Hanging idle ???

When I out riding and come to a stop. the idle hangs for about 10 secs and then drops. what is causing this? also I get hesitation and stumbling at constant low(1/4) throttle. The bike pulls strong when rolling on but backfires a little when rolling off. I'm thinking it is running rich because I haven't adjusted the jetting since the winter.

Thanks is advance



Post your main and pilot jet numbers and needle position. Then check your hot start and choke boots for cracks.

Then check to see if your accel pump stream is operating correctly.

And of course, make sure the throttle is really closing down like it should.

If all is well, you may have some timing issues or a faulty carb. Well chat about that next.

Anyone else?



The Idle

Does it do the Hang thingy even if at idle and you just rev the motor ?

If yes Possible air leak See DaveJ reply

Poping on Decel is normal but can be cut down by uping the pilot from the stock 42 to a 45. It willnot iliminate it but will reduce it

426's are netoriuos for BOG at bottom, search for BKMOD in the threads this should help or just go to http://motoman393.forreal.net

look at Tech Articles and BKMod

[ June 02, 2002: Message edited by: EgoAhole ]

The hanging idle is from a lean pilot circuit. I had the same problem when I bought my bike. I did everthing from cleaning the carb. about 10 times to test the TPS sensor. After about a week of working on my bike I found the fix.

I live at ocean level, but I did have the stock pilot jet in my 99 Yz400 set at 1-3/8out when i was having the problem. I now have it at approx. 2-1/4. It funs fine and i'm not fouling any spark plugs. :)

The idle hangs after reving the motor, but it seems to olny do it whaen I'm riding not whaen it on the stand.

I have already change the pilot jet to 45 and the main jet ( don't remember what maybe 160??) I also changed the needle but don't rememebr what it is either. I get you guys really numbers if I can't fix it I will check for leaks, play around with the mixture screw which I think is 2 1/4 out already.


Mike, if you are already 2 1/4 out on the screw maybe you need to go one step up on the pilot jet? :)

I ran into this problem after servicing my carb. I had an air leak where the air box connects the carb adaptor. I noticed that the adaptor was secured by two Allen bolts that put pressure on the adaptor's mounting bushings. As it turned out)check yours), the bolts only pulled the bushing to the cylinder head. This is fine and dandy if the bushings don't lose their integrity. Mine lost the integrity and the adaptor slipped away from the head and allowed air to enter. Can you say "intermittent air leak"? This air leak is very difficult to pin point. I found mine by using an oxygen bottle set at a slow emmittance and waved around the intake tract until found. I simply set the engine at high idle and then waved the oxygen nozzle around the intake and listened for an idle increase. Of course, I did this in the drive way with a fire extinguiser handy. Use only a miniscule oxygen emittance to do this test. Don't set yourself on fire.

[ June 03, 2002: Message edited by: Boit ]

Having too large of a slow air jet will cause it also.

I had the hanging idle thing too, but I dropped the needle and it went away. Ive got the 45 pilot, and Im 2 turns out on the screw. Needle in the middle, and a 168 main.

Mike go check out the post for Boit, Hick, or Taffy. There is a link to a document for Tuning the FCR. Its bitchen. Click it, print it, jet it!

The part about that doc is, it doesnt tell you what to do, it tells you how to figure out what to do and why what your doing works. This is invaluable information.

The old saying "give a man a fish....

I did a little work on my bike tonight before it got dark and here is what I came out with. I was running 168 main and 45 pilot, postion 4 on the needle. I did not check the accelerator pump. I played with the pilot screw adjustment, but It didn't seem to make a difference, from 2 1/2 out to 1 in the bike still stumbled at constant throttle. It would backfire more on decelleration at 1 (to lean?) . I changed the clipon down one and up one. Down it was worse, but up one from stock wasn't better than stock. The bike pulls very well at 1/3 to full throttle and now idles well, it just constant throttle that causes it to jerk and hesitate. I was thinking of changing to the stock pilot jet but turning the pilot screw in made it to lean wouldn't the smaller pilot jet do the same thing? I will also make sure the carbs are clean. I also added the dakar dual sport kit the other day. It initial worked but after a short ride it died, but the bike would still run great but with the hesitation even though the lights did not work could this cause the hesitation?

I appreciate everyones help, I'm trying to learn as much as I can about jetting so pleae bear with me


I tried for 2 years or so to git rid of the same problem and the only thing that fixed it was a smaller slow air jet.It was not as bad as yours,more of a off idle stumble but it did also have the hanging idle.

I tried different idle jets, needles, played with the idle mixture screw and nothing worked.

That slow air jet is just as important as any of the fuel jets and it often goes overlooked.

I will agree that an air leak will cause the same problem but if that has been ruled out I would go after the slow air jet next.

EJR has a great point, that slow air jet is key to the overall. Go get the document from that link. It addresses everything in the whole carb, and why. It awesome. Its making me rethink my whole setup.

Thanks for all the suggestions and links. I have been reading the manual and the links. where is the slow air screw? I don't see it in the manual

Here the lastest on my bike. The hanging idle is gone, probably a leak. The bike idles great and pulls very strong thru out the range but still stumbles at constant throttle at 1/4 to 1/3 opened. I tried a 48 pilot and alot of pilot screw setting. I tried 42 with stock settings and different needle and pilot screw settings. No matter what I did it still stumbled. I will try at 45 next. I also need to check the acclerator pump. One other thing I thought of was I have a WR magento that doesn't seem to working correctly. I am only getting 5 ohms with the power off to the Dakar dual sport kit and 2 with it on. But if this was causing a problem wouldn't it get worst when the revs got higher.

Thanks for the help

the saga continues

[ June 04, 2002: Message edited by: Mike Walsh ]

If you take off the intake adapter that the air box connects to you will see two jets, the one on the right is the main air jet, the other is the slow air jet. In some cases there might be a screw instead of a jet which is better because you can adjust the circuit simply by turning the screw. Another advantage is you can turn it with the engine running and listen for changes. I have had good results turning the screw out until I get the highest idle speed. I have also went a quarter turn more even though it had no effect on the idle and it caused a hesitation.

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