Carb Technology 2-Smokes vs. 4-Strokes

[WR400_Jay]

Ok guys maybe I'm getting a little far out there in my quest for the ULTIMATE ? setup on my 00'WR400 but every little bit counts.

Clark you might have some insight on this one.

You can change the strength of the signal the carb is receiving from the motor by changing the spacing between the carb and the motor. In the world of 2-smokes the closer you move the carb to the motor the harder the "hit" the motor has to offer. The motor comes on hard and revs quickly. Great for Motocross. Move the carb away and the signal strength gets weaker and it reduces the hard hit characteristic and tends to build more torque and low end power.

Will increasing the space between the carb and the motor via a spacer change how the motor performs on a thumper? Will it produce more torque? Has anyone tried this? I remember back in my muscle car days longer runner intakes always made more torque and pulled real hard off idle.

I noticed and article on Jimmy Button's bike in the latest issue of Dirtbike Mag and they placed a magnesium spacer on the intake side of the carb. What for?

What do you think??????????????

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00'WR400 75'Yamaha 250 enduro

White e-Series S-bend 12 discs, Yz timed, rejetted to Clark specs, throttle stop cut, lid removed, Topar Racing top clamp, Tag Metals T2 bars, Scotts damper, Devol disc guard and frame guards, lights removed, Cr routed Fastline brakeline, Acerbis Rally pro hand guards fastened to the top clamp, Gel grips Live in Southwest Ohio

[Mitch_from_Oz]

Jay, my guess, (just a guess guys.. ?) is that the carb/motor is more succeptible (is that the right spelling)to air velocity, or rather dead air velocity.

If you increase the size of the Airbox such and lengthen the funnel effect into the carb, then that is how you modify how the bike performs...

Anyways, i am getting the Doug Henry Airbox and Velocity stack this week and will be installing it on Saturday my time. All the carb settings will need to be changed. I will give a full report on it by the end of the weekend as I will be practising for the super motard series this sunday at a road circuit.

Mitch

[MotoGreg]

Our carbs (Keihin FCR) when used on roadrace bikes have different length adapters (the part between the carb and manifold) the shorter it is the more top end, the longer it is the more it shifts to the bottom. They also sell three different length velocity stacks, same thing, shorter for top end. F1 cars and some of the top factory Superbikes have velocity stacks that shorten mechanically as the rpm rises.

Just like everything else on a four stroke, it's a compromise. Intake length and shape, port size, valve size, cam lobe specs, exhaust, ect... You can put the power wherever you want it, the key is getting a good spread of power of a big rpm range and that is why there are things like EXUP, VTEC, variable valve timing, tapered exhaust headpipes...

If you have a big carb, ports, valves, cam lobes, and a large diameter exhaust pipe you can make as much HP as you want. Just don't expect any power until you really get some rpm going. You need intake and exhaust VELOCITY, that is the key.

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'99 WR400

'92 GSXR 7/11

Visit my photo album AT YOUR OWN RISK!! My photo album

Anyone here a sportbike fan also? Then visit us here at www.insanespeed.com

[RodH]

Considering that the early development of the YZ/WR400 engine had a large amount of input from the F1 engineers, it's my opinnion that Yamaha would have close to the optimum set-up, and all we do with our re-jetting and fiddling is getting it right for different pipes, altitudes etc.

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RodH Canberra, Australia<A HREF="http://www.400thumpers.oz-au.com" TARGET=_blank>

400 Thumpers Australia</A>

2000 WR400F See Photo's and Modifications

[Clark_Mason]

MotoGreg has hit the nail on the head and I do not believe that Yamaha has the otpium setting for all conditions. I believe that Yam did a magnificant job of estimating the world wide conditions but it can always be improved upon for your particular local conditions. I have proved this many times across many manufactures of equipment. You can through testing good note taking and a little general understanding always improve on stock performance. Now admittedily I have taken this personal passion beyond the average and spend a lot of time and personal resources on these topics but I feel I have reported the facts as I see them based on my particular riding conditions.

Clark

[Matt_Porritt]

The FCR on the 98-99 was taken directly off a roadbike and put on the YZ/WR.

The '00s have an updated vesion which preforms better.

In a short answer.. YES.. changing the distance between the carb and barrel will effect things to an extent..

BUT not as much as changing the dead air volume.

You will not get alot for changing the distance.. but a larger dead air volume will give more torque and will feel like an extra 50cc on the bottom.

***MITCH- Are you getting you airbox in aussie??

If you havn't already paid for it, I'll do one at COST for you!

Send us an email anyway..

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--

**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Rubber Chicken Racing

[jj]

Hey!!!! No fair!!!! (I learned that from my 7 year old)

JJ

[James_Dean]

I had a very interesting discussion with a Tech support rep at Sudco Int. (Paul) a week ago about Keihin FCR and PJ/PWK carb jetting. His perspective was that Yamaha grabbed onto th ZX7R streetbike jetting when they were developing the YZ400 and used it as a baseline to start from. The accelerator pump was designed to feed 4 barrels with 4 nozzles and the YZ/WR single pump nozzle doesn't flow that fast. This makes it continue to flow a couple of seconds longer than originally designed. They limit the pump stroke to avoid wasted fuel by shimming the diaphram with a washer when setting up carbs for top level racers. They are the primary source for aftermarket carbs in the country (I believe). He seemed surprised that I was finding better performance with more carb needle taper, and was unaware that the YZ426 was using this currently, unlike the YZ/WR400's.

We discussed the use of single, double and triple taper needles in 2-strokes also. This is an area where the FCR is going to evolve in the future. 2-strokes use triple taper needles to "load" extra fuel in the lower cases at low settings and gives that instant snap that thumpers can't match. They don't feel an accelerator pump can match the 2-stroke response due to the drop in air velocity when the throttle is wicked open. I would suspect additional tapers will help in addition to accelerator pump changes.

From my perspective,Yamaha has been pretty consistent with YZ jetting but not with WR's. The settings have been up and down with the non-California/ Europe/ California/ models. Clark has landed very close to the middle of all this with the DVP needle but richer pilot and main. For those looking to maximize performance you should look at the jetting change the YZ426 got despite the fact it is a FCR2. My '00WR makes more power with an EKP needle clip#4 consistently thruout the throttle range than any other setting I've found after trying a spectrum of settings. Having the airbox lid off and getting more air won't do much unless you get the fuel to go with it. I proposed this 2 years ago to Karel Kramer at Dirt Rider and it was ignored (probably round filed). Now the YZ426 runs it standard.

Suggested setting is - EKP#4 or #3 and #172 main (Sudco has equivalent EMP#5 or #4)

James Dean

[James_Dean]

See you soon Jay!

[MotoGreg]

They limit the pump stroke to avoid wasted fuel by shimming the diaphram with a washer when setting up carbs for top level racers

This sounds like the same idea as the P-38 lightning. How thick of a shim did he mention? Maybe I'll try this out to see if I notice any difference, it's a free way to experiment without spending the bucks for a P-38.

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'99 WR400

'92 GSXR 7/11

Visit my photo album AT YOUR OWN RISK!! My photo album

Anyone here a sportbike fan also? Then visit us here at www.insanespeed.com

[James_Dean]

He didn't say the shim thickness, I'll check back with him. Any other questions for him?

[WR400_Jay]

Thanks for the feedback guys.

Mitch You have to let us know what happens once you install the new airbox and velocity stack and also what your jetting changes are with the airbox and then with the stack.

Hey James Dean! I'll be out your way tomorrow night (21 June) I'll call you when I get there. I'm sure we'll talk about this more indepth over a couple of glasses of the finest hops and barley. I might be making some spacers before the next ride.

Thanks Guys!

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00'WR400 75'Yamaha 250 enduro

White e-Series S-bend 12 discs, Yz timed, rejetted to Clark specs, throttle stop cut, lid removed, Topar Racing top clamp, Tag Metals T2 bars, Scotts damper, Devol disc guard and frame guards, lights removed, Cr routed Fastline brakeline, Acerbis Rally pro hand guards fastened to the top clamp, Gel grips Live in Southwest Ohio

[Clark_Mason]

As I have posted before this is exactly what the P-38 does. 1) it shortens the stroke of the diaphram shortening the the duration of the accelerator "squirt" and 2) it has larger oriface leaving the accelerator pump bowl making for a stronger but shorted duration "squirt" vs stock.

Clark

[This message has been edited by Clark Mason (edited 06-21-2000).]

[Clark_Mason]

James Dean

I thought the 426 was using a EVR needle and a smaller main jet? What have you found out?? Maybe e-mail me.

Clark

[WR400_Jay]

Clark I was out here in Seattle last night with James Dean drinking some suds and talking about jetting. He has a great chart made up that covers all the aspects of the different needles used in this carb. The 00'426 uses and EKR needle which is the same as the EVR but 3 clip positions richer. The E taper needle is the one I'm going to try next. I'm still having what seems to be a lean problem farther in the rpm range.

James Dean was pretty cool. He had me meet him at the local bike shop in Renton before we went out to eat. That really broke my heart having to look at new bikes.

James is a class act and was very helpful. If your ever in Seattle you should look him up

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00'WR400 75'Yamaha 250 enduro

White e-Series S-bend 12 discs, Yz timed, rejetted to Clark specs, throttle stop cut, lid removed, Topar Racing top clamp, Tag Metals T2 bars, Scotts damper, Devol disc guard and frame guards, lights removed, Cr routed Fastline brakeline, Acerbis Rally pro hand guards fastened to the top clamp, Gel grips Live in Southwest Ohio

[MotoGreg]

I used to know a James Dean that rode sportbikes and I think ocassionally raced at Willow Springs... same dude?

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'99 WR400

'92 GSXR 7/11

Visit my photo album AT YOUR OWN RISK!! My photo album

Anyone here a sportbike fan also? Then visit us here at www.insanespeed.com

[James_Dean]

-This James Dean has always lived in Seattle...

-MotoGreg,the fuel pump shim was approximately .5mm thick according to Sudco. Might be worth a try.

-Jay was fun having a beer with, too bad we couldn't of had that pitcher of Red Hook after a days ride though. Maybe next time.

-And as of today another WR has YZ cam timing. Yes, mine too!

-Found it interesting that I didn't need to loosen the intake cam cap or lift the intake cam to shift the exhaust cam timing. Just moved the exhaust cam sprocket adjacent to the intake sprocket and had enough slack to shift the 1 tooth. The sprocket has 32 teeth, and makes for an 11 degree change on the cam or 22 degree change on the crank. This seems like alot!

-The change in cam timing seems to have richened the idle circuit slightly. Turning the pilot screw in from 2 1/2 to 1 1/2 with the #45 pilot/#75 air jet seems to have cleaned it up and brought back the response. Extended riding will tell. The midrange with the YZ426 carb needle rips. I will continue giving feedback on the performance of the EKR and EKP carb needles and compare to 400 carb settings.

James Dean

[This message has been edited by James Dean (edited 06-25-2000).]

[Kevin_in_New_Hampshire]

James,

I am surprised to hear you mention Red Hook. It doesn't seem possible to have made it all the way up to the NW corner of the U.S., but we have the Red Hook Brewery just down the road from me here in New Hamphire (please know that it is pronounced New Hamp-shuh). Could it be the same brew?

Do you have any idea how the miracle needle would work on a '99? You are riding a 2K bike, correct? I could always figure out jetting on my 2 strokes, but I will have to admit my WR just has me wondering. I was hoping to find a performance shop to bring my bike to. I wanted to put it up on the O2 or exhaust monitor or whatever it is. I heard that our carbs will give an inaccurate reading on the monitor and won't work to correct our jetting.

Kevin

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99 WR, all YZ mods, de-octopused, OEM YZ tank and seat, jetting by Clark, fork job by Pro Action.

AMA, NETRA

For clarification: WE'LL be having a KICK ASS time in Moab!! Right Mitchy & Bill?!!

[James_Dean]

Kevin,

The '00WR came with a DRS which is nearly the same as the YZ400 DVR. Both carbs are using the same components so the tuning results should carry over with slight adjustments.

The reason I dwell on the taper is that all the D-- needles have very flat taper profiles. Only .75 degrees. This makes the needle fat at the tip and interferes with main jet tuning. At full throttle the diameter in the nozzle is roughly 2.35mm and the nozzle itself is 2.90mm. The resulting difference in area for the fuel to flow is less than a 175 main jet!!(1.75mm diameter, you can do the math too) The fuel must pass through both the main jet and nozzle/needle which means both are influencing full throttle jetting. It's no surprise that main jet selections are made all the way up to 190 for YZ/WR400's, the needle becomes the primary restriction and not the main. Could just take out the main jet all together. Did it myself to see, rode 20 miles no problem, just doesn't rev out as well.

Looking at 3/4 throttle is where my emphasis is because the transition in this area is far more lean than it needs to be. It makes for smooth power but lacks the torque and response that it could have. There is no magic here, just common sense and experience. Thumpers don't give the feedback of 2-strokes with sputter and ping. They just slow in their response either way. I just test and test and test... knowing exactly which way I'm going at each throttle position by graphing needle diameter versus throttle position. It works great but still takes time to find the boundaries of rich(slower) and lean(slower too). I'd really like a few others to try the 426 needle, it's only $12. I just ordered another to double check the code,

more will follow,

James Dean

[Kevin_in_New_Hampshire]

I'm all ears James. Looking forward to your results!

Kevin