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View Full Version : DIY: 2stroke jetting (the easy way)


nutty professor
8th August 2005, 07:58 PM
First, Nuttys golden rules of jetting :

1) The factory know what they're doing (if you're a long way from standard jetting, something's wrong) This does not apply to RMXs.

2) Most problems that feel like jetting, aren't.

3) Always jet the pilot first, then the main, then the needle.

Pilot jet
OK, after checking the float height is ok and the air screw is in the factory position, start the bike and warm it up, a rag on each radiator helps.

If the pilot is too lean the bike will takes ages to drop into a low speed idle after the throttle is blipped. If it's too rich it'll go " gung gung gung" when closing the throttle while rolling in gear.

Try to get it as lean as possible with an immediate drop to idle. A good indicator of a too rich pilot is that the choke can be pushed in almost straight away.

Dont try to tidy up the pilot with the air screw, any more than 1/4 turn away from the standard setting at the air screw will affect the carbs' operation

Main jet
Jetting the main is usually done by plug chop, ride the bike at full revs loaded in say 4th gear,hit and hold the kill switch in till the bike rolls to a stop.

Look for a milk coffee colour on the porcelain, use a used plug that has already coloured up, (note KX85s and 100s will be a lot leaner than the plug chop looks, be careful!)

Grey or white plugs are too lean, black coffee is too rich.

Jet Needle
Most bikes wont need the jet needle changed, generally bikes with shorter mufflers than stock will require the needle to be richened by one clip. (raise the needle,drop the clip) this will only richen the bike as it comes onto the main, giving a nice transition and not a wild hit.

Other things
On older bikes make sure the slide and body are a nice fit, not worn and make sure the needle has no freeplay in the needle holder. Remember, FMX jetting is about a nice rip through the mid-range as you hit the up ramp not max HP.

I jet Cody Mackies YZ250 for max hp but I jet Maddos CR250 to work any where any time, so it's little rich with a hot plug. Lastly, oil/fuel ratio has little to do with rich or lean jetting. Quality fuel and oil comes first. Nutty.

1madgerman
8th August 2005, 08:09 PM
Cheers nutty im just playing with my jetting atm did a plug chop and she was running ritch i dropped the main from standard 178s to the next smallest i had 172 and the pilot from 50 to 48, needle is on 2nd slot from the top what do ya reckon i will see how this runs next ride. Got next weekend off the bike while i wait till i have the money and go see the boys at TEKNIK to get my suspension done.
Lars

stavross
8th August 2005, 08:16 PM
use a used plug that has already coloured up,



you mean run a plug that has already being used when your testing jetting?... thats funny

nutty professor
8th August 2005, 08:26 PM
you mean run a plug that has already being used when your testing jetting?... thats funny

Why? a new white porcelain won't show an overlean motor thats about to hot sieze, whereas a used plug will re-colour very quickly under load. It makes the whole process quicker to use a used plug. A used plug will come to true colour much quicker than a new one. Nutty

stavross
8th August 2005, 08:35 PM
cause your bringing in more variables. if you start with a clean plug, you can see what happened to it at that altitude, the terrain, and time of day

hawwwwwwt
8th August 2005, 08:36 PM
Holy hell man.

My jetting is off it's tits on my 04 rm125. I dont really understand most of what you wrote. So the next best thing was to call the bike shop and ask what the go is, they got me some jets and im yet to install them. But i hope they work well.

I really do shocking in racing. And the last thing i want to be doing is blaming the bike. But im starting to think from all the things people do to their bikes, there's gotta be a good reason why they do so well.

Nutty i'd be so greatful if you were willing to answer a few questions of mine since you pretty much know everything i want to know. Thanks.

nutty professor
9th August 2005, 09:23 AM
Just p.m. Dude and we'll sort it. Oh, and Stavross, so far, two world records (Maddo), one world championship (GP80) and last saturday at Brissy, Cody Mackie came out in his first time ever in a Pro Open National SX on the 250 and passed Andos factory Honda to win the heat. So I'll stick to used plugs when jetting. All views welcome on these boards.. and props given.Nutty

guz530
9th August 2005, 09:59 AM
hey nutty thanks for the great thread..just a small detail ..when your turning the air screw a "turn" is actually a totally 360 degree rotation of the screw huh? my 03 cr250 is a piece of shit..i can never get it quite right.! :Z105:

mulishatrooper
9th August 2005, 10:28 AM
hey nutty can we get some pic's ? to help understand what your saying thanks for the help

malc :D

stavross
9th August 2005, 02:02 PM
Oh, and Stavross, so far, and last saturday at Brissy, Cody Mackie came out in his first time ever in a Pro Open National SX on the 250 and passed Andos factory Honda to win the heat. So I'll stick to used plugs when jetting. All views welcome on these boards..

well pitty cody didnt make it past the finals. bike must not have being running to good

stavross
9th August 2005, 02:04 PM
a "turn" is actually a totally 360 degree rotation of the screw huh?

yes..........

nutty professor
9th August 2005, 05:29 PM
Hey Guz, pull the pipe and replace the o' ring on the barrell flange. Clean it up real good and silastic it as well with a good oil proof silastic.

CR250s are critical on this seal, if it's leaky the jetting goes off. Worth a try.

And Stav, the bike was cool, Codys only 63kg and it was his first National level pro open SX. If anything I overpowered it coz his Dad said he struggled to hang on to it. He'll come good over the series and as he gets bigger and stronger. Nutty

guz530
10th August 2005, 08:47 AM
thanks heaps nutty

i just rebuilt the old girl and the seal your talking about was F*(ked!
so yeah its heaps better now...seems to be okay jetting wise when im flat out all day at black duck, but when i try race it , she goes all weird..i think i will just have to play with it for a bit...

thanks again

guz...

wayno
9th September 2005, 11:30 PM
hey man ive got no idea about jetting so i need some help, i just put an fmf fatty pipe and powercore2 silencer on my 05 yz 250 , i did a hot test and the plug core came up white , should i change the main jet or the pilot jet and increase or decrease

hoppa24
9th September 2005, 11:43 PM
hey man ive got no idea about jetting so i need some help, i just put an fmf fatty pipe and powercore2 silencer on my 05 yz 250 , i did a hot test and the plug core came up white , should i change the main jet or the pilot jet and increase or decrease


im pretty sure that if ur plug is white, its running lean. and u should go up a main jet size. (correct me if im wrong).

wayno
10th September 2005, 09:35 PM
is there any diference between 178 and 178s main jet

FoXRaCiNG.2
10th September 2005, 10:21 PM
well pitty cody didnt make it past the finals. bike must not have being running to good
bloody hell man, nutty had made a thread to help people about their jetting out of his own time and all you can do is diss him. props nutty for the thread and lets stay on topic.

Spudman
16th November 2005, 10:41 AM
Cheers nutty im just playing with my jetting atm did a plug chop and she was running ritch i dropped the main from standard 178s to the next smallest i had 172 and the pilot from 50 to 48, needle is on 2nd slot from the top what do ya reckon i will see how this runs next ride. Got next weekend off the bike while i wait till i have the money and go see the boys at TEKNIK to get my suspension done.
Lars
gota ride the thing lars dont b scared 2 give her sum champ.havent heard anything about u riden did u drop out or sump"m?spudman.

KORUPT-FMX
13th January 2006, 08:29 PM
ok hmm, i ride 05 cr 250, full fmf system, for riding fmx perfect jetting i found after playing for hours was one leaner on everything, clip, main, and pilot!!

nutty professor
23rd January 2006, 09:41 AM
is there any diference between 178 and 178s main jet

'S' stands for short, all 178 jets are the same jet size. 'S' style jets are for use on carbs with shortened float bowls. If 'S' jets are used in stock body carbs the motor will lean off on the main because the float height is decreased. Nutty

GStylEr
30th January 2006, 04:33 PM
Hey Nutty just a quick question in regards to the above... Do you think a high float level could cause the main to run rich? Only really askin cause of what you said above, thought it might be possible seeing as I adjusted my float level a while ago when I was havin trouble and the main doesn't run too good at all now. I think I raised the float height... Think it could be a problem? Just hopin to find out soon so that if it is the case, I can adjust it now while there's no fuel in it before I refuel to go ride. If not, then I won't worry about it just yet, it's not causin any problems just runs crappy at full throttle.

nutty professor
31st January 2006, 01:39 PM
Yer Grant it will, the float height increases the capilliary effect and boosts the fuel flow up the through the main jet.

Since the pilot is on a seperate air and fuel circuit it's not affected.

Low speed running isn't affected much because the jet needle is blocking the emulsion tube, therefore the main effect of high float level is at wide open throttle. Nutty.

Crosser_250
3rd April 2006, 08:04 PM
Hey Nutty

My bike i think is running to rich it does that Gung Gung Gung thing as it trys to idle down. So i was just wondering with the mixture screw is Turning it inwards more air or is winding it out more air(on a 88 CR 250)
Thanx Buddy

nutty professor
4th April 2006, 02:33 PM
Winding it in makes the pilot richer, and vice-versa.

This bike should have 1.5 turns out on the air screw, go no more than 1/2 turn away from this.

Check the powervalve is returning to its stop by removing the side hatch, sometimes this was the cause on this model.

They were a great bike when new.

Crosser_250
4th April 2006, 04:27 PM
Thanx buddy
It has been a really good bike soo far ill tell ya if it works for me
How hard was it for you to get into small engine mechanics, im a bit interestsed in this career
Thanx

Crosser_250
4th April 2006, 08:03 PM
Yeah it stopped the Gung Gung Gung thing but it feels like its got a bit weaker of a band now. Any ideas would be great thanx

nutty professor
5th April 2006, 07:46 AM
it feels like its got a bit weaker of a band now.

That's probably because the bike is pulling evenly through the revs,......good thing. Nutty.

p.s. You can't go to any TAFE and learn 2 stroke performance, read 'Two Stroke Tuning' by A Graham Bell and go from there. I learnt from Bede Parkinson, the godfather of kart racing and Tony Hatton.

Crosser_250
5th April 2006, 04:07 PM
Ok cool thanx
Cya

KORUPT-FMX
6th April 2006, 03:44 PM
hey mate, da 05 cr still does the gung gung running ur specs that i said in the 06 cr thread... time 2 lean the air fuel mixture?

nutty professor
7th April 2006, 03:07 PM
hey mate, da 05 cr still does the gung gung running ur specs that i said in the 06 cr thread... time 2 lean the air fuel mixture?

We're seeing a fair bit of variation in jetting between different bikes, try this:

To check the pilot, roll down a big hill in top gear, no throttle. If it fires, gung, gung, then the pilot jet is too rich. If it takes a long time to die down into a nice idle then it's too lean.

Before doing this test, remove the reed block and make sure the reeds are in good nick and not overlapping the reed seals. Some Hondas come with reeds that are too long, (its easy fixed with a rat tail file).

peedee
7th May 2006, 08:11 PM
hey dude
i ride a kx250 2001, my bike when i first start it up when its cold it idles really high and when i rev it in nuetral it takes ages for it too go back to the high idle..but after i ride and its all nice and hot it idles alot slower and sumtimes cuts out because its idleing so slow....is that normal?
catch ya

M.r Mulisha
7th May 2006, 08:32 PM
That's probably because the bike is pulling evenly through the revs,......good thing. Nutty.

p.s. You can't go to any TAFE and learn 2 stroke performance, read 'Two Stroke Tuning' by A Graham Bell and go from there. I learnt from Bede Parkinson, the godfather of kart racing and Tony Hatton.
where can u read it ? is it a book or something ?
is there one for 4 bangers?

nutty professor
8th May 2006, 04:41 PM
hey dude
i ride a kx250 2001, my bike when i first start it up when its cold it idles really high and when i rev it in nuetral it takes ages for it too go back to the high idle..but after i ride and its all nice and hot it idles alot slower and sumtimes cuts out because its idleing so slow....is that normal?
catch ya

Yer, it's normal, the high revving is the aromatics in the fresh air filter oil burning off. nutty.

nutty professor
8th May 2006, 04:44 PM
where can u read it ? is it a book or something ?
is there one for 4 bangers?

Yer, there is a 4t version of Bells book. You can also read 'Tuning for speed' by Phil Irving. You will find them in a big building called a library.

yz125_troop
8th May 2006, 09:07 PM
hey man who can i contact other than the honda shop to get my bike goin hard (jetting etc)

luke

Roasted
24th September 2006, 06:10 PM
hey nutty just wondering if you could help me out i have a 05/06 cr125 it runs super good on track my mate jetted it when he had it btu when i go to freestyle or jump it hates low rpm's and fowles plugs! as long as i keep it in high rpm it is fine but just half throttle actiono is no go!

could you sugest any thing or a complete carb set up for me thanks!

p.s i also have a pc shorty fitted!

nutty professor
29th September 2006, 06:48 PM
hey nutty just wondering if you could help me out i have a 05/06 cr125 it runs super good on track my mate jetted it when he had it btu when i go to freestyle or jump it hates low rpm's and fowles plugs! as long as i keep it in high rpm it is fine but just half throttle actiono is no go!

could you sugest any thing or a complete carb set up for me thanks!

p.s i also have a pc shorty fitted!

It's a needle or pilot issue man, re-jet it and get the correct needle/pilot set-up. Not up on 125s atm,sorry.

azzfox
11th November 2008, 01:09 AM
Hey nutty if your still around this area of the forum, whats your knowledge on jetting 97-98 CR250's? I was reading through a service manual and it confused the hell out of me. It appears to have a needle, main jet and air screw as standard, but also a slow jet, power jet and solenoid valve (correct me if Im wrong). WHAT THE SHIT. Once I get the thing running again I wouldnt mind having a bit of a play around with it, where would one start on this contraption? :p

Cheers.

nutty professor
11th November 2008, 05:12 PM
Hey nutty if your still around this area of the forum, whats your knowledge on jetting 97-98 CR250's? I was reading through a service manual and it confused the hell out of me. It appears to have a needle, main jet and air screw as standard, but also a slow jet, power jet and solenoid valve (correct me if Im wrong). WHAT THE SHIT. Once I get the thing running again I wouldnt mind having a bit of a play around with it, where would one start on this contraption? :p

Cheers.

The SJ is built into the carb and never needs changing, the PJ is connected to the SV and drops fuel flow off at high rpms for more overrev. It shouldn't need touching either. the SV can be disconnected if you jet the main jet to compensate for it. All the factory bikes at the time left the PJ/SV system inoperative.

patch1234
11th November 2008, 05:15 PM
hey nutty reckon u could do a thread on four stroke jetting?? ( i searched and couldn't find one... :p )

azzfox
11th November 2008, 05:23 PM
The SJ is built into the carb and never needs changing, the PJ is connected to the SV and drops fuel flow off at high rpms for more overrev. It shouldn't need touching either. the SV can be disconnected if you jet the main jet to compensate for it. All the factory bikes at the time left the PJ/SV system inoperative.

Cool, cheers for that!