Non CV Carbs for the 950 Adventure?

[funandadventure2002]

Directly Actuated Slide Carbs for KTM 950?

Two things to chew on and throw into the discussion:

1. Has anyone yet adapted directly actuated slide carburetors to the KTM 950 motor? Direct slide carbs are not as smooth but are much more responsive than CVs. Perhaps there is a shortage of down-draught choices that could be adapted. I would like to consider this.

2. What do the desert race bikes use? CVs? Bigger ones? Or fuel injection?

03 950 Adventure S, "Karl der KTM"
92 KLR 650
00 Super Sherpa
96 R/T Yamaha
72 Duc 350

[KTMax]

Originally Posted by funandadventure2002

2. What do the desert race bikes use?

The stock CV carbs...

I'm pretty sure the LC8 will become an unridable beast with slide carbs and that this is the reason KTM doesn't even change them for desert racing with 'reasonably skilled hands' at the helm I would say...

[Goss]

I am no carb expert but I thought I have heard many times that a CV carb needs fewer changes or tweaks to deal with altitude. If that is true I would stay with the stock carbs so I do not have to make changes when going to higher elevations.

Goss

[kevbo]

CV's at altitude:

I've never lived below 4,000'MSL. I've never seen a factory jetted CV machine that wasn't fat (and usually way fat) at altitude, just like direct slide carbs...even ones that are notorious for being "overly lean to meet EPA"...or whatever. Note that High altitude jetting kits are offered by some OEMs for thier non-EPA certified (off highway only) CV equipped machines. (quads, watercraft, etc)

Because every carburated bike I've ever owned was jetted too rich, over the years I've put WAAAAAY too much thought into what makes a carb good at altitude....which I will now subject you to. Lets see if I can explain this without setting off Rattus Chopper's propeller head alarm...or at least with no math:

A CV carb controls mixture well over a broad range of RPM and throttle opening, but does nothing to compensate for air density. That said, CV carbs DO offer advantages vis. altitude variations. Niether a contridiction, nor a fine distinction.

The venturi part of a CV carb is virtually identical to a directly actuated slide carb. (there are several substantially identical versions of each, though I don't think there is a completly flat slided CV made) This is the part of the carb that establishes the mixture, and the intake air doesn't care if a cable or a diaphram is holding the slide up.

There is no reason a rider with nothing better to do couldn't connect a vacuum gauge to a direct-acting carb, and maintain the vacuum at a constant level....that wouldn't control power at all, but up until the crash, it would deliver the same mixture as a CV carb would have.

A CV carb limits the venturi opening to maintain velocity through the throat area. This provides more accurate mixture control....bogging (going lean due to excessive throttle at low RPM) is, in particular, prevented.

Without this improved mixture control, a direct-acting carb needs to be jetted fatter because going lean causes more rideability issues than going rich...The required rich jetting hurts emissions and fuel economy, and THAT is why OEMs have all but abandoned direct acting carbs in applications that require EPA certification.

Therein also lies the advantages of a CV carb at altitude.

1)They didn't need to be jetted too fat at sea level, so they
have some wiggle room between stoch. and way too rich as you climb that pass. Both get richer, but the DA had a head start.

2) The lower air density decreases mass flow, much as reducing engine displacement would. If the carb was on the hairy edge of being too big for the engine at sea level (and with HP wars, most bike carbs fit that shoe) then it will be much easier to bog the bike at altitude, even at mid-range rpm. A CV carb won't allow that. If the carb is oversized, it will compensate by never fully opening. A plug check will still show that the mixture is fat, but you won't have bogging related rideability issues.

CV's vs direct acting: pros and cons:

As noted above, the "guts" of both are essentially the same. The CV carb automatically prevents the rider from bogging the engine.

In other-than-thumper applications, CV carbs have an extra variable working to disturb cylinder balance...spring tension on the plunger. Doesn't seem to preclude thier use though. Apparently it isn't too hard to get the springs matched well enough.

IF a direct acting carb is jetted a bit fat, then a skilled rider can get a wee-bit more torque by just starting to bog the engine making it go lean just enough to offset the the rich jetting. This provides slightly more "responsivness". If the carb is equipped with an acceleration pump ("pumper") then this is greatly enhanced, and the machine can really "jump"....tricky to get it all set up right though, and better to error rich than lean, so you can forget EPA certs.

The CV carb also has a butterfly valve cluttering the passage downstream of the venturi...which can allow a skosh more top end IF the DA carb is sized at the bleeding edge of being too big.

Note that on $$$ HPN ralley bikes, CV carbs are universally fitted, even on oilheads. Apparently, in a $$ no object project, the CV has advantages. My guess is that they give the rider one less thing to worry about (bogging) allowing a little more concentration on the terrain or not getting lost.

Reliability: I've personally had 3 CV diaphrams fail, fixed one for a fellow stopped on the roadside (bandaid from first aid kit). IMO this is the achillies heel of the design, and the only thing I don't really like about them...otherwise they are an elegant way to make a variable venturi carb work quite well. That said, throttle cables are less reliable than the diaphrams, and common to both types...and you can still "limp home" even with a badly ripped diaphram (ain't no fun though) and maybe other brands have better diaphrams than the bings I've ripped.

Getting the jetting "close enough" is probably easier with a CV....getting it "optimal" is probably easier with the DA (easier to know which jet is active when) There is a fairly broad range of mixture where an engine will "run good".....it may have poor economy, or less than optimal power, but it isn't unpleasant to ride.
With a CV carb, you just need to fall somewhere in the "happy zone". Because a DA carb allows the mixture to vary quite a bit, it can step out of this happy zone if it's mixture variation isn't well centered....and if it is over sized, the variation may exceed the "happy zone" even when well centered. Another point for CV's with the OEMs...bolt them on and call them good, no need to fuss with jetting on each machine.


Note that I didn't say one is better than the other. When set up correctly, both work quite well. When set up wrong, either will make you lust after the other type. I lusted after flat-slide mikunis for my airhead for a long time. When I eventually understood the ins and outs of CV carbs, I was able to get the bings working right, and became faithful to them.

One final thought: If you stick with the OEM carbs there will be a wealth of experience from other owners and tuners about how to make them behave. Your dealer will have parts, and won't be able to blame the non-OEM carbs when the bike isn't running right, and you can't figure out why. It can take a fair bit of futzing with jets to get a new carb (no data for that engine) just to the point of "not too crappy" If your not racing, do you really want to do what it takes to milk out the few% gain that you MIGHT get by swapping carbs?

[merlin]

[Flanny]

Really well put.

Overall, I don't see why anyone would look for more power, or even more response from the 950 if it is being used off road at all....

As it comes from the box, it is already a handfull off-road to get the back end to really hook up.

So far, my riding has greatly improved using the 950 because it ABOSLUTELY REQUIRES smooth control of the throttle for anything even remotely resembling traction management. I can;t imagine what it would be like with a pair of pumper carbs for example...it would be an uncontrolable beast off-road, and would ruin it.

Beyond the airbox mod (to smooth power=good thing), I would forget about engine mods alltogether.

The key to getting more performance out of this bike (if that can even be a requirement) is to focus on weight and suspension in my view...

[Boon Booni]

Thanks for subjecting me to that Mr. Kevbo


That was very informative...

[Goss]

Good Lord. Where have you been all our lives Kevbo ?

That kind of knowledge is what makes this site what it is. Now back to your books.

Goss

[Wayne Weber]

Originally Posted by kevbo

CV's at altitude:

I've never ?

Great post, I learned a few things for sure. We've always debated going to direct pulls on the Rally Twins, but now I think we'll stay w/ CV's!

We ride from 5000 to 13000 feet regularly, so the ability to adapt to altitude really matters to us.

Thanks for the download kevbo!!!

[funandadventure2002]

Thank you kevbo! That will take a while to digest. I will stick to the CVs also. I would suggest in the unlikely event of someone running a 950 Adventure in a Mod Prod roadracing class, direct actuation slides would be a good option. So far I know of no such efforts out there.

[kevbo]

Thanks for the positive feedback.

[Stuka]

You just picked the meat of my life. Having built several thumpers and Japadenso street bikes I would have to say for overall performance, the flat slides have it hands down. They, equipped with 'pumpers', will, when properly tuned, blow away both fuel injected engines and CV equipped engines with gusto. My preference is for the Keihin units, they come in two flavors, 39mm and 41 mm venturi sizes respectively. I'm VERY sure that Keihin is hard at work building the appropriate apparatus to join together two 39mm Keihin Flatslide FCR carbs for the Baddest Ktoom of them all. Lets see, 950/2=475cc per hole, which translates into a 39mm carbs, #47 pilot, a #172 main jet, tune with appropriate needle for your power delivery needs and WALLA, You have just fed your bike and easy 15 to 20 horsepower. Add the pumper units and these gizzies will absolutely astound you with responsiveness and smootheness of operation. The BIG downside in setting up the pure flatslides, read non-electronically controlled ones, is that they offer little to no altitude tolerance. They absolutely enjoy 29.92Mb of pressure @about 52 degrees to be optimum. I have noticed, read guys riding the Yamaha WRF450's, that these Yamaha flat slide Keihin units are electronically controlled for mixture, so essentially they are electronic CV's using flat slides and they are far more altitude tolerant than the pure Keihin FCR series of flat slides. Racing at altitudes above 5000' agl was a night mare with my XR660 as I had to extensively re-jet and re-needle the carbs. But in the end, and if I had my druthers, I would opt for the performance package of Flatslides and keep the CV's on the bench ready to go if the BIG TOUR ever popped up. IMHO!!

[Desmofan]

I converted my DRZ400S from it's puny stock 36mm CV to a 39mm Keihin FCR ("E" model carb), quite a boost in power, but mileage went WAY down

The idle circuit is a PITA too...

[funandadventure2002]

Thats the kind of response I knew must be out there somewhere! Years ago I exchanged the 26 mm CVs on both of my GS (750 and 1000) Suzukis with 29 mm direct slide units. It was the easist bolt on horswpower you could ever have asked for!

[Pilbara]

Originally Posted by Stuka

You just picked the meat of my life. Having built several thumpers and Japadenso street bikes I would have to say for overall performance, the flat slides have it hands down. They, equipped with 'pumpers', will, when properly tuned, blow away both fuel injected engines and CV equipped engines with gusto. My preference is for the Keihin units, they come in two flavors, 39mm and 41 mm venturi sizes respectively. I'm VERY sure that Keihin is hard at work building the appropriate apparatus to join together two 39mm Keihin Flatslide FCR carbs for the Baddest Ktoom of them all. Lets see, 950/2=475cc per hole, which translates into a 39mm carbs, #47 pilot, a #172 main jet, tune with appropriate needle for your power delivery needs and WALLA, You have just fed your bike and easy 15 to 20 horsepower. Add the pumper units and these gizzies will absolutely astound you with responsiveness and smootheness of operation. The BIG downside in setting up the pure flatslides, read non-electronically controlled ones, is that they offer little to no altitude tolerance. They absolutely enjoy 29.92Mb of pressure @about 52 degrees to be optimum. I have noticed, read guys riding the Yamaha WRF450's, that these Yamaha flat slide Keihin units are electronically controlled for mixture, so essentially they are electronic CV's using flat slides and they are far more altitude tolerant than the pure Keihin FCR series of flat slides. Racing at altitudes above 5000' agl was a night mare with my XR660 as I had to extensively re-jet and re-needle the carbs. But in the end, and if I had my druthers, I would opt for the performance package of Flatslides and keep the CV's on the bench ready to go if the BIG TOUR ever popped up. IMHO!!

Hello Stuka, I was searching the net for 41mm FCRs and came upon this thread. I am finding as I get to know the SE better and subsequently going quicker that I can't get the response I need to turn the bike when in higher gears, hence I was considering ditching the CVs.

I am interested that you have mentioned 39mm. I was worried about going too big, I measured the hole in the CVs and it was 43mm, I estimated that the butterfly would probably equate to about 3mm and hence decided on 41 cause I can get some of those. I guess that what I am reading above you are of the opinion that 39mm will be good enough? Would not want to sacrifice any top end

I am thinking that my thinking would be better off with the smaller ones after reading your note.

Cheers
Pilbara.