Hey Guys,


I am having some low RPM problems with my 250. The problem is that when the bike comes down off throttle it will make a kung kung kung noise. I have checked all the usual suspects, leaky main seals, rod bearings, jetting, but nothing seems to correct the problem. My bike virtually has no idle at all and increasing the idle only makes the problem worse. I tried jetting richer on the PJ, but does not help at all. I have also read other posts, but their problem went away with the addition of the PWK Carb. Mine didn't. It also did this when it was all stock as well. At the point where I am sick of throwing money at this thing trying to fix it.


Here are the specs of the bike.


07 CR250


PWK Airstriker Carb - 45pj, 180 mj, needle in 3rd clip JD blue needle


Pro Circuit Head and Cylinder mods - runs on 110 sunoco mixed 32:1 with Maxima Castor 927


BR9EG Plug


V-Force Reeds


OEM Piston


Plug is blackish to dark brown and dry when doing plug check.

Sounds like when you try to rev a cold two stroke on idle circuit?


What is your air screw setting?


I think your jetting is rich all around. Race fuel will run richer than pump gas.

Typically this sound or symptom is from an overly rich idle circuit, but can easily be confused with detonation.


-When you do a bunch of downhill or coasting in gear letting the engine make a fair bit of the "gung gung gung" symptom, does it load up with fuel?

-Did you check your reeds? leaking reed valve can cause weird conditions like this.

-Is your JD needle a PWK needle, or for a TMX?
-What is your a/s setting? have you used the a/s to determine pilot range?


Also...I gave up on the JD needles once I went PWK, I have had much better results with the stock 2000 cr250 needle. The JD's were too rich at 0 - 1/8 throttle forcing me to run in 1rst or 2nd clip, which caused lean spots up higher... never did get either red or blue to run just how I wanted. Lower elevation guys may have had better luck.

It sounds like you have always had this problem with different setups, but whenever I end up stumped with something like this, I start from scratch at the basics.

-leakdown test

-compression test

-ensure clean spark

-start jetting process with pilot circuit and float level, then main plug chops, then needle, re-check pilot if large needle changes were made.



Could you post a vid of it ?   Also running an 03 with the PC cyl / head mods & running a Denso IW24 spark plug per PC's recommendation.  I've ran it since day one with the mods & have had good results on the engine running clean off idle & thru the revs with optimum jetting.  The Denso's are a little on the pricey side, but it would be worth a try..   

My A/S setting is 1.25 turns out. Turning the idle up only makes it worse, so I run virtually no idle at all. The needle I am running is the JD blue needle in the 3rd clip position. The bike has brand new main bearings, seals, gaskets, everything and still cannot figure out what its problem is. 

I still think you are rich....especially since you are running race gas. Try backing the air screw out

Have you tried running a different needle ? I didn't have good success with the JD needles on the mikuni & feel their not as good as advertised. Running an 2000 715/289R needle with very good results with the PC mods.

But then again...in my mind Kung Kung is a rich sound......ignore me if you know its a lean sound.

Have you tried running a different needle ? I didn't have good success with the JD needles on the mikuni & feel their not as good as advertised. Running an 2000 715/289R needle with very good results with the PC mods.


I could try the Red Needle that came with the Airstriker carb. 

I will try leaning the bike out a little on the bottom. But I will make a video of it running so you guys can further diagnose. 

I could try the Red Needle that came with the Airstriker carb. 

I will try leaning the bike out a little on the bottom. But I will make a video of it running so you guys can further diagnose. 

Try the red needle 3rd clip for sure.  I bet your problems will be solved or at least better.


In my experience the Sunoco 110 doesn't change jetting as much as I thought it would.  Most summer pump gas has a specific gravity between roughly .735 and .74 but it does vary.  Winter gas is normally lower in the range of .71-.73.  Sunoco 110 is .728 so it really isn't that far off from pump gas.  It also has a somewhat similar evaporation rate to pump gas so jetting shouldn't change all that much.  If you step up to Sunoco Supreme or some VP they typically have lower SG which would require a bit leaner jetting.  What it does do is give you a huge window where you can run lean without any detonation.  Many people go leaner because of this.  The biggest issue I actually see is the Vforce and the porting.  From what I remember Vforce instructions are to lean out the pilot and richen the main.  I am not familiar with PC mods so can't speak to that. 


One thing I have learned is that testing out going 1 or 2 steps leaner won't blow up an engine like most people are scared of.  You will be able to feel if it's too lean and the 110 gas should keep you safe.  Just make sure to not ride it if it is lean for too long a period of time. 

What altitude, temp, and humidity are you at?


Have you pressure tested the motor?


That VF3 is known to leak fairly easily...

Did some work tonight on the bike. Switched to the red JD needle for the Airstriker in the 3rd position. Cleaned the bike up some on the bottom and did not kung kung kung as prominently, but was still there. I did check my spark plug gap. Turns out it was waaaaaaaaay over tolerance, so I adjusted that. Ran the bike on the stand and the works pipe turned a nice purple color. It has never done that before so the pipe is getting some heat through it finally. Usually it would just stay brown and be cold to the touch 5 mins after a moto. I think we may be on to something here. I will have to order a smaller pilot jet. I think it is getting too much fuel on the bottom side of jetting causing the noise. Before I did all this tonight I ran it and pulled the plug and it was cinnamon brown on the insulator, but wet and black everywhere else. Now with a new plug, the plug came out nice and drier than before.


My altitude is 610ft above sea level and today it was about 86 degrees out. 


I do not think it is the reed block, it was just rebuilt 2 motor hours ago. It did this even with the stock cage too...

Going to try something new tonight. My exhaust might be sucking in air on the down stroke through the expansion chamber. Going to buy some larger diameter o-rings from the hardware store and some copper RTV and seal her off good and see how she runs.

Hey guys, 


While I am waiting for the RTV to cure, here are some plug pictures I took of what my plug looked like after I took out of my bike from this past weekend. Looks like my jetting is about spot on, what do you guys think? I set the spark gap to spec and now she likes to eat eat eat that Sunoco!


Edited by Speeddemon73

Plug looks ok, but I like to run my 250 2t a tad on the rich side since I'm in the upper rpm range.  It looks like your headed in the right direction with it & would suggest trying some VP C12.   

Is that a plug indexing washer I see there?

Update: Sealing off the exhaust pipe did not work. Did not help the situation at all. Inspected the ignition side crank seal and it looked a little wet. Replaced that with an extra seal I had and still did not fix the problem. I'm stumped and tired of throwing money at it. I just want to ride already!!!

Is that a plug indexing washer I see there?

Its a 1mm plug spacer that pro circuit included when they did my motor. Because they cut the head so low, I have to space the plug up or it will hit the piston.

Read some other forums posts and most people have ran their bikes will no idle at all as this is a result of trying to get the bike to idle. Well, going to replace the intake seal and set the idle back to zero and see if that helps. 

I'm no expert but I'll throw this in:

My 04 cr250 does that when I forget the fuel valve. Maybe you have plugged lines or aftermarket filter?

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