Pipe vs Intake velocity

[Mac]

I had my bike jetted perfectly with my stock pipe, great throttle response and coco brown plug. I installed an FMF REV pipe on my Y2K KDX 200 over the winter and had it out for the first time this weekend. The bike absolutly rips when I hit the midrange and lofts the front end when holding WOT :) But the off idle throttle response has disappeared. I understand that a REV pipe will steal some Low end for improved top end but I think this is more of a pilot circuit jetting issue. I had the a/f screw @ 1/4 turn out for best performance which indicated that I need a larger pilot. To help me jet the bike I would like to know exactly what caused my pilot circuit to lean out with a pipe change. The only thing I can think of is the intake velocity @ low RPM has dropped off requiring a richer pilot to compensate. Is this correct or is there something else I'm missing??

Thanks,

------------------
Mac
00-KDX-200

 

[Mikeb]

YUP the REV will steal "a little" low end. Some of that lost low end can be recovered by installing the Boyeson PRO series reeds and removing the airbox lid. You are correct on your assumption - More air out needs more air in. What jets are you running now and what elevation are you riding.

------------------
Mikeb
1998 KDX200  
http://www.justkdx.dirtrider.net

 

[Russ]

Just to play the devil's advocate here, I told my dealer that I wanted more low end response and the first thing he asked is if I did anything to the airbox. I said that I removed the snorkle and drilled holes in it and he said THAT was why my initial response was so lean. Makes sense in a way...kind of like not putting oil on your filter. Over oil the filter and the bike runs rich. I put the snorkle back in and put duct tape over the holes (took it back to stock) and I DID notice that my A/S settings had more of an effect. I was able to run 1 full turn out and never could before. Probably will open it up again though as I noticed some top end loss. Only takes a few minutes to go back and forth...try it! So much fun to play with this stuff!

------------------

 

[Mac]

I agree that reinstalling the airbox lid, over oiling the airfilter and installing a larger pilot would all have approx the same effect of richening the circuit. I am pretty sure that increasing the pilot will fix my problem I was more interested in understanding what physics were causing the carb to lean off idle but maintain perfect jetting from 1/4 to WOT by changing only the pipe. Would switching from a stock pipe to a REV pipe increase or decrease the intake velocity??? If so then would this change only occur at certain RPM's?? Maybe this is a question for Mr. Gorr.

My current jetting is:
Main 155
pilot 42
a/f screw 1/2
stock needle 4th clip
This jetting was perfect with the stock pipe but seems way too lean on the pilot with a REV pipe.

My bike has all mod except carb divider and porting.

I think this is why the carb divider mod works so well. I believe the intake velocity drops off with some of the performance mods and the divider gives some of this velocity back improving throttle response. Sorta like running a smaller card = faster airflow through carb = better low end response. Make sense to anyone or should I just go have another beer.

Just trying to understand some of the physics involved with carb jetting.

------------------
Mac
00-KDX-200

 

[Canadian Dave]

Check out the JustKDX home page, click on tech tips then on KIPS Power Valves. Read through the 4 links in the first paragraph.

They should answer your question.

David

------------------
Let's bust big . . . I don't think so. Let's bust bark! Now you're talking!

www.justkdx.dirtrider.net