bad idle dies when u blip the throttle

10 replies to this topic
  • rpm

Posted September 06, 2015 - 02:40 PM


OK I need help, I been riding and fixing sense 1972 but I got a problem with my 03 YZ450F

Here's the story it was running great and we took it to the desert about 4000-5000 above sea level, this also is the same altitude I live at, my buddy was riding my YZ450F when he passed me an I noticed blue smoke coming off the engine on both sides..( Presumed oil leak) I stopped him and yes a small pin hole had developed in the steel oil line letting oil run on most the back side of the engine. I sent another friend to camp for a quart of oil we filled it and rode back to camp carefully,, It still ran great but was done for the trip.

I installed a new line an changed oil and filter than took a ride with my daughter up in the mountains, The idle was rough but other than that it ran great, but soon developed a problem when getting into technical stuff with throttle closed and clutching it up some trails as soon as I opened the throttle it would die. 0 to 1/4 throttle than it would rough idle for any where from 3 seconds to 1 minute than just die. Also it was very hard starting and would only start with no throttle which is normal but sometimes I would have to use the hotstart than it would fire and rough idle but as soon as I touched the throttle it would just die, one time I got it running and ran it down the fire road at high RPM an it was missing like crap, I came back up it died, started it again and ran great at high rpm's  but still rough idle and would die if I barely blipped the gas. I thought maybe the altitude but when I got home it ran the same. 

I took the carb apart and completely cleaned everything and adjusted the accelerator pump but still no different. I'm thinking something electrical but it came on so quick.... I personally don't think the oil leak caused it as it ran fine back to camp just smoked a lot from the leak.

I have no mechanics I trust and in fact I have received many dirtbikes and ATV's from neighbors to fix an repair because the dealers couldn't find the problems.

But this is stumping me almost seems like a carb/electrical problem but it came on like instantly......

Any help or ideas would be appreciated


  • defilmcrew

Posted September 07, 2015 - 02:42 PM


You can check almost all the electronics with a volt meter to see if something is funny there. Or maybe check the valves and cams for lack of oil issues? Plug while you are there

  • rpm

Posted September 08, 2015 - 05:47 AM


Thanks, I checked accelerator pump yesterday and it gave a good spray of gas when throttle was turned so I'm ruling that out. I'll get out the meter and start checking the electronics and take a look at valves  and cams... I hope no damage but can't be sure.


  • defilmcrew

Posted September 08, 2015 - 08:03 PM


Check electrical first. When the carb was off did you mess with the ap timing? They can spray too much (long spray) or too late which will kill the bike on small blips of throttle. And when you cleaned the carb on the pilot jet hole (in the carb above the jet and into the throat) is an angled hole, make sure that is clean with a needle or piece of thin wire because you cant see through the angle usually to visually check. Also the plate gasket on the slide make sure its new or in really good shape and make sure its not upside down which lots of people do. Also did you take the tp sensor off during the cleaning? I would ohm the stator windings after the carb especially if its missing randomly. hot is usually when they show signs of failing

  • rpm

Posted September 09, 2015 - 06:18 AM



Thanks I didn't take the TP sensor off during cleaning but checked it yesterday and doesn't show the proper resistance as shown in manual,  (ordered new 0ne) also I'll check the the plate gasket it got put on correctly but didn't get a new one....

I did adjust the ap timing so I'll look at that again cause it puts a long steady stream about 3-4 seconds worth,,, the angled jet hole is clean.  I also checked the spark plug cap which in this case seems to be a coil ( not like a 2 stroke bike) as well an it did not meet the called for specs out of the manual,  (ordered a new one also) 

I don't know but the ap spray seemed to long in my opinion seems like that itself would flood it out. 

I'll check the stator today hopefully if I can get the time.... 


 for the ideas, I'm OLD school and this is only my 2nd 4 stroke among  

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  • defilmcrew

Posted September 10, 2015 - 07:44 PM


The stream should barely miss the slide. Video it and watch in slow motion is what i used to do. 3 to 4 is long but shouldnt really kill it on a slow blip. Get the electronics squared away then play with ap timing usually you have to speed the up

  • defilmcrew

Posted September 10, 2015 - 07:47 PM


No problem for the advice hope it helps. Had an 02 426 that acted up until i got it pinned down. Drove me nuts

  • snowdave

Posted September 24, 2015 - 07:23 AM


Did you ever get this issue resolved?  I am suddenly having a similar problem on my 2003.  I have torn the carb apart multiple times, but still can't seem to find the real problem.

  • rpm

Posted September 24, 2015 - 10:27 AM


No not yet I to cleaned the carb more than once than adjusted the accelerator pump lever, and put on a new Throttle position sensor and a spark plug cap/COIL when I noticed that there's a hole just above and to the right (if your sitting on the bike) of the exhaust pipe coming from the head, and the hole goes into the head. It seem as it was used for a manual decompression system. When they went to the exhaust cam decompression set up they just plugged it rather than casting new heads. Well the plug was missing and it's not shown on the parts list, I found out about it on Ebay and ordered a plug which should arrive today I will install it an see what happens. 03 yz450.jpg

Attached Thumbnails

  • 20150917_193904_resized_1.jpg03yz450f.jpg

  • grayracer513

Posted September 24, 2015 - 12:13 PM


The plug is listed in the parts fiche on the camshaft/chain page, PN 90338-18064-00


It's a piece of shit, though.  Use an 18mm brass "freeze plug" (cup plug, core plug, etc.) from an auto parts store instead.



  • ThumpMe

Posted September 25, 2015 - 08:15 PM


I have an ' is a WR but basically the same motor and carb.


 Mine did a similar thing a few years back...I remember it cause it drove me nuts for a while.


 I doubt it is electrical but you never can tell. The TPS can be reset using an ohm meter. By loosening up the screw that locks it in place and then turning it to get it back in the range called for in the manual.


 My problem was actually carburetion and two fold (well I think!). First I found the tiny little hole that goes down into a fuel well in about the middle of the slide was blocked off. This is one of the areas that gas in the carb first evaporates away in and it clogs up that tiny hole. This was most noticeable in that it would die when the  throttle was blipped. I had to take a real small wire, de bur it good,  and force a little plug of varnish out of that hole. The wire has to be real small and then forced though the hole which has a slight bend to it down into a little fuel well to clean it out. 


 The REAL culprit turned out to be that while installing the air adjustment screw in the bottom of the carb I had somehow pinched or tore a little bitty piece of the o-ring that goes around the air screw off. It would run fine at times and then that little piece of rubber would shift into a different position and clog things up. When it did it would die constantly and be real hard to start too. Even after I pulled it apart probably 5-6 more times that little chunk of rubber stayed up in that screw hole and it was not till I noticed the little "chunk" missing off the o-ring and looked up in where the screw threads in with an eye loop that I saw it. It was real pain to clear it out too. Ended up flushing it out using WD 40.


 After that it was back in good condition, but what a nightmare to find the culprit.


 Because yours seems to have started all of a sudden I am not sure if it is a similar problem, but pretty easy to check once you know what to look for.


 Good luck, keep us posted.

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