CRF's Only Forums

Just had my 08 450R ported,and new top end, and got it out Sat. for a quick break-in.
It runs good and strong, but was getting a little back-fire on decel. So I put a richer (178 from 175) main jet in and it helped a little. Question is: Does a port/polish generally require richer jetting? The guy that ported it told me he though the jetting should remain the same. I'm going to check the plug tonight for a true indication, just thought someone with port experience could shed a little knowledge. AS?? or any one else??

My thoughts are that if you increase the flow without adding a little more fuel, that it will be drawing more air, in turn leaning the mixture.

You are correct if the porting is done well and you haven't sacrificed velocity for volume.

The decel pop is a pilot jet issue as the main is closed on decel.

What pilot are you running?? 45?

Yep 45. Needle clip 2nd down. Temp was a bit cool at 45, so I richened up the fuel screw a bit. it didn't pop very much after the main jet change, only on the stand while I blipped the throttle, and maybe once on the track, minor, but it's been so perfect in the past.
Thanks for verifying that I should need the adjustment.

I went 1/8 - 1/4 turn in on the fuel screw and that seemed to fix my backfiring issues.