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YZ400F help!

Joined
May 16, 2007
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#1
I had a problem with the way my yz400f was running so first I will tell you what it was doing. First off it wouldnt idle without the choke on no matter what I did to the idle adjustment but it would run with the choke off if I kept on the throttle pretty heavily but it was popping like a popcorn machine so I killed the engine took it back to my garage pulled the carb and cleaned it out then I reinstalled the carb and when I go to start the bike I can kick it over without using the compression release and that was never possible. So my first thought was that there was something wrong with the compression release valve but it seems to be working fine. Does anyone know what would be causing this? If its something wierd you will have to explain it to me im not a 4-stroke mechanic. Thanks alot.
 
Joined
Jan 3, 2007
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#2
I do not know alot about 4 strokes but it could be you need a new piston-rings if the comp is that low or it could be something with the cam.
 

Patman

Pantless Wonder
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#3
When was the last time the valves were adjusted? Top end serviced?
 
Joined
May 16, 2007
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#4
I have not adjusted the valves since I bought the bike about 1 year ago because it was running great until recently. Its got its compression back just from me kicking it over many times and it is running pretty good with the choke on its not backfiring anymore and its seems like itself again but it still wont run with the choke off. Is the carb still dirty? Or is there a fuel screw I need to adjust? I tried playing with the idle adjustment but it does not effect it with the choke off it still dies.
 

lwsmithjr

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#5
How long had it been sitting before it started acting up?

Needing the choke on and popping usually indicate a lean (too much air, not enough gas) condition. However, the loss of compression for a short period of time suggest you may have valve train issues. Or it could be lean and the compression release is sticking.

I would check first for air leaks. Check the intake boot where it attaches to the carb, and the manifold between the carb and head. Make sure its all tight and no cracks in the rubber. Also check the exhaust where it attaches to the engine. If it is not seated properly, air leaks there can cause popping. You could also have some blocked fuel passages inside the carb, not allowing enough fuel to flow. The hot start plunger could also be stuck open.

Having said all of that, after owning the bike a year, it's time (past time!) for some regular maintenance to the top end. 4 strokes require a little more than "Gas and Go!"
 
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May 16, 2007
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#6
It only sat for a few weeks before it acted up. The reason I havnt adjusted the valves or done any work except changed filters and the oil is because I only get to ride it once or twice a month. The compression problem could have been because I took the carb off and didnt have the air intake clamped on tight couldnt it? Seems to be fine now. I am going to do some more work on it tonight and see if I can get anywhere with it.
 
Joined
Jan 27, 2000
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#7
A bike that won't idle usually indicates a plugged pilot jet or idle jet. Remove your carb and remove the pilot jet, (the little jet that sits next to the main jet) and make sure you can see daylight through the hole. If not, clean it or replace it. Also, remove the fuel screw and blow air through the passageways. Remove the little plate on the bottom of the float bowl and clean the sediment and make sure the accelerator pump diaphragm is in good condition. Make sure the little rubber boot on the accelerator pump rod is in position. These can become dislodged and water and sediment can get into the accelerator pump.

Like others have suggested, check your valve clearance. This is very important because if your valves are too tight, your bike won't run right no matter what you do. Also, will make it hard to start and could cost you a lot of money to repair if you let it go too long.

One more thing. Check the breather hose that comes from the top of the engine. If this hose is partially clogged with dirt, it will make the bike hard or impossible to start and it won't idle correctly. That hose can be re-routed to the airbox and the problem will be eliminated.

Just my $ .02
 
Joined
Apr 6, 2007
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#9
mine did the same thing it was the hot start was missing check the hoses and make sure there tight
 
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#10
95xr250 said:
mine did the same thing it was the hot start was missing check the hoses and make sure there tight

That's another think to check. Make sure your hot start button is closing all of the way. If the rubber boot becomes dislodged, it will prevent the hot start button from closing all of the way. Also, remove the hot start button and clean it. Make sure the rubber seal on the end of the shaft is not damaged.