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2003 YZ450F "ColdBlooded Issue"
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Posted by: ashzeigler---------------------
Hey guys I just bought a 2003 YZ450F. This bike is in absolute pristine condition and has very low hours. Everything is in tip top shape and it rides like a complete dream! It has unlimited power and is perfect in every aspect except the whole "Coldblooded" Part. Now this bike does have a Full FMF Exhaust System and an Aftermarket Air Filter. The bike is fairly easy to start however it usually dies when it is first started. After that I try and kick it again and it just will not cooperate. It takes countless kicks and it even does this when it is warm. If I take a break from riding on the track and sit for a minute or two, then it still is a jerk and will not start right up. Now I have done some research and I guess adding an aftermarket exhaust and air filter can cause these kinds of problems. I guess my question is, how do I fix it? Oh and I had a 2003 YZ450F BEFORE and it was in horrible condition compared to this one that I just got and it would fire right up. Granted it was stock, so maybe it really is the exhaust and air filter. Anyway thank you all for your time and if any of you have a link or some advice on how to get this bike working right it would be greatly appreciated. Thanks a ton! Oh and one more thing, it also over idles.. I tried to mess with that turn nob, but it did not want to work for me. If I even touch the throttle it usually over idles for 15 ish seconds before it calms down and dies or barely stays alive. Thanks!
-Ashton
Posted by: whenfoxforks-ruled---------------------
Where is the mixture screw adjusted to, how many turns out? Do you know what size leak/power jet is in it? Have you checked the ap squirt/timing? Vintage Bob
Posted by: ashzeigler---------------------
I am pretty sure everything is all stock. I think it has a size 42 (Stock) Jet in it and I will have to check on the mixture screw part. I am pretty sure these are all stock setting though. Any ideas? And I will get back with you on the mixture screw. Thanks!
Posted by: _JOE_---------------------
Sounds like a pilot circuit jetting issue. If it has a pipe and stock jetting you're pissin in the wind. You need to get comfortable with tuning the carb as it will change with conditions/engine wear. You'll most likely be able to get it to start decent with feul screw adjustments but if you end up more than two and a half turns out or less than one you should change the pilot jet. You might as well grab a couple main jets while you're at it and experiment with them too because you might have alot more potential for top end pull. The needle can really help how smooth the midrange is.
Most bikes(4 strokes) come stock with really lean jetting. You can check FMF's website for a starting point with the pipe.
Posted by: whenfoxforks-ruled---------------------
There are some really small washers and o rings atop the pilot adjuster/mixture screw. Missing or cracked o rings will make it real hard to start and bog. Stock, its a small slotted adjuster out the bottom through the bowl. The after market screws make it easier to adjust. The leak/power jet looks real close in proximity to the main and pilot jet, on your carb, and a #72 is stock. When you hit the throttle, the AP circuit squirts near raw fuel in, its crucial the squirt/timing just misses the slide, or it will hesitate/bog. The leak/power jet provides the fuel for that circuit. I would check the AP diaphragm also, cracked or deteriorated. Here I thought the 05~06 Honda fcr carb was expensive 5~600 dollars, what the heck is your carb made of? Unobtanium priced at near 1500 dollars! Be careful! Basically the same carb, why can't they call the parts the same thing, because its priced more? I remember the 05 and 06 really benefited from tying the AP adjuster together, when you hit the throttle wfo the squirt did not always shoot. By the adjuster it floats sometimes. I have seen some after market ones that are solid, might have been Boyseen. I did double check Eric's book on Performance, I sure thought that year had a specific issue with bogging. All he said was get a bigger header pipe, or a flywheel weight, the power hits to hard, and the crank is weak. Vintage Bob
Posted by: ashzeigler---------------------
Well guys me and my friend messed with my bike. We moved the Main Jet down one, messed with the pilot screw in every aspect and it wouldn't run right. Then we ran to the main jet up one (1 above normal) and then messed with the pilot screw in every aspect and it ran like crap. Lastly we returned the main jet to its stock settings and messed with it in every aspect and it STILL backfires like there is no tomorrow. I do not know what to do so hopefully one of you guys will have an idea. Im going to go up to my local shop tomorrow and hopefully they can give me some pretty detailed ideas. Anyway, thanks for the help guys, I guess my bike is just being dumb. Stupid Full System FMF Titanium4 Exhaust... Lol.
Posted by: ashzeigler---------------------
Oh and guys we got the right point on the pilot screw so that the bike will start on the first kick every time. However on Deceleration this bike will Back Fire LIKE CRAZY!!!! Any idea how to fix the backfiring?!?!?! It is pretty bad...
Posted by: _JOE_---------------------
How's the valve clearances? Exhaust leaks can cause the popping on decel as well as lean pilot circuit. Have you tried a larger pilot jet?
Posted by: ashzeigler---------------------
No I have not tried a larger pilot jet. Do you think if I up'd the size that would solve the problem? And I do not think there is a leak on the exhaust. Is there an easy way to check for an exhaust leak? Thanks Joe, I appreciate the help. And I think the clearance on the valves is good.
Posted by: _JOE_---------------------
Thinking will always bite you in the ass. You should at least check them periodically, say every 15 or 20 hours, or when you get a bike that's new to you. Here's a jetting link that should help you understand how it all works......http://www.dirtrider.net/forums3/sh...315#post1340315
Posted by: ashzeigler---------------------
Well Joe I decided that I am going to go up to a 48 Pilot and a 170 Main. They should be in by Friday so I will work on it this weekend. I am hoping that this will fix everything! After I put in the 48/170 should I put all the jets back to their stock settings or should I tinker with it? Thanks! 
-Ashton
Posted by: ashzeigler---------------------
Hey everyone. I have an 03 YZ450F. I am having some major backfiring out of my bike so after a lot of research I have decided to up to a 48 Pilot and a 170 Main. They will be in on thursday. Today I bought a "Sunline Aluminum Calibrated Fuel Mixture Screw." Does anyone know what this is like compared to a 48 pilot? I think I might of just wasted $20 by buying this Sunline thing because I really want to take care of this crazy backfiring and everyone told me to go 48/170 to do it. What do you guys think? Like I said the 48/170 will be here on thursday, so should I try and take back the Sunline? Also my bike has a full FMF exhaust system, so that is why I have to rejet and fix this backfiring. Any help would be greatly appreciated. Thanks!
Here is the link to that Sunline thing: http://www.sixsixone.com/Catalog_Su...ba-282df3a4ea1f
Posted by: ellandoh---------------------
a fuel mixture screw and a pilot jet are two completely different things
Posted by: Matt90GT---------------------
I think you are wasting your money buy not buying a manual first!
Posted by: ellandoh---------------------
i think you are wasting your money not buying a subscription to DRN first 
Posted by: Rich Rohrich---------------------
If the exhaust is sealed properly it will always pop a little on decel when these bikes are jetted CORRECTLY. Guys who set them up so it never pops are running them way too rich.
Fooling around with the main jet won't fix a closed throttle decel popping condition.
As Matt pointed out, getting the service manual and understanding which circuits in the carb correspond to various throttle settings would be the smartest place to start.
Aftermarket pipes are notorious for sucking in air at the mating surfaces which will make for a lot of popping on decel. It would be worth your time to pull pipe apart and properly seal each joint with copper based high temp silicone.
Posted by: ashzeigler---------------------
Well guys I checked my exhaust to make sure that there were no leaks. Then I installed the 48 pilot / 170 main, and now it hardly ever backfires and it seems like it got a massive performance boost. I am scared to open this bike up now!!! On top of that I messed with the pilot screw and now she starts on the first kick every time and idles very very strong! Thanks for all the help, I really appreciate it! I am a dirt bike noob so thanks for being understanding. Im off to ride, have a great memorial day weekend guys! :D
Posted by: oldguy---------------------
Messsing with the main jet will do nothing for starting as it doesn't kick in until the revs are up. The backfiring on decel more then likely means you are to lean on the pilot. If you have to turn the fuel screw more than 3/4 turn above or below 1 1/2 turns out you need a new pilot jet sized that direction. We had a similar problem with an 07 250F where the pilot was way to rich and the bike would only fire if cold and then had to be bump started once hot. The power was perfect as were the vlaves
Posted by: whenfoxforks-ruled---------------------
Any thumpers running the adjustable leak jet? Vintage Bob
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