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Andrew_in_OZ

FCR carb tuning

69 posts in this topic

I found this site on fcr tuning it has some great info on fueljets, air jets, needles & emulsion. check it out it will give you a better understanding of what is happening in your carb at different throttle positions and will help in getting almost if not perfect jetting. www.lifenet.com/brm/carbkei.htm :)

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Andrew

"00"WR400

YZ timing,EKN needle, VORTEX dualcurve ignition

SF triple platinum s/plug

open box -unifilter

98oct fuel

Staintune s/s open exhaust & header......

Somebody stop me !!!

[This message has been edited by Andrew in OZ (edited 03-02-2001).]

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andy

it's a good article & one, to be fair, JD put up a few weeks ago.

i've never touched my floatbowl either...

Taffy

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check this out

(URL=http://www.lifenet.com/patrick/dyno.txt) www.lifenet.com/patrick/dyno.txt (/URL)

bits on dyno work & on MAJ's for the FCR carb

****** hell hick it worked!

Taffy

[This message has been edited by Taffy (edited 03-02-2001).]

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Sorry JD, not trying to take credit for finding this site, I know a lot of you will have already seen it and I didn't see your post untill Taffy pointed it out. Just trying to help others who may not have seen it

------------------

Andrew

"00"WR400

YZ timing,EKN needle, VORTEX dualcurve ignition

SF triple platinum s/plug

open box -unifilter

98oct fuel

Staintune s/s open exhaust & header......

Somebody stop me !!!

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Taffy,

What are your thaughts on this one, In the above manual the float hight should be set at 9mm and in my "00"owners manual says 8mm slightly richer, also the float valve size for a 39-50 hp/carb should be a 3.2 yet my manual says 3.8 even richer again, do you think this could be causing any jetting problems for a range of different aplications, for eg in one of your previous posts you explained how tuning up the engine required leaning the jetting. your experience and thaughts are always apreciated

------------------

Andrew

"00"WR400

YZ timing,EKN needle, VORTEX dualcurve ignition

SF triple platinum s/plug

open box -unifilter

98oct fuel

Staintune s/s open exhaust & header......

Somebody stop me !!!

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the carbs are designed to work at their best around a common or middle jet. i would suggest that the 165/170 MJ is about the middle. we're operating up at 170/180 so they need the fuel.

clark & others have changed their float height & reported no problems & on one of my only visits to the KTM TT site they were saying that the fuel economy was improved on the 400/520 with this mod.

the fuel valve is something else though. 2.8 i can understand, but 3.8!!!!

there is usually a reason for everything so have a good think on it over a while, you know-chew on it! & see what comes out.

i got that strange surge on my sixth run & the above can be added as a question mark.

this is most definately a case of "you go looking for trouble &..."!

i would only want to run a mod off the back of a bike that had remained untouched for a while & i knew well for all it's glitches & cough's while jumping logs & rocks etc.

why don't you buy a fuel valve & change the float height & see what comes out?

you'll need to run it agterwards 'as is' just to be sure.

Taffy

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Andrew,

The float valve needs to be bigger when there is not a fuel pump pressurized fuel supply. The YZ/WR's are gravity feed and near the upper end of the HP range for a 3.2mm. When riding thru the whoops or over big rocks it might be a factor though. A lower float height by 1mm would help. My '89 RM had a TMX with this problem, rocky sections made it run rich.

As the article says, 3psi feed on a larger float valve area would "overpower the floats bouyancy, flooding the fuel bowl". Thus the "not recommended" status for 3 psi feed.

Taffy,

Good article you posted, thanks. Look at paragraphs 4 and 5 in the response. He talks about the main air jet (MAJ) and "the FCR main air correction circuit really sucks .... if you're already at a #200, you might have to pull the main air jet right out... . (and so on)

---

You mentioned drilling out the #200 to a 220 and I have been thinking the same. There have been alot of indications on this for the YZ426 and the E-- needles. Bigger mains (172-175) pull good in the mid-upper RPM range, but smaller mains rev better (165).

So, I took out the MAJ and went to a bigger main - #172. It feels like its BOTH pulling harder AND revving harder!!!

Would someone else please try it too? Am I just imagining this? :)

James

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if this train leaves town without me i'm gonna kick your arse (ass)!

can't get there soon enough JD.

Taffy

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JD,

I pulled out the MAJ and drilled it to a #250(2.5mm) the air gallery in the carb is approx 2.2-2.3mm limiting the jet to approx a #225, I've used a #170 MJ(was#168), std EKN in clip 4(was 3) also changed PJ from #45 1 3/4 turns to #48 1 turn out, AP 1.5mm stroke, untouched start-stroke.

1st run slight miss then roars to 1/4 throttle then a slight stumble at 1/4 and power comes on strong to WOT (minor rough "rich" feeling from 1/4 to near 2/3 throttle). from WOT to redline is much much better than before.

2nd run changed to clip 5, pilot screw 3/4 turn, closed to 1/8 better (no/minimal miss felt)but stumble sill there at just before 1/4, 1/4 to around 2/3 throttle felt strong but rougher, from there to WOT to redline is fantastic(same as first run).

tried clip 3 and turned in AP stop screw 1/4 turn (approx 1.25mm stroke) no missing at all, good smooth power (pick up not as good as clip 4 or 5) I am un able to do any more runs today (goto go out) I,ll do a couple more runs tomorrow with different MJ's and clip posie's, I'm borderline on the #45 & #48 PJ's, with Taffy's pilot air screw (which I will order)I should be able to get the #48 working better than it already is. sugestions welcome

------------------

Andrew

"00"WR400

YZ timing,EKN needle, VORTEX dualcurve ignition

SF triple platinum s/plug

open box -unifilter

98oct fuel

Staintune s/s open exhaust & header......

Somebody stop me !!!

[This message has been edited by Andrew in OZ (edited 03-07-2001).]

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Andrew,

The passage behind the jet was the same on mine too, about 2.3mm. Doesn't do much to drill the air jet bigger, so I just took the air jet out. Was considering cleaning out the passage next. It looks like they don't drill it cleanly through.

Suggest you stick with the #48 and 3/4 turns on the pilot screw using clips 4 & 5. With clip #3 the pilot screw can be a little more out, 1 - 1 1/4.

Where was the stumble at in clip #4? about 2/3 throttle?

Looking forward to next results. :)

James

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Andrew in OZ and James Dean,

Pardon my ignorance if I am making a stupid suggestion.

The main air jet is not the pilot air jet, correct?

If this is the case and your bike still has the #75 pilot air, and you are deciding on a 45 or 48 pilot, then maybe the 100 pilot air might be a better choice with the 48 pilot.

Yes?

Where is the main air jet anyway.

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The main air jet is a #200 and is located right next to the pilot air jet.

The bigger #100 pilot air is a good idea with the #48 pilot jet. The #45 could go either way from my experience, with a #75 or #100 pilot air jet.

[This message has been edited by James Dean (edited 03-05-2001).]

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JD,

the stumble was at 1/4 throttle in clip #4, turning in the PS to 3/4 turn out and AP stop screw in 1/4 turn(approx 1.25mmstroke)cleaned it up nicely.I found #3 clip a little lean but #4 a bit rich so today i put the #168 main in and needle on clip#4 and it absolutly screamed from closed to WOT but is not quite as strong from WOT to redline which is OK cause VERRY VERRY rarely the bike is ridden at over 11,000 rpm (red line is 11,800rpm with my setup) to solve this though I could go two ways (1)buy a #220 MAJ or (2)cleanup/drill main air gallery to a true #230 or #240 flow and use #170 MJ & clip #4. Most likely the 2nd option but it can wait till i fit the pilot air screw.

------------------

Andrew

"00"WR400

YZ timing,EKN needle, VORTEX dualcurve ignition

SF triple platinum s/plug

open box -unifilter

98oct fuel

Staintune s/s open exhaust & header......

Somebody stop me !!!

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Andrew and James,

So if I follow this discussion correctly the main air jet affects the 1/2-3/4 throttle more and the main jet influences the 3/4 to WOT more?

Am I reading this info correctly.

Andrew I am using the EKN#4 with the 168 main, and while it is very early in my testing to talk about the 3/4- to WOT aspects of the set up. My thoughts were similar (maybe?) that the power tapered off at the upper end. I wasn't sure if this was just because it isn't a good idea to blast down my neighborhood full throttle or the power was so great in the 1/4 to mid range that the upper just didn't feel as impressive.

Thanks for posting your results.

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John,

You need to read the articles posted by links above.

The Main air jet is for near or at full throttle. It and the main fuel jet can be used to get more torque under load below peak RPMs and still get max HP at higher RPMs. The 2 jets need to work together.

The author suspects the fuel comes out in blobs if not enough air is coming through the main air circuit.

Andrew,

Thanks for the clarification. Moving the needle 1 clip should not be much of a factor at full throttle. Meaning the main should be selected with no impact down lower.

They don't sell a #220 main air jet. #200 is the biggest. If you go too big boring the passage, a #200 could be drilled to size as needed.

For a reference, I have a half dozen 38-40mm Mikuni needle jets that measure 2.5-2.55mm at the main air inlet. Very close to your option 2. :)

James

[This message has been edited by James Dean (edited 03-05-2001).]

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Greetings gents..... been following the conversation you all have had over the last month or so on this jetting... I have a 2000 Wr, yz timed ekp needle #4 position, 45 pilot, and 172 main, white brothers e series exhaust with the hi-flo header, I run no disks and just an open end cap, anyways I yz timed my bike on friday of last week, and boy what a differance! it pulls hard and the front wheel will come off the ground clear though 4th and even into 5th, I did get it up to 5th gear WOT and it did seem that I was not building as much power, could this be caused by not having the 100 pilot air jet (I have one on order it will be here next week as well as a 48 pilot) will the 100 air and the 48 pilot improve things? (Iam in spokane James D) and in your previous post James you stated that you removed the main air, does that mean you have actually removed it completly? if so would you suggest I try this? I would be more than happy to try these mods you all are fooling with... anything to make more power!

shane

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shane

so you've been following the posts closely for a month?

& you're willing to try things?

i think you should have bought the PAJ-SCREW & saved yourself a load of money ready for the changes about to be made?

all

last night i had the carb down as i can't work on anything else at the moment (the engine is away). you can drill the MAJ hole out by using a drill that is 34mm proud of the chuck. take your ET out first of course!

Taffy

[This message has been edited by Taffy (edited 03-06-2001).]

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shane,

Yes, the #200 pilot MAIN jet was removed completely. The hole behind it is not that much bigger.

The #100 will not make a noticeable difference at WOT.

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So based upon reading the posts. It seems like at WOT and higher revs there is a rich condition.

Correct?

Main air jet:

If you find that the engine runs perfectly at WOT near the torque peak, but becomes lean toward redline, select smaller main air jets. Conversely, if the engine runs perfectly near the torque peak but richens toward redline, select larger main air jets

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The rich condition is a product of looking for the torque peak and ending up with a bigger main jet.

Example:

With a #200 MAJ and #165 main jet the performance may be perfect at WOT & redline.

However, it takes a bigger main to get WOT to perform best at the torque peak. (#172 possibly)

There will be a rich condition with a #172 main jet toward redline unless the bigger main air jet is used. Thus a need for a #220 or #230 MAJ.

Problem is they don't make one. :)

So we come up with a work around. :D

Ever wonder why NONE of the FCR carbs out of all the latest thumpers use anything smaller than a #200 MAJ? and if they could have gone bigger would they have?

James

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