Bike not reaching the redline
Q. I’m still trying to figure out what the hell is going on with my bike… It won’t redline. It gets to about 11,000 and then the sound of the engine changes and then it will hardly move from then on. Plugs are good, air filter is good. Bike runs good all around it just wont pass 11k for shit. Anybody got some info for me? Plug wires are brand new. Yeah it happens in all gears. It is not the rev limiter kicking in, the sound is TOTALLY different. The engine goes from the normal smooth “roar” until it hits 11k then it goes right to a “buzz” and the sound of the engine changes dramatically, but it is not the on off on off sound of the rev limiter, it is a constant sound. That is what I meant by falling on it’s ass. I doesn’t sound as drastic as an engine not hitting on all 4, but it does sound kinda unharmonious, if that makes any sense. On the main jet being lean, how would the main jet become lean. It ran like a champ when I first got it, hitting the rev limiter without hesitation, so I’m guessing the jetting was good then.
A. BROKEN VALVE SPRING, resulting in the above mentioned valve float. All ZX7s use 2 springs per valve, an inner and a outer spring. When the inner spring breaks, the outer spring has enough tension to allow the valve to function at idle and during revving up. When you get up to 9,000 plus RPMs, the spring is then to weak to control the valve and it floats, which basically means it never fully closes. That’s why it runs OK at idle but dies on top. I would not run this engine any more as you are subject to dropping the valve. I do not know the history of your bike, but this is not uncommon on 91-95Zxs that have been raced or abused hard. I had one brought to me in 94 that had won a race on Saturday but was totally uncompetitive on Sunday for a valve job. It had 6 broken inner valve springs. Its hard to tell by just looking. You will need to pull the head. When I do valve jobs I always check spring pressure at both installed height(CLOSED) and at maximum lift on a Rimac Machine. I recently had a ZX-11 head brought to me that had 40,000+ miles on it for a valve job. I was brought new springs as well as valves and guides. The old springs checked as good as the new ones, because this bike had never been run hard.
Yes, the engine would run below max rpm as the stronger outside spring would handle it up to a point. The Kawasaki race kit springs are a single high RPM spring and do not use the smaller inner spring. Interestingly enough the race kit spring measures out at 40lbs of seat pressure at installed height or the amount of pressure exerted by the spring when the valve is closed. The dual stock spring measures out at about 35lbs, so you see the race single spring is actually stronger than the dual stock spring. If you measure the stock outer spring only it checks out at about 25lbs of seat pressure and only 85 lbs at max cam lift, where the stock dual spring measures at about 110lbs at max lift. The single kit springs measure out at 112lbs at max lift. So you see there isn’t much difference in the spring pressure between the stock and kit spring at max lift, but the kit spring does it without the inner spring. This allows the 91-95ZX7s to rev to 14,000rpms reliably, when using the single high rpm spring. APE makes a replica of the very rare and expensive kit spring along with some nice Titanium retainers. This will cost you about 400 dollars total. Plus labor, about 500 dollars and parts. Ouch, not cheap. There is not a easy way to check for broken springs but by diagnosing the problem. What is the history of your bike?? You mentioned valve job. Is that valve JOB or valve adjustment?? The main reason I say BROKE SPRING, is because I’ve seen this happen alot! As the engine is building RPMs the stock outer spring has enough pressure to control the valve open and closing until the rpms reach a point were it cant keep up. Then the valve never fully closes resulting in valve float and a loss of power and the engine changing its tune. I hate this for ya!!!, but this wasn’t caused by wheelies, I’m pretty sure.