YZ450 setting idle/pilot screw, popping on decel

16 replies to this topic
  • motorcross242

Posted September 06, 2011 - 02:54 PM


To start off, this bike is 100% stock other than PMB S/A endcap. I ran my bike recently after installing a hour/tach meter and I noticed that my idle speed was low compared to what the manual calls out for. I figured this was due to old gas, probably 3-4 months old. I drained the fuel and replaced with fresh gas. Still low idle speed, as low as about 1400 rpms and then dies. After the bike is fully warmed up the bike stumbles as it idles. So I figured I could increase idle speed but it seems I've created a problem. I adjusted the screw in this picture #1. Posted Image
I think this raised my idle speed to around 1900 rpms. I turned this screw about 1/4 turn clockwise. The manual says to adjust this knob #3 in this picture to increase idle, although I do not have it. Posted Image
I thought everything was fine but I noticed that popping on decel was more than normal and listening to a video shows its popping more than I ever recall.

This isnt normal right?

I've read that popping on decel is somewhat normal and some richen the fuel screw to cut it out. Is this the same screw that changes idle speed?

Edited by motorcross242, September 06, 2011 - 03:15 PM.

  • grayracer513

Posted September 06, 2011 - 04:55 PM


If the bike sat for 3-4 months while it was aging the gasoline, it probably has a film of varnish dried into the pilot jet. A #45 pilot has an orifice of 0.45mm. A film of varnish only .002" thick will reduce that by .004", and make it a #35. A bit small, if you ask me.



  • motorcross242

Posted September 07, 2011 - 07:26 AM


Well I've read that whole thread many times and I have no idea how to clear the passageways with carb spray. I don't even know how to pull the jets out. I've also read the thumperfaq on jetting and I'm still unsure how to work on the bike's carb. I rode the bike again last night and I noticed that between idle and 1/4 throttle its chugging.

All of the carb tuning videos on youtube shows just four screws to remove the bottom of the carb to get access to the jets. Is this all that is required on the YZ's? I think im going to just buy a new pilot and main jet and try that first.

Edited by motorcross242, September 07, 2011 - 09:13 AM.

  • grayracer513

Posted September 07, 2011 - 09:17 AM


So, what? You want a magical solution to the problem? Dried varnish will not yield to carb spray in the first place, and the film will probably have to be removed by the more aggressive means I listed. The only way to accomplish that is to remove, disassemble, and clean the carb.

If you are a competent mechanic, you should be capable of this, particularly if you download a manual from the links in Common Threads. Otherwise, you'll need someone to do it for you. There's a slim possibilty that the problem can be cured with a small dose of a commercial automotive injector cleaning additive. STP makes a good one.

  • motorcross242

Posted September 07, 2011 - 09:48 AM


I can see how you took my previous post as a request for some magical solution. :) I was hoping someone has gone through this problem and posted a "how to" with pictures. Or even a step by step procedure? I have my manual and I have the downloaded manual as well. I'm just nervous. I am planning on replacing all of these stock jets: main jet, pilot jet, needle jet, starter jet, pilot air jet and leak jet. This bike has sat for more than 3-4 months sometimes even close to a year at a time and I think it would be beneficial to just get all new jets. Do you have any helpful information for the task ahead of me gray?

  • mikewrf18

Posted September 07, 2011 - 09:58 AM


Buy a new Pilot and install it, to eliminate that possibility first. Only $4-5. If you look in the carb section in your manual, you will be able to identify where it is located. I find it easier to just remove the float bowl so you can get a better look at it, but many just remove the bottom float bowl drain bolt and access the main jet and pilot jet that way. Just loosen the intake and air boot hose clamps and rotate the bottom of the carb towards you. Might consider installing an adjustable fuel screw while your at it. There is a thread in the stickies I believe showing you how to install an after market fuel screw. If I see it, I'll post it up. And yes, carb work itimidated me at first as well, but found it to be rather easy, provided you take your time, and document everything you removed, and how it goes back in.

Edited by mikewrf18, September 07, 2011 - 12:20 PM.

  • motorcross242

Posted September 07, 2011 - 11:23 AM


I will be buying a new pilot jet and main jet and jet needle.

Is this correct ?
Pilot Jet - Stock is 45
Main Jet - Stock is 160

  • grayracer513

Posted September 07, 2011 - 12:05 PM


Unless you think you need to change the sizes, I see no reason to replace any of the jets, just clean them. There's certainly no need to fiddle with the starter, leak, or pilot air jets. The bike's not old enough to have any significant wear on the needle or the needle jet orifice, either.

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  • motorcross242

Posted September 12, 2011 - 10:13 AM


Something still isn't right. I replaced the main jet, needle jet and pilot jet. My bike feels extremely hard to start ever since I touched the idle screw. I messed around with the fuel screw this weekend and can't seem to dial it in correctly. The bike also doesn't feel as crisp off 1/8 twist of throttle. If I snap the throttle fast it sort of bogs. Riding it feels ok, no noticeable delay in throttle. I've got probably a little more than normal popping on decel. Starting the bike hot sometimes I don't need the hot start lever.

  • mikewrf18

Posted September 12, 2011 - 10:46 AM


Compliments of Thumperfaq.com for setting the fuel screw. I would set your fuel screw to 2 and a half turns out, then get your idle screw set properly, then start over with tuning the fuel screw again following these steps:
(You may want to put a fan directing air to your radiator when doing this to keep your bike from overheating)

When the pilot circuit is jetted properly, starting the bike should not be a problem. You will need the choke to start cold and it won't run well off of choke for 30 seconds or so. When hot you will probably need the hot start. The bike should idles smoothly and have no hesitation of idle and minimal backfiring on deceleration. For more info about the pilot screw, see this article that recently appeared in Motocross Action Magazine.

Setting the Fuel Screw / Pilot Jet "by Ear"

Adjust the idle with the black knob until it is too fast. Then adjust it back down until it is around 1900-2000 RPM or if you don't have a tachometer (see below) until it sounds just a little high.

Before you start adjusting, count the turns required to tighten it up lightly.

Then start the bike with the slightly elevated idle and turn it out 1/4 turn, 1/2 turn, 3/4 turn and so on until you get to 2 turns. Listen for best RPM and best response to a quick 1/4 turn tweak of the throttle at each position of the fuel screw.

Now turn back in 1/4 turn at a time doing the same thing. By now you should have been able to distinguish the speed of the idle and the responsiveness to tweaking the throttle.

If it gets better between 3/4 and 2 turns out, set it at the best location and leave the rest of the pilot circuit alone.

If it is getting better turning it in or is best less than 3/4 turns out, replace the pilot jet with a smaller one and go through this procedure again.

If it is getting better as you turn it out or best at more than 2 turns out, replace the pilot jet with a larger one and go through this procedure again.

Setting Fuel Screw / Pilot Jet with Tachometer

Warm the bike up by riding about 10 minutes. Place it on a stand, have it idling. If you have a fan, direct it into the radiators (A YZF will start to boil out if you take too long to do this, WRFs have a nice catch tank).

Turn the fuel screw 1 1/2 turns out. Read the RPM for about 10 seconds (on my tachometer, cause it bounces around). If the avg RPM is not between 1700 and 1900, adjust to about 1800 with idle screw knob on carburetor Write down average RPM.

Turn the fuel screw 1/2 turn out. Write down the average RPM.

Turn the screw 3 turns out. Write down the average RPM.

If #2 is greater than #3 or #4, you have the right pilot jet. Usually the difference will only be 50 to 100 RPM. Go on to step #7.

If #3 is greatest, you need less fuel. Install the next smallest number pilot jet. Go to step #3 and repeat.

If #4 is greatest, you need more fuel. Install the next largest number pilot jet. Go to step #3 and repeat.

Adjust the fuel screw in 1/4 turn increments around 1 1/2 turns out and find the maximum RPM fuel screw position. If the idle is now above 1900 RPM, turn it down to be in spec.

If you get a little deceleration backfiring on closed throttle, try adding another 1/4 turn out. Remember you will need to redo this if the altitude or temperature changes significantly.

  • yzf1999

Posted September 12, 2011 - 04:05 PM


Do yourself a favor...take a 17mm wrench and remove the drain bolt and drain the gas at the bottom of the float bowl after EVERY ride..that way you can let it set as long as you like until the next ride and not have to worry about your jets gumming up...:)

  • motorcross242

Posted October 10, 2011 - 01:48 PM


What is the best way to start the bike after draining all the gas out. I turn on the gas, pull the choke, try priming the motor with a few twists of the throttle find compression stroke, steady kicks and it still doesn't start. Sometimes I have to twist the throttle a little while kicking. Could this possibly be related to the starter jet? I've replaced the pilot jet, main and needle jet with stock sizes and the bike idles right at 18-1900 rpm's when warm, starts easily when warm..

All stock jets with zip ty fuel screw 2-1/4 turns out.

I understand this is a race bike and its not meant to "cruise" but there seems to be a bit of surging when I give about 1/8th throttle at steady rate. Is this normal?

  • MrN2OBelvedere

Posted October 10, 2011 - 09:57 PM


Something still isn't right. I replaced the main jet, needle jet and pilot jet. My bike feels extremely hard to start ever since I touched the idle screw. I messed around with the fuel screw this weekend and can't seem to dial it in correctly. The bike also doesn't feel as crisp off 1/8 twist of throttle. If I snap the throttle fast it sort of bogs. Riding it feels ok, no noticeable delay in throttle. I've got probably a little more than normal popping on decel. Starting the bike hot sometimes I don't need the hot start lever.

If the bike hesitates when you snap the throttle, maybe the accelerator pump isn't working properly. This would usually be worse when under a load though. Try doing the same thing while going slow in a high gear and see what happens.

In my opinion, if you aren't well-versed in carburetor tuning, then you should either take it to a shop or find some books on the subject and start learning. If you set up a carb wrong, you can actually damage the engine.

By the way, improper carb tuning isn't the only cause of exhaust popping. An exhaust leak can also cause this.

  • motorcross242

Posted October 11, 2011 - 06:44 AM


The bike does not hesitate anymore when I snap the throttle. I believe I worked this out with clean jets and adjusting the fuel mixture screw. Under load in a high gear this thing pulls extremely hard. No bog. After riding a friends 09 yz450, this bike is in great shape.

My question now is on cold starts after draining all the gas, could the starter jet be giving me the problem of having to twist the throttle while trying to start? Leaving it in choke and giving a few prime shots does not seem to do anything...? If I don't drain the gas, cold starting is not too hard, usually less than a few kicks and it starts right up?

  • grayracer513

Posted October 11, 2011 - 07:06 AM


You specify "after draining the gas". Does it cold start reasonably well if the gas has not been drained?

  • motorcross242

Posted October 11, 2011 - 07:49 AM


If I do not drain the carb after the bike has run, it'll start the next day cold no problems.

After draining the carb completely, what is the method of starting? I can't start with the choke alone or priming the engine. I have to twist the throttle a little while kicking and it'll usually fire right up.

  • grayracer513

Posted October 11, 2011 - 11:43 AM


It sounds as if fuel flow into the bowl is restricted in some way. The filters on the petcock may be clogged or the float may not be dropping far enough. If it were a starter jet problem, it wouldn't matter whether the bowl was drained or not.

Usually, when it's necessary to crack the throttle to start it, it's a sign that it's slightly rich. If that's the case, it could indicate that the float is sticking in the down position until the level in the bowl is already higher than the shut off point. Or just not always seating as it should.

Odd situation.

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