Low RPM, 1/4 throttle bog

notenuftoys · Post by **notenuftoys** » 06:06 pm Nov 23 2008

After getting the RB carb and head mod, my KDX doesn't climb near as well because it will bog down at low RPM and less than 1/2 throttle. I've searched through here enough to try some fixes, but I'd like to understand better what's happening in the engine.

Based on the other suggestions I've seen, it looks like the engine is getting too much gas, not enough air when it bogs. Is that correct? Or is it the other way around?

I'm gonna figure out this jetting mystery come hell or high water!

Post by **fuzzy** » 02:48 pm Nov 24 2008

Is it worse when you roll the throttle on, or when you just wick it open?

Need to adjust your air screw. Most of what his mod does is make the air screw (pilot circuit) better, and more tunable. That being said you have to tune it. Out is lean, in is rich. Start richer for safety and go from there.

canyncarvr · Post by **canyncarvr** » 04:45 pm Nov 24 2008

Re: 'Need to adjust your air screw.'

Yep.

Start at 2 1/2 out. Run a bracketing sequence to find out where you have the best throttle reponse. You need make changes only in the 1/16 of a turn, or about one width of the red slot in your brandy new finger adjustable air screw.

You DID get one'a those, right?

The general 'rule of thumb' (that's what makes it general, 'eh?) that if you're more'n two turns out means you need a smaller pilot jet, less than 1/2 a turn out and you need a larger pilot jet...that does not apply to an RB-modified carb. The 2 1/2 out (or so) area has come to be called the SSS..the Second Sweet Spot.

It's there..and you won't believe what you've been missing when you find it: 3rd gear becomes your 'new' 1st!

Nope. Not even kidding.

The finger adjustable screw is very helpful. A temp change of 20º will definitely have you wanting to give the air screw a tweak.

It runs real good.

notenuftoys · Post by **notenuftoys** » 05:53 pm Nov 24 2008

I did get the air screw.

It's about 2 1/2 turn out right now. It has a CEK needle, and is jetted 152/40. I've ordered new reeds, they should be here in a couple of days. I'll play around w/ the air screw on Friday when we go riding.

notenuftoys · Post by **notenuftoys** » 07:22 pm Nov 27 2008

I put in new reeds and checked the air screw when it was on the stand. The screw was only out 1 turn!

Now wonder it's rich!

I backed out 2 turns and rode it down the street. Much better. Hopefully rain will hold off tomorrow and I'll play around with it some more.

Post by **fuzzy** » 10:06 am Nov 28 2008

Keep in mind what CC said about the small increments. You'll have to get used to making MINOR adjustments for peak throttle response. You'll get real good at it, and if like me won't stand for less than perfect.

jc7622 · Post by **jc7622** » 12:53 pm Nov 29 2008

Mine bogged really bad when I put the RB head on. I needed to re-jet. I went up on the pilot jet and played around with the needle position. It runs really good now, but I'm pretty sure there is more potential there if I really put some work into getting the jetting just right.

RBD · Post by **RBD** » 12:59 am Dec 04 2008

YEP!

Fine tuning is the key....

The carb set-up is a little rich out of the box, hoping that I got the jetting close for your riding needs (a little fine tuning maybe necessary especially with the mixture screw). I did and do supply you with optional jets and instruction sheets on fine tuning your RB modified carb for you individual needs, temps and altitude.

Please contact me with your RB carb mod problems (the RB carb and head mod are for people looking for additional performance enhancements, and does need some knowledge in basic fine tuning)?

Thank you, Ron

notenuftoys · Post by **notenuftoys** » 10:18 pm Dec 05 2008

I went riding last Sunday, and after adjusting the air screw, the bike is really running awesome! I'm still getting a little hesitation when I twist to almost full throttle, so I might need to back off the main jet. I want to run through this tank of gas, get some fresh gas. But right now it's really running good. Giving it full throttle coming out of second gear turns the front wheel comes up. Hopefully I'll finish this older gas this weekend.

canyncarvr · Post by **canyncarvr** » 04:20 pm Dec 27 2008

...where did it end up? You're 'after adjusting the air screw' comment could refer to your '2 turns out' post earlier...or not.

Not that where you end up with your air screw means anything to anyone else, but I'm curious about it anyway.

notenuftoys · Post by **notenuftoys** » 08:05 pm Dec 28 2008

I've ended up 2 1/8 turns out. that seems to be the best spot right now.

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Low RPM, 1/4 throttle bog

Low RPM, 1/4 throttle bog