New thumper owner have a question

I bought an 08 yz450 bike was clean no grease or leaks on it, previous owner claims 30 hrs on it, and been sitting for 9-12 months, when i got there he had it running, road it up and down the street had monster power and gears seemed to be bottomless. I cleaned out the carb and replaced fuel line it just crumpled was dry. Road it after carb clean ran great, took it to milestone mx and 4-5 laps in just started popin and banging really bad and when i let off gas it turns off. It has stock header and fmf factory 4.1 on it i tried looking on fmfs jetting section and it seems to have every yz but the 450 WTF?!?!!! Anywho wanna start with jetting and and work my way into the more complex like possible timing issues or valves, whats the best jet settings for what ive got, i also read here on this site that the cdi for this bike isint the best to use an 06 or even a 03 cdi box, is this true any info or feed back would be a great help or point me with any other routes or issues any of you have had and worked out for you

Based on your information presented, chances are good the jetting is correct. If the bike is that old with such little use there's a really good chance you'll need to clean the carb again, and probably again. And then maybe one more time. The last low time, older bike I bought needed the carb cleaned at least 5 times before it could make it through 30 minutes of riding. The reason being is all the varnish and junk in the tank and carb was flaking off and plugging the jets and other orifices of the carburetor. 


So I'd say pull the carb off, again, and clean every hole and jet in it and be prepared to do it again. Take the tools and clean it at the track too so you don't lose a day of riding.

Edited by WouldsAssassin

I think if you dump the stock crappy 08,09 muffler you are supposed to go up a couple sizes on the main (from 160 to 165 I think) and one spot lower with the clip. That being said I bet your pilot jet is plugged or the fuel screw circuit is plugged. I would put in the right jets while I was in there.

I ride at mile stone once in a while. I have an 09 with factory ti mega bomb and 4.1 muffler as well. I run stock jetting, no problems.

Like others have said check carb and fuel screw adjustment again. On the fuel screw I'm usually around 1.5-2 out.

The 06 and 03 cdi give the bike an extra kick in the pants in the bottom end department but do not really make them run better. I'm 230 lbs and the 09 has a perfect balance of power for me. It has really made me faster in turns due to the fact I feel I can control the breaking free point of the rear wheel better. I tend to steer with the rear end standing at speed and with the front setting down on anything over 90 degrees. If you steer with the rear in tight corners I would recommend the cdi swap.

Another thing to check is the exhaust. Make sure it is tight. I put a power bomb on mine and the screws that go into the motor loosen up on me allot. It makes the entire exhaust shift and it starts to backfire like crazy. If it is loose. Take it apart and use high temp copper rtv at the joints. Make sure you check the bolts often.

Yamaha carberated 450s are know to have the most finicky jetting on 450s- the only real good fix is to buy a boyseen quick shot 2 with adjustable leak jet. Run a 45 pilot and a 165 main. Ive had this set up on two different yz 450s and it fixed the issue perfect. ImageUploadedByThumper Talk1394419904.339763.jpg

Also keep in mind the valve clearance issues can also cause popping and shity running. I recommend to check your valve clearance first then go to the jetting.

Welp i thank u all for your input, there was a number of things i had to do i removed the petcock pressure sprayed the tank out let it dry out a day or so when i removed the bowl this last time the gas looked like whiskey brown and smelled something awful. There was nada one clamp on the exhaust other then the one to the top end. Call me cheap for now but it works i went to auto parts store and got two 2" exhaust clamps for $6 instead of paying $25 a piece a local bike shop lol A small jet in the bowl was clogged and almost fell apart, replaced it, moved the needle down one and put a 165 in and the biggest issue of them all was the air jet on the bottom was no longer there. Guess it blew out while at the track It would idle tits but give it gas and she died, so I'm hoping I've solved all my issues for now, just gotta get accustom to the whole engine breaking that the 2 smokes lacked lol

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