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Jetting Help With Mx Porting


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I recently ordered some ported cylinders with pistons off ebay. My main jet was 290, stock pilot with toomey t5s and removed air box lid before the port. What is a good place to start after the port? I know it is port specific but I would like to know what jet range to order.

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I recently ordered some ported cylinders with pistons off ebay.  My main jet was 290, stock pilot with toomey t5s and removed air box lid before the port.  What is a good place to start after the port?  I know it is port specific but I would like to know what jet range to order.

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man, your gonna have to be careful jetting those in. Porwork can be from one extreme to the other in regards to jetting.

 

You could be from 340 to 380, so I'd say start at 360 and hope you get some good readings. You may be on the high side, but I dunno, this is where plug checking really comes in handy.... :)

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man, your gonna have to be careful jetting those in.  Porwork can be from one extreme to the other in regards to jetting. 

 

You could be from 340 to 380, so I'd say start at 360 and hope you get some good readings.  You may be on the high side, but I dunno, this is where plug checking really comes in handy.... :)

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I emailed the guy I bought the cylinders from and he was running 270 mains with the porting, dmc pipes, and holes in the air box lid at an elevation of 550 feet. To me it seems as though he was running lean. But can porting actually make your jetting go down?

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I emailed the guy I bought the cylinders from and he was running 270 mains with the porting, dmc pipes, and holes in the air box lid at an elevation of 550 feet.  To me it seems as though he was running lean.  But can porting actually make your jetting go down?

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I wouldn't imagine that porting makes jetting go down, as the ports are being opened to allow more air and therefore more fuel into the cylinders.

 

I guess loco's correct when he says ... "man, your gonna have to be careful jetting those in. Porwork can be from one extreme to the other in regards to jetting."

 

I have felt the same as you with regards to thinking my set up is lean. ... MX race port, skimmed head, Doma Racing exhausts, airbox lid removed, and K&Ns with outerwares. Elevation 600ft. The setup with the std. carbs on this bike is Pilot-25, Mains-270, and the jet needle on the bottom clip (richest). This is the way I bought the 'Shee and I believe it was set up on a dyno after the engine work.

 

With the above setup, I've no problems in any range of the throttle from idle to WOT and have had no overheating issues. I'm just about to put on a DynaFS, so I'm going to get stuck into plug chopping to get the most out of the whole package. I've ordered 300s and 320s as I believe this is where I should at least be, but we shall see.

 

Rare.

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