To jet, or not to jet


17 replies to this topic
  • basinite

Posted July 14, 2009 - 11:32 AM

#1

I have a 08 WR450F with almost 3,000 miles on it. The only mods I've did to the bike are the free mods to unkork the bike that you find here on thumper talk. I also modified the stock exhaust.
I ride trails that range from 4,000' to 13,000' elevation on a regular basis. The reason for the elevation extremes is because I live in Colorado. I have never touched the jets on the bike and neither did the bike shop where I bought the bike. These are the stock jets. I have never had a problem with the bike. It doesn't smoke black smoke, it doesn't sputter, it only over heats and boils over on parts of trails that are extremely difficult and slow, where you would expect it to get hot. The bike also starts very easily no matter if I'm at 4,000' or 10,000'.
I guess my question is why change the jet's? Am I missing out on something? Any help would be apprecited. :worthy:

  • JSanfilippo

Posted July 14, 2009 - 12:07 PM

#2

If you're satisfied with the way the bike runs leave it alone. Do you have an adjustable fuel screw?

  • rkwfxd1

Posted July 14, 2009 - 01:05 PM

#3

^x2

If I rode at your altitude I doubt I would have rejetted mine.

  • William1

Posted July 14, 2009 - 01:39 PM

#4

The changes you made leaned the bike out somewhat. The altitude cause the mixture to be rich. Who knows, it might be accurate though it is probable it could be better. You should swing by OTD on Colfax and let Eddie touch your bike for ten minutes.

  • KJ790

Posted July 14, 2009 - 06:55 PM

#5

I have a 08 WR450F with almost 3,000 miles on it. The only mods I've did to the bike are the free mods to unkork the bike that you find here on thumper talk. I also modified the stock exhaust.
I ride trails that range from 4,000' to 13,000' elevation on a regular basis. The reason for the elevation extremes is because I live in Colorado. I have never touched the jets on the bike and neither did the bike shop where I bought the bike. These are the stock jets. I have never had a problem with the bike. It doesn't smoke black smoke, it doesn't sputter, it only over heats and boils over on parts of trails that are extremely difficult and slow, where you would expect it to get hot. The bike also starts very easily no matter if I'm at 4,000' or 10,000'.
I guess my question is why change the jet's? Am I missing out on something? Any help would be apprecited. :worthy:


If you are happy with the way it runs, that's great. However, you would be amazed at the power difference a change in jetting can make, even though the bike will start the same, rev up the same, and sound the same.

I recently did some dyno tuning with a YZ250F (I know it isn't a WR450F, but the effect is the same). I discovered that this bike performs the best when it is way rich according to the A/F meter. The meter showed that during WOT runs a 170 main jet gave the "optimal" A/F ratio (around 13:1). However, as I kept increasing the main size, I kept gaining power up until I got to a 185 main jet. The meter showed that it was rich (just below 12:1), but this is what the engine performed the best with under my conditions. Here is the difference:

Posted Image

These were WOT runs. As you can see, the 170 main made a tiny bit more peak power, but with the 185 main the area under the curve was much much greater. The engine was making around 8 more hp at 7000 RPM and almost 5 more hp at 8000 RPM with the 185 main jet.

  • Krannie McKranface

Posted July 14, 2009 - 07:59 PM

#6

If you are happy with the way it runs, that's great. However, you would be amazed at the power difference a change in jetting can make, even though the bike will start the same, rev up the same, and sound the same.

I recently did some dyno tuning with a YZ250F (I know it isn't a WR450F, but the effect is the same). I discovered that this bike performs the best when it is way rich according to the A/F meter. The meter showed that during WOT runs a 170 main jet gave the "optimal" A/F ratio (around 13:1). However, as I kept increasing the main size, I kept gaining power up until I got to a 185 main jet. The meter showed that it was rich (just below 12:1), but this is what the engine performed the best with under my conditions. Here is the difference:

Posted Image

These were WOT runs. As you can see, the 170 main made a tiny bit more peak power, but with the 185 main the area under the curve was much much greater. The engine was making around 8 more hp at 7000 RPM and almost 5 more hp at 8000 RPM with the 185 main jet.


WOT dyno charts wont' tell you much for single track IMHO.

  • KJ790

Posted July 14, 2009 - 08:05 PM

#7

WOT dyno charts wont' tell you much for single track IMHO.


True, but that isn't my point at all. I was just using that graph since it is what I had at my disposal to show. I was demonstrating how much jetting can affect power. Changing the pilot and needle will also create big power differences, of which would be very relevant to single trail riding.

  • basinite

Posted July 17, 2009 - 03:51 PM

#8

Thanks for all your replies. Basically, if I understand you all correctly, even though my bike runs good, if I rejeted it would have alot more power.
Right now as it is, if I am in 1st or 2nd gear I have to really set back and pull on the handle bars as well as give it alot of gas before I can even attempt to ride a wheely. If I change out the jets will it give it enough power, so that all I have to do is give it alot of throttle and it will lift the front tire up on it's own? I weigh about 180LB and I expected this bike being a 450 to have enough power to wheely over if given to much gas, but this is not the case. Maybe it's because of improper jetting? Any advise?

  • KJ790

Posted July 17, 2009 - 05:01 PM

#9

Thanks for all your replies. Basically, if I understand you all correctly, even though my bike runs good, if I rejeted it would have alot more power.
Right now as it is, if I am in 1st or 2nd gear I have to really set back and pull on the handle bars as well as give it alot of gas before I can even attempt to ride a wheely. If I change out the jets will it give it enough power, so that all I have to do is give it alot of throttle and it will lift the front tire up on it's own? I weigh about 180LB and I expected this bike being a 450 to have enough power to wheely over if given to much gas, but this is not the case. Maybe it's because of improper jetting? Any advise?


I can't tell you if your bike is properly jetted right now or not. However, it is possible that the jetting is not perfect, but close enough to start fine, sound fine, rev up fine, and generally run fine, but be rather significantly down on power through areas of the RPM range.

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  • JSanfilippo

Posted July 18, 2009 - 12:33 PM

#10

Right now as it is, if I am in 1st or 2nd gear I have to really set back and pull on the handle bars as well as give it alot of gas before I can even attempt to ride a wheely.


What elevation was this at? In my experience at sea level a basically stock WR 450f should easily raise the front tire in first and second (even third)

Here is a wheelie vid of the second or third time I was out with the bike. The only mods on the bike at this point was a cut throttle stop and gray wire. I weigh alot more than you and sitting in a neutral position the front end comes up in second with just throttle.

http://s65.photobuck...nt=6d39697d.flv

  • basinite

Posted July 18, 2009 - 03:14 PM

#11

If I was to give my bike full throttle it still would not wheely in second gear like yours did in that video clip. I would have to really lean back and work at it to do what you did with little effort.
I usually ride between the 5'000 to 13'000 foot elevation range here in Colorado. One thing I forgot to mention. One problem I have noticed with the bike is that there is a bog when I first apply the throttle while going slow on a trail. This is a real pain when trying to wheely over a fallen tree, big rocks and etc. The bog doesn't always happen, but the fact that it does happen, even occationally, is annoying. I heared a Quick Shot by Boyessen would fix this, but I also heard that the quick shot is a rip-off and jetting is the sollution. Any suggestions?

  • KJ790

Posted July 18, 2009 - 03:23 PM

#12

If I was to give my bike full throttle it still would not wheely in second gear like yours did in that video clip. I would have to really lean back and work at it to do what you did with little effort.
I usually ride between the 5'000 to 13'000 foot elevation range here in Colorado. One thing I forgot to mention. One problem I have noticed with the bike is that there is a bog when I first apply the throttle while going slow on a trail. This is a real pain when trying to wheely over a fallen tree, big rocks and etc. The bog doesn't always happen, but the fact that it does happen, even occationally, is annoying. I heared a Quick Shot by Boyessen would fix this, but I also heard that the quick shot is a rip-off and jetting is the sollution. Any suggestions?


The quick shot doesn't fix any problems, it just covers them up, it is a bandaid for incorrect jetting. A bog right off the bottom is incorrect pilot jetting. This could probably be fixed by just turning the fuel screw, at most you will have to change the pilot jet if you have the wrong one in. Lack of power at WOT means you have the wrong main jet.

  • basinite

Posted July 18, 2009 - 06:46 PM

#13

The quick shot doesn't fix any problems, it just covers them up, it is a bandaid for incorrect jetting. A bog right off the bottom is incorrect pilot jetting. This could probably be fixed by just turning the fuel screw, at most you will have to change the pilot jet if you have the wrong one in. Lack of power at WOT means you have the wrong main jet.

You are a life saver! I took your advice and wen't online and found a sight that gave instructions on how to adjust a fuel screw and adjusted mine. Wow! Major difference. I am now able to do a wheelie in 1st and 2nd by just giving it gas. I think I have a problem though. When I adjusted the fuel screw I turned it in all the way and the bike didn't die. The website I went to said that if the fuel screw is turned in all the way the bike shouldn't run. My bike sputtered, but it never quit. My fuel screw was turned out two turns. I now have it only a quarter turn out. Runs way better, but maybe I have the wrong fuel screw? The fuel screw I am using is for a YZ. It's what the part store gave me. I (my friend actually did everything and I watched since I'm still learning about working on dirt bikes) replaced the screw when I first bought the bike and did the free mods. Any suggestions and thanks again for all of your good advice. I love this website!
I CAN NOW DO WHEELIES IN 2ND GEAR BY JUST USING THE GAS, HELL YA!

  • basinite

Posted July 18, 2009 - 06:57 PM

#14

Just got done trying the bike with the fuel screw half turn open. I think it might run better with quarter turn open, but that might just be in my head. Hard to tell, they both seem about the same.

  • JSanfilippo

Posted July 18, 2009 - 08:33 PM

#15

You are a life saver! I took your advice and wen't online and found a sight that gave instructions on how to adjust a fuel screw and adjusted mine. Wow! Major difference. I am now able to do a wheelie in 1st and 2nd by just giving it gas. I think I have a problem though. When I adjusted the fuel screw I turned it in all the way and the bike didn't die. The website I went to said that if the fuel screw is turned in all the way the bike shouldn't run. My bike sputtered, but it never quit. My fuel screw was turned out two turns. I now have it only a quarter turn out. Runs way better, but maybe I have the wrong fuel screw? The fuel screw I am using is for a YZ. It's what the part store gave me. I (my friend actually did everything and I watched since I'm still learning about working on dirt bikes) replaced the screw when I first bought the bike and did the free mods. Any suggestions and thanks again for all of your good advice. I love this website!
I CAN NOW DO WHEELIES IN 2ND GEAR BY JUST USING THE GAS, HELL YA!


:worthy:

Is the fuel screw brass or aluminum? I guess the brass ones are easier to dial in. The fact that your FS is for a YZ doesn't matter. With the exception of the ACV and different sized jets the YZ and WR have the same carb.

Sounds like you need to go one step smaller on the pilot jet if you're only 1/4 a turn out. I think the normal adjustment range is 1-3 turns with 1.5-2 turns being optimal.

  • basinite

Posted July 19, 2009 - 02:07 PM

#16

It's aluminum. Last night I turned the fuel screw out 3/4 turn and rode the bike today. Still running great at 3/4 turn. Tommaroe I'll be going on a trip to Taylor Park with a friend. Never been there before, but were supposed to ride 120 miles! This will be a good test for it, since on some parts of the trail I was told we would be above timber line (well above 11,000' elevation). As time goes on I will play with it and see how far out I can go on the fuel screw and still get good power without the bog.
Today I was able to get the front wheel in the air while in 3rd gear while only using the gas. Now I just have to work on my technique.
I read somewhere that if you have your bike jetted to lean you will melt pistons. Am I ever at risk by having the fuel screw set to lean? I think mine should be fine, but want to make sure before tommaroe.

  • chevytrkn1

Posted July 19, 2009 - 05:24 PM

#17

the JD kit i installed on my 07 450 works perfectley. I also ride from 4000 to around 12000 here in Montana and never a bog anywhere

  • KJ790

Posted July 20, 2009 - 06:47 PM

#18

It's aluminum. Last night I turned the fuel screw out 3/4 turn and rode the bike today. Still running great at 3/4 turn. Tommaroe I'll be going on a trip to Taylor Park with a friend. Never been there before, but were supposed to ride 120 miles! This will be a good test for it, since on some parts of the trail I was told we would be above timber line (well above 11,000' elevation). As time goes on I will play with it and see how far out I can go on the fuel screw and still get good power without the bog.
Today I was able to get the front wheel in the air while in 3rd gear while only using the gas. Now I just have to work on my technique.
I read somewhere that if you have your bike jetted to lean you will melt pistons. Am I ever at risk by having the fuel screw set to lean? I think mine should be fine, but want to make sure before tommaroe.


If you set the pilot circuit (fuel screw and pilot jet) too lean then the bike will pop excessively when you back off the throttle, and it will tend to idle very poorly. It will start very hard, if at all and won't idle if you go way way to lean on the pilot circuit. The other circuits, such as the main circuit, can cause damage such as melted pistons if they are excessively lean, but you have to go very lean with the main to cause damage like that.




 
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