A little help with tuning

Roy Dietsch · Posted: Sat Mar 03, 2007 5:40 pm Post subject: A little help with tuning

The solo season is going to start before I get a chance to head to the local
track and I want to drive the shifter I just bought. Its jetted for C12 fuel
but I would like to run something more readily available maybe like a 110
Sunoco its also going to be pretty cold and I think I have to rejet it.

How do I jet it?
How do I know what size to use?
Where do I buy jets?
I was going to take it down to the kart shop near by and have them do it
but cash is getting pretty tight and I think I should learn how to do it on
my own.
Its a 2001 Road Rebel with a KX125 and I thought it carb read Vishu or
something like that. maybe you can tell a little more from a pic

Any help or information you can give will help greatly!

Thanks,
Roy
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Nathan Fowler · Posted: Sun Mar 04, 2007 12:34 am Post subject:

Can we get a better shot of the carb? From that picture it looks like Keihin PWK, but I am not 100% certain because I see some odd things (or its a distortion of the picture that I see).

For jets I use: Chaparral Motorsports (Motorcycle Performance Dealer)
For information about motorcycle carbs: Sudco
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#96 - 2000 Trackmagic Demon / Stock Honda

Roy Dietsch · Posted: Mon Mar 05, 2007 7:28 pm Post subject:

The kart is being stored out of town and the soonest I can get to it is friday, but I will post back with more info about the carb.

Thanks,
Roy
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Bob Monday · Joined: 21 May 2004 Posts: 91

That's what I thought due to the screw-on black top and venting.

Jetting is unique to each engine due to porting, pipe/silencer and other carb components. My kart will not run with jetting typical for most 125s. It was WAY too rich!

Start rich and lean it down until you get the right jetting, realizing that barometer, temp and humidity will affect jetting when you get close to optimum. It is NOT the kind of thing that you get right once and stay teh same thereafter.

Your best bet is to run some laps with people who will help you get it jetted right, then teach you what to do after that. Once you get the hang of jetting, you have a ride that's a lot more fun!

Bob Monday

Nathan Fowler · Posted: Wed Mar 07, 2007 8:59 am Post subject:

Another big question is: Where are you at? You just might be close enough to some of us "established" karters that might warrant a trip out to your area for an event. We never can have to many Shifters in SCCA! Twisted Evil

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#96 - 2000 Trackmagic Demon / Stock Honda

Roy Dietsch · Posted: Wed Mar 07, 2007 12:27 pm Post subject:

I live in milwaukee but might go to the Peru tour(big if, I'm a little scared of
the surface) and I will for sure be at the milwaukee tour and nationals(if
really suck I proly won't be driving a kart at nats.) Is there a large
concentration of F125's near milwaukee? We typically have 1-5 shifters at our
local events but I don't know anybody's contact info. If I get an EGT what
should I be looking for for a range?

Thanks,
Roy
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Bob Monday · Joined: 21 May 2004 Posts: 91

You'll have to weld the bung into the pipe to add an EGT sensor. My recollection is 1100 degrees is max at full throttle. I noticed that you've got the biggest silencer I've ever seen on a moto Wink

, probably having come from an ICC somewhere along the way.

Bob

Roy Dietsch · Posted: Wed Mar 07, 2007 5:09 pm Post subject: Re: temps

Nathan Fowler · Posted: Wed Mar 07, 2007 7:20 pm Post subject: Re: temps

Roy Dietsch · Posted: Wed Mar 07, 2007 8:43 pm Post subject: Re: temps

Bob Monday · Joined: 21 May 2004 Posts: 91

These engines are very sensitive to backpressure. You pretty much want the pipe to control that due to the engineered tuning for the pulse.

With the carb you appear to have (it has a longer body between the slide and the reeds), plus the additional pressure in the longer silencer plumbing and muffler (plus much more weight back there!), my guess is that you have a lot of very low-end torque and then it runs out of RPM quickly.

Its one of those things that when you experience "right" for the first time, you say "Oh yeah!" and then continue to hunt for it every day thereafter.

Bob

Roy Dietsch · Posted: Sun Mar 11, 2007 4:40 pm Post subject:

The carb reads Kirish I think...

Thanks,
Roy
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Larry Andrews · Posted: Sun Mar 11, 2007 10:59 pm Post subject:

Be careful about advice you get regarding EGT temperatures - the numbers people get are highly dependent on the installation of the temp probe. The further away from the engine, the colder the gas will be. The shorter the probe, the colder the reading will be. There's a lot going on...

The highest I ever got on my stock CR125 with a Mychron3 Gold was about 1120F. That was with the probe mounted under the pipe pretty close to 6" from the piston.

hth, la

Roy Dietsch · Posted: Mon Mar 12, 2007 11:39 am Post subject:

I think I found the jets here http://www.sudco.com/keihin_mj.html#335 in the right column based on what you guys are saying, I looked up the pics of those carbs and I think I do have the PWK. From the part diagrams it appears I would have to take the bottom off and the jet is in there, is this correct?

Thanks,
Roy
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Bob Monday · Joined: 21 May 2004 Posts: 91

You can get to the main (high speed) jet by going through the plug on the bottom. To get to the pilot (low speed) jet, you have to pull the bowl off. Be careful not to tip it when you pull it off or the pin that holds the float in can fall out. This especially applies when reassembling it!

The low speed jet affects how it comes off the corner as you roll into the throttle. It's only the first 1/4 throttle. Frankly, you seldom mess with it once set (unless you are running both sprint and long tracks: "long track" = sustained full throttle, like 30-45 seconds or more in 6th gear).

If you pull the bowl off, watch how the brass tube goes down into the body of the carb. It's a challenge to figure out how to reassemble the bowl the first time or two.

Bob