Deceleration Knock--Normal?

[Brian]

Is the deceleration knock on a 250 2-smoke a normal thing with these bikes? Is it because I run premium pump gas vs. race gas or what? Every time you slow down, or the rpm returns to idle, the thing knocks. I'm running a 45pilot, 175main, stock slide, stock needle 2nd from top. A.S. ~2 turns out.

 

[whyzee]

Not good! Sounds like you got something seriously wrong. I'd start with a complete tear down and rebuild. KTM's only do that when the crank is about to fail. :|

 

[biglou]

Stanley, what are you doing to my bike?! :scream: When you say "knock", do you mean a detonation, a "clunk" or what? If you think about a reving 2-stroke going "ring-ding-ding", that bike would give a "ding" or two on decel. You can also hear and/or feel the powervalve opening/closing if you are "in tune" with the bike and listening to it.

 

[fremontguy]

My 300 will give a knocking noise on deceleration & idle when running rich on the pilot, low throttle area. It usually also makes that sound before stalling. If I lean it out, the bike will idle & decelerate smooth and clean.

 

[Patman]

OK Stan let's see if we can try to help sort things a little bit.
-How "old" was the fuel mix?and was it kept in the bike or in a metal can?
-Are these your revised jetting specs or the same ones that had you spooging?
-What kind of color is plug showing? BTW what kind & range of plug are you using?
-with all the problems you've been seeing I'd REALLY suggest pulling the carb and giving it a good once over to be sure nothing is "gumming up the works" (check float height, seat, clear passages....

 

[Brian]

I guess I made it sound worse than it is. I got it running right, and it was doing this since day 1 I rode it at CL. Lou described it better, more of a ring-ding-ding-ding. Fresh fuel, I found out spooging was only becasue oil was leaking out of the can. I've pulled the carb, it looks good, but then again, what do I know? I know there's not something seriously wrong because Lou would have noticed, and it was doing it the day I got it. Just a wierd quirk, I guess.

 

[fishhead]

Stan

decel knock on a katoom sounds like someone hitting the head with a good sized ballpeen hammer. ring a ding is not decel knock.

 

[copyguy]

I may have the same problem but not sure. Sometimes, the instant I let off the throttle I hear what sounds like one knock. It doesn't continue doing it as I decelerate just the one time noise.

I have just attributed it to drivetrain lash or the chain slapping but it's real hard to tell where the sound is coming from when your riding. Does the deceleration knock typically happen just once or would it continue to knock continuously through deceleration? Any ideas would be greatly appreciated.

It's an 02 250 EXC, the spark plug is a nice dark tan color, I don't remember my jetting settings but I know I have the N3EJ straight taper needle out of a YZ. I've been running this for about a year with no other problems.

Thanks

 

[fishhead]

try turning in your air screw a turn or so or go up one size on your pilot and see if it makes a difference.

 

[trakkerman]

I've had the same thing on my '96 300exc in the past. I seems to be a jetting / gas problem. It also seems to happen more before the engine gets to a good running temperature.

 

[Brian]

As good as it runs, I won't worry about it, especially if its a common thing. Maybe I'll try some race gas at DW.

 

[Jeff Gilbert]

Lou, I told you the JB Weld wouldn't hold :p

 

[yarbonwick]

Ahh I see that the lil' (probably now bigger than me) Stanmeister converted over to the pumpkins. What you're hearing Stan is from cheap fuel. Take that stuff out, put it in the weedeaters/edgers....dads bike...or what-have-you and get the good stuff. What you're learning is the typical characteristics of pumpkin-jettison. Notorious creatures that would seem happier with a Edelbrock self-programming fuel injection instead of antiquated neeed/jet tin carbs. BTW....did I mention that mines for sale? :)

 

[RanMan]

My '02 200 EXC would do that. I tried different things because I
knew I was in the ballpark on jetting, so I raised the needle (drop clip)
one notch and mine went away. That's easy enough to try it anyway.
Raise needle one or two notches and see how it works.

 

[georgieboy]

It is a mixture thingie. I ve got it with mine huskie sometimes. When i drove it fast for couple of minutes and i close throttle it goes banging in the pipe.
I am planning to it solve by increasing my pilot jet one step, see if that cures it.
I have trouble starting and revving when cold, need the choke too long. So i gues it is the pilot jet area.

 

[MotoGyro]

Turn your idle down where it won't idle an dit will stop doing that

 

[georgieboy]

I don't want that when offroad riding. Maybe with mx it is fine.
Although i feel you not curing the symptom, merely make it unnotice.

 

[john stu]

piston slap?

 

[70 marlin]

Sounds like a slight lean condition @ 1/4 to half throttle range with the needle or maybe the slide. I'd start with one clip down on the needle.

 

[Brian]

I'm hop9ing someone can help me dial it in at DW. I really think the triple-taper needle needs to go, first, and then maybe someone could guide me form there? Anyone willing to help gets to test ride it;)

 

[yarbonwick]

Stan when I had mine, a good all around jetting was a 42p 172 m and NOZE 2nd from top. With this I didn't have to jet for the summer, jet for the fall, jet for the winter, jet for the spring, jet just to be jetting, etc. I had the same scenerio and going to that plus using good quality fuel 103 50/50 with pump produced great results. Good luck!

 

[Brian]

Lloyd, hanks for the help. Is the NOZE a single-taper? I'll pick up at least a needle and go from there, I think the needle is the source of surging, too. I had a 42p, and after a WFO section, it would surge at idle, changed it to a 45, and it helped. I wonder if a 42p/NOZE would eliminate the idle surge vs. 42p/NOZG or even 45p/NOZG(still surges on occasion). Will jetting alone help the decel knock, or will I need to run race gas, too? BTW, do you have a dirt bike anymore?

 

[yarbonwick]

The NOZG is actually a leaner needle than the NOZE. These are not single taper needles. A lot of people go to a #7 slide instead of the stock #6. I've resisted in doing that since I didn't want to pay $60 for a slide. Others chop theirs down to a #7. For where you live, the #6 can be had. Here's a couple of things I've come up with over the years. On needles:

N85A = NOZE ---> Richest
N85B = NOZF
N85C = NOZG
N85D = NOZH
N85E = NOZI ---> Leanest

In relation to that, there are five positions in the needle. Top is leanest, bottom is richest. The needle effects Being I had a rich needle, I placed the clip in the 2nd from leanest postion allowing me to adjust it and it only to suite my enviroment. Hey when I got the bike, it came with a NOZI !!

A couple of really good reading materials are http://justkdx.dirtrider.net/carbtuning.html and Eric Gorr's "Motocross and Off-Road Performance Handbook"

Good Luck!!

 

[fishhead]

Stan,

A lot depends on the type of riding you do.

Yarbonwick has some good advice but before I make any suggestions could you let me know what clip position you are running when you get the decel knock, what the trail or track conditions are and what class you are running.

 

[Brian]

needle position is 2nd from top. I really need to try some race gas. I am hesitant to change needles, etc because I love the power delivery, don't want it any smoother. I'm in a class all by myself;)(don't race)