more snap & 3/4 throttle response

28 replies to this topic
  • Taffy

Posted March 04, 2001 - 12:49 AM


thought i'd keep it going but make it easier for some with slow screens etc.

i got that info on the MAJ by going to the 'FCR tuning' & going to con rod length & reading "this" & "this" at the bottom.

JD, you were quick off the mark!

it seems that we only have to get the middle of the needle right & the rest can be done with a needle, air jet or fuel jet either at the top or at the bottom.

i would recommend a MAJ drilled out to 2 1/2mm or 3/32" (2.4mm) because old "dyno rhino" isn't going to be doing 10,000 miles like clarke is he? & a hole like that is going to be like a venis flytrap. something large & horrible is going to fall down there eventually!

i didn't come back at you the other day because your maths had been loose & mine was wrong. well i did leave school in 1977!

yes a larger ET will make a difference higher up the revs as would your 'mistake' of lifting the needle to help taper start.

the difference is that i get a longer taper by about three mm. so an EKT is still in the ball park. i've been given a price over here of £12 ($20) for the ET if i need it.

i will double check my snap starts & then see where that leaves the needle. i want to get the bottom end right THEN the top. i know, i know, the wrong order. but i'm talking only one needle clip + or - so it's not quite the same.

looking at your test results i think you're very close to a needle on clip No7 & APJ off completely. i bet you can't wait for that PAJ-SCREW! i think you've done well to run the bike without it.

the other thing that interests me is something i mentioned earlier;

i've spent this evening checking how far into the ET a needle sits. i measured my EFP needle & the straight sits 6mm down into the tight throat of the ET. the tight part measures 11mm long.

although a restriction is a restriction; if it is a very short restriction as opposed to a long drawn out one, more fuel must flow at tickover

standard ET
an EKT needle on clip 7 (or EDT clip 4)would sit 1 clip higher than my EFP in the measurements did =.9mm

the taper starts on a _ _ P & _ _ T are 1.71mm apart. add the .9mm form above & that's 2.6mm higher taper start than my armchair measurements.

so now we have an extremely tight fitting _ _ T needle but it's sitting only 3 1/2mm from the top of the ET. can i presume that this minimalist gap will still allow more fuel through over such a short distance?

a smooth tickover is always my goal.

tough one.


[This message has been edited by Taffy (edited 03-04-2001).]

  • James_Dean

Posted March 04, 2001 - 10:28 PM



In the latest FCR tuning thread Andrew and I both found the passage behind the MAJ (on the '00WR)to be only about 2.3mm. Drilling the MAJ bigger will not help more than removing it. Is the '99 the same?

The EKT#4 with a 2.95ET is so close to the EFM#5 or EFN#5 with a 2.90ET that it's hard to imagine anyone could tell the difference. The lengths will only be .9mm difference at the tip, nearly the same fuel flow area, and nearly the same taper start. Just my $.02

With a standard ET the largest (EKT) needle is certain to feel lean at 1/16 - 1/8 throttle despite the higher taper start. If the tapers relationship is the same as further down (as in EKP), the needle is just adding material in the fuel passage. The REALLY confusing part is when the leaner needle is used with a richer clip. Such as EKT#5 vs EKP#4.

My experience says the diameter closest to the nozzle crest is more important than length (11mm) along the nozzle below. What you are looking at in this 11mm is something that has been of much interest to me for many years. Tight woods riding at low throttle settings on big bores makes this a critical zone. It may be the upper half of this 11mm that is where the action occurs. I'm curious about your thoughts on this too.


  • Taffy

Posted March 05, 2001 - 02:56 AM


& i thought that the needle would sit considerably higher in the ET!

the 11mm throat in the ET is there as you know for a wide range of carb applications. the 5mm below & next to the tapering needle is doing nothing.

one can only guess at which point more fuel flows past a narrow gap as the needle is withdrawn. it would be very easy to do a home test using parrafin pouring into the ET from a measuring jug. liquid is as good as air in this scenario.

but what does it all add up to?

you could drill the ET from the top end down 4mm & you still have a 2.9mm throat; but it is now only 1-2mm overlapping the needle straight. touch the throttle & bam!!! instant taper & fuel!

i really have to say that i have the snap now & that maybe on saturday you got it. how can you have even more snap when it's already snapping!!!

the idea of the ekp on clip 7 THEN changing to EKT which starts the taper 1.7mm higher means that the needle is barely in the ET.

the MAJ tests sound to have gone well. well done!

there is clearly something wrong at keihin's end with the carb. our attitude should be as long as it works for us that's ok.

what do you boys say "more than a mouth full's enough"! if we only need 230MAJ then that's good. otherwise someone has got to work out where all the restrictions & blockages are & it stops being fun when you have to bore & reblock your carbs MAJ passageways.

like i said before, another way of looking at my thoughts with the EKT is to consider the length of the needle in mm from the beginning of the taper to the tip (not forgetting that the needle would be on clip No7 & thus have 5.4mm missing off the bottom).


  • James_Dean

Posted March 05, 2001 - 08:32 AM


If you drill the emulsion tube (ET, needle jet), the diameter and depth are very important.

A large diameter (3.0mm ?) will act like moving the clip by the same depth (~4mm) more or less.

A close diameter (2.92mm ?) will be acting in a more smooth manner, maybe like an extra taper of the same length as the depth of drill? :)

I have a few Mikuni VM type ET's and the main air passage is cast into these. I'll do some measuring and compare. One of these was drilled in an experiment about 10 years ago, but it didn't center right (looked too ugly to use).


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

  • Taffy

Posted March 05, 2001 - 03:05 PM



i've re-measured the needle in the ET today & have to say that the straight diameter only sits in the ET throat by 4.5 mm when the needle clip is holding the needle up at No7.

i reckon there is more to come, by getting the needle right near the top of the throat.

putting a _ _ T needle in will see this down to 2.2mm + /- .5mm.

you know how the top racing teams are known to just run every needle on clip No4 & instead of going up or down a clip they change needles. i'm just wondering if they don't run the needle taper right near the top of the ET & COULDN'T lift the needle w/o the taper 'coming out'!!!

i'm talking just one or two mm of overlap.

while i was in the carb i checked what the starter jet did. that is weird. underneath the SJ when holding the carb upside down there is a little 1mm hole & all they seem to do is feed each other!!!

i then had a look at the MAJ orifice & noticed that it feeds directly to the outside of the ET (which i guessed it would). yes the 98/99 carb have only a 2.3mm hole so while i was there i drilled it out to 3mm by having the drill bit 34mm proud of the drill & with the ET out it was a doddle. ready for future action.


  • James_Dean

Posted March 05, 2001 - 03:59 PM


That 2.2mm higher taper start on the EKT comes with the reduction of CSA from:
6.605 - 5.875 = .730mm2 (EKP#7/EFP#5/na)

down to:
6.605 - 6.048 = .557mm2 (EKT#7/EFT#5/EBT#3)

A 24% drop in area for fuel to flow at the narrowest point. Below 1/8 throttle it will be lean. Some of the YZ400's with DVR go as high as #50 on the pilot jet to offset a lean condition and you are suggesting leaner yet but a richer taper and taper start, hmm... :)

Remember in the tuning article: "mix it up there, and it's super easy to really botch things up with everything being a band-aid for the next."


You read my mind on the MAJ passage. Easy fix and 3mm sounds close enough to 1/8 inch to work in this time zone. Wonder about Andrew's time zone? :D


  • Andrew_in_OZ

Posted March 05, 2001 - 08:18 PM


theres a big difference in power between a #200 & #230 I might try a 2.5mm drill first and see if it feels like it could use more

  • Taffy

Posted March 06, 2001 - 01:19 AM



andy the idea is that you control things with the jet & don't have any other equations to worry about.

by having a 3mm or 1/8 (3.175mm) hole & then putting the MAJ back in all you have to worry about is the jet changes you make.

needle straight/CSA's
i did a load of figures on that very thing JD lastnight. i want to get my bike running first, try the MAJ changes by taking a box of drill bits to those jets that i have (170/180/190/200). i need to get this EFP needle in the right position & sorted.

IF i think for any reason i can use a higher start of taper, i will buy the EFT or EBT needle, a new ET & use the new drill bit to bore out to 2.95mm.

a 'T'/2.95 combo will have about the same 'space' as a 'N'/2.90 would.

it's accademic at the moment.

as for 1/4 throttle =i'm in control 100% with the PJ/PAJ-SCREW so can lean off.

1/2 throttle = the tough one!

3/4 throttle = MJ/MAJ has more control & not the ET. ???

the idea is to see how near the top you can start a taper & it's reaction thereof.

thing is JD, you've got that APJ squirting all over the place. like a headache you don't need. i've said it enough times. the bike accelerates really hard without it.

my enebriated delight on saturday night wasn't so much your discovery but the fact that you were prepared to try someone else's idea's.

take's a big man to do that especially when our idea's are so public.

once you've tried No 125 PAJ (screw) with the 48 you'll realise that everything else was wandering in the dark.

i know that even on your first ride you'll say that the screw needs to be here or there (No 140 /50PJ was a guess) but i'm not one for the minutae.


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

  • James_Dean

Posted March 06, 2001 - 10:24 PM


Taffy and others,

I am always open to trying other carb settings. It just takes time and the right components to make a good comparison. Exploring the possibilities and understanding the variables gradually refines performance and helps everyone.

For each of the trials of jetting that I've posted there are 5-10 tests that were not posted. The idea of dropping the AP was kicked around before, but set aside for a mainstream answer with an AP that all could just plug and play. The focus generally has been on diameter, clip(L1), and taper on standard needles. Results from last summer were the EKP and EKN needles.

Now after a winters break you have refueled the non-AP idea and the easy adjust AP stroke stop opens the door.

The adjustable pilot air has potential to make starting easier, more torque, less stalling, and influence the need for AP too.

You will find limited participation because not everyone has the time or the desire to understand the interactions. Fortunately I have a small track at the house and road plated bike to broaden the testing.

Before a setting is thrown out for everyone to use it needs to be ridden at least a few hundred miles to capture the variations in conditions and at least a few types of exhausts. To do less would be a dis-service and leave the patrons frustrated and lose the fun of it. After all, it's just for fun. :)


  • Taffy

Posted March 07, 2001 - 12:44 AM


i cannot believe you wrote that!

i cannot believe it.

no comment.

but, as usual, the truth will out in the end!


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  • James_Dean

Posted March 10, 2001 - 02:28 PM



Fouled a plug today :) using EKN#5, #48/1 1/2 PFS/1 1/2 PAS etc., no accel pump.(45F at 500ft elev.)

Backing down to #48/1 1/4 PFS/ 1 1/3 PAS, EKN#5, this idles faster than anything to date and after idling back down the bike will still roll on the road (slightly uphill) with no hand on the throttle in 1st gear. Just THINK about touching the throttle and she speeds up.

Turning on and off the APJ is a close match. The #5 clip favors having an APJ by a small margin (1.0mm stroke, 1 1/2 turns delay). Takes some stupid hard wrist movement at low rpms either way.

I don't have a good measure to compare the MAJ. Running a #220, with the passage behind = 2.80mm and #172 main jet. There must be something going on here because it feels like the front end lost about 20lbs. :D The rev limiter also feels like it comes too quick (like 8k not 11k). The full throttle stuff needs someone with a better measure to check out. Even if they would just pull out the main air jet and go up 2 on the main jet. (Hick or ?, would you be willing to go that way?)

Andrew? any news?


  • Taffy

Posted March 11, 2001 - 12:45 AM



i'm pesuming that you haven't got the 50PJ out yet for your own reasons. you also haven't been to clip No 7.

anything that speeds up the revs on an engine is good news of some sort.

the oldest rule is "to give the engine what it wants" it just depends which circuit we use.

this rev limiter thing sounds whacko.

i don't think you should mix these top & bottom end tests. the piggy-in-the-middle is the needle. unlike my tests, you're mucking about top & bottom. (ps i only changed my MJ at the beginning & at the very end).

i see you describe the pilot screw as a pilot fuel screw. this is going to be fun.

the PAJ & the PJ mix, you then control the overall mix with the PS. i've said enough, you know.


  • Andrew_in_OZ

Posted March 11, 2001 - 01:17 AM


James, check FCR carb tuning for latest tests.
Taffy do u have a part no for the Pilot air screw & is it available thruogh local bike shop or sudco or ?.? I've rang a couple of bike shops here and they are un able to find it without a P/No.Thanks and anxiously awaiting your MAJ test results, Hope everything goes OK on the G/Box side of things.


[This message has been edited by Andrew in OZ (edited 03-11-2001).]

  • James_Dean

Posted March 11, 2001 - 09:31 PM



I'll work the #48 first. The #50 is on the big side from testing last year. Even at #50 and 3/4 turns it was rich. The PAJ screw will need to be out a good bit to work. The different model carbs use a different pilot jet type, they do not interchange from your carb to mine. Wouldn't be too surprised if your '99 might like a #50 and not the '00.

Clip #7 is off my list, fouled a plug after playing with clip #6 & having bad sputter on downhills. Too rich in the middle.

The rev limiter comment was just saying it hits the limiter easy.

So far have about 4 steps of PJ/PAJ screw settings with a #48 alone.

See the FCR tuning thread for the lastest detailed list of jetting attempts. The APJ is becoming less and less needed with the PAJ screw tuning. The bike has good snap either way (APJ on or off) and there is little to complain about. The only thing now is to give it some time to be sure that all conditions have it working well. Starting is easier too within the limited time riding. When stalling with no APJ there is no fuel thrown in the dead engine from the pump. This helps on the restart and should not need the hot-start as much. No worries if somebody twists the throttle when you're not looking too!

#48 pilot
1 1/6 turns on PS
1 1/4 turns on PAS
EKN#5 (modified with 2mm higher taper start)
#172 MJ
#220 MAJ


[This message has been edited by James Dean (edited 03-11-2001).]

  • MN_Kevin

Posted March 12, 2001 - 12:41 AM


James and Taffy,
What years are your bikes, for reference sake?

'99 WZ with all YZ mods, FMF PowerBomb header, Stroker SX-1 silencer, SS front brake line, forked over by Pro-Action, OEM YZ tank and IMS seat.

  • Taffy

Posted March 12, 2001 - 03:57 AM



james is on a '00
clark & i are on '99's
aussie andy '00 or '01.

mine will be a '99, taff cam timing, white bros air filter, flywheel weight, magura juice clutch, powerbomb header & stroker tailpipe. the carb... watch this space.

if you're going to do MAJ testing, i think you should all be running at least 15/50. you gotta f-e-e-l it.

you don't want to do any of it on the dirt either the road is the place. no bumps = no stutters & no mind games.


  • MN_Kevin

Posted March 12, 2001 - 05:00 AM


How do you like your exhaust combo??

P.S. I went one tooth smaller on the countershaft. rear is standard

[This message has been edited by NH Kevin (edited 03-12-2001).]

  • Taffy

Posted March 12, 2001 - 12:52 PM


the exhaust is imminent!

it must come tomorrow surely?


  • James_Dean

Posted March 20, 2001 - 09:41 PM



Are you up on 2 wheels again?

Latest search is leading me towards an FHN/FHP#5 needle. Remember that great response in clip #7? Much the same. This is running cleaner down low and increases fuel in the middle and with a #170 main/#230 MAJ clean up higher. APJ is optional with #48PJ/1.3PS/1.5PAJ.

This is a reasonable progression with the reduced APJ and improved top end MAJ.

Keep thinking beyond the standard factory settings. :)


  • Taffy

Posted March 21, 2001 - 02:20 AM


where are you feeling the difference then jd?

how does it compare to your 'e' needle?

i know that you said you had an FMN & an FMP could you not conduct your tests with these. remember about trying to get back to the 4th clip all the time.

the bike is in bits still & i hope to start on it this weekend. i've nothing else booked up.

this foot 'n mouth has robbed everybody over here of the inspiration to do much. my mates & i aren't phoning up coz there's nothing to say. end up talking like well.... you know.... GIRLS!!!

does my bum look big in this...!

as i understand it the MAJ's have leaned off the bikes at 3/4 to WOT correct?

the thing is, the needle is completely out of the ET & up & away. so where the MAJ has helped has nothing to do (or very little) with the needle we're running it would seem.

in my own mind it would appear that the middle of the needle to the tip are controlling a far smaller area of the bike than i used to think when doing flat out riding. the needle's true raison d'etre (reason for living)is roll on isn't it.

so the 'f' will help 1/2 throttle snap the same way a 'd' was replaced by an 'e'.

at the end of it, how rideable are these things going to be for sludgy enduro. the boys next door will love this coz they're all 22 & put their brains in their underpants years ago!

imagine the little 250 with the wrong jetting & cam timing. they say it's quick std, what would it be like after we'd had a go at it!


[This message has been edited by Taffy (edited 03-21-2001).]


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