Full FMF exhaust system.. need to re jet?

2 replies to this topic
  • newfie37

Posted May 27, 2014 - 12:12 PM


Just ordered the SS Megabomb and the Powercore Ti for my 03 yz450f.


Heard the titanium dents a little easier so I went with the stainless, I don't race so didn't think the titanium was worth it for the megabomb. Any truth to this?




Im wondering now that I have a megabomb and the powercore if the exhaust is going to be pulling so much air out that im going to need a bigger pilot jet or just turn up my idle screw?


Any suggestions or any good jetting kits I could buy?

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  • grayracer513

Posted May 27, 2014 - 12:38 PM


I never recommend anyone's jetting kits.  I think you're better off using OEM Keihin parts, whether through Yamaha or through a dealer such as Sudco.


Have you taken the time to prowl through the jetting DB? http://www.thumperta...tting-database/


Also read: http://www.thumperta...b/#entry2289879


It is a misconception on two counts that the FMF pipe will automatically require different jetting.  The first assumes that the pipe is so much more efficient than the stock pipe that it will need it.  The fact is that stock YZF pipes (not including the "shorty" on the '08-'09 models) are really pretty good pipes top begin with, and the FMF will probably add no more than 1 full HP.


The second error in thinking is that the pipe "is going to be pulling so much air out" that you'll need to go richer.  Carbs work based on air velocity and the vacuum produced by that air flow on fuel passages with fixed orifice jets in them.  If the air flow through the carb increases, more fuel will be delivered, simple as that.   If the increase in air flow is great enough, it will increase the strength of the vacuum signal at the main jet so much that it will result in the mixture becoming too RICH, not too lean. 


Put the pipe on the bike, see how it runs, and go from there.  Don't get ahead of yourself.

  • newfie37

Posted May 27, 2014 - 08:52 PM


That's a few things to think about. I just ordered the pipes in dont hVe them yet, just wanted some other input incase I would need a bigger pilot jet but the fact that I think it would
Be pulling it out faster would lead to more
Fuel going in. I'm going to place them on when I get
Them maybe turn up the idle screw just a tad and play with it see when the bike sounds better.

Apologies for poor grammar and English laptop is currently down and not the best typed on my phone.

Thanks for the information.

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