Pilot jet setting

23 replies to this topic
  • ea844c

Posted September 11, 2008 - 08:36 AM


My Son's 03 yz450 engine was completly rebuilt and after breakin and took it to the track I noticed that it was poping from decelling. When you ajust the pilot srew, do you turn the knob all the way in and then turn the big red knob to 2 1/2 or turn it starting at #1 and turn it 2 1/2 turns out?

  • William1

Posted September 11, 2008 - 09:00 AM


Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method.
Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.
*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***
Turn the screw in until the idle becomes rough or the bike stalls.
if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.
If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.
Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.
If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.
If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.
If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

Be sure to keep notes of your settings. You may find you need to make a change, say if you're riding in the mountains or down at the beach. Returning home, it is a simple matter of referring to your notes to restore things.

However.......... I suspect the problem is not the pilot/fuel screw rather it is an exhaust leak.

  • grayracer513

Posted September 11, 2008 - 10:28 AM


Check the exhaust for air leaks, check that the hot start has free play, and completely closes, then read this:


  • sherco67

Posted September 11, 2008 - 01:35 PM


had popping in 06 after rebuild, as per this forum which is really great went with 45 pilot jet, 168 main jet, 3rd clip down, 1.5-2 turns out, but with full dr. d exhaust i went with 45 pilot, 170 main, and 4th clip and 2 turns out and seems to run alot stronger, good luck

  • ea844c

Posted September 12, 2008 - 06:35 AM


The only place that I can find an exhaust leak would be where the Mufler meet the exhaust pipe at the front of the Muffler. There are holes where Revits should be. I have not checked the Hot Start yet, but I will. I know that after the breakin my Son said that the Hot Start was stiff but losen up. Thanks for all your help Guys.

  • chad30

Posted November 12, 2008 - 09:31 AM


I have the same issue and i have a slight leak where the header pipe meets the tailpipe... I have a slip on exhaust and the manufacturer said its no big deal... just curious if this is something I should address or is it ok if the popping doesnt bother me?

  • grayracer513

Posted November 12, 2008 - 10:57 AM


It's harmless unless the popping is so forceful that it threatens to blow your muffler off or start a fire. But if it's loose enough to leak significantly, it will likely loosen itself further due to vibration caused wear.

If you feel ambitious, smear a bit of high temp silicon on the header about a half inch back from the end and let the assembly sit 12 hours before you start it.

Otherwise, don't let it worry you.

  • chad30

Posted November 14, 2008 - 01:59 PM


thanks for that info! ya maybe one day i'll put some sealant in there, its
not bad at all and the leak is pretty minor, not enough to wear worse..
thanks again!

  • chad30

Posted November 18, 2008 - 02:33 PM


sorry to hijack thread :thumbsup: I got another question, i pulled out my fuel screw to
dbl check and clean things, I didnt see an o ring, i pulled the float bowl to make sure it didnt fall in there etc. Any particular place I should look that it may have fallen? I can't find it anywhere, hopefully I just dropped it and lost it... I ordered one of those nifty fuel screws with the spring oring and washer from Zip Ty but just want to make sure that o ring isnt hiding somewhere in my carb ready to cause me issues.......

  • grayracer513

Posted November 18, 2008 - 03:39 PM


> It's lost

> It's stuck up in the bore in the carb body where the screw lives.

Did you find the spring and washer?

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  • ea844c

Posted November 19, 2008 - 07:06 AM


Well here we go again, I got the poping taken care of buy reajusting the air fuel mixture screw. But here is the deal, as I was adjusting the fuel mixture screw, fuel started to leak from the shaft of the screw. The FCR carb I think is a 39 when we bought the bike has a big red knob with numbers on it for eaiser adjusting. I ordered another air fuel mixture screw from Haynes racing and took out the Red mixture screw and I noticed that ther was not an O ring. I replaced it with the new air fuel screw with the new one with an O ring and the motor would not start and would Back Fire. I replaced the new screw with the Old mixture screw and it back fired once and then started. What is the problem? Does the old scre supposed to have an O ring or not?
Thanks for everybodys help!

  • William1

Posted November 19, 2008 - 08:26 AM


The old oring was probably stuck in the carb.

  • grayracer513

Posted November 19, 2008 - 08:57 AM


If it was leaking fuel from around the pilot screw, I suggest that the O-ring was either altogether, having been previously lost, or was torn.

There needs to be, starting at the bottom with the screw, the pilot screw, a spring, a washer, and an O-ring. Oddly, these items are not listed separately in the YZ parts list, but are in the WR list:

O-RING 3TJ-14562-40-00
WASHER 3TJ-14952-40-00
SPRING 3TJ-14160-40-00

  • ea844c

Posted November 19, 2008 - 09:08 PM


The first fuel mixture screw was a aftermarket one with a long shaft and a big red knob with 1-4 numbers on the knob. Their was not a O-ring or a washer or spring that came out when I unscrewed the knob to replace it with a new one. the new fuel mixture screw did not have a spring or washer with it but it did have an O-ring. Do you think that the O-ring is still in the carb?

  • grayracer513

Posted November 19, 2008 - 10:16 PM


You'll have to look and see.

  • chad30

Posted November 19, 2008 - 11:20 PM


is it possible for the oring to fall into the bowl etc.? I can't find it but I'm hoping I just dropped it cause I bought a new set anyway and if possible, I'd like to avoid taking everything apart again... but of course will if I need to..

I do have the spring and washer just no o-ring... I didn't keep an eye out for it the first time I removed it because I didnt know there was one so it is very possible I just lost it... Will check out further this Saturday....

  • William1

Posted November 20, 2008 - 03:58 AM


No, no chance of it being inside the bowl. It does get stuck agains the fuel screw seat. Removing the bowl does make it easier to be absolutely certain if this is the case.

  • grayracer513

Posted November 20, 2008 - 07:36 AM


You have to understand that the fuel screw passes through the float bowl and threads into the carb body. The hole in the bowl that it passes through is basically like a tunnel, and the top end of that tunnel is sealed against the carb body by the float bowl gasket. As William says, the only way it can fall is out, and they will stick up inside the carb quite often.

  • ea844c

Posted November 20, 2008 - 10:11 AM


Guys thanks so much for your help! I'm trying to understand everything. So when I unscrewed the Fuel Mixture Screw, with regards to the old aftermarket fuel screw, it is suppose to have a O-ring and if so the O-ring could be in the body of the tube. Also when the O-ring comes out will the spring and washer come out too? I noticed on the old aftermarket fuel mixture screw, their was not a grove for a O-ring but with the Haynes racing one it has a grove and O-ring but no spring and washer. Should there be all three, O-ring, spring and washer with the fuel mixture screw?
Thanks Guys

  • William1

Posted November 20, 2008 - 02:36 PM


No, not all replacement screws come with the little parts.

The groove on the shaft is for a second oring, used to provide additional friction to keep the screw from vibrating out of position. Sometimes, you need to drill the float bowl out with a 1/4" drill to make use of this.

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