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polarbear

Who's the Engine Expert???

36 posts in this topic

Who's the Engine Expert???

I put the White Brothers slip on the good ol' 07 YZ 450. The power down low is lots better and feels better overall. The only thing that I need help figuring out is why the bike doesn't rev out like it should. It was like this before. I rode a 08 after mine and the 08 has way more top end power. The 07 used to rip up top and lately its not. It pulls great down low but as soon as it hits the mid through the top its not as powerful as it used to be. Its better to lugg it around then it is to rip it up high. My two guesses is that it needs a valve adjustment and/or the jetting is not right. Can anyone give me some Ideas???

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It's hard to say since I don't know all of the circumstances, but it sounds to me like your main jet is too rich. Try dropping the main a little and see if it revs out better.

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It's hard to say since I don't know all of the circumstances, but it sounds to me like your main jet is too rich. Try dropping the main a little and see if it revs out better.

it has a 160, my altitude is 4500 ft. I never got a reply from eddie on this but I don't know what one I should have for this altitude. Should I go with a 158 for now???

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I'd agree with that. WB slip-ons are known for fattening the jetting somewhat.

Grayracer, you are always good at answering my questions. Did you read my review on the 08 yz 450? Well my question back is, what do you mean by the above statement. Fattening the jetting??? Does that mean the Slip-on makes the 160 act like a 162 or higher???

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Yes. They tend to run richer.

Thanks. So what jet would you say would be the winner? Any other jetting adjustments?

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I'd say go smaller on the main a little and then just adjust the fuel screw (depending where your fuel screw is now). If your fuel screw is too far in or out then you need to change your pilot. The main jet controls the jetting at high RPMs though, so that is probably your problem.

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I don't know about you guys, but usually when I put an exhaust on an ATV/bike it tends to allow the motor to breath better. If the motor can breath better, it will want more fuel -- more air needs more fuel. I don't think I have every found a race exhaust that restricts the air more. I may be way off base here, but this has been my experience.

Maybe someone could explain to me why the pipe would cause the bike to need a leaner jet??? Thanks

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I don't know about you guys, but usually when I put an exhaust on an ATV/bike it tends to allow the motor to breath better. If the motor can breath better, it will want more fuel -- more air needs more fuel. I don't think I have every found a race exhaust that restricts the air more. I may be way off base here, but this has been my experience.

Maybe someone could explain to me why the pipe would cause the bike to need a leaner jet??? Thanks

Quite often, systems which increase the core diameter of the muffler to a large degree will cause the engine to run rich. And the reason very often IS increased air flow through the carb. It is not true that an increase in flow through the carb will require a larger jet to compensate. Carbs will compensate for this on their own to a large degree simply because the fuel is delivered by the airflow. The flow of air over the main nozzle is what generates the vacuum signal that draws fuel into the airstream. If air flow is increased, or as often is the case at lower speeds in smaller, high powered engines, stabilized by some change in the engine, the vacuum signal can become so strong the the jet has to be reduced in size in order to correct a too rich condition.

WB systems and slip-ons were known for this for a long time, particularly on 250F's.

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I agree, this can be the case. But that would assume the bike is jetted perfectly for the stock setup. I have done alot of dyno work and have found the the majority of the bikes come very lean. I know the 2007 YZ450F comes especially lean stock - at least for sea level. I have done some dyno work on some machines and had to increase the jet sizes more than 10 sizes to get the bike to have the correct air mixture.

I do admit that we did not have a white brothers exhaust to test at the time so it could be very different as you mention.

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I agree, this can be the case. But that would assume the bike is jetted perfectly for the stock setup.
From direct experience with a WB R4, no it doesn't. I dropped one on a YZ250F with OEM jetting, and while the main was OK, the pilot was 2 sizes too big with the R4.

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Hey, guys, I am still having problems with the bike's jetting since I put on the WB slip on exhaust. As I stated earlier, it had a 160 stock main jet. I went ahead and put on a 155, tried it out, 158, and even went to 165 just too see the difference. With the 158 being the one that seemed to work the best. The main problem with all of them is lots of black smoke coming out the exhaust. It still puts out black smoke. Its like a badly jetted two-stroke! I don't have the expertise of some of you, so please help me figure out this problem! The bike is really slow and sluggish. All the other 450's that my buddies have are faster then mine, and all of theirs are stock! Even the 08 YZF 450 with stock pipe is faster then mine! HELP!!!

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With all that rich jetting after putting on the pipe, how's the spark plug? Might wanna throw a new one in there to see if you've "gunked" it up with a rich mixture.

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When we put on the pipe it started smoking. We have not changed the spark plug. I am going to do that today. After that, if its still smoking and has low power what would be the option? I am tempted to take it to a Dyno and have them jet it for me. This is frustrating as HELL!!!! What should I do?

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Before the pipe it wasn't smoking, but it did lack power compared to other 450's. My cousin got an 08 yzf 450 that is supposedly slower then the 07 and his is obviously faster then mine, and it is all stock. So I got the WB MX 4 slip on exhaust , it did liven up the low end a bit, but the mid and high are very sluggish, there is not the hit that it should have. Also, like I said, since I put on the slip on it smokes a lot, and the Low seems to have more power, but the mid and the top has less power then before. Hopefully this will help all you experts out there to come up with some possible solutions.

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Sounds like your burning oil. Is it that kind of smoke? Valve stem seal/guide worn? Did someone through some pre-mix in as joke? Just kidding, but it does sound like your getting some oil in the combustion chamber. Have you done a compression test or leakdown test? Is the breather tube routed into the airbox? I know that the hondas blow alot of black smoke if they take a tumble and spew the oil all over the air filter. I would try putting the stock silencer back on and call white brothers about their exhaust. I don't know if that helps any but just some of my thoughts. You have probablly already checked all of those things. Best of luck and let us know,

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Have you done a leak down test? If you have a lot of hours on it you might need to re-ring it.

If the bike ran good at one time and began running poorly (before you put the exhaust on) the top end just might be tired.

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