2009 yz 450f hanging idle

So about a week ago i bought a 2009 yz 450f and today was the first day i actually got to ride it for like an hour. After it is warm and you let off the throttle the rpms stay high and it takes 5 sec for it to idle down or i have to let the clutch out so it almost stalls so the motor can slow down again. Does anybody know why it would be doing this?

Hanging idle is common

First you must correctly adjust the fuel screw to see if it is just not correctly adusted.

 

..but most likey for hanging idle that bad, you probaby have an air leak.

 

Most common places are a corroded hot start plunger, or broken hot start cable cap, worn out or missing slide plate seal, loose air boots, cracked air boots, badly leaking header crush washer or mid joint seal, or even a missing or worn out fuel screw oring ( very common)

 

You adjust the fuel screw with a hot motor, starting at 1 turn out and then lowering the idle as low as it will go

Then you raise the idle with the fuel screw.

If it goes to high, lower it with the idle screw and check to see if you can raise it again with the fuel screw.

Your goal is apporx 1.5 turns out, if you have the correct pilot size.

If your start with a high idle and try to adjust the fuel screw, you will get a hanging idle that way too.

Alright ill try and do the ajusting and if that doesnt work you are saying i should pull the carb and the boots off and check for any of the things you listed above?

I agree with the remedy posted above. But go that route only after you confirm that the throttle tube and throttle cables are clean and lubed.

Hanging idle nearly never has to do with the cables hanging up and the slide not dropping.

You can of course confirm this by tilting up the subframe, marking the slide and intake with a sharpie, then viewing the slide while idle is hanging.

Another place to look is your valve clearances.  Tight intakes will cause this.

So i took off the valve cover to check the valve clearences as i had not done it since i got the bike and the first thing i noticed was that the cam lobes were not facing oppisite directions as you can see in the picture. I believe the timing is set up correctly as the two dots are the in the same place?

1473536672712.jpg1473536676483.jpg1473536679182.jpg1473536683438.jpg

The intake cam has two dots as well is that ehy they are not facing the oppsisite of each other? Why would there be two dots?

1473536909793.jpg

You need to download the service manual from Yamaha's website

Alright well i looked at the manual and i can see that the two marks are normal i jjst couldnt see it on the exhaust cam. So the timing is alright but it didnt show where the cam lobes should be. I thought they were supposed to be facing the exact oppisite from each other so correct me if im wrong. 1473545121964.jpg1473545124249.jpg

You are going down the rabbit hole with this. Pay attention to where the valves are and you will see the open at different times even when the look like they are in the same position. Think about it, if your intake and exhaust valves were opening at the same time the bike would run at all. Just check the clearances and move on to air leaks, jetting, and adjustments.

Edited by jwleonard

I also got the carb apart and there is alotta rust in the hot start plunger whole how would i go about cleaning that?

Use a brass pistol brush on the carb.

 

Looks to me as if you seized the intake cam and made it slip in its sprocket.  Replace the intake cam, the timing chain and tensioner, and have the head repaired if the cam bore is galled.

Hanging idle nearly never has to do with the cables hanging up and the slide not dropping.

You can of course confirm this by tilting up the subframe, marking the slide and intake with a sharpie, then viewing the slide while idle is hanging.

I disagree. Frayed cable, filthy gummed up cables, or dirt jammed in the throttle tube causes hanging idle or even worse with a quickness.

Use a brass pistol brush on the carb.

Looks to me as if you seized the intake cam and made it slip in its sprocket. Replace the intake cam, the timing chain and tensioner, and have the head repaired if the cam bore is galled.

So if the cam were to siez up and make the lobe slip in its sprocket it would still run and cause the hanging idle? It runs perfectly through all gears with all the power it just has the hanging idle when i let off the throttle. I would think that if it slipped on its sprocket that the bike would not run at all?

I disagree. Frayed cable, filthy gummed up cables, or dirt jammed in the throttle tube causes hanging idle or even worse with a quickness.

When i took out the carb and pulled the throttle cable the return cable was frayed a little underneath the carb cover. Would that be a cause?

Replace it. Also do a spark plug reading. If the plug indicates a lean condition, check for vacuum leaks and confirm valve clearances are within specs.

So if the cam were to siez up and make the lobe slip in its sprocket it would still run and cause the hanging idle? It runs perfectly through all gears with all the power it just has the hanging idle when i let off the throttle. I would think that if it slipped on its sprocket that the bike would not run at all?

 

It depends on how far it slips, but the intake lobe in the picture is too far toward horizontal (they normally look like the picture), and retarded intake timing will cause an unstable idle, among other things.  But you weren't wrong; If it slips far enough, or the chain jumps, it can not only stop running, the valves can end up hitting the piston and as much as destroy the engine.

 

Cam seizures aren't always, or maybe even usually total.  A lot of them are just momentary events that drag on the cam just long enough to leave a mark.  If this happens at the wrong time, it can cause the sprocket to slip on the shaft just that much.

 

Have you had the cam out?  Did you check for any indication of seizure/scoring damage in the cam bores?

lobes.jpg

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