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Discussion Starter · #1 · (Edited)
I had just purchased a 1996 ZX6R, my first super sport. Got a really good deal on it. It was in good shape.

When I drove it on the highway I released it wont go above 45-50 mph. It started losing power at that point. Jerking, hesitating, popping, etc.

I took it to a mechanic and we checked everything (hoses, fuel cock, fuel pump, carbs, cleaned and synced them, timing, spark plugs (whitish, showing lean signs)) everything worked fine as it should.

It had a K&N air filter, but everything else was stock including the exhaust.

After spending 5 hours and 200$ on labor, we figured it was the lean fuel air mixture that was causing loss of power.

At around 45-50 mph, the fuel air mix would become lean, since the wind would enter at high speed in the air ducts which are in front of the fairing. I wanted laminar air flow going into the air box. Then engine would run proper.

I tried to solve it with stock air filter, but it was no good. It gave me the same problem. So I went back to the K&N air filter and decided to make it work for me.

So I installed Scotch-Brite foam and it resolved my problem for good. The foam worked as a damper for the wind and provided a laminar air flow within the tunnel to reach the air box and air filter.

Now I could touch speed of 90 MPH. But still there was loos of power at higher speed. So I closed one port with aluminum foil and put the Scotch-Brite on top of the foil for mirror image.

Now it runs like a rocket. I don't feel any loss of power at any speed. I can feel a lot of power even in the 5th gear and I feel I can easily go over 100 mph.

This really worked better then I had imagined.

I am planning to let it stay like this till a long time.

I know I could install a bigger main jet and schim the needle, but right now I dont want to spend any more money on the mechanic than I already have.

This will also keep my K&N air filter pretty clean. And I don't have to replace it ever I guess.

I would like to know any comments on my hack. I would really appreciate.

Attached is a picture of the Scotch-Brite power hack.
 

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Its good to hear you have it running, but this sounds like a band-aid to whatever problem you are actually having. I doubt is an issue to the type of flow, but rather the fact that you have restricted the flow. Mind you, by closing off one port, there is going to be more air trying to go through the open port. If you assume the mass flow rate is the same, you get double the velocity. Higher velocity makes turbulent flow more likely. Total flow rate is likely to go down to some extent due to the restriction of only having one port, but its not going to be cut in half. There could also lower pressure in the intake with the restriction which could be helping draw more fuel.

One thing you didn't say is if you checked out your spark plugs and your ignition system. As plugs wear out the resistance across the plug gap becomes higher. At loads, there is more air in the cylinders so the density of the air in the fuel goes up which also raises the resistance. Lastly, at higher RPM's the saturation time of the coils is smaller so voltage across the gap may be smaller and unable to overcome the resistance. Once you have the plugs out, you can also "read" the plugs. There may be evidence of the engine running lean or rich, along with a host of other conditions. Plenty of guides to reading plugs can be found in a Google search.
 

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Discussion Starter · #3 ·
Thanks for your comment. You are right, its a band aid fix for now. Info on air gap resistance also helps.

Forgot to mention that I had checked the spark plugs and it showed signs of lean mixture as well. It was whitish color which is indication of lean.

Closing one input port is actually cutting the airflow, and somehow adequate flow is reaching the carbs. As I experience very good power across the range of throttle.
 

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I would expect that you have vacuum leaks which are messing up the mixture.

There are a lot of little rubber vacuum lines which are used to tell the carburetors how much fuel to supply for a given set of conditions. If they are missing, or have cracked due to age, the leaks will surely hose it up.

Same is true of the rubber intake hoses, between the airbox and the carbs, and the head of the motor. Any leaks there will cause all sorts of issues.

On a 20 year old motorcycle, there is nearly a 100% probability that some previous owner has 'fixed' the bike. The chance of that going well, through the 3-8 owners that have preceded you are not good.

the other factor that you have to look at is what is the state of tune of the bike? When were the spark plugs replaced last? When were the clearances on the valves checked?
 

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Discussion Starter · #5 ·
The vacuum tubes were sealed and we looked for any leaks. But none were found. We found evaporator system was completely missing too. But it's harmless to motor or performance I heard.
Yes there were 11 previous owners, and they did change the above.
The air box is quite securely installed on the carburetors we checked.
 

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So how about the valves? Intake and exhaust? Do you have the shop manual for the bike? It can be downloaded here

Restricting the airflow, and that gets you to more power, and a much more even power delivery? That should tell you something is seriously amiss. Without knowing how long that went on for it's difficult to be optimistic that it's a simple fix.

Running a motor lean is far, far worse than running it rich. Easy to burn the valves which will make everything that much worse.

I admire the improvised solution -- always a fan of initiative. That's just a tool to identify what's going on.

Now that you know there is a problem, spend the time to fix it before it gets worse.
 

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Discussion Starter · #7 ·
We did not check the valves, intake or exhaust. We just assumed they would be fine. At that point we did not suspect there could be issue there. But I am not sure how valves can be culprit in this situation.

Yes I do use the service manual when in doubt. I have a copy. Thanks for the link.

With the Scotch Brite hack its incredible how I can now go up to 110 MPH easily and there still more power left to go faster. But I don't dare to go beyond that as its very scary.

One thing I noticed is that there is a little more vibration then before across the whole range of speed. I wonder why it would be so. The bike is extremely responsive as it should be, very smooth acceleration. I am seriously loving it and I am so exited.

I am afraid about what you said, I need to fix it before it gets worse.

I really wonder how can air flow restriction damage the engine in anyways. I guess spark plugs would be worn out more often, which are cheap anyways. But logically if engine is running lean it would never give out so much power. So I am inclined to think its getting a near perfect fuel air ration somehow.

Would there be anything else that would hurt bad ?
 

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We did not check the valves, intake or exhaust. We just assumed they would be fine. At that point we did not suspect there could be issue there. But I am not sure how valves can be culprit in this situation.

Yes I do use the service manual when in doubt. I have a copy. Thanks for the link.

With the Scotch Brite hack its incredible how I can now go up to 110 MPH easily and there still more power left to go faster. But I don't dare to go beyond that as its very scary.

One thing I noticed is that there is a little more vibration then before across the whole range of speed. I wonder why it would be so. The bike is extremely responsive as it should be, very smooth acceleration. I am seriously loving it and I am so exited.

I am afraid about what you said, I need to fix it before it gets worse.

I really wonder how can air flow restriction damage the engine in anyways. I guess spark plugs would be worn out more often, which are cheap anyways. But logically if engine is running lean it would never give out so much power. So I am inclined to think its getting a near perfect fuel air ration somehow.

Would there be anything else that would hurt bad ?
Restricting the air flow should result in a loss of power, as that would make the motor work harder to breathe.

The bike was running lean, to the point you could see damage to the spark plugs, right? Any idea how long that has gone on for? The damage that has already possibly occurred is not going to go away. If someone gutted the muffler, and opened up the air box, but did not change the jetting in the carbs, it will run lean. The bike was already on the edge of being jetted lean to meet emissions requirements when it was sold the first time.

Richer (probably where you are now) is better...... but you don't really know anything at this point; just that it runs better than it did. Best to get everything working as it is meant to. Then, you only have one problem at a time to deal with.

Get the vacuum lines sorted. Seal any holes in the intake and exhaust that are not meant to be there. Get the valves adjusted so they open and close when they are meant to. THEN adjust the carburetors -- get them jetted properly, then get them synchronized. Your bike will be smoother, faster, and get better MPG. It may also run cooler.
 
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