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Olds455guy

High Altitude Jetting 08

9 posts in this topic

I've got an 08 YZ450, stock besides an Lexx pipe. I bought it used and its never ran exactly right. Basicly it runs very poorly in lower rpms, it will occationally completely cut out at mid throttle. It runs great at high rpms just really rough low to mid. So I tore the carb apart and here is what I found, Main Jet 160, Pilot Jet 45, Leak Jet 72 and another jet that is 38 Im not sure what that ones called. I live and ride between 6k and 10k feet above sea level. I can't find anything at this high of an altitude in the jetting database. Im not sure where to start, any opinions would be helpful. Thanks in advance!

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To begin with, it looks like you could use a manual:

http://www.yamahaownershandbook.com.au/?r=0

The 72 is more than likely the starter jet. The leak jet is a 55, and located in the float bowl. The only thing that should be in the carb with a 3 and an 8 on it is the needle seat, a 3point8 (3.8), and not removable. The likelihood is that none of these is involved with your problem.

Since it's apart, be sure that the float level is correct, and be sure no one has put the release plate (on the back of the slide) in upside down. The "square" end with the hole near the edge goes down.

Try adjusting the pilot screw according to this guide:

http://www.thumpertalk.com/forum/showthread.php?t=327405

You could be running too rich a pilot jet given the altitude. When you look at the jetting others use, you can roughly compensate for altitude by going one percent leaner jet number for each 1000 feet increase in altitude.

Something else to look at is the throttle position sensor (TPS). If it's faulty, or it's been moved out of its correct position, it can cause something like what you describe. Unplug it and run without it once to see if things improve.

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I have been dealing with the same thing on mine. It feels like the bike isn't getting enough air. Someone told me to remove the screen on the plastic piece that holds the air filter. I have already tried a high variety of different jettings with no change.

Edited by Rewker

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Removing the screen will do nothing whatsoever to improve performance. Nothing.

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Removing the screen will do nothing whatsoever to improve performance. Nothing.

Not true. I removed my screen and it greatly improved the problem I was having. I thought the problem was only jetting and have been playing with it for months. Now after removing the screen I have much better performance in the area that Olds455guy is talking about. Now that I look back on it the problem seams more like the bike was struggling for air than anything. Not sure where you ride, but I start riding at almost 9000 feet and we struggle in the thin air up here

Olds455guy, I also suggest trying an air filter that lets more air through than the stock one. It helped mine as well. After doing these two things I now feel like I can at least feel exactly where I need to adjust the jetting rather than struggling to pick up exactly what the problem is. Good luck.

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Not true.

It's not only true, it's true by actual measurement, as opposed to what something feels like. Your point regarding the filter itself is valid. There are any number of elements available that are less restrictive than the OEM Yamaha unit, but the flash screen on '99-'09 models has been proven on a dyno a couple of times not to restrict as much air as the filter itself, and to offer no benefit when removed. In one case, Doug Henry's supermoto team did exhaustive testing on several different intake configurations and ended up running the air box in a very nearly stock setup with a White Bros filter and the screen in place.

When you analyze the matter, it makes even less sense. The problem is that you have an engine that was set up to run at a nominal 2000 feet, and it's been moved to an environment where the air is less dense. It is a fundamental fact that the less dense the air is, the more easily it will pass through a filter element, screen, or other such restriction. On that basis, the screen would be more of an impediment to operation at sea level than in Colorado anywhere. If the bike is jetted right, it will run right

Another thing that you really can't change very easily that also contributes at high elevations is ignition timing. The lower air density means that the fuel octane and ignition advance requirements will change. If you're still running the original CDI, try to find one for an '06, or even an '04. The '06 will rev better, but both improve low end more very significantly. Failing that, you might want to look into a multi-programmable unit like the Power Commander, Vortex , or MSD boxes.

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Thanks everybody for the help. I unhooked the TPS and didn't notice must of a difference. I am going to try a different filter and jet and see if that can remedy my problem.

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It's not only true, it's true by actual measurement, as opposed to what something feels like. Your point regarding the filter itself is valid. There are any number of elements available that are less restrictive than the OEM Yamaha unit, but the flash screen on '99-'09 models has been proven on a dyno a couple of times not to restrict as much air as the filter itself, and to offer no benefit when removed. In one case, Doug Henry's supermoto team did exhaustive testing on several different intake configurations and ended up running the air box in a very nearly stock setup with a White Bros filter and the screen in place.

When you analyze the matter, it makes even less sense. The problem is that you have an engine that was set up to run at a nominal 2000 feet, and it's been moved to an environment where the air is less dense. It is a fundamental fact that the less dense the air is, the more easily it will pass through a filter element, screen, or other such restriction. On that basis, the screen would be more of an impediment to operation at sea level than in Colorado anywhere. If the bike is jetted right, it will run right

Another thing that you really can't change very easily that also contributes at high elevations is ignition timing. The lower air density means that the fuel octane and ignition advance requirements will change. If you're still running the original CDI, try to find one for an '06, or even an '04. The '06 will rev better, but both improve low end more very significantly. Failing that, you might want to look into a multi-programmable unit like the Power Commander, Vortex , or MSD boxes.

Thanks Greyracer this is really good info. Do you know the testing procedure when putting something on a dyno? Is the engine run at specific RPMs and then tested, or does the dyno test take in to account acceleration? The reason I ask is because the only time I really felt the issue was during large changes in throttle opening. For instance when I would go from almost zero throttle to half or three quarters the bike would bog down and occasionally I felt as though I was going to get thrown over the handlebars. I did cut my screen out and the problem has been greatly reduced. There was also some plastic spill over in the stock screen that I have and I cut that out as well so maybe that is what made the difference. All I know is that I have struggled to find the right jetting since I bought this bike and now that I have taken out the screen I feel like I can really tell what is happening as far as jetting goes and am able to make adjustments.

Can you also go into some detail about how a CDI works? How does it help adjust the timing in the bike and why does that make a difference because of the octane of the gas? I know that at altitude, because of the density of the air it is possible to run a lower octane gas, but I don't know why that has anything to do with timing. Thanks in advance.

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The type of dyno most often used these days is an inertia dyno. The bike accelerates the dyno roller through the engine's whole effective torque range in a higher gear against an inertia mass intended to simulate the mass of the bike, and then the software calculates the torque that was required to produce that acceleration. Sudden bursts of throttle are not actually counted, but the torque and horsepower throughout the whole rev range are.

The bog you mention is properly corrected by tuning the accelerator pump, pilot, and early needle circuits.

The CDI uses 3 pieces of information to adjust the timing: Crank position, which it gets from the trigger coil signal, Engine RPM, also from the trigger coil signal, and throttle position, from the TPS. In general, since fuel burns at very nearly the same speed regardless of engine speed, the faster the engine turns, the sooner (more advanced) the spark has to happen to be sure that the flame has built up an effective amount of pressure as the piston begins moving down the bore on the power stroke. Conversely, the slower the engine turns, the more the spark must be delayed (retarded) in order to prevent too much pressure from building before the piston starts down, as this can lead to detonation, first noticed as "pinging".

The progression from retarded to farther advanced as the engine accelerates is called the "advance curve". One of the things you can do with this curve is use it to control the torque output of a large, very powerful engine by retarding it from optimum levels at points along the curve. The '08 and '09 models had this done by the factory to reduce low speed torque because it was thought that making the engine aggressive would make it more rideable. Since the lower air density at altitude has the effect of changing the spark advance requirements of the engine, changing the advance curve can be a way to compensate for high altitudes.

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