Explain RB carb mod for me?

[skipro3]

"relatively little condensation of the fuel which does pass through the exhaust system."

Wow! I never thought of it like that; if the unburned fuel doesn't condense (because the pipe is warm enough to keep it vaporized) then it can't spooge but instead leaves the tailpipe as vapor, not liquid. Hmmmm
But if the unburned fuel is vapor, then why didn't it burn up in the combustion chamber?
Could there be improvements in spark duration or timing?

"The first ss with the CEK on the boy's 200 is not a big hit as it is currently set up."
I agree, it's not so much as a "hit" as it is an extremely sensitive throttle control at just off idle up to mid rpms. That's at any ground speed for me anyways. The result is; The throttle feels much like a light switch when traveling over flat ground, either on or off, but on hills where there is a load on the motor, it's very much an appreciated performance characteristic. A slight crack of the throttle takes most minor grades, and up to 1/3 throttle or so will suffice for greater demands. If I find I need more throttle than 1/2, then I'm probably in the wrong gear and need to downshift. Please consider that most, meaning almost all, my riding is single track with speeds typically under 25mph.

"think how much fun my 130 Lb son will have!!!!"
14 is not too old to give the old man a hug for the effort you are putting into his ride. Heck 24 (my oldest son's age) is not too old for that matter!

" until your satisfied nothing is better for the riding style you have."
Isn't that the main trick. Of course the riding style constantly improves if one is riding as often as they should.

"If the bike starts to lug down and then the engine's power drops off very fast I'm lean normally in the needle."
That's exactly where I was before CC advised me to move the needle clip. Huge difference. I'm glad to hear someone else describe the same sensation.

BTW have you tried a flywheel weight with the KDX or the other bikes for that matter?

[IdahoCharley]

Skipro - "Could there be improvements in spark duration or timing?" I believe this is the nature of the 2-cycle beasts with carborators. Some unburnt fuel will pass through the system. IMO - Trying to jet so close that you eliminate all the excessive fuel that passes from the cylinder and through the system may have you burning down the engine. Too many variables with the engine control systems we are talking about on the KDXs.

Concerning the hug from the 14 year old - I agree. Also my older boys (28 and 25 yrs of age) better give me a hug and thank you as my reward for working on their houses or toys. LOL

Concerning Flywheel weights - Have installed flywheel weights in the past (94 WR250, 98 380 EXC, 01 CR 500, 00 YZ 250) and have removed them from my personnal rides. i.e. 94 wr 250 and the 98 380 Exc. I really would rather jet/pipe/gear for the riding we do and keep the snap of the 2-cycle engine. I'm sort of a horsepower junkie and when the trail opens up, or there is an area to play, I really want the quick snap and screaming rpms If I were to race the 380 though I would add additional flywheel weight because it would make it less tiring and centainly make it easier in mud, roots, and other slick areas. IMO at 5000 foot elevation and above the KDX 200 feels fine with the flywheel it has. I watch the boy ride it later this season and get his input but if it was my main ride I would not add weight.

[IdahoCharley]

[[/quote]

IC, Call me a stick in mud but its still not clicking for me. How are you verifying that your slide is cut to a 7? Inquiring minds want to know. Your descriptions of it having a harder hit before you recut the slide a second time make it sound like you cut the slide too far (well past 7) and are having to compensate with a overly large PJ. There is a lot of people who run a 7 slide with the RB carb mod and don't run a PJ near that large.

-Maurice[/quote]

Maurice - First let me clarify that I DO NOT have a RB carb mod.

To answer your question. I have a factory cut 6.5 and 7.0 for my 39 mm Keihin carborator. I just place the appropriate slide on a flat surface with the cutaway standing on the flat surface. Measure the depth of the cutaway with calipers. Then I cut the slide being worked on down to the same depth as the factory cut slide. And smoothly blend the depth into the existing slide cutaway area. You do need to keep the same angle on the cut but your depth changes while the overal width of the slide cutaway remains very close to the same. Make sense?

If you really want the exact numbers for the cut I believe that Sudco or Carb Warehouse use to have the numbers posted in their catalog. Are the hand made cuts exactly 6.5 or 7.0. NO!! It really may be closer to a 6.4 or 6.6 but what you are aiming for is really a carborator that works.

Does it really matter if you are a little off? The numbers are referenced more for communication of what works with various jetting. Back in the early 80's when I was riding a IT 400 and communicated that "I cut the crap out of the slide before I could get it to work" verses " I cut the Holy crap out of the slide" or "shaved about a 1/16 inch out of the slide". I think I stated in a previous post that I likely went past the 7.0 slide cut. I measured the first cut to a 6.5 but the second cut was just cut into the slide and I did not measure it. SWAG

All the carb circuits need to work together. Variety of ways to get there. There is nothing extremely magical about a slide cutaway other than how it relates to the combination of jetting it is being used with. Please note that many riders/tuners that ride the higher elevations run anywhere from 6.5 to a 8.0 slide. It seems like the higher riding in Colorado usually call for 7.5 or 8.0 slides. But then again it could just the combination of jetting that has become "normal set-ups" for that area. Hope this is food for thought!

[RBD]

IdahoCharley and everyone,

Slide cutaway;

Slide cut-a-ways are more than just hacking off material, although cutting on the slide will make a change to how it works. There how ever needs to be some accurate measured areas to maintain a consistency from one project to another or bike to bike.

Different slide cutaways have different angles as well and different heights of cut at the small "D" shaped area around the needle. The radius edge of the "D" shape is where the slide is measured.

If you set the slide on a flat surface and measure the height to the cutaway at the "D" shaped area, a # 5 slide would measure 5.5 mm. A #6 slide would measure 6.5 mm and a # 7 would measure 7.5 mm.

Also the angle of the radius cut of the cutaway would be at a steeper angle on the larger numbered cut away slide.

As you can see in the attached drawing the actual measurement of a cut away and where it is measured (it's not at the flat bottom part of the slide). Again, if it is measured from the flat surface to the radius of the "D" you need to add .5 mm and this is why a 6.5 will measure 7 mm .

Anyone confused yet? No it's not rocket science, but there is more to it than just getting out the Dermal or the old Rat-Tail file.

Cheers!
Ron

[KDXer]

Thanks for that explanation Ron...

[IdahoCharley]

RBD - I agree whole heartly with your assessment/description of the cut on a carb slide. If I were modifying carborators for others and wanted to send out complete units with proper jetting to go with the modifications the precision of the machine work would need to be consistent, accurate, precise, and reproducable. Anything less may cause the business to falter. The word of mouth on all the forums where your business has been mentioned all indicated to me that your stuff works well and you have lots of satisfied customers. Seems most of your customers believe that your products absolutely rock. (One of those satisfied customers (air screws/ head mod) is me FWIW)

However, for an individual working on his own bike who has time to tinker the exactness of the additional cutaway may not be as important. If he cuts it a little too large (like I did on the 200) he may have to go up on the pilot, go to a different needle, or replace the slide. If he cuts it a little less than intended he may have to make jetting adjustments to balance the circuit(s).

Do I dare say this? It may be possibly to even improve slightly on a published carborator combination/set-up for a particular bike at a particular elevation: Whether by chance or tenacity aimed at the project.

Many years ago I use to think only the needles, needle jets, TVs, main and pilot jets that were listed for a carborator would work. Over time and especially since so much information became available via the internet my opinions have drastically changed.

NOTE - Where I have measured or compared the slides (factor cut slide for a 39mm keihin carb to a hand cut one) has been with relative measurements. i.e. the reference measurement of .6 mm is added to the numerical designation on the slide is what we have been doing on the 39mm keihin slides for some time. Sounds like that is a wrong reference for the 35 mm slide and it should be a .5mm. And yes the angle of the hand cut is not exact or as uniform as a factory cut or precision machined slide.

Bottom Line - I probably have too much time on my hands.

[canyncarvr]

Idle away in gear is normally a good sign. If there is a bog ..... then the lower circuit may benefit from changes.

You should be able to tune the airscrew to a 'no bog' throttle response when you flick the throttle from off-idle to around 3/8s-1/2 or so, bike under load (moderate grade), in a bit too high a gear (say 2nd) from a fairly slow speed (walking pace). Most hesitations in such a case will be from a too lean low speed circuit, usually fixed with a small change to the airscrew.

An easy starting choke-cold bike changing to a PITA hot starting bike can usually be resolved with a tad tweak 'in' to the AS, too.

re: What works best when hot.

That IS where the riding is...when it's hot. Some folks deal with a bit too lean a pilot (runs great when hot...choke-cold start is a bit of a bother) by adjusting the airscrew for the cold start...changing it back to what works hot. Ron's finger adjustable airscrew is great for that.

I'm sure your kid appreciates the tuning you do for him. It sure makes riding a lot more fun when the bike is responding as it should.

Yeah..having the front end just sit there when you flick the throttle right before that log can be painful! ;)

That happened to me last weekend. Well..it wasn't a log, but a small bit of lake (you might call it a puddle...but 'lake' sounds better). I got a faceful of mud and bootful of water 'cuz the front-end didn't come up like it 'should have'. Of course...I was altogether in the wrong gear when I came 'round the corner and saw the puddle..oops..lake. I could've hit the brakes..which would have put me smack in the middle of the thing...probably lying down.

I settled for some mud up my nose.

Tinker away!! 'Ya never know what works the best until you try something that doesn't!

Cheers!

[IdahoCharley]

I solved my Air Screw problem yesterday!!

Took the carb completely apart (second time) and using a 5 power magnifier glass found my problem. The very small hole on the down stream side of the TV which gives the vacuum signal to the PJ was semi-blocked. The first time I checked the carb I sprayed carb cleaner through all passages and followed up with 100 psig air blast. This time I cranked the air up to 150 psig and still could not blow out whatever was caught in the passage and although it would pass carb cleaner it was partically blocked and would not dissolve. I had trouble finding a small stiff wire to run though the passage but finally found a small enough wire to push through it. That small passage is about 1/4 inch long but what a PITA.

Able to now drop the PJ to a 45 and the AS is working like it should!!

CC - Puddle is a LAKE when you end up in it: Stick is a LOG when it throws you over the bars: Drop-off is a CLIFF as you slide off it: Large rock is a BOULDER as it deflects you into a crash: Small gully is the Grand Canyon when you bottom so hard it knocks you off the bike: Small steep high traction Incline is a VERTICAL WALL as you loop out. Silt build up in a stream is QUICK SAND as you bike stops and sinks.

[RBD]

TIP for cleaning small jet passages,

Try using guitar strings, they work great. The smaller strings are small enough to push through the smallest jet passages and jets. They are also stiff enough to do the job.

You probably know some one that has and plays a guitar.... and they will no doubt have old sets of strings laying around as they brake all of the time. I'm sure they would be more than happy to you give you a old set for free

Ron

[Indawoods]

The string clippings should be good enough if your gettin' em from a friend....or the local music shop should give you all you want for free... just ask.
Strings range from .009 to .042 for a extra light electric set. Ultra lights range from .008-.042... They are cheap to buy too...from $5-$8 a set...should last ya a lifetime for that!

[m0rie]

Able to now drop the PJ to a 45 and the AS is working like it should!!

IC - Glad you found the problem. Having the AS working correctly is a good thing, especially at the altitude that your riding at!

-Maurice

[canyncarvr]

I'll sell you my brand new set of Ernie Ball Earthwood Lites for my 12-string!!

Twice the fun of a 6-string set!! (at only slightly more than twice the price! Unused only because I spent the money to fix the split bridge and broken nut (OW!!) on my dirtbike!!

That's a great tip!! Thanks Mr. Black.

IC: Great!!! Man...you gotta feel good about that!!! Great detective work!!

Great tech tips, tool advice, anecdotal fix-its...all in one place!!!

You guys are good!!!

..no wonder I have such a hard time riding...all my encounters are the CAPS ones-a-those!!

[IdahoCharley]

Ron thanks for the tip. I have a boy that plays a guitar and I never even thought of those strings.