08 jetting


7 replies to this topic
  • lt1air

Posted May 23, 2008 - 02:50 PM

#1

I just installed an MRD complete system on my 08 450f , when riding at a steady throttle, maybe 1\4 the engine surges and pops big time on decel.
I changed the main jet to a 165 from a 160, still does the same symtoms
any suggestions??
thanks

  • grayracer513

Posted May 23, 2008 - 03:36 PM

#2

The '08 typically runs well with a 160 main, but as I recall, most have had better luck bumping the pilot jet up one size.

Anytime you change or work on the exhaust and then have a decel backfire, the first thing you should check for is air leaks in the system. Use a new copper gasket at the head, and be certain that the pipe fits tight at the midpipe joint. If an air leak is the underlying cause, and you chase the problem with jetting, you'll end up with a bike that is way too fat.

The pilot jet is the component of the carb that would affect decel popping. However, it's important to understand that a properly jetted YZ450 will pop a little on decel. It's only a symptom of a problem when it's excessive.

Check the exhaust, then do the following to get the pilot right.

http://www.thumperta...ad.php?t=327405


The 165 may be OK, but you might want to go back to the 160 after the pilot is corrected. If you get the pilot dialed as in the link, and it still stutters at a steady speed, try raising the needle one and see what effect that has.

Here's a couple of posts from the Jetting sticky:

http://www.thumperta...050#post5380050

http://www.thumperta...512#post6293512

http://www.thumperta...501#post6305501

  • Big Black

Posted May 23, 2008 - 10:06 PM

#3

I had the same issues, i just changed the pilot to 48 and that fixed it.

  • lt1air

Posted May 30, 2008 - 02:57 PM

#4

So , I contacted the manufacturer of the pipe, he informed me to change the pilot jet to #48 , move the clip down two grooves and keep the 165 main in it,

the popping on decel is gone, however the surging is still there, I found if I leaned the fuel screw it improved the surging, I realized the fuel screw is almost all the way in , there's about 1\8 of a turn left.

The surging happens when you at abut 1\8 throttle in any gear just cruising.

I don't want to pop the engine due to a lean mixture, any suggestions are very welcome.

Thanks guys

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  • Family Man

Posted May 30, 2008 - 03:49 PM

#5

Try setting the fuel screw about 2 to 2 1/2 out.

  • MX926

Posted May 30, 2008 - 06:54 PM

#6

I also had/have the same issue withmy 08. I'm running
168 main
45 pilot
needle 3rd position
fuel screw 2-1/2

Right now I have just a minor pop on decel.
I probably also need to bump the pilot up to 48. I've just been waiting on the weather to warm up before I change it. I'm sure I will have to drop the main back down also once it gets warmer and stays warm.

  • MRD RACING

Posted June 02, 2008 - 06:47 PM

#7

The '08 typically runs well with a 160 main, but as I recall, most have had better luck bumping the pilot jet up one size.

Anytime you change or work on the exhaust and then have a decel backfire, the first thing you should check for is air leaks in the system. Use a new copper gasket at the head, and be certain that the pipe fits tight at the midpipe joint. If an air leak is the underlying cause, and you chase the problem with jetting, you'll end up with a bike that is way too fat.

The pilot jet is the component of the carb that would affect decel popping. However, it's important to understand that a properly jetted YZ450 will pop a little on decel. It's only a symptom of a problem when it's excessive.

Check the exhaust, then do the following to get the pilot right.

http://www.thumperta...ad.php?t=327405


The 165 may be OK, but you might want to go back to the 160 after the pilot is corrected. If you get the pilot dialed as in the link, and it still stutters at a steady speed, try raising the needle one and see what effect that has.

Here's a couple of posts from the Jetting sticky:

http://www.thumperta...050#post5380050

http://www.thumperta...512#post6293512

http://www.thumperta...501#post6305501


thanks for the help :thumbsup:

  • MRD RACING

Posted June 02, 2008 - 07:02 PM

#8

So , I contacted the manufacturer of the pipe, he informed me to change the pilot jet to #48 , move the clip down two grooves and keep the 165 main in it,

the popping on decel is gone, however the surging is still there, I found if I leaned the fuel screw it improved the surging, I realized the fuel screw is almost all the way in , there's about 1\8 of a turn left.

The surging happens when you at abut 1\8 throttle in any gear just cruising.

I don't want to pop the engine due to a lean mixture, any suggestions are very welcome.

Thanks guys


it sounds like the neddle is too rich if you lean the fuel screw and the surging goes away ,so maybe if you lower the neddle it will help , the ones I have jetted ran good with the stock neddle , but your atmospheric condishions could require a leaner neddle, also you cant hurt the engine by running the fuel screw closed





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