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Discussion Starter #1
OK. I've got a Micron Al. can on order! I don't really want to pull the carbs and rejet...I could end up with a mess like Emrah!:yell:

Do you think adjusting the mixture screws will be enough to make the engine happy?

Philip
'00 ZR-7, Stainless Steel Brake Lines, Corbin Seat, Fairing, Taller Factory Blue Al. ATV Bars, Cobra CB Radio/FRS Radio, Arai Quantum/f with J&M Headset.

"Put the credit card down. Back up and move away from from the credit card...":D
 

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Its been my experience that re-jetting is required when using a pereformance slip-on. Many slip-ons say that re-jetting is not required, but it usually is for optimum performance. Adjusting the mixture settings probably will not be enough. Thats just my opinion, I re-jetted mine and I have no complaints. Good luck :D
 

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Dirty Harry.... Moderator
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But what are the consequences...

I've had the Yoshimura round race slip-on on my ZR for about 600 miles of hard riding and haven't re-jetted. What are the consequences of not re-jetting, am I screwing up the engine? Like I said before, it runs like the tail light is on fire and I haven't noticed any problems, (yet). And I also have a K&N filter installed. Any help here? Tom ;)

Kelly, didn't you have the same question?
 

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Wheelie for Safety
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Hi Brent (Perljm),

Didn't you mention when we talked that your gas mileage was not as good because of the re-jetting. Have you calculated what your MPG is now.

When are you going up to Alice's or HWY 9 and Skyline. I'd sure like to meet up with you and go for a ride.

Andy
 

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Discussion Starter #5
Andy

Did you just rejet or shim the needles and rejet?

Did you go with a K&N air cleaner?

Philip
 

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Wheelie for Safety
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Hi Philip,

My bike is completely stock. All I did was adjust the idle mixture screws when I first got it. It runs great. I did get a Micron slip-on a while back, but I hated the loud sound it made, so I sold it.

Andy
 

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The Commander
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Eddie Vedder is right, even when the manufacturer says rejetting is not "required", that doesn't mean it's not a good idea. The risk with a wide open can and a K&N filter is that you'll be running too lean. Lean is MUCH worse than rich as far as potential damage to the engine. And since we all know that the stock bike is set a little lean from the factory, it can get nothing but worse. Turning out the mixture screws will help the off-idle response, but once you open the throttle past 1/4, the needles and mains take over. I rode my bike for a while with the Muzzy pipe before I rejetted. After installing the K&N and the Holeshot jet kit, there was no comparison. It's a very noticeable increase in power across the rev range and much smoother. It's definitely not difficult to do. If I didn't screw it up, that's a good sign! I suspect Emrah's problem was shims that were WAAAAYYYY too thick. The ones that come with the Holeshot kit are 1mm, and I used two of them on each needle. If you buy your own shims rather than getting a kit, just make sure they're measured with a micrometer to ensure the size. My 2 cents.
 

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Hey Andy,

I haven't computed my gas mileage yet, but i think my gauge may be off. I'm going to try the gas gauge mod and see if that helps. I've had other bikes and have ridden all day without draining the tank, and with the zr-7 i seem to always be filling up. Maybe its because this bike is so much fun to ride :)
I'm probably going out this weekend if your up for a ride andy. The weather should be sunny and cool. Let me know.
 

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Wheelie for Safety
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The Pros and Cons of rejetting:

Pros: More power. :)
Cons: Worse gas mileage and hotter running engine. :(

Andy
 

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Dirty Harry.... Moderator
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Hey Andy, when you put the Micron on your bike did you re-jet, and if you did was there any difference in performance when you put the stock can back on the bike? Tom
 

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Andy Bajka said:
The Pros and Cons of rejetting:

Pros: More power. :)
Cons: Worse gas mileage and hotter running engine. :(

Andy
Andy you re right about the power gain and the worse mileage, but the engine will run hotter on a lean mixture!
For the engine it's best to have the correct stoichiometric ratio, but between rich and lean, it's better to run a little rich.
Overly lean mixture can cause serious damage, because of overheating, pinging etc.


Aris
 

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The Commander
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Good point Aris. Also, the term "worse mileage" is relative. I'd say my bike dropped maybe 2-3 mpg, but that's about it. To me, that's pretty insignificant. Don't think of it as rejetting because you want more power, think of it as trying to optimize the fuel delivery to match the airflow through the intake and exhaust. If you free up the airflow, you need to add more fuel to keep it in the optimal lean/rich balance that Aris spoke of.
 

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Wheelie for Safety
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Hi Aris,

Yes the stock bike runs at 14.7:1 air/fuel mixture which is the correct stoichiometric ratio. Putting on a slip-on doesn't change the air/fuel mixture. That is why manufacturers say you don't need to re-jet.

When you re-jet you are bringing the air/fuel mixture to 12-13:1. Now you will get a bigger explosion and more heat. You also get more pollution and worse gas mileage.

Jarel, I agree that if you change the air filter you will need to re-jet.

Andy
 

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The Commander
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"Putting on a slip-on doesn't change the air/fuel mixture."

That's a debate that was waged here as well as on SportbikeWorld a while back. I used to think the same thing. Here was the conclusion as I recall (I'm paraphrasing, since I don't remember it all exactly): While it doesn't make intuitive sense that adding a free-flow exhaust would affect the mixture, in effect it does. It has to do with the efficiency of the exhaust gasses leaving the cylinder. There is always residual exhaust mixture left in the cylinder after the exhaust stroke, which limits how much fresh mixture can come in. So a free-flow exhaust allows more of this residual mixture to exit the chamber, effectively increasing the amount of fresh mixture allowed into the chamber on the intake stroke. Since carbs (as opposed to fuel injection) are driven by the vacuum created by the intake stroke, you ARE getting more air through the carbs, which increases the leanness of the mixture. Then when you combine that with a free-flow air filter, you're now freeing up even more airflow to be sucked through the carbs, making it leaner still. Someone please correct me if I left anything out.
 

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Wheelie for Safety
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The scavenging effect is not really changed by just changing the slip-on. The 4 into 1 exhaust header is mostly responsible for that.

I can see why there is such a debate. The manufacturers of slip-ons say you don't have to re-jet. But they don't really know anything and have never done any testing. :)

Jarel and Aris, this thread has been very interesting and I have learned from your comments. Thanks...

Hear is a very well written article on the subject on carburetors, re-jetting, slip-ons, cam, big bore kits, and more.

http://www.f6rider.com/VRCC/tech/carb101.htm

Andy
 

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Hi Andy,

That's a well written article, and the info is good also. Did you notice the note on how bikes (it means engines like the ZR7's really) built before 90's were adjusted lean all across the rev range?

Aris
 

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The Commander
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Yea, that's a good one, I think I'll put a link to it on ZR-7.com.

I think this excerpt from the article pretty much sums up the conversation:

"Generally, a slip-on muffler system will work acceptably with simple mixture screw adjustments and possibly some slight shimming of the main jet needles. A couple more horsepower may be obtained by changing the main jets and needles as well."
 
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