04 RM-Z decel pop...why?

Hi all, I'm new to the forum, but I have a puzzle that has me a bit perplexed. I have a Keihin FCR carb in a 2004 RM-Z 250. As of now I've got a decel pop that ranges from a couple small pops at about 60 degrees air temp to a downright insane series of pops at about 45 degrees. I have a feeling this is my idle circuit, but here's a spec list of my current jetting;

-40 pilot jet

-1 and 5/8 turn out on the fuel screw

-178 main

-3rd clip from the top on the needle

-these are Team Green settings for the 04 kx250f, same bike


-valves are freshly shimmed and within range

-carb just cleaned

-no vacuum leaks

-altitude of 1500ft or so

The big question is, is this something i should be concerned about? and if so, what is a remedy to the problem? If I overlooked a thread related to this, my bad, i did try and found nothing relevant to my problem. Any help would be appreciated.

Leak at the exhaust header? Forget about the specific setting for the fuel screw. Set it up properly (loads of threads/stickies on how to) and go from there.

Header is fine, gaskets are in good condition. i did use the set-up techniques outlined in many online forums and even by the advice of a local shop's owner who had the same bike, they just happen to fall in line with the team green settings. Again, I just am wondering how big of an issue this is, and what circuit causes it, and what, if any, remedies are available.

Pilot cicuit on most. EPA on others....

Here's my opinion on decel popping: if a bike is set up to idle right, at say 1200 RPM, then when you are decelerating at 3-4000 RPM, with the throttle completely shut, it's gonna be lean, no matter what. Actually, I've heard of some carbs that have a decel jet, that supplies a little extra gas when the pull is there, but not when normally idling. The only way to get rid of decel popping is to richen the idle circuit and then, you will be too rich on a normal idle.

And when did you last repack your silencer????

Silencer had just been repacked johnny, like first ride out on a fresh repack.

OlderHuskyRider, That's what i was thinking, carb is set up properly to idle normally, and the popping happens when i chop throttle from a significantly higher rpm, but doesn't do it if i roll off. So in your opinion, is it a bad thing on the engine, like something i should be concerned with? On the other hand, i don't want an overly rich idle circuit either.

Not a bad thing, it is normal on a race bike.

Your bike is not designed to be trail ridden, it is designed to be at WOT, then hard on the brakes, not coasting. It does not have an ACV (Automatic Coast Valve) nor does it have an efficient muffler, like a trail or woods bike does. Correct idle speed is not 1,200, more like 1,850 rpm.

Thanks for the input guys, I think William nailed it though.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now