jetting - pilot circut - idle question

3 replies to this topic
  • jrutkows

Posted June 29, 2010 - 12:57 PM


On my 07 Wr450 i was attempting to adjust the pilot circuit and I was able to turn the fuel screw in all the way with no obvious change. I was expecting that the bike would die with it all the way in. The screw is not easy to get to so perhaps it wasn't totally bottomed out but shouldn't the bike have stalled with it turned all the way in, even if the pilot jet was too big?

My main question is: is something else wrong that I need to fix before I go changing the pilot jet and adjusting that circuit? I haven't had the bike that long and the previous owner didn't seem all that knowledgeable so i could expect anything to be fouled up. I'm kind of confused because when I let go of the throttle I get occasional popping, not every time and not all the time, probably more so when the bike is cold which I would expect to be from the pilot circuit being too lean, but maybe the needle clip position?

Bike seems to run strong, but I have no frame of reference as the only other bike I've had or ridden is my WR250. I'm riding between 5-9000 feet so I would expect to notice some change in how the bike is running with altitude unless something is really far off? Popping doesn't seem to get better when I get up high?

Right now I'm running a 160 main, and the JD blue needle, somewhere around middle clip, set it to the recommendation in the kit.

Any ideas would be a big help, thanks

  • William1

Posted June 29, 2010 - 01:34 PM


The bike should stall with the fuel screw gently bottomed. If the pilot jet is too large, the bike will not stall with the fuel screw in. The fuel screw fine tunes the pilot jetting. Based on your altitude, I'd expect to see a #40 or #38 pilot jet.

Popping on decel is normal but... should be minimal to nonexistent on your bike. Do you have an open exhaust? Open pipes, being less muffles, let more pops out. Is the AIS still in the bike? This injects air into the exhaust, mimicking an exhaust leak, great for total emissions, but makes for an ideal popping environment. Is the ACV (Auto Coast enricher Valve) still operational? This is supposed to add fuel at high rpms with the throttle closed. All of these can contribute. Though I have to wonder at the mentality of AIS and ACV. Seems to me that they work against one another.

Below is the correct procedure for adjusting it and confirming pilot jet size.
Fuel Screw/Pilot Jet
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. Do it with the bike fully heated up.
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.

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  • jrutkows

Posted June 30, 2010 - 06:40 AM


Thanks, that is very helpful. I did remove the AIS as the previous owner gave me the kit. I've never hear of the ACV, so I will look into that. This is on/in the carb I assume?

Sorry for lack of details on the original post, I have all the free mods + FMF megabomb.

I decided to look into the pilot circuit because of the popping, the bike seems to run ok, but then again who knows since I don't have a good frame of reference.

So I'm still a bit confused, sounds like the pilot jet is probably way to big, but why then all the popping in decel? Is is possible for it to be so rich that I get popping? Bike seems the idle fine and off idle engine response seems good. Could it be the needle position? Seems like that wouldn't make much difference till 1/4 throttle? I will try to get the pilot circuit sorted out and go from there.

thanks for all the help.

  • William1

Posted June 30, 2010 - 08:00 AM


Unless you are getting expolsions, popping is a way of life. You can jet away the popping but at the expense of rideability and performance.

Check the ACV - Read about it in your manual.


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