loosing my mind---help

21 replies to this topic
  • wrdatk

Posted November 12, 2010 - 04:48 PM


I have a 2003 yz450f, I was out riding on my street today for like 15 min, I'm coming back to my house and the bike starts loosing power and dies. I assume its running out of gas so I cross my fingers and it fires back up, I have to keep feathering the throttle but it runs long enough to get home. I get back and check the gas its good, check the oil once it cooled down some and it was good, then the coolant again it was good. I spent about an hour trying a few things and couldn't find anything. So today I started taking it apart, I tore down the whole carb and cleaned it and installed a full rebuild kit(figured might as well while its apart). Then before I tried to start it again I pulled the valve cover and such off and checked valve clearances and all is within spec. So I get it all back together and same thing, it runs all choppy and i have to hold the throttle open some and feather it to keep it running and it will die under any kind of load.

I have also checked and cleaned the air filter while the carb was off. I also checked fuel supply and it has a steady flow from the tank. I also pulled the spark plug checked it, and checked the gap (all good)

Any ideas guys? I find it very strange that it started doing this all a sudden like this, the bike was not and has never been overheated or run low on oil, oil was changed about 20 hours ago with new filter and I cleaned the screen. Bike also has great compression......I'm lost.

  • wrdatk

Posted November 13, 2010 - 09:02 AM


any ideas guys?

Any help would be great, thanks

  • daniel605

Posted November 13, 2010 - 11:38 AM


I had a 2003 crf 450 i know you are saying it didnt overheat. But it did the same exact thing and i would have swore it blew up. but after cooled off some it was fine. the only thing i could come up with was the thing overheated. I talked to a couple extremely good mechanics about it and they said the same thing that it probably got a lil warm on me. if you were on the hard top i would say the same thing. just try it again and see what happened. i know that it didnt do ne damage to the bike. dont know if that helped you ne.

  • daniel605

Posted November 13, 2010 - 11:56 AM


just talked to one of my mechanic buddys and he said to check your impellar on your water pump. if the bike started up fine and ran for a couple of minutes then started acting up then it sounded like not overheating just getting a lil warm and the mixture would get lean. he said next thing he would check would be the impellar.

  • wrdatk

Posted November 13, 2010 - 02:26 PM


The bike wasn't run hard at all and for only 10-15min so no overheat, I just flushed the cooling system and replaced the impeller last week to get ready for some winter riding haha. Plus today it is doing the same thing as soon as you start it. So IMO it couldnt be that. Thanks for the response though.

  • sacfelix

Posted November 13, 2010 - 02:39 PM


I had a buddy have the same thing happen and it ended up being the rod bearing.

  • brentn

Posted November 13, 2010 - 08:05 PM


If it was overheating you'd be losing coolant and it would be spewing out of the hose.

  • lumpy790

Posted November 14, 2010 - 05:26 AM


pull the hose off your gas cap and re-test. Sounds like it may be vapor locking.

  • wrdatk

Posted November 14, 2010 - 07:11 AM


Ive already made sure it was getting sufficient fuel flow, would a failing cdi have these symptoms? If so how could I check it?


  • grayracer513

Posted November 14, 2010 - 08:00 AM


Ive already made sure it was getting sufficient fuel flow, ...

I know you said that, but next time it acts up, loosen the gas cap. You might be surprised.

You might also want to check that fuel actually flows through the needle seat at an adequate rate.

If the symptoms are more running out of fuel than like the engine is tying up, it could be a CDI. Unfortunately, the only way to check that is to substitute a known good unit.

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  • daniel605

Posted November 14, 2010 - 09:59 AM


actually yes a failing cdi would because the cdi controls alot of that stuff. if you have ne buddys witht the same model bike i would take his cdi and try it maybe might get a suprise there

  • lumpy790

Posted November 15, 2010 - 10:20 AM


A CDI rarely fails. IF ......... its ignition related its more than likley the stator.

  • grayracer513

Posted November 15, 2010 - 10:35 AM


Stators are the most common ignition component to fail, but generally, they will cause a total, hard ignition failure, or at the least, severe misfiring. Here, the OP states the bike "starts loosing power", and made him think it was running out of gas. I may be misreading that, or he may not be articulating the symptoms well, but that sort of behavior is more typical of something that can control the power output in the first place, and so sounds more like a CDI. But the stator could be the culprit. The good part of that is that the stator can actually be tested.

  • bblueox

Posted December 12, 2010 - 06:50 AM


I would have to agree with the others about a vapor lock problem. I have seen this, and it is very hard to trouble-shoot unless your looking for it. The bike will run great/good for limited amount of time, then it will act like it is running out of gas and die. Buy the time you get to check the fuel rate it's flowing fine again. When trouble shooting always start with the simple stuff first. Good luck.

  • Aka.Goose

Posted December 12, 2010 - 10:49 AM


I had a buddy have the same thing happen and it ended up being the rod bearing.

Sorry to say, but this would be my guess also...Sounds to me like the rod bearing is binding...Is there a little more resistance when kicking it over than you're used to? (Like one kick may be fine and smooth, then the next is kinda hard?)

Edit: crap, didn't realize this was from a month ago...any updates?

  • jbrooks26

Posted December 12, 2010 - 11:38 AM


If this is still happening, I had a very similar issue several years back. Turned out that the clip came off of the needle, and the needle was still resting in the needle jet when the throttle was opened. I would also think that maybe the main fuel jet or needle jet may be partially plugged. Just a few other things tha that have not been mentioned before. Let us know how it turns out.

Hope this helps.


  • zukboy789

Posted December 12, 2010 - 11:43 AM


Well my guess would be the rod bearings also. And by the way, 20 hours on and oil change is way too much. My 2 stroke get over 4 oil changes in that amount of time.

  • Yzf_Buz

Posted December 20, 2010 - 01:00 PM


Well ?

  • Lil Red1

Posted June 18, 2011 - 04:50 AM


I know this was from a long time ago but was the outcome of this?

I have the exact same issues. Bike just started losing power and died, I thought it was out of gas but no. I kept kicking and in my fit of rage I didn't notice that it was getting more difficult to kick over until the kickstarter didn't budge. Left alone for a day or two and tried kicking it again, still budged. Somehow when I was about to give up it kicked over.

But it runs pretty rough, only for a second or two and dies. No matter what just dies.

I know two of my exhaust valves are not the right distance, getting the parts to fix here soon.

But what is this rod problem that you guys are talking about? I am not mechanically inclined just yet so please have some patience while I ask stupid questions and get this fixed. :ride:

  • RiderX

Posted June 18, 2011 - 12:50 PM


They're speaking of the rod bearing at the bottom It would make sense that thermal expansion would cause a bad bearing to start seizing, then after it cools and shrinks a bit it fires up like nothing was wrong-until it heats up again. of the connecting rod, sometimes referred to as the "big end bearing." Is your bike hard to kick, like something is seizing immediately after it dies?

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