total newb question re exhaust install

3 replies to this topic
  • zedthumper

Posted April 07, 2008 - 09:28 AM


Have mercy on me...
I just installed an fmf factory 4.1 exhaust system (powerbomb header) on my 04 yz450.
How can I tell if the engine is running lean/hot vs normal? The paperwork that came with the exhaust said that the system was designed to run with the stock jetting.

I just installed it yesterday, and took it out for a couple of very short test rides through all the gears at fairly good pace. There is no hesitation, and the bike starts on the 1st or 2nd kick every time.

Am I good to go, or do I need to test this some other way?

Also please take this into consideration, I am strictly a recreational "older" rider that doesnt need max performance, only wanted to freshen up my ride. I just dont want to be doing any damage to the bike that I cant detect until its too late.

Thanks in advance.

  • William1

Posted April 07, 2008 - 10:18 AM


Key change due to a less restrictive pipe are to the main opny, 99% of the time. I'd run a main two sizes larger, and take a flat out test ride. If all is good, then you have a happy bike. Is your airbox opened up?

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  • grayracer513

Posted April 07, 2008 - 11:06 AM


There is no need to go to a larger jet when installing the FMF 4.1 on an '04 YZ450. If the bike is running well, you should leave it. Going two sizes larger will show up as rich, and you will most likely notice this as a sooty exhaust, and a tendency to stutter slightly as the throttle is rolled off from wide open at speed.

The stock '04 YZ450 exhaust is a very good system, not at all restrictive, and the FMF will only add about .75 to 1.0 hp over the stock system. What it will do is add power to both ends of the power curve, making the power more linear and useful.

  • zedthumper

Posted April 07, 2008 - 11:17 AM


Thanks for the quick replies to both of you.

Gray, I will go with your recommendation to stick with the stock jetting, as long as the bike is running well.


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