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Damn stumped on this yz450

By jackiefresh69,
in YZ400F/426F/450F/450FX

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jackiefresh69

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Hello guys . I have a newly purchased yz450f 2008 model and I bought it knowing that it wasn't quite running right. It does seem to start but it will not stay running very long. I can keep it running if I keep it revving but choke on or off it refuses to stay running. Things I've tried are:

Turned idle up but it will still stall out after a while.

Adjusted feul screw

Swapped pilot jet with a new one

Cleaned carby various times (thoroughly)

Checked slide wasn't it wrong way

Adjusted and checked valves are in spec

It will run sometimes well for say a minute or so then just die like someone hit the kill switch.

What direction should I head now? Thanks guys

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Grand Master

FORMA MOTO

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You have either fuel starvation or spark loss I would say

 

First things first:

 

- no idle means a bad pilot circuit. You may have changed the pilot jet, but you need to ream the passage above it with fishing line and spray cleaner. There is an elbow up there that clogs

- no idle can also mean a contaminated stator pick up, though usually this shows up at higher rpms.

- no idle can also mean an upside down slide plate, which should have the hole on the bottom

 

You say if acts like a kill switch after a minute or so. Will it re-start immediately, or do you have to 'wait' a bit?

 

Does it run 'cleanly' at other rpms, no backfiring ?

 

Do you have a gas tank vent valve on the hose? If so remove it.

 

Have you changed the plug?  Cleaned and checked the electrical connections, especially the coil ground, plug cap, and main ground ?

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Grand Master

grayracer513

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Check the stator physically to be sure that it's bolted down to the cover securely, and that none of the coils appear loose on the coil frame.  Then check the resistances per the manual.  Assuming that you have pilot circuit passages cleared (verify by blowing carb cleaner up into it and watching for it to exit into the crab throat just to the engine side of the slide), that would be the next place to look.

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jackiefresh69

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You have either fuel starvation or spark loss I would say

First things first:

- no idle means a bad pilot circuit. You may have changed the pilot jet, but you need to ream the passage above it with fishing line and spray cleaner. There is an elbow up there that clogs

- no idle can also mean a contaminated stator pick up, though usually this shows up at higher rpms.

- no idle can also mean an upside down slide plate, which should have the hole on the bottom

You say if acts like a kill switch after a minute or so. Will it re-start immediately, or do you have to 'wait' a bit?

Does it run 'cleanly' at other rpms, no backfiring ?

Do you have a gas tank vent valve on the hose? If so remove it.

Have you changed the plug? Cleaned and checked the electrical connections, especially the coil ground, plug cap, and main ground ?

Thanks for the help I will check these things today. With the restartin it's pretty unpredictable . Sometimes it will start right up but then it won't run longer than 5 seconds this time. Yes runs great if you keep it revving. A little backfire but nothing real crazy. I'll get back tonight with what else I've tried

Check the stator physically to be sure that it's bolted down to the cover securely, and that none of the coils appear loose on the coil frame. Then check the resistances per the manual. Assuming that you have pilot circuit passages cleared (verify by blowing carb cleaner up into it and watching for it to exit into the crab throat just to the engine side of the slide), that would be the next place to look.

Okay thanks I will check today

When the bike is stone cold and you start it will it run longer than after a few starts?

Yes it does seem to be that way

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jackiefresh69

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Okay so I've taken carb apart again . Slide is in right way. Sprayed through all the passageways and watched the carb clean go all the way through . Hot start is free and All the cables have free play and stator looks good and is secure . Havnt tested it yet though.

Maybe I have a valve leaking? Thanks for the help guys . What else should I try?

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Grand Master

FORMA MOTO

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Okay so I've taken carb apart again . Slide is in right way. Sprayed through all the passageways and watched the carb clean go all the way through . Hot start is free and All the cables have free play and stator looks good and is secure . Havnt tested it yet though.

Maybe I have a valve leaking? Thanks for the help guys . What else should I try?

 

Is the hot start plunger dropping all the way and sealing in it 's chamber ?

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jackiefresh69

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It really dosnt seem to have a pattern in it starting. Just started then and ran with choke but choke off it stalls and backfires a little . Might have to go up on pilot . Fuel screw at 2.5 out and a 42 pilot. Might have to go to 45? What do you guys think[/quote

Now it runs without choke. can't get any sort of pattern. Either way it will run but soon die out? Has spark but seems kind of weak

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jackiefresh69

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Your info is starting to be contradictory

You should use a 45 pilot

If it runs rough past idle, it's not the pilot jet

If it cannot reach higher rpms, it sounds like the stator or a mis-timed cam or cam timing jump

It basically won't idle at all . But if I keep revving it it's goes fine . I just can't let it idle or else it will die out
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jackiefresh69

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Have you checked to see if the valves are tight?

Yep I just adjusted the valves so there all in spec .

Do a leakdown test. Check timing. Install an automotive spark tester. It is basically a plugwire with a light built in. You'll be able to see the strength of the spark and if it is eratic or going away.

Okay cool thanks I'll look into both
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