Jetting after EG Mods

hoosierf

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#1
Eric ported our KX125 concentrating on more bottom end. I asked for a stump puller. The motor really is great, but the jetting has gone sideways on me and I need some guidance from the experts. I'm running MR2 and I've got the main up to a 180, the bike is still running poor, and my plug has no indication of a fuel ring at all after cutting the threads away. This is not a big bore, Is that size main possible? It acts like it could take a 190!

The bike pulls like mad until a sustained run a WOT. At WOT after about 1 second the bike gives a big bawaaaah and drops RPM's dramatically and sort of surges. One other thing I should mention is that the bike has a one-off exhaust manifold that Eric machined. Could changing the shape and size of the manifold dramatically effect the airflow, requiring the large main jet? I would have guessed the bike wanted a smaller main with the smaller manifold opening.

Any other clues on where I should look? The bike runs well up through the RPM range, so I don't think I have an air leak. Could it be a reed problem? They looked good when the motor was assembled three weeks ago.

I'm concerned that I could be going the wrong direction on the main, but the damn thing ran really well at 95 degrees and 90% humidity when we first assembled it and there is not a hint of spooge anywhere.

I'm thinking of draining the MR2 and running some C12 to see if that improves the condition. I know it's almost impossible to diagnos from afar, but any suggestions would give me something to try.
 

marcusgunby

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#2
You can go upto a 200 main jet-KTMs used to come with that kind of size main-should you need to? i cannot answer.
 

hoosierf

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#3
Thanks Marcus,

I know Eric is out of pocket, but perhaps is Rich will see my post and weigh in. The potential of the mods Eric performed are apparent. I've just got to finish the project and I'll have a ripper.
 
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#4
Changing the fuel is the easiest, safest and I think most logical approach. You'll richen the fuel curve across the board and know for sure if you are going in the right direction.
 

bclapham

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#6
dont forget also, that if the porting was stock before the head/squish/piston clearances may have also been stock too- if eric changed the head and tightened the squish then this might mean a bigger main.
 

Faded

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#7
Originally posted by hoosierf
The bike pulls like mad until a sustained run a WOT. At WOT after about 1 second the bike gives a big bawaaaah and drops RPM's dramatically and sort of surges.
Did you check your float level?
 

hoosierf

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#8
The tuner (me) is an idiot. The switch to C12 had it barely running. I knew it ran bad on a 170 main, so I dropped a 160 in and it rips on top, no mid and no bottom. Leaned the needle one clip position and doubled the power spread. Now I need to pick up a few pilot jets and we're set. I think I'll need to go one step leaner on the needle, but I thought it smarter to work the pilot out since the needle is in the ballpark and then go back to the needle for final tuning and finally tweak the main a step or two and voila, race motor! I'm real happy with the work Eric did on it and I learned a thing or two along the way.