new top end = running rich?

dmstarr · 03-14-2007

I don't want to beat a dead horse, but here is something kind of intriguing me:
My 97 KX250 has always smoked a bit. Since I've replaced the top end, it smokes more and now leaves a little black residue on the rear fender (shorty silencer). The silencer was repacked, and the plug still looks good. Could worn rings have been concealing a rich condition? I have no problem playng with the jetting. I'm just afraid I'm getting blow-by or something now. Compression checks good.

griffbones · 03-14-2007

Most times a fresh top end will run a little bit richer, because the better sealing piston is able to more efficiently draw the air fuel mixture into the cylinder.

KDXGarage · 03-15-2007

Is your new top end fully broken in? Mine was smoky until it was fully broken in, then didn't really smoke at all.

dmstarr · 03-19-2007

yeah, I've got about 7 hours in it now. Griff, that's kinda what I thought - better vacuum from the new rings.
thanks guys.

dmstarr · 03-28-2007

I am trying to tune my jetting per Spanky's jetting guide. However, when adjusting the pilot circuit, turning the air screw seems to have no effect on the idle speed, until I close it all the way (fully CW). Then the bike dies. Pilot jet too rich?

griffbones · 03-28-2007

Sometimes on two strokes it is hard to get much speed difference out of the air screw, unlike the fuel screw on four strokes. I would start out at stock pilot jetting and air screw settings and see how she acts. If it bogs from low end - turn the air screw in a little or go up one size on the pilot; if it girgles (rich) then open the air screw up or lean the pilot. Generaly speaking if you have to open the air screw up very much over two or two and one half turns out you are too rich on the pilot; if you have to turn the air screw in much more than one turn out to achieve best performance then you are probably too lean on the pilot jet. The pilot jet is best sized by sound and feel of the bike, it is next to imposible to jet the pilot circuit by plug readings or idle speed. IMHO

dmstarr · 03-28-2007

thanks. I'll pay careful attention to the initial throttle response!

03-14-2007	#1 (permalink)
dmstarr Tree Magnet BTK Expert Join Date: Dec 2005 Posts: 390	new top end = running rich? I don't want to beat a dead horse, but here is something kind of intriguing me: My 97 KX250 has always smoked a bit. Since I've replaced the top end, it smokes more and now leaves a little black residue on the rear fender (shorty silencer). The silencer was repacked, and the plug still looks good. Could worn rings have been concealing a rich condition? I have no problem playng with the jetting. I'm just afraid I'm getting blow-by or something now. Compression checks good.

03-14-2007	#2 (permalink)
griffbones Another old dirt bike guy BTK Expert Join Date: Sep 2006 Posts: 565	Most times a fresh top end will run a little bit richer, because the better sealing piston is able to more efficiently draw the air fuel mixture into the cylinder. __________________ '05 KX85 w/103cc engine '97 DR407 '83 CR60 '05 TTR230 '07 250XCW

03-15-2007	#3 (permalink)
KDXGarage Hey, lemme ride your bike BTK Expert Join Date: Dec 2005 Location: AL, USA Posts: 1,849	Is your new top end fully broken in? Mine was smoky until it was fully broken in, then didn't really smoke at all. __________________ DO NOT e-mail me asking for a pdf of a manual!! Get a Freaking JOB like I did, and buy one!

03-28-2007	#6 (permalink)
griffbones Another old dirt bike guy BTK Expert Join Date: Sep 2006 Posts: 565	Sometimes on two strokes it is hard to get much speed difference out of the air screw, unlike the fuel screw on four strokes. I would start out at stock pilot jetting and air screw settings and see how she acts. If it bogs from low end - turn the air screw in a little or go up one size on the pilot; if it girgles (rich) then open the air screw up or lean the pilot. Generaly speaking if you have to open the air screw up very much over two or two and one half turns out you are too rich on the pilot; if you have to turn the air screw in much more than one turn out to achieve best performance then you are probably too lean on the pilot jet. The pilot jet is best sized by sound and feel of the bike, it is next to imposible to jet the pilot circuit by plug readings or idle speed. IMHO __________________ '05 KX85 w/103cc engine '97 DR407 '83 CR60 '05 TTR230 '07 250XCW

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03-19-2007	#4 (permalink)
dmstarr Tree Magnet BTK Expert Join Date: Dec 2005 Posts: 390	yeah, I've got about 7 hours in it now. Griff, that's kinda what I thought - better vacuum from the new rings. thanks guys.

03-28-2007	#5 (permalink)
dmstarr Tree Magnet BTK Expert Join Date: Dec 2005 Posts: 390	I am trying to tune my jetting per Spanky's jetting guide. However, when adjusting the pilot circuit, turning the air screw seems to have no effect on the idle speed, until I close it all the way (fully CW). Then the bike dies. Pilot jet too rich?

03-28-2007	#7 (permalink)
dmstarr Tree Magnet BTK Expert Join Date: Dec 2005 Posts: 390	thanks. I'll pay careful attention to the initial throttle response!

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