RB-modified head . . . for Race Gas!!

Questions and Answers about the best carb and Head mods available for the KDX.
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RB-modified head . . . for Race Gas!!

Post by Jeb »

Couldn't wait 'till tomorrow, so I snuck away and went riding to put the RB race gas head to the test.

No airbox lid, RB'd carb, RB race gas head, rev-profile pipe, PC S/A
148MJ, 38PJ, CEK w/clip @ #3; A/S range between 2&1/8 and 2&1/2 generally . . . started with a clean air filter and repacked silencer. Retarded timing just shy of 1/2 the distance between middle and top mark (I didn't measure how much that was and the degree of change was somewhat arbitrary). 12/47 gearing. 50/50 VP 110/premium with premix @ 32:1.

Rode hard for most of four hours at "the farm". 90 degrees and humid. The smiles outweighed the sweat . . .

Biggest differences: Noticeably quicker to rev, and the higher revs were more meaningful. Right about 4-5k the revs would come on fast. The power would drop off @ maybe 10k (that's a guess) but it would pull all the way up to that point. I would be approaching landmarks - jumps, logs, turns, etc - and would already be topping out in the gear that I normally wouldn't. First few times I it gave me the impression that I was in a gear too low and I shifted up, only to find out I was going faster, almost too fast. On some of the field edges I could rip through all 6 gears and the bike pulled STRONG through them, especially noticeable difference was 4th through 6th. I could feel each gear pull stronger than the first, awesome pickup. Exhilarating!!

Midrange power is flat out awesome. I found myself switching down a gear where I would usually roll on the throttle and lug around, just to feel the improved snappiness. On several occasions I found myself having to stop in a hurry.

Low end seemed strong - pulled plenty hard - but not as significant of a change. Seemed a bit fat off idle so I dropped the CEK to #2 later and it helped.

SSS - easier to find: more prominent and wider (at least I think I was there, I need to fiddle some more to make sure). "Good" range seemed to be roughly 2&1/8 - 2&1/4 (yeah, about an 1/8th wide)

Throttle response - off idle was improved with the needle clip change, everywhere else it was superb! I exaggerate not to say that as long as I wasn't in a gear too low, a snap of the throttle meant a snap in the bike's revs. Granted, that was there with the premium-gas-cut RB head (w/ carb). There's no denying that it's more prominent now. More wheelspin, and the back wheel was wanting to come out easier around turns.

Jetting: I did a chop while there and after I cut my plug up when I got home it shows I could drop the main a size. I might change the main for this Thursday's ride unless it cools significantly.

The bike idles differently now, a bit rougher . . . what ever that means!

Oh yeah, NO spooge. I'm tempted to say it's the clean packing - and that certainly helped - but I've changed packing before and had spooge after a relatively short period of time. NO spooge - interesting given that jetting could be leaned.

Things to try:
-Fiddle with jetting a little more.
-Different gearing. I'd never miss first gear if it disappeared (didn't climb any steep hills, though). I'm going to try stock gearing and if that doesn't suit things I'll try a 13/49 combo.
-Retard the timing a little more, just to see.
-I might fiddle with the race gas % some more but I'm going to wait. What I've got seems great for power/throttle response.
-Gotta spend some more time with the suspension. I need to revalve up front and respring in back. I felt the difference with the increase in speed.

Bottom line: power was improved everywhere, particularly midrange. I believe the timing change helped to get more out of the higher RPMs. When I rode the bike this past week after installing the race gas head the low end seemed a bit more punchy than it did today, and I believe the timing change was responsible for that. HOWEVER, the low end is STILL stronger . . .

I'm thoroughly impressed with Mr. Black's work.
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Post by 2001kdx »

I think stock gearing is a good idea. I might have a twisted view on this now because of my current ride, but I had my KDX geared 12/48, and I notice I was shifting all the time. Sure, I had a granny first that would climb a tree, but the fact is is I could rip through a section without shifting as much, like I could use 3rd all the way through, whereas with 12/48 I'd hit fourth in a hurry.

"The power would drop off @ maybe 10k "

Wow, that's a wild guess :lol: If I had to guess, I'd say the 200 w/Rev would be out of steam way before 9500RPM, and you have a 220.

Never ran race gas in any of my bikes. Just can't justify all the extra $$$ for the type of riding I do. Happy testing!
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Post by fuzzy »

It's a beautiful thing! After messing with karts I'm going to have that done to all my 2-cycles during rebuild time. I never ride where I'm not getting gas out of the trailer so it's no big deal on availability. It's not that much more either(especially since I ride about didly squat these days)....Especially when bought in 5-gal steel drums...Just mix ONLY what you need. I usually mix 5gals of 93 fromt he pump, run half, and then run the rest in my tractor after it sits around for months. :neutral:
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Post by canyncarvr »

It's good to hear good things about the little green machine. Ron's work is a treat...I get a big kick out of just looking at it when I open a boxful!

Re: '(I didn't measure how much that was and the degree of change was somewhat arbitrary).

You had asked about this earlier..I was going to answer..got sidetracked yakking about something else.

Figure the degree of change (if you care) with simple math. Measure the size of the stator ring, pi times diameter, divide by 360.

Like:
5" diameter stator ring
Pi D gets you 15.708"
Divide by 360 to get .043"/º

Point of reference: 1/16" is .062"

Figuring where you started from is more problematic. Timing spec is given in mm BTDC (piston, not stator) or a given degree figure. The KDX 200 spec sheet lists the 'H' model as: 21º or 2mm BTDC.

So...hook up a good inductive timing light (the stator/coil type ignition on the KDX probably won't fire most series lights), make a mark at 6K (if you don't have a tach..the 21º is a max figure before the KIPS drops it)..measure piston postion with a dial indicator in the spark plug hole, see if it matches up.

From the spec sheet you know that 1mm of piston movement = about 10 1/2º. You could make some marks on the case/rotor that way, too. 'About' meaning piston movement isn't the same throughout a revolution, but as long as you're measuring things within a mm or two from the top, it'll be pretty close.

Fact is, it would be a good idea to time the bike 'correctly' in the first place. Just 'cuz the spec is 21ºBTDC..who says MaKaw set yours up that way?

Mine? Ha! I dunno. I've advanced it, retarded it..set it to where I like it. Where is that? Uh...I dunno. :wink:

ps
Check all the math to suit yourself. Right or wrong..the general tack is correct.

Too bad the ignition coil isn't mounted outside the rotor. With the KDX you have to be taking the flywheel off everytime you want to change sumthin'.

Check that woodruff key when you do! It'll slip out the 'back' of the rotor and you won't know it if you don't check that it stayed stuck in the crank.

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Post by fuzzy »

Too bad the ignition coil isn't mounted outside the rotor
Indeed...They should all be like that. Maybe I'll convert the WR over to a PVL Magneto...LOL. Got a few laying around.
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Post by canyncarvr »

The Banshee's mounted outside. Pole gap adjustment? A snap! Timing adjustments till require rotor removal..but wouldn't have to with the coil outside ('natch).

..then...the cover would have to be bigger...

Yeah..put your 13 CSS back on! With enough power, you will STILL be in too high a gear with even more speed when you come up on one'a those 'landmarks'.

Then we'll hear complain'n about how you need a larger rotor and larger pucks in your caliper to haul yur butt down!

Well...after you get the suspension sorted out.

One good turn deserves another!! :wink:

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Post by Jeb »

Great details/info on the timing piece - thanks!

I have another reason to be interested in the timing, specifically how the KIPS changes things:

I've ordered/received the spring that you find in the exhaust advancer assy. The idea is to measure the spring rate and then acquire spring(s) of similar proportions but with different rates. I'd like to try a spring that's less stiff initially in an attempt to get the KIPS to open a little earlier.

Note that I'm NOT referring to the detent spring at the end of the rod that moves the valves. If I go through with this I'm gonna keep that unchanged with the expectation that the detent spring's resistance will continue to give the valves their quick-acting response once things start moving down in the advancer assy. So the hope is the exhaust advancer starts opening at a different (lower) RPM with it's spring being different, but the detent spring continues to do what it does now (I hope this isn't too confusing).

So I've made the measurement and I've found a couple of sources of springs that should work. In fact, a relatively small reduction in the spring's rate (5%) should net a 500-600 RPM drop (roughly, not actually linear but at that small of a change it's linear-like).

My concern (and thus my hesitation) is when the timing changes in relation to the preferred KIPS opening timing. Since timing change @ 6k would not coincide with the KIPS opening I'm not sure what to expect. Any potential harm to the motor? Any ideas on how performance would change? I don't imagine there's a way to modify the change in timing, but is there an aftermarket device with programmable or adjustable timing?
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Post by IdahoCharley »

>|<>QBB<
Jeb wrote: My concern (and thus my hesitation) is when the timing changes in relation to the preferred KIPS opening timing. Since timing change @ 6k would not coincide with the KIPS opening I'm not sure what to expect. Any potential harm to the motor? Any ideas on how performance would change? I don't imagine there's a way to modify the change in timing, but is there an aftermarket device with programmable or adjustable timing?
I don't think the change your discussing is going to do any harm to the motor. KTM models for years have had adjustable power valve settings without the ability to change the timing curve. The (1998 ~ 2004) 250/300/380 models had an adjustable PV setting based upon a screw adjusting the spring pressure either increasing or decreasing the PV spring's preload. (Sounds very close to what your attempting IMO)

The newer 250/300 come with a couple of optional springs which helps you 'select' the point where PV opens. Again no change in ignition curve is associated with the changes in PV opening.

PS You asked if there was progammable or adjustable ignition curve timing. There is a couple of companies that make them; although I'm not aware of one for the KDX. (I believe Factory Connection is one supplier of the ignition curve boxes.)
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Post by fuzzy »

You really just want the ign set for the fuel your using, and then forget it...Minor changes can be made to fine tune for the environment, but experts need only apply, and the result is only MINOR. IMO, I would fine-tune to get the most low-end on C-12, then never touch it.
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Post by canyncarvr »

Re: '(I hope this isn't too confusing).

Perfectly clear. Not everyone here rides a summer squash or punkin.

Timing can be relatively easily changed with the use of a different timing capacitor in the CDI. The problem is getting to it!

From HERE:

jaguar wrote:]Just looking at the schematic (see my other post on CDI repair) I'd say the basic design of this CDI tends to cause it to advance the spark timing more as revs go up until around 6000 when the timing capacitor doesn't have enough time to discharge completely before the next cycle and the effect is that above 6000 the spark timing starts to retard (or at least stop advancing) which is really needed.
For reference, this from jag:
Image


He's talking about the 4.7uf electrolytic.

I suppose it's 'best' to have the two (pipe effect and timing) happen at the same time..they being related. Retarded timing transfers heat to the pipe, enhancing pipe effect..at the same time flow (pressure) and volume adjustment (Helmholtz resonator 'off') takes place.

Don't know that separating the two would actually damage anything. Combustion pressures should increase a good bit at effective 'pipe time' with the scavenging/supercharge effect..but the air fuel mixture won't. As long as sufficient octane fuel is used to prevent detonation that could be associated with increased cylinder pressures and advanced timing, you will be fine.

I would expect a power dip if the pipe/timing got too far apart..yanno...like the given lousy response when the KIPS is busted? Not exactly the same issue, but an inference is there. As little as you would be spreading apart the coincidence of the two, I doubt you would notice it.

...Ha. How do you 'spread apart' a coincidence? It's either concident or it's not, ain't it? Once 'spread' it, it ain't one no more!

Still...you get the idea? :neutral:

Well....I do!! :wink:


Re: Aftermarket CDIs

I don't know of one for the KDX ('cept jaguar's). You might call these guys. They may know of a cross that will work. Yeah...fat chance of that, but they DO make programmable ignitions for a lot of stuff.

Have fun! ...and share the wonderful new improvements!

**edit**
And whilst I was searching, linking and prognosticating...fuzzy beat me to it! Yeah...what he said. Didn't take him long, did it...... :wink:

Oh...note post #40 in the above thread..an Electrex (likely) hot rod sparky box.

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Post by fuzzy »

Very cool info on the CDI.
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Post by canyncarvr »

I've got one'a jag's modified sparky boxes. Have yet to mess with it...the wire colors are different, so are the connectors. I don't know for sure what goes to what.

I 'spose I'll get to that someday..........

Maybe he'd sell you one, Jeb. He's back on the street...somewhere.

M0rie might know where he's at.

I mean...other than right before the at....



Oh...note in the afore posted link that Jag figured the timing cap to be saturated around 3KRPM. I hadn't read that far when I wrote the 'coincidence' part above. From what Jag writes, I'd say changing the relationship 'tween the spark and the KIPS 'timing' (ha! Isn't that funny!!??) won't make a speck of matter damage-wise. Notwithstanding fuzzy's comments on fuel suitability.

If you didn't read through the above thread...spend some time doing so.

For those that don't know...jaguar is a sharp cookie. He messed around a good bit with his bike (a KDX)..epoxying ports, I think at one time messing with his own sleeves..all in the name of making things work on the other end of the planet (south...WAY south...I mean almost Antarctica south..). He's a good gringo!

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Post by Jeb »

More great stuff on this thread, thanks . . .

rode again yesterday (Thursday) pretty much all day (9-to-5) and a good bit of the riding was low-RPM crawl-speed. STILL no spooge dripping out (I see a tiny bit on the inside of the discharge end) so the spooge-less phenomena seems to be holding true whether I'm rippin' or gruntin' with this head. And to reiterate - throttle response is fine. I'm inclined to say it's improved over the pump-gas-cut head but I think what I'm detecting is the increase in power . . . maybe. I'm not sure to be honest, but I AM sure that I'm a satisfied RB-designs customer. 'Not sure how many times a grin popped up on my face as I rode but my lips are chapped today for some reason.

The snap is fabulous!

Think I'll give a slightly less spring a try keeping all else constant at least at first. I'm measuring a spring rate on the stock piece of 48 kg/mm. IMO it's fascinating when you consider that 4 small steel balls generate enough centripetal force to crack the advancer assembly open and compress this quite stiff spring by something like 1/2"! Interesting how mom nature works. Anyway, I was a bit off in my initial thinking about how RPMs would change with spring rate (got my squares back-asswards). Here's some better figures (48kg/mm = 27 lb/in BTW):

27lb/in ---> 6,000 RPM (or whatever stock ends up being)
25lb/in ---> 5,775 RPM
23lb/in ---> 5,550 RPM
21lb/in ---> 5,175 RPM

Please note this is an estimation and unproven as of yet. I'm going to obtain 25lb/in, 23lb/in, and 21lb/in springs as best I can. It will not be exact so I'll measure them with my high-tech methodology. I'm interested in the 5,500 or so mark and thus select a spring accordingly. I'll tach w/ the stock spring and then remeasure after changing the spring. I'll use the handy-dandy "observation port" on the right side of the motor to ID KIPS movement timing. Should be relatively straightforward - I already have a mechanic-helper lined up (my 7 yr old has taken an interest!).

'May be a while to get to it to order springs, etc.

I'm may also going to follow up with the CDI stuff eventually.
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