CR125 Advice and Jetting (noob...)

Evan York

Working on a CR125 project. Got a motor at a pretty good deal and completely rebuilt (new crank, seals, etc.). I want to make sure I go into this project understanding everything about CR125's. This package is NOT going to be the fastest motor out there...it's only a training tool for me while I am not racing cars. Something to keep me sharp and in shape.

I have an open filter, but I'm also getting an airbox with 4 holes and mesh covers. The jetting is something that is new to me...so I need some advice on the theory behind the carb setup.

I live in the Charlotte, NC. So we're pretty humid, and generally warm. But it can get cold, and dry.

Also, any advice to anything Honda CR125 related is hugely appreciated. Give me your 2 cents! When to rebuild top and bottom end, good spark plugs to run, gas/oil mix, etc.?

Thanks for your advice!

P.S. - I'm looking for MyChron's (with sensors), radiators, and radiator mounts still. Make me an offer!
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Regards,

Evan York

www.evan-york.com

@EvanYork

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"That which you manifest is before you" - Enzo the dog

Steve Buckner · Joined: 22 Apr 2003 Posts: 839

Read thru the tech section for shifters. srsengines.com

If you want a new Rad. www.ellisracingproducts.com

al nunley · Joined: 13 Nov 2006 Posts: 3038

Few, it would seem, will agree with me, but if it was me.
An EGT for getting the jetting right.
An air density gauge so you know what jetting to start with.
A compression gauge. Compression is The Holy Grail.
CHT and water gauges are OK, but I want to know what’s happening “in” the engine. Detonation, and it’s prevention, are the key to going fast.
Testing jetting on the track is OK, but I want to know what jet to run before I hit the track.
What can I say, compression, compression, compression.
Running high octane fuel is OK, but do you need?? If you can run lower octane fuel, with no detonation, it’s faster.
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If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting

Jim McMahon · Posted: Wed Aug 29, 2012 6:45 am Post subject:

Evan, is your CR meant to be modified or stock? That will influence some of the answers. Especially jetting. Do you know what year the parts are? With stockers the '01 cylinders tend to like a slightly bigger jet. As pointed out, Johns SRS site has a lot of good information. As does swedetechs.
Best thing however may be to get a starting point from the previous owner on jetting.

Airbox vs filter will also change jetting. It will probably run richer with the airbox.
Gas:Oil ratio, stock or mod 24:1 is the most common one. Around 6oz/gal.
Motul Kart Grand Prix Oil is used a lot in SKUSA (synth). Maxima 927 is nice if you like castor.
Fuel: VP MS98 is the fuel used for stock hondas (96MON), for a mod VP C12 (MON106 I think) or Philips T111 are both very good.
Using pump gas even with a stocker is a risky business.
If you are looking to save a few bucks on fuel you could run 100LL AvGas but you will have to figure out jetting. Start big, work down.
Leaded fuel is preferable but not essential.
Spark Plug: NGK *10*. B10EG and B10EV are cheap and cheerful.
Air density gauge or whether station is a good idea. 2.5% in air density will usually warrant a jet change if you are chasing peformance. Once you have a base jet select practice laps I dont think I'd worry about changing the jet until theres a 7% change or so.

Checkout classifieds and of course eBay for MyChron stuff.
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Live in the midwest and have a TaG, Shifter, KPV, KT100, Animal\LO206, enduro or superkart?
Click here to join the karting festival at Blackhawk, IL June 1st-2nd.

Jim McMahon · Posted: Wed Aug 29, 2012 6:57 am Post subject:

Here's some links that might help you on your way...

Guide from Duncan Racing on Mikuni Jetting (Motorcycle)
Keihin tuning guide from Ian Williams Tuning
Slide carb tuning (John Denman\Kartweb
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Live in the midwest and have a TaG, Shifter, KPV, KT100, Animal\LO206, enduro or superkart?
Click here to join the karting festival at Blackhawk, IL June 1st-2nd.

andy graham · Posted: Wed Aug 29, 2012 10:19 am Post subject:

@Jim--I have the motor Evan is talking about in this post...it's a stock `95 CR125, stock porting, rebuilt to stock specs...kick start gear and idler gear were removed and hole plugged...new oem crank assembly was installed and centered...gears and shifter forks looked good, no metal shavings in the cases anywhere....clutch basket had the usual chatter marks for an engine this age, we were able to clean that up and keep it within spec...oem top end parts were also installed...came apart and went back together just like a Honda should...this engine was tired when we started, and the feel of this new assembly compared to the old one is night and day....so he's gonna use it as a practice kart as I understand, I suggested keeping it simple, I suggested using an airbox as that is the way the original application of the engine is designed...back in my days of racing sea doos I saw many a burnt cylinder due to open style filters and improper jetting...I have little experience with the CR on a kart, but know the engine well from my many years of running them on the MX track...all powervalve parts were removed in the rebuild as well..power valve ports were plugged and blended...I'm interested to hear your suggestions on jetting and air box vs. open style for karting

Matt Stephenson · Joined: 29 Jan 2012 Posts: 22

All those are good comments.... Stay on the rich side till you get a handle on the jetting. The jetting guide someone mentioned is a great place to start.... I run the *6 slide, DGK needle, 55 pilot. I run the RLV V4 pipe, PWM pump around carb. If you run SKUSA rules get the skusa timing plate and run full advanced.

Like they say, compression is KING. keep yer cyl nice, replace with a stock top end kit at around 6-8 hours depending how much speed yer loosing. Piston itself can last a while, but you will get a handle on it after a few rings. You can do the top end without taking the head off, it saves a few mins of your time.

Spike Products out of Indiana I think, they make a great radiator for not very much money. Summer you may want the 10x18 ish, winter get the smallest they make. 8x16 I think. Optimum water temp is 120 INTO the motor.
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Matt Stephenson
S2 Shifter
www.StephensonRaceTech.com

Evan York · Posted: Thu Aug 30, 2012 1:46 pm Post subject:

All,

Thanks for the responses! I appreciate it immensely. Looks like I'm getting a new Birel here soon. I know this kind of depends on the track, but what kind of rear gears are good to start with?

As for what motor I am running, refer to Andy's post. Pretty much a stock moto that's as old as I. Hah!
_________________
Regards,

Evan York

www.evan-york.com

@EvanYork

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"That which you manifest is before you" - Enzo the dog

Evan York · Posted: Thu Aug 30, 2012 1:48 pm Post subject:

Oh! And, how do I know what Keihin carb I have? If it helps at all, the number on the carb is PJ15kajf
_________________
Regards,

Evan York

www.evan-york.com

@EvanYork

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"That which you manifest is before you" - Enzo the dog

Steve Buckner · Joined: 22 Apr 2003 Posts: 839

Check out this web site. Click on the PJ section. If you dont have any luck call them. http://www.sudco.com/

John Reikes · Posted: Tue Sep 04, 2012 8:44 am Post subject:

I just got my first kart and went out for a practice day a couple weeks ago. The mechanic who was helping me said to just run pump gas for a practice day. Was that bad advice (per Jim McMahon's post in this thread)? The kart is a stock Honda setup.

Secondary question... if that is the case, how do I get rid of the 3 or so gallons I have left? It already has oil mixed in, so it's not like I can pour it into my car.
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- John

Jim McMahon · Posted: Tue Sep 04, 2012 8:56 am Post subject:

Use it in the snowblower/lawnmower/weedwacker. You could also add some octane booster to it I guess. Whatever you need to bring it up to around 96MON.

The CR125 sees much longer full throttle periods with much higher sustained RPMs that it does in an MX setting. Also, the exhaus pipe that is on a kart is pretty aggressive as a result running pump gas is asking for trouble.

You might get away with it. Depends on how tight the track is, how aggressive of a driver you are, how much ignition advance you have and how rich you run the thing. It's just not a practice I'd recommend if you can avoid it. It'll take a lot of gallons of race fuel (or even 100LL) to offset the cost of a piston. If you stick it bad enough you may even need to replace the cylinder if you want to race it in a stock moto class.

Short answer: Todd Belew, one of the founders of the SM "concept" recommends race fuel, I would do what he says Smile

_________________
Live in the midwest and have a TaG, Shifter, KPV, KT100, Animal\LO206, enduro or superkart?
Click here to join the karting festival at Blackhawk, IL June 1st-2nd.

John Reikes · Posted: Tue Sep 04, 2012 10:06 am Post subject:

[Deleted -- just noticed there was already a response about how to get rid of it...]
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- John

Evan York · Posted: Mon Oct 22, 2012 10:50 pm Post subject:

So, I just found out that the 1995 CR125 has a Keihin PJ Carburetor. Does that change jetting at all? Sounds like the only difference (in regards to "jetting") is the fact that the choke and idle screw are the same jet.

What about the important stuff? And, where should I find a good jetting kit for the Keihin PJ?

Thanks again!
_________________
Regards,

Evan York

www.evan-york.com

@EvanYork

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"That which you manifest is before you" - Enzo the dog