpm gear /range.
Im orientend to a ignition problem but i cant understand what.
Cdi swapped and nothing
Tried two set of plugs d8ea and d9ea nothing change
Coils pickup readings are in the spec
The only strange thing on the bike is that the exhaust have not the center pipe that connect the silencers togheter. The PO for some reason has it cut and then capped.

Spark advance moving freely?
Steve

no problem except when i am in 5 or 6 gear.
The bike hit only 7000 rpm and then wont go over this point.

In 6th gear at 7000rpm you'd be at about 170kph - I would expect your acceleration at that speed to be pretty poor on a 34 year old bike.

Does it cough and sputter at the max RPM?
It could be a vibration causing the kickstand switch to get flakey and turning the ignition on and off. It will act like a rev limiter... coughing and sputtering.

I would bypass the safety interlocks and run power direct to the ignition coils etc.

SWest wrote: Spark advance moving freely?
Steve

He should have electronic advance on that bike I would imagine.

Yes. No advancer. Maybe the air gap on the pickup coils?
About the sidestand switch i can say im sure it work because there is a light for it on the gauge and to switch off the engine i have to completely release the stand..

Irish Yobbo wrote:

no problem except when i am in 5 or 6 gear.
The bike hit only 7000 rpm and then wont go over this point.

In 6th gear at 7000rpm you'd be at about 170kph - I would expect your acceleration at that speed to be pretty poor on a 34 year old bike.

Why? This bike can easily pass 220 kph when in order..

Vibrations at speed can do a lot of strange things. In a troubleshooting situation like this, you want to remove as many variables as you can. I am saying run a temporary to power wire from the battery to the ignition (and coils) to bypass the ignition switch, kill switch etc., and a temporary interlock ground wire which is what the side stand eventually connects to (through logic diodes etc.). If the ignition is like the Zx550 and 750's, there should be one wire on the igniter that just gets jumpered to the ground wire. That bypasses all of the interlocks for the ignition.

Once that's done you can rule out the ignition as the cause and move on to other possibilities.

When you say you have new coils, do you mean new factory coils specifically for your bike? Or did you get some aftermarket coils? If the coils have resistance that's too high, that usually indicates the coils are too slow for your application. That can cause spark to cease under heavy load... high RPM and large throttle opening. You would notice this at lower gears as well, though, unless it takes a sustained throttle for the coils to heat up or something.

I will try and i let you know. I purchased aftermarket coil, not stock or dyna..

Coil tested again 2.6 ohm cold or hot and 13kohm output resistance cold or hot.
I had the wires tested and no resistence under a load with a 2amp lamp light.
Resistance at the multimeter with no load is 0.00
The bike start instantly and is very very responsive at the throttle. Alot of stuff is brand new but the problem is not fuel related since when i try to open tge choke lever under load nothing change....

shamal wrote: Alot of stuff is brand new but the problem is not fuel related since when i try to open tge choke lever under load nothing change....

That does not rule out a rich situation. If the engine is too rich and sputters out, then applying the choke won't make much difference since it's already too rich.

But the description of how it behaves needs to be more precise. Does the engine run smooth at 7000 RPM in 5th but just won't go any faster? Or is it sputtering and surging and backfiring?

Also, I'm not trying to be funny, but are you sure it's running on all four cylinders. I only ask because there have been many times where someone asks me to tune their inline-four bike or balance the carbs and it turns out they've been riding around on 2 cylinders the whole time.

Nono.. The bike is running on fourcylinders. Very angry and responsive. I have a thermal termometer and all the exhaust manifold are the same temp. For precision i have after a couple of minutes from cold 1234 90° 92°88°94°
Fuel level is set.. And in fact the engine is running smooooth until hit 7000rpm range. Then it show some symptoms like when you are out of gas. It try to pass the 7000rpm range and then come back to 6800rpm tgen again.. Sometimes i can reach some higher rpmthan 7000 but veeery slowly and with this on off effect
Sprry for english

Do you have any inline fuel filters? Those can flow ok for around town, but just slow enough to run out of fuel on a long hard acceleration on the highway. I've had that exact problem on the 550. It took a while to figure out. The paper element would flow sometimes and other times would slow down. It needs a fuel pump to have enough pressure. The symptom was just like someone turned the ignition off. All four carbs would run out of gas at the same time.

After a while, the filter would not flow at all sometimes, which is how I found it.

Here's a video where it was not flowing until I pressurized the gas tank. Then it flowed fine.