All Things Moto! Dirtbike Forums

Go Back   All Things Moto! Dirtbike Forums > Dirt Bike Knowledge Base and Build Threads > 2-Stroke Dirt Bike Knowledge Base

Reply
 
LinkBack Thread Tools
  #1  
Old 09-17-2004, 08:05 AM
mtk's Avatar
mtk mtk is offline
Pro Class
 
Join Date: Apr 2004
Last Online: 01-24-2008 08:41 PM
Location: Finleyville, PA
Posts: 1,630
Default Baseline jetting info: '02 CR250 with ProCircuit or Fatty

     

Hey everyone,

I recently acquired a 2002 CR250 that came with a FMF Gnarly pipe and ProCircuit Shorty silencer on it. Since the Gnarly pipe is advertised as a "low end" pipe that, by their own admission, takes away some of the top end hit, I decided to check out some other alternatives.

Thanks to the wonders of Ebay, I've now got an FMF Fatty pipe, Power Core 2 silencer, and Turbine Core 2 silencer to test out on it. I also acquired a ProCircuit Platinum pipe to go with the Shorty silencer. On top of that, I've also got the stock pipe and silencer to play with, if I so choose, but I admit I'm partial to the bling bling of the nickel-plated pipes over the black stocker.

I opened up the carb and the bike was running a 410 main, which explains the very poor top end performance it had with the Gnarly pipe (it was clearly WAY rich on top, judging by all the raw fuel in the pipe), the stock needle, and a 30 slow jet (stock is 32.5), which probably also explains why it didn't foul more spark plugs on me (lean slow jetting and way rich main jetting). I ordered a jet set from Honda (12 different needles, 4 slow jets, and 12 main jets) so I'd have plenty of jets to play with, rather than nickel and dime myself to death buying a few jets at a time and making this process drag out for months. The bike also has V-Force reeds in it.

What I'm looking for is some baseline jetting suggestions for the bike with the FMF Fatty/Power Core 2 and also with the ProCircuit Platinum/Shorty combo. The Turbine Core 2 will only be used when I need a spark arrestor, in all likelyhood. I realize that set-ups vary due to lots of factors, I'm just trying to get a decent starting point and save myself a few iterations.

And yes, I know the hot ticket is probably to get a Keihin PWK carb from a 2000 model, but I've got to beat my head against the wall for a bit with the stocker before I jump ship to the Keihin. I'm stubborn that way.

Thanks for the help.

Last edited by mtk; 09-17-2004 at 08:08 AM.
Reply With Quote
  #2  
Old 09-20-2004, 11:51 AM
Mecrazyperson's Avatar
Banned
 
Join Date: Nov 2003
Last Online: 05-12-2008 04:26 PM
Posts: 1,302
Default Re: Baseline jetting info: '02 CR250 with ProCircuit or Fatty

Hey MTK, Well here is what I run in my 02 CR250. I have V-force2 reeds and FMF fatty/shorty combo. I run Red lines premix at 40:1 and a Denzo iredium spark plug. "Yes the spark plug helps alot" I have my jetting stock except for the main is at a 400 main and 2 1/2 turns on the fuel screw. depending on your elavation and oil you might want to run different jets then me. I would start with a your main's first, then start changing pilets if you dont get a good setting. Just keep in mind 380 is the stock main.


P4L Good luck with the bike, I love mine you should like yours im sure.
Reply With Quote
  #3  
Old 09-20-2004, 03:28 PM
Faded's Avatar
ATM! Pro Class
 
Join Date: Oct 2003
Last Online: 09-25-2019 01:17 PM
Location: Idaho... No, U-da-ho.
Posts: 3,394
Default Re: Baseline jetting info: '02 CR250 with ProCircuit or Fatty

mtk,

A quick search showed that you're @ approx. 1240ft above sea level. I've found that elevation, fuel/oil type, premix ratio and weather all affect the CR's Mikuni carb more so than "other" carbs. My first piece of advice to you (if you haven't already) is to set the float level. Stock spec is 15mm. I'm sure you know how, but if not a search here on ATM will help you find a post on how to set it.

A few quick thoughts on this carb to help fill you in. The triple taper on the needles sucks. They make for a bad transition from just off idle to around 1/4 throttle and again at 3/4 to WFO. Most end up with a rich pilot to compensate for a lean spot around 1/8-1/4 when the other circuits are dialed which results in weak bottom end, but a screaming top end. The rest end up with a main jet that is too rich to compensate for that same bog which helps keep the low end in check, but makes for terrible top end performance. Yes, the main jet on this carb can have an affect all the way down to just off idle (just below 1/8 throttle).

The optional needles for this carb come in two different "families". You'll have the 6BEY30-XX and the 6BEY31-XX families. The difference is that the 31's are ½ clip leaner than their respective ‘30’ counterparts. The last two digits in the needle number designate the straight section diameter. I.E. - The stock 6BEY30-74 has a straight section of 2.745mm. The larger the straight section diameter of the needle, the leaner it will be. The taper and length of all the needles are the same. In all reality all those needles are only going to be good for tuning @ 1/4 throttle.

You do have another variable if you choose to use it. Honda doesn’t specify anywhere in their literature, but the needle jet, or nozzle (not to be confused with the jet needle) is removable. The stock needle jet is an S-9. If you’re looking to lean out or richen around 1/4 throttle while not affecting the main jet strength too much then you can use a different needle jet. Most have good results going to the leaner S-8 for an improvement in the midrange and low-to-mid transition. The needle jet is the part that the main jet is threaded into and can be removed with an 8mm socket with or without the main jet in place. Yamaha has assigned part numbers to a couple different needle jets, or any reputable carb parts house should know what you’re talking about if you desire a different one.

On mine I ended up running a leaner pilot, a fatter needle, the S-8 nozzle and the appropriate main jet. My bike, elevation, etc. are vastly different then yours, so my specs really won’t help you much, but hopefully some of this information will.

Last edited by Faded; 07-19-2007 at 07:07 AM.
Reply With Quote
  #4  
Old 09-20-2004, 06:48 PM
Novice Class
 
Join Date: Aug 2004
Last Online: 06-09-2007 11:33 PM
Location: Mentor, Oh
Posts: 55
Default Re: Baseline jetting info: '02 CR250 with ProCircuit or Fatty

Faded,
I have an 01 with the Mikuni, do your references to jetting and issues with the transition take place on the old style engine as well?

I just bought the bike and haven't really started dialing in the jetting, but right now it seems a little flat on top, even with the SST/Shorty combo (and vforce).
Reply With Quote
  #5  
Old 09-21-2004, 08:21 PM
mtk's Avatar
mtk mtk is offline
Pro Class
 
Join Date: Apr 2004
Last Online: 01-24-2008 08:41 PM
Location: Finleyville, PA
Posts: 1,630
Default Re: Baseline jetting info: '02 CR250 with ProCircuit or Fatty

Faded,

Thanks much for the info. I actually ordered one of the S-8 "nozzles" from a YZ125 when I ordered my jet collection. I'll check out the float level bit before I start swapping jets and pipes. Thanks for the input.
Reply With Quote
  #6  
Old 09-22-2004, 11:16 AM
Faded's Avatar
ATM! Pro Class
 
Join Date: Oct 2003
Last Online: 09-25-2019 01:17 PM
Location: Idaho... No, U-da-ho.
Posts: 3,394
Default Re: Baseline jetting info: '02 CR250 with ProCircuit or Fatty

Quote:
Originally posted by mossman
Faded,
I have an 01 with the Mikuni, do your references to jetting and issues with the transition take place on the old style engine as well?
Yes, they seem to be affected as well. I'm not up to par with the needle #'s used on the '01's like I am the '02's though, so it would take some looking into.
Reply With Quote
  #7  
Old 10-11-2004, 02:50 PM
mtk's Avatar
mtk mtk is offline
Pro Class
 
Join Date: Apr 2004
Last Online: 01-24-2008 08:41 PM
Location: Finleyville, PA
Posts: 1,630
Default Re: Baseline jetting info: '02 CR250 with ProCircuit or Fatty

Faded,

I've had a chance to get the bike out a few times since I last posted and have been playing with the jetting a bit. It's getting better, but it is still far from perfect. What I could really use is a "quick & dirty" listing of what tuning parameter does what. For example:

=======================
Float Bowl Level: Proper float bowl level is 15mm. This MUST be set correctly prior to making any jetting changes.

Main Jet: controls fuel flow from 1/8 to full throttle. Stock 380.

Slow Jet: controls idle-speed mixture and 0-1/8 throttle. Stock 32.5

Air Screw: fine-tunes idle-speed mixture and 0-1/8 throttle.

Needle Jet (Nozzle): Controls mixture at roughly 3/4 throttle. Stock is S-9, "optional" S-8 nozzle is leaner in this region.

Jet Needle (Needle): Controls mixture from 1/8- 3/4 throttle. Stock Needle is 6BEY30-74. Optional needles are 6BEY31-xx and 6BEY30-xx. The "30" or "31" indicates ________ while the "xx" indicates ___________.

Slide Cutout: Controls mixture at _______. Stock slide cutout is 4.0. The 2003 CR250 uses a 4.25 slide cutout, which is an option.
===========================

The biggest ones, for me, are the slide cutout effects, the nozzle effects, and what all the different needle numbers mean. Since I'm sure I'm not the only one who's been puzzled by such things, I'm sure this would be a prime addition to the Knowledge Base.

Thanks much for your help.
Reply With Quote
  #8  
Old 10-11-2004, 04:19 PM
ProAMA725's Avatar
Pro Class
 
Join Date: Sep 2004
Last Online: 03-25-2011 01:09 PM
Location: Big D, Texas
Posts: 1,262
Default Re: Baseline jetting info: '02 CR250 with ProCircuit or Fatty

Try these couple of things. It may help alittle. This is kinda how I jet my bike.
Step 1, put it in neutral, let it idle for a second, and then crack it wide open. If it hesitates, your pilot or airscrew needs adjustment. It should go from idle and rev without any hesitation.
Step 2, cruise around 1/4 throttle for a couple of minutes, and then crack it wide open. If it hesitates, it is lean, if it takes off it is perfect, and if it boggs, trying to clean out the engine, it is rich. That can be partial airscrew, needle and pilot.
step 3, put it in first gear, go wide open all the way up to top rev. You will feel it do something. If it wants to flip over on you, it is probably pretty close on the main jet. But the main thing, is in first gear you will feel the problem much more. If you feel the bike top out, and it falls off the power, you are rich or lean. It should rev out without really feeling a fall off in power. If it just goes, and runs out of juice, but no drastic drop in power, or feels like you hit the brakes, it is probably pretty close.

Obviously there is much more involved, but those things will give you a starting point, if you are way off.
Reply With Quote
  #9  
Old 10-12-2004, 07:50 AM
Faded's Avatar
ATM! Pro Class
 
Join Date: Oct 2003
Last Online: 09-25-2019 01:17 PM
Location: Idaho... No, U-da-ho.
Posts: 3,394
Default Re: Baseline jetting info: '02 CR250 with ProCircuit or Fatty

Quote:
Float Bowl Level: Proper float bowl level is 15mm. This MUST be set correctly prior to making any jetting changes.

This will also affect all jetting circuits. The volume of fuel allowed to exist in the float bowl will dictate what is basically fuel pressure. The more fuel present the greater the pressure which will result in richer jetting.



Quote:
Main Jet: controls fuel flow from 1/8 to full throttle. Stock 380.

The main jet is most effective at WOT, but in these Mikunis its presence can be felt as low as 1/8 due to circuit overlap. It won't have as much of an effect as the other components in the slower circuits, but understanding circuit overlap is a step in the right direction.



Quote:
Slow Jet: controls idle-speed mixture and 0-1/8 throttle. Stock 32.5

Spot on.



Quote:
Air Screw: fine-tunes idle-speed mixture and 0-1/8 throttle.

The air screw will also have an effect on throttle response during large transitions in throttle position. Not a big deal, but something to be aware of.



Quote:
Needle Jet (Nozzle): Controls mixture at roughly 3/4 throttle. Stock is S-9, "optional" S-8 nozzle is leaner in this region.

Needle jet effectiveness starts @ about 1/8 to 1/4 tapering off after that.



Quote:
Jet Needle (Needle): Controls mixture from 1/8- 3/4 throttle. Stock Needle is 6BEY30-74. Optional needles are 6BEY31-xx and 6BEY30-xx. The "30" or "31" indicates ________ while the "xx" indicates ___________.

The "30" and "31" indicate the physical location of the clip positions on the needle. The "31" family of needles is 1/2 clip position leaner (the positions are machined 1/2 position closer to the top) than its respective "30" counterpart. The last two numbers on the needle indicate the straight section diameter of the needle and have the greatest effect at 1/4 throttle, but the circuit overlap will extend up until the main jet. Once the needle has been lifted up and out of the way then the main jet will be the primary metering component. The number on the needles, 74 (in the case of the stock needle) means that the needle's straight section diameter is 2.745mm. Sadly the taper of the needle, taper location and length of the needles is all the same.



Quote:
Slide Cutout: Controls mixture at _______. Stock slide cutout is 4.0. The 2003 CR250 uses a 4.25 slide cutout, which is an option.

The slide cutout controls the mixture mainly @ 1/4 throttle, and like the air screw will have an effect on throttle response during large throttle transitions. This is an expensive option especially when there are other tuning components to use when tuning for 1/4 throttle. Needle straight section diameter and clip position can be considered as substitutes. By varying the straight section diameter you're allowing more fuel past the needle as soon as you lift the slide. Also, by lowering the clip position and raising the needle you're allowing the needle taper to be exposed at a smaller throttle opening which will also bypass more fuel and richen thing up.


Hope this helps.

Last edited by Faded; 03-22-2006 at 02:15 PM.
Reply With Quote
  #10  
Old 10-12-2004, 08:25 AM
mtk's Avatar
mtk mtk is offline
Pro Class
 
Join Date: Apr 2004
Last Online: 01-24-2008 08:41 PM
Location: Finleyville, PA
Posts: 1,630
Thumbs up Re: Baseline jetting info: '02 CR250 with ProCircuit or Fatty

Andrew,

That's a big help, thanks much.

Michael
Reply With Quote
Reply

  All Things Moto! Dirtbike Forums > Dirt Bike Knowledge Base and Build Threads > 2-Stroke Dirt Bike Knowledge Base


Thread Tools


Similar Threads for: Baseline jetting info: '02 CR250 with ProCircuit or Fatty
Thread Thread Starter Forum Replies Last Post
1987 CR250 Re-jetting Carb. With FMF Fatty pipe? luvvnlife 2-Stroke Dirt Bikes - Honda 3 01-26-2010 06:09 PM
fmf fatty jetting? rockymtnclimber 2-Stroke Dirt Bike Forums 4 08-11-2008 10:57 AM
89 YZ250 Jetting baseline cruzmystar 2-Stroke Dirt Bikes - Yamaha 0 07-28-2008 01:01 AM
More jetting info. 2fun General Discussion 1 01-17-2005 04:26 AM
ProCircuit Pipe for 2004 CR250 gene19 2-Stroke Dirt Bikes - Honda 7 01-31-2004 10:31 AM


All times are GMT -6. The time now is 08:41 PM.


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright ©2002-2019, JDub Consulting, Inc. All Right Reserved.