Clipping the carb

Questions and Answers about the best carb and Head mods available for the KDX.
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Piston
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Clipping the carb

Post by Piston »

Hi guys Im just wondering if anyone has come across this, to cut along story short I brought a Kdx 250 1991 from a friend, he had the topend modified to a 300cc. Only other performance upgrade is a prociruit system.
I now have to run it in, other than when its on pipe running it runs like a dog, wont idle very well keeps on bogging its self down and if it sits too long on idle it fouls the plug, this i put down to a oily ratio at 30:1 i will be going back upto 40:1 once I have run it in.
That part I understand the only trouble I have is that he "clipped the carb"
aparrently by clipping the carb depending on where you clip it, it lets more or less fuel through. Apparently it was a benificial thing to do for running in the new topend. The problem is that when the bike has not been started for say longer than one hour it has very high idle then it manages to settle it self down, he put this down to when he clipped the carb. as it was fine before he done that.
Only trouble is Im not sure where the clip should be on the cable for factory settings.
I would not have thought I have a airleak other wise it would not settle its self down after 20seconds of idleing.
I would be greatful for any help given on this as i do not want to do it any damage.
Thanks Andy
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canyncarvr
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Post by canyncarvr »

This is in the RB section 'cuz...why?

The term 'clipped' referring to changing the clip postion of the needle in the carb, right?

Generally, anything having to do with modifying the fuel to the carb during a break-in is specious.

You might want to 'modify' the AIR intake into the carb....as in no WOT right off the bat on a choke-cold 'new' topend...but changing the fuel delivery....is to be of what benefit?

Another generally, but a high idle on a cold start is (other things OK...no air leaks and such) is the sign of a too small pilot. A bike that does that may indeed run perfectly well once it IS warmed up...but can be a pain 'cuz it will generally be hard to start (choke cold) and high revs on a choke cold start isn't generally going to do you any favors. A high idle in that case can be 'regulated' using the kill switch..or you may turn the air screw in when you start it...out when you run it.

That's not uncommon when using a 35-38 pilot in a 36mm carby on a 200/220 btw.

If it fouls plugs at idle, that is an indication of a too rich idle (slow) circuit..not too lean.

I don't have a 250..and they are not 'like' a 200/220 in a lot of regards.

The 'modified to 300cc' is problematic. How? Presumeably the cylinder was sleeved? What else? If the head was not machined to 'match', all sorts of things will be off. The static compression for one will be too high. It may not breathe as well as it did at idle, either.

Watch the static compression..as that can be a bike killer. Well...engine killer..bike or not. This:

'I used to have 150psi at crank speeds...now I have 180psi!!'

....is NOT necessarily a good thing.

An overbored bike falls into the 'All bets are off!' when it comes to 'standard' jetting practices. It may need 'modified' fuel! Even when the 'expert' builder says it don't, that doesn't mean anything. Quite a few riders have had their bikes 'modified' in a big-bore fashion, specifying 'pump gas' to be used...only to find that it is NOT a workable solution.

The reason for changing to 30:1 was...what? You leaned it out (air/fuel-wise) because....why?

Changing premix ratios for the sake of a 'better' break-in process isn't generally considered a good idea, either.

Hang on a bit...some of the 250-ers on this board (no KX-ers need reply! ) may chime in.

Re: '..wont idle very well keeps on bogging its self down..'

Notwithstanding the overbore issue, that is a tuning (jetting) issue.

But...like I said...all bets are off with an overbored engine.

Good luck! Have fun tweaking!!

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Piston
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Post by Piston »

Hi Sorry about it being posted in the RB section was one of my first couple of posts and i made a mistake in the section where I posted it.

Thanks For the info I changed the clip position back to standard and it still idles very high on start up, thanks you very much for the tip about regulating the high idle with the kill switch, its proven to be very helpful while i try to solve this problem.
Iam beginning to get the feeling that I may have been misled when I went to pick the bike up, I have been thining that it must of been somthing that I have done, as when i went to pick it up it started up and ran fine. But like I said if the bike is say started within half of an hour to the last time it started the idle is fine, it only goes onto a high idle after its been sitting for 1 hour or longer.
So it might of been started before I went to pick it up. A good lesson to be learnt for myslef is to feel the engine while im looking over it to see if its hot or cold.
Anyways if ive been ripped off ive been ripped off just have to learn from it. Im going to try to persavere with it and along with the help from this forum hopefully i will get it fixed. I want to do it myself though as the main reason for buying a old two stroke it that I can learn to repair the bike for myself as its a bit more simpler than my 06 XR 650, which just runs perfect fullstop.
So after reading your post is it not a good idea to change the oil ratio? should i run it in at 40:1?
The only thing I changed on the bike when i got it here is the mixture of fuel I changed it to 30:1 after following some advise from a bike shop.
Maybe that might be contributing to my high idle?
Anyways thanks for your input sofar.
2006 XR650 commuter
KDX250 Wolf in sheeps clothing
IdahoCharley
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Post by IdahoCharley »

The change in premix ratio may be the reason you idle is a little high. If it was 40:1 and you went to a 30:1 ratio - the bike is running a little leaner on the fuel (gas) portion of the premix.

The other thing to remember is the air screw on the carburator is meant to be used to fine tune the low speed idle circuit. Have your friend point it out to you (if you don't know which screw it is already) and turn in clockwise about 1/2 turn and see if this helps settle down the idle a little quicker. (It is best if you look at the screw and then screw it in clockwise until it LIGHTLY seats counting the number of turns it takes to LIGHTLY seat the screw THEN back it out the same amount less one half turn. ex if it takes 1 3/4 turns to close the air screw then open it back up 1 1/4 turns. If you are at 1/2 turn or less then you should install the next larger Pilot Jet) Get use to using the air screw to fine tune the idle and just off idle response - it is one of your best friends in getting the right pull off the bottom with the 2-stroke engine.

At this point in time nothing you have said sounds like you've been ripped off. Or that anything has been done to hide anything. FWIW - I like to warm up my bikes prior to selling them just to make sure the guy coming to ride it doesn't cold sieze the engine :roll:
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