Does porting require new jetting?

[SpectraSVT]

Just received my mo better cylinder from Eric Gorr and I was wondering if I need to mess with my jetting again? There wasn't any jetting recommendations with the cylinder so I guess I just want to know what to expect. Any experiences?

 

[NO HAND]

I've had to rejet to get most out of it. It was a 00cr125 cylinder. I had the mid-top porting and needed to go 2-3 sizes richer on the main and leaner on the pilot. 125s are more picky on jetting... especially hondas'. Perhaps your YZ250 will need to be very close to stock, but I'm sure you will have to make some adjustments.

 

[gonzo843]

I just had my 125 ported. I had to go leaner on all of the jetting by about 1 size.

 

[Rich Rohrich]

Having to go leaner on the jetting is the most common result.

 

[SpectraSVT]

thx guys

 

[Studboy]

Spectra, fill us in when you finally get to ride it!!!

 

[mobrown]

Curious...

If you get a cylinder ported doesn't that mean more air? If you get more air wouldn't you need more fuel (i.e. larger jets)?

Just thought I'd ask for my own edification (sp?). Have never changed jets and am curious.

Thanx

 

[Rich Rohrich]

Proper porting (as well as some other mods) will to a large degree,improve scavenging . When you improve scavenging of the cylinder during the exhaust event there is less residual exhaust left behind in the cylinder and it's easier for the fuel in the chamber to find an oxygen molecule to partner up with. As a result you tend to need to take in less fuel for best power.

Every engine is different , but this is pretty common with japanese two-strokes.

 

[SpectraSVT]

I should be getting my new top end kit tonite. I wasn't expecting to get my cylinder back so fast! I have done a complete teardown of the bike and in the process of regreasing and clean up. I'm hoping to have it back together so I can test it out this weekend. I will definitely post a ride report.

I figured better airflow would cause too much fuel to be sucked in and lean was the way to go. But it was good to have it confirmed. I hate jetting stuff so hopefully it will go smooth.

 

[ML536]

Interesting.

What about with ported four strokes? How does head work typically affect the jetting on them?