Sputtering at low RPM

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smuggler
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Sputtering at low RPM

Post by smuggler »

Hi,
Im new to this forum, but from what I can see it looks great.
I have a 2001 kdx(of course) 220 that is a little sputtery at low rpm, up to 1/4 throttle. I just bought the bike, its in excellent condition, and from what I can see everything is stock except for a air filter. The engine has never been taken down, lots of power and runs smooth except for the first 1/4 throttle. Previous owner used Amsoil intercept faithfully at 40:1.
Is the top of the air box normally fully open or is this a mod.

also compression is about 120 psi

Any ideas would be great. Thanks
2001 KDX 220
2006 Polaris Sportsman 500 EFI
1996 Polaris XLT SKS
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Colorado Mike
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Post by Colorado Mike »

First thing, clean the carb really well, and make sure the float level is set right. Look at your pilot jet, make sure it's clean and sized reasonably. also might want to try changing the position of the needle.

Also make sure you reeds are in good shape, not frayed, and they are closed when you look at them.

120 seems low on compression, but there's lots of variables in how people check that.

A 220 with a stock piston is a time bomb. Change to a Wiseco before you hear the BANG!
Mike

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Re: Sputtering at low RPM

Post by Kurt Franz »

>|<>QBB<
smuggler wrote:Hi,
Im new to this forum, but from what I can see it looks great.
I have a 2001 kdx(of course) 220 that is a little sputtery at low rpm, up to 1/4 throttle. I just bought the bike, its in excellent condition, and from what I can see everything is stock except for a air filter. The engine has never been taken down, lots of power and runs smooth except for the first 1/4 throttle. Previous owner used Amsoil intercept faithfully at 40:1.
Is the top of the air box normally fully open or is this a mod.

also compression is about 120 psi

Any ideas would be great. Thanks
Is the following occurring:

If you take off quickly and give it normal throttle it runs fine. But lets say you get up to speed and want to "cruise" at a set speed and you close the throttle down to about 1/4 th and try to hold it there and keep your "cruise" speed the engine starts running a bit rough - kind of like surging and if you give it a tiny bit more throttle it is OK, but then you cant hold your cruise speed where you want it? It won't stall and it runs OK, but it constantly surges between sputtering and smooth running. Also it will idle fine.

OR is it just plain running poorly at 1/4 throttle and will stall if you don't give it more throttle? Also it wont idle.

Which one?
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Post by canyncarvr »

You're familiar with what a 2-stroke engine sounds like when it's 4-stroking?

Or...kind'a what Kurt said.......... :wink:

Oh...taking the lid off the airbox is a common modification. As is removing the snorkel and keeping the lid, or keeping the snorkel AND the lid..and drilling the rearward flat part full of holes. The point being, the OEM airbox is restrictive air-flow-wise.


Read again what CO Mike said about the OEM piston. Do yourself a favor...get it out! I don't want to hear...'But it's been in there since '01 and it's been OK.' I'll have Guido come over and slap you upside the haid for a response of that sort.

:grin:

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Post by smuggler »

Hey guys,
Thanks for all the advice, the bike is doing what kurt said. When cruising at 1/4 throttle its sputtery, a little more throttle and its ok. Idles fine and Otherwise the bike runs great, you call this 4-stroking? I had a 1994 kx 250 that did the same thing, is this normal for a 2-stroke?
Also I guess I will have to buy a wisco piston for spring.

This forum is #1.......
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Post by canyncarvr »

>|<>QBB<
smuggler wrote:When cruising at 1/4 throttle its sputtery, a little more throttle and its ok. ...you call this 4-stroking? .. is this normal for a 2-stroke?
This forum is #1.......
Yes. The bike is firing every 720º (like a 4-stroke) as opposed to every 360º (like a 2-stroke).

Yes. It is normal.

I think it is a happy 2-stroke sound. :grin: :grin: :mrgreen:

If you find your bike doesn't every DO that...chances are excellent it is way too lean (or way too hot)..and it's going to stick pretty soon.

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Post by smuggler »

Thanks guys thats good to Know, now to buy a new piston&rings.....
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Post by Kurt Franz »

If I recall correctly, I thought I heard somewhere that the shape of the exhaust pipe has a lot to do with causing 4 stroking. My 1973 Yamaha CT3 which is a factory dual sport never does it, but the size and shape of the exhaust pipes are very different. On the CT3 the expansion chamber is much narrower and has less area.

Maybe , maybe not..
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Post by canyncarvr »

>|<>QBB<
Kurt Franz wrote:If I recall correctly, I thought I heard somewhere that the shape of the exhaust pipe has a lot to do with causing 4 stroking. My 1973 Yamaha CT3 which is a factory dual sport never does it, but the size and shape of the exhaust pipes are very different. On the CT3 the expansion chamber is much narrower and has less area.

Maybe , maybe not..
As much of nothing as I know about that, I'm still absolutely sure you are correct. The pipe shape being a major factor (if not THE major factor) in the 2-stroke exhaust scavenging/supercharging effect, it obviously has an impact on whether there is (or is not) enough of a charge in the combustion chamber to fire on every crank revolution.

I've not owned dozens of 2-strokes...but a handful. Every one has had that 'happy 2-stroke' yippee skippee effect.

Interesting that your experience is 180º out from that.

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Post by Ogre »

None of my 2 stroke trials bikes skipped or sputtered like my KDX does.
They were all very smoooooth.
They also had little to no expansion chamber and it was after a very long straight section.

Has anyone noticed if a torque ring tames this on the KDX?
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Post by Jeb »

My experience (FWIW) with the KDX220 I used to own and the two 2-strokes now in my garage is the snappier the throttle response, the more 4-stroking. The 4-stroking was most predominant on the 220 when it seemed to run best come to think of it. For a time I was tried to "clean it up" thinking that something wasn't right and all it seemed to get out of the effort was a decrease in performance.
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Post by canyncarvr »

Agreed. For the KDX anyway, and dirtbikes in general, if four-stroking isn't happening a good bit of the time in a light-to-no load situation and a 'maintenance' throttle position, something is wrong.

Now...that 'something' could be anything from a dragging brake to a clay impacted chain guide to an off-jetted carby, but it IS 'something'.

Besides..that skipping and sputtering noise from your KDX lessens (masks) the engine racket!

It's a good thing!!

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Post by smuggler »

Wonderful info guys, now I understand. The bike is as I mentioned a 2001 with original front tire, sprockets and chain. I know the fellow personally and he babied the bike. Was $2200 Canadian too much?
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Post by Colorado Mike »

I don't know what the conversion rate is now, and it really doesn't matter. If you like your bike, that's what counts. I bought mine new and then sunk a lot into modding it. Did I spend too much? sure! Do I like it? you bet.

You spent less a lot less than me for whatever that's worth. Enjoy your bike. :supz:
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Post by fuzzy »

2 stroke trials bikes.....little to no expansion chamber
I think there is your answer right there. A lot of this '4-stroking' has to do w/ the 'modern' pipe, and also part throttle cruising w/ the PV possibly closed.. Whether it's a 'low end' pipe or not make no mistake these big 'new' pipes are designed to have a high-rpm supercharging effect, and to do their best job w/ the PV open...Granted the make the low end compromise pipes that help. Tune the bike to not blow up on the pipe, and it will 4-stroke when not. Also, the thing is trying to get on the pipe by nature...

The trials motor being the opposite w/ no regard to top end performance in the design of the ports and the pipe. Nice and smooth low end performer. I kinda wish someone would make a real 'low end' pipe for the KDX w/ no regards to open PV/high RPM performance whatsoever. I think I lot of tractor-like torque is still in there hiding, but what's 'normal' is a great balance of all the worlds which makes the KDX great.
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Post by Kurt Franz »

>|<>QBB<
fuzzy wrote: I kinda wish someone would make a real 'low end' pipe for the KDX w/ no regards to open PV/high RPM performance whatsoever. I think I lot of tractor-like torque is still in there hiding, but what's 'normal' is a great balance of all the worlds which makes the KDX great.
If someone made a real low end pipe that would make my KDX run like my CT3 I would buy it right now. A lot of my riding is across flat spaces at 45-50 mph - prime 4 stroking territory on the KDX....
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Post by Ogre »

Kurt,

Have you tried a 14T front sprocket?
I recently slapped one on my 220 and found that it "loads up" the engine more when just cruising along and cut the 4 stroking back quite a bit.

Other than being a bit fast in real tight stuff, my bike pulls the higher gearing quite well and makes the bike more enjoyable to casually putt thru the trails on.
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Post by fuzzy »

COuld basically use a straight pipe....LOL. Carb would need vastly retuned.
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Post by Kurt Franz »

Here is a picture comparing the exhaust pipes from the KMX200 and the KDX200. The KMX was a street version of the KDX200 that was available in England and other countries, but not here in the US.

Pretty radical differences in the designs of these pipes which most likely lends credence to the 4 stroking discussion.

Image[/img]
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Post by fuzzy »

Looks interesting
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