I read somewhere in a Mikuni manual or similar that a step leaner needle jet equals roughly the same as your needle clip on the leanest is the same as the leaner jet on the middle clip.... Wow I worded that terribly.
Let's try this way. P5 jet with the leanest needle clip would be the same as the P4 (leaner) jet with the same needle in the middle clip or there abouts.
Assuming that's true. I have 389 P4s on the leanest needle position the next leaner needle I can find is a P2.
Is this going to be roughly the same fueling at mid clip position?
Not exactly.
At idle and just above idle the thick parallel part of the needle sits in the needle jet , so moving the needle up or down makes no difference at small openings, but increasing or decreasing the needle jet size will make a difference.
Except that at low throttle openings, almost zero fuel is being pulled up through the needle jet, so the actual effect will be three fifths or five eighths of sweet F all.
Once you get up into the midrange, raising the needle or increasing the needle jet size will achieve a similar result. Not exactly the same on a dual taper needle but pretty close.
The short version: go ahead and try P-2s. On your bike we have no way to determine how much it's too rich, so we would be guessing, but that really is the simplest option unless you look at needle profiles to see one that is thicker where you need it to be.
Thanks Teazer, I looked at needle profiles and my head starts to hurt trying to figure it out...
The real question though is where is it rich i.e. at what throttle openings.
If you post the needle number you now have, we can see what else might work but it all comes down to where it's rich. Lower mid range it's easier to change the needle jet, mid to upper midrange the needle has more effect. Post the needle number and we can put that into simple excel spreadsheet software and compare the annular area with different jets versus different needles.
I'll run through my progression so far as it pertains to carburation.
Pilot jets seem to have little effect from 45 to 32.5 on any sort of open throttle. I can tell when it's lean just by the way it settles down to idle and rich by how quick it will fizzle out when warm.
After that (1/8 throttle or better) it feels rich to my novice wrist and when cold on the 32.5 pilots it will take throttle really well after 1/8-1/4 throttle. When it's warm it doesn't really matter. It can feel a bit boggy when snapping open the throttle like 1/4 of the way, then snap open to 1/2 all the way to full it being 50 years old and 350ccs, me being 200lbs this might just be how she goes though.... This has improved slowly as I progressed from P8s, P6s, now P4s and it's pretty close but the cooler weather has me motivated to get it right. The biggest issue is the 1/2-full throttle flat spot before the powerband, around 6k or so. Which is mostly gone.
FWIW I never see head temps (measured with infrared gun) above 300f even on hot days and hard running.
This is a Chuck stage 2 top end w/TM32s.
Excel sheet? I'm very interested...
Here's the charts from the Mikuni manual that shows how the needle jet and the needle clip positions change things
(https://i.ibb.co/b169cw7/needlejet.jpg) (https://ibb.co/WnHJm8s)
(https://i.ibb.co/m0Gcp1R/needle.jpg) (https://ibb.co/Lr8ZKb0)
Your still too rich on needle jet and its over riding the pilot circuit. It masks all your efforts to adjust. But you tried and thats the important thing cause if you don't go to school you can't get a diploma. When you get closer the mixture screw and pilot jet will start responding like their supposed to. Beware as when you get to the correct needle jet or leaner all of a sudden its extremely important that the other stuff is on the rich side and your paying attention, shut it off and reevaluate. Notes are good. I bet in your adjustments there were times when Damn its pulling good there for a thousand and crap all around. That crap's valuable. Yeah needle jets steps are 0,2,4. OK needles and clip positions. Just because the needle's tapered doesn't mean when its idling and you start cracking the throttle that the taper is doing anything. No the straight part above taper is controlling flow, the annular gap. What % of throttle taper starts is one of your goals. The graphs show an optimum and rich lean line banana. I'm thinking your banana is still smooshed out the top of graph a bit
Primary needle jets with the hood up?
P0s are too lean. It's interesting how needle clip on the richest it would rip up to about 50% throttle than bog real bad before the powerband as I went leaner the midrange would start going flat even as my rich bogging would start to go away. Either way it was worse than with the P4s. I've got a P2s and a slightly leaner needle on order.
Anybody need some 389 P needle jets?
Sorry Edgefinder, I'm not sure what you meant...
If it rips better with a P-0 with needle at the richest setting, that is confusing. Taper starts at 35mm and at idle (closed throttle, the needle is around about the 25mm mark, so still on the root diameter. Raise it fully and the taper will now start at around 30mm, so the needle starts to have an effect from say 1/8 onward. By 50%, the needle will be at around 45mm ie where the needle jet sits in the metering part of the needle jet and will be much richer by then and with the double taper, will be way richer at the top end.
I'd try a pair of 6FJ41 needles from Jets R Us and see if that cleans up the top half. You may have to change to a single taper needle like a 6F9.
I may need those needle jets when you have it all dialed in.
Thanks Teazer. I only did one test run on the richest because it was running so bad before the powerband I wasn't confident it was rich so I shut it down early. In other words I'm taking that run with a grain of salt. Once it started getting better I gave it longer runs being confident it was rich in that area. Also it would take a good long while to get up to operating temp. I'm going to try the 5 series (5FP96) needles first that I ordered. I appreciate the advice.
Let me know if you need some P jets I've got 8s,6s, 0s and possibly 4s or 2s depending....
Theres some needle charts at Ian Williams Tuning. They are copies of copies but they are drawings of carb throat with needle jet and common needles with taper exaggerated.
http://www.iwt.com.au/MIKUNIjn.HTM
A 389 needle jet has a hood about 8mm tall and I don't have a clue what your bike might want but mx bikes that get the throttle twisted a lot and often seem to work good like that. Street bikes tend to not want it up higher than the slide cutaway at the throttle opening you ride around a lot at.
You can work it out yourself. Here's a TM32.
Then get the needle dimensions and compare to other needles to see what changes where. Sometimes you have to buy a few needles to measure just to work out which might work.
What's interesting is how much difference a small change in diameter can make. Had a guy a few years (decades ?) back with a midrange missfire at Gratten. I was able to find a pair of needles in my stash that were only slightly different that I lent him and they completely cleaned up the rich miss. Made him a very happy camper - and he even remembered to return them at the end of the race weekend.
I checked the dimensions the other day and the differences were so small.
That simplifies things a lot. I get it now... :patriot:
I finally had a chance to throw in some leaner tubes. I went with the P2s on the 2nd leanest clip and it is definitely the best it's ever been. There might be the slightest bit of a flat spot but I'm not changing anything until I get a longer run out of it.
Does anyone find that a bigger needle jet makes the main jet start engaging earlier in the throttle?
I was reading on dirt bike forums and a guy mentioned that claim. Not sure what type of needle jet he was using though.
I felt that with a bigger main that the powerband was earlier and the flat spot before the powerband was for sure more pronounced.
What I find strange with changing needle jets is my flat spot was not effected by a main jet change even on full throttle. I tried going up and down on mains and the only way to start eliminating the flat spot was with needle jets.
I just can't leave well enough alone.... Every time I think I've got this flat spot before the powerband solved my second test ride proves otherwise.
I went back down to P0s and after multiple test rides it seems pretty well gone :celebrate: for real this time. Now I've got this bad off idle stumble that improves when I crank in the idle air screw but by the time it's gone (the off idle stumble) it won't idle.
This is at 32.5 pilots 1.25 turns in or more. Even at 1-.75 turns in it still isn't perfect.
So I think I've finally gotten to a point where I can feel what's happening with this thing.
Here's my jetting followed by my hypothesis:
32.5p
240m
P0
Leanest clip on needle
If I got to the second clip position, off idle (1/8 or less throttle actually) is fine but my flat spot is there. I can get a decent idle too. With my current settings no flat spot hard to idle and no flat spot.
If I go to yet another leaner needle jet (emulsion tube) I'll be able to raise the needle and likely go to a richer pilot.
How's my logic?
Who wants some needle jets?
Quote from: Czakky on June 13, 2021, 04:56:40 PM
Now I've got this bad off idle stumble that improves when I crank in the idle air screw but by the time it's gone (the off idle stumble) it won't idle.
I have been in this spot a couple times. Pretty sure you need a needle that the taper starts earlier. Start of taper is closer to bottom groove of needle. What needle is in it? Your trying to richen with the pilot and mixture screw.
Quote from: Czakky on June 13, 2021, 04:56:40 PM
So I think I've finally gotten to a point where I can feel what's happening with this thing.
It sounds like your much closer where adjustments make a difference. When you started it was so rich small adjustments wern't noticeable.
Quote from: Czakky on June 13, 2021, 04:56:40 PM
Here's my jetting followed by my hypothesis:
32.5p
240m
P0
Leanest clip on needle
If I got to the second clip position, off idle (1/8 or less throttle actually) is fine but my flat spot is there. I can get a decent idle too. With my current settings no flat spot hard to idle and no flat spot.
If I go to yet another leaner needle jet (emulsion tube) I'll be able to raise the needle and likely go to a richer pilot.
How's my logic?
If the air bleeds are 1.8 you should try 1.5 and 1.3 to lean it a little. They are cheap and do make a difference in how it accelerates from a stop. Sometimes enough that you can get the needle towards the middle clip.
shim the needle. put it between the slots.
I'll try both and report back. The difference between the two clip positions is pretty dramatic.
At what revs and/or throttle position are you seeing/feeling that difference?
In theory, the needle and needle jet are there to moderate the main jet. Normally the pilot jet and pilot air jet/adjustment control idle and just off idle. As the throttle is cracked open, the cutaway becomes the main control system and from there up to say 3/4 it's all about the needle and needle jet. Main jet does impact all the way down but main jet has to be set of full throttle operation and the needle jet spray bar height and main air jet control the fuel slope.
What the heck is a fuel slope? Well we tend to conflate revs and throttle opening and that's understandable, but we usually get things wrong because of that. At full throttle the air:fuel ratio changes as revs rise. Two strokes typically require a richer mixture at full throttle & full revs than a 4 stroke, so the air jet or spray bar are used to change how much richer or leaner it gets aas revs rise. That's what they mean by changing the fuel slope.
That's also why people say to get idle right first and then main jet at full revs. After that the needle and needle jet are used to tweak that fuel slope at lower throttle openings.
In this case, it sounds like a wrong cutaway or wrong needle jet taper or the point at which that taper starts.
Which is why I asked where is the issue in terms of throttle opening.
The problem ive been trying to tune out originally is around 5500-6k rpm in higher gear mainly when I'm at 1/4-3/4 throttle. Full throttle it'll zing right past the flat spot. I previously had it pretty well cleaned up with P2s when temps were cooler. Now it warmed up it is pretty bad again. Went to P0s on 2nd clip, minimal change now I'm at 1st clip (leanest) and flat spot gone, but now it seems to lost any signal from the needle jet at closed throttle. So now I will either wind in the air screw and it won't idle for longer than a few seconds or if I go the other way on the air screw it wont take throttle until it's like 1/8-1/4 open.
That indicates that it was marginally rich with cold dense air and has become too rich with warmer less dense air. Try O-8 needle jets to clean that up.
The idle is now too lean, so try going up two sizes in the pilot jet.
Needle jet size doesn't usually have that much impact on idle but circuits do overlap. Try that and see what changes.
needle should have zero effect on idle if the slides are set correctly. if they are too high on the idle screws it it will affect it. :twocents:
I will check to see how far my slides are at idle. That was definitely on my radar during tuning and idle has always been okay with these pilots with other needle jets. Now it's just super picky with the air screw where at fatter njs it didn't have much effect and small throttle openings were never an issue.
I measured slides at fully closed and I was able to get a regular aerosol straw in there with some resistance. I don't think my slides are set too high. I threw fresh plugs in and went for a very quick ride with the needle spacers (.5mm). Checked plugs, thought they looked a bit lean, but maybe just not enough mileage. I intentionally threw them in warm and didn't use choke to start it.
BTW the bogging is back with these spacers, although not as bad and idle is very touchy. I'm able to get her to warm up and pull from small throttle opening but only idle for like 10 seconds before dying. :bang:
Can you make a video and take us for a ride?
I'd have to tape my phone to my chest...
I believe it was edgefinder who said that if my needle jets were too big I'd get more signal from them than the pilots. I have to believe this to be the case as when I get down to near correct on the needle jet the pilot is too big. I played with what needles I have and i was only able to get her to idle at 3+ turns out..
Second issue I have is my bogging thing. This seems completely dependent on needle position. I tried raising the needle (richer) two positions and my small throttle position 0-1/8) lean condition is gone. I measured my needle and it makes sense as the taper is beginning right where that slide position is. Looks like I've got to go leaner on the needle jets again... Who ever said jets are cheap can go to hell!
Quote from: Czakky on October 04, 2020, 09:27:25 PM
I read somewhere in a Mikuni manual or similar that a step leaner needle jet equals roughly the same as your needle clip on the leanest is the same as the leaner jet on the middle clip.... Wow I worded that terribly.
Let's try this way. P5 jet with the leanest needle clip would be the same as the P4 (leaner) jet with the same needle in the middle clip or there abouts.
Assuming that's true. I have 389 P4s on the leanest needle position the next leaner needle I can find is a P2.
Is this going to be roughly the same fueling at mid clip position?
I recently switched from p8 to p6 on TM34's. With the p8's it was slightly rich on the leanest clip and would splutter just before getting on the pipe. P6's with the needle on the middle clip ran almost the same. So at least in my case, your info was accurate.
agreed. with the 32s, i was at p4 at richest setting went to p6 and was at middle. very good info to know that there's a degree of 'overlap'
Hey Czakky, I knew this would be a good thread when you started it. You seemed determined to go all the way and make the carbs work like they should. As a 15 year owner of TM 28's on an rz500 I got to laugh off a lot of bottled up pain at your expense. I'm not an expert at jetting and my motor is different but many hours and $$$ later its running pretty dang good.
Your using 389 needle jets. They have a hood that sticks up 8mm under the slide. My bike won't run until cut to 4mm high, its too lean. As I cut them lower, I stop at 2.5mm, the low speed running gets richer and the needle can be a clip higher. How high are you cutting the hood? Or are you?
Ha! I'm glad you enjoy a good jetting thread as much as I do.
No hood cutting yet. I ordered the next leaner needle jets after looking at needle tapers for at least an hour and not finding one that would do what I need. When I get my root issue taken care of I'll start considering the hood shortening.
The root issue is the 3/4 throttle issue, which is showing the problems from incorrect needle position. I think... I just keep thinking that I've got it then it goes sideways.
What is the purpose of the hood?
What is the purpose of the hood?
Don't try to get me lying but its supposed to reduce fuel picked up when airflow is backwards. Many will say cut them off so I threw out numbers I cut to. I'm a machinist so I have to have numbers. I've been working on mx bikes and they are taller than what RD folks use. That's why I tried testing and am surprised how much difference .5mm makes. I'm curious what others say but I think 3mm is good and I don't think 8mm will work.
I have a mill if you want some cut.
Luke
Thanks for the offer Luke. I will get the fun part of the throttle fixed then head to the early part. I've got my throttle marked up again in 1/8 increments and am totally in tune with when each part of the carb is coming into play.
Believe me though it just keeps getting better!
i think chucks std practice was to cut them to 2mm but i cant remember 100%.
According to an old Mikuni book, the spray tube is supposed to be called the "primary choke". And that changes fueling from just past 1/2 way to the top end.
IIRC, the taller the Primary Choke, the more it richens up the top end.
"The lip generates turbulence which creates a greater vacuum behind the lip where the needle is located. At high RPMs, the increases vacuum helps to pull more fuel up from the float bowl. The height of that lip (primary choke/spray tube) determines the amount of turbulence and consequently the amount of extra depression at the venturi exit of the needle jet. A taller primary choke will create more turbulence and more suction acting on the fuel, leading to richening the mixture as revs rise. Use of a taller primary choke allows the use of a smaller main jet to achieve the same fueling at high RPMs, but leans out the mixture in the medium throttle position." That was basically a straight quote from an old manual.
Late model RDs with the #284 series tall primary choke and aftermarket copies are presumably designed so that at full throttle. the mixture slope is tipped to make them richer than a low primary choke at top revs.
So if you shorten the primary choke and get the main jet right for top RPMS, it will be richer at mid revs - assuming that the main jet is actually correct for top revs. We all tend to confuse revs with throttle position, so take care.
If your issue has an event horizon that is basically throttle position related, it most likely needs a different needle which either different slope or more likely a different position at which the taper starts.
Excellent info Teazer.
I got bored waiting for jets so I ground off 2mm from the hood of the P0s. It didn't seem to change much for my problem areas. This was before I read the info Teazer posted so I wasn't really paying attention to RPMs. Either way it's still either too rich to idle at 1.25 turns or less or lean below 1/8 throttle.
My theory is that if I have a leaner needle jet I can raise the needle enough for the 1/8 throttle lean-ness. If not I'll try and guess at the correct needle.
FYI, JET TUBE AND NEEDLE HAVE ZERO TO DO WITH IDLE CIRCUIT. (SORRY CAPS.)
Mark is correct, that the needle and needle jet should not effect idle or just as you crack open the throttle, unless something is way off.
Is the problem just as you roll on the throttle? That could be the slide cutaway, but I'd want to get idle and mid range closer before buying slides. I have taken a file to slides to correct that situation before but I had a pile of slides I could offer as sacrifice.
Try a smaller needle jet and see if that's enough to clean it up. Jet's aren't cheap but cheaper than slides.....
But the big issue is 1/2 or 3/4 throttle IIRC. Did you check those needles I suggested earlier? I think I may have a pair here that I bought for some obscure reason and were never used - yet. They're longer so I'm not sure if they will work, but worth a try.
At 1/2 throttle it's mainly about needle and jet, but by 3/4 the main jet is exerting more influence.
M I'm totally not ignoring you just getting the meat of the throttle correct first.
The idle has always been pretty rich, but livable until I went down to P0s. Which it wouldn't even start at first until I shut off the fuel and kicked a hundred times, opened up the air screws to 2 turns or better. By then small throttle openings were too lean and it wouldn't take throttle. Screws go in and it will not idle and is very hard to start when warm.... Somebody mentioned that overly rich nj's could effect pilots? That's what I was going with as nothing else changed and my idle game sucks now.
Yes the first part of throttle just after idle, it's okay when the needle is raised but then I get the four stroking at higher throttle position. I'm with you Tzr on that process just not there yet.
Leaner nj's on the way.
I take your advice to heart and was hoping to try those needles I just couldn't find the taper/size of those on any of my charts if I remember correctly.
3/4+ she's running like a scalded cat!
Quote from: m in sc on July 01, 2021, 10:09:11 AM
FYI, JET TUBE AND NEEDLE HAVE ZERO TO DO WITH IDLE CIRCUIT. (SORRY CAPS.)
I'll have to disagree. My tm28's came prejetted from cpw for my bike rz500 with P8 needle jets. I gave them the benefit of doubt and went down to 17.5 pilot jets to get it to run. After 5 or 6 needle jet changes i'm at O-0 or N-8's which is what the stock Mikuni 28's used. Pilot jets went progressively bigger to the 30's in it now. The groove between jet and needle feeds fuel from idle up in my experience.
The one thing that stayed the same over dozens of jet changes was where the needle taper starts in relation to throttle position. On my bike if it doesn't happen at about 1/10 throttle it bogs. Even when everything else was on the rich side and that was very perplexing. The stock carb's needle taper starts early and there are only 2 needles available but they don't start early enough so i'm running modified needles.
even on carbs where the pilot is fed off the main circuit, ie: some older hondas for example, it , the needle taper and jet tube sizing, still has zero to do with idle. you will maybe see some effect as low as 1/8th throttle but not idle. Nope. never. the ONLY carb that is affected by any of this, i think, are the lectron style carbs (needle). But pwk/tm/tmx/vm, nope.
tms:
(http://www.2strokeworld.net/wp-content/uploads/general-tech/tech-jetting/tm-tuning-diagram.jpg)
lifted from here:
https://www.mikunioz.com/tuning-tips-and-manual-downloads/?v=7516fd43adaa
Quote from: m in sc on July 02, 2021, 10:16:49 AM
even on carbs where the pilot is fed off the main circuit, ie: some older hondas for example, it , the needle taper and jet tube sizing, still has zero to do with idle. you will maybe see some effect as low as 1/8th throttle but not idle. Nope. never. the ONLY carb that is affected by any of this, i think, are the lectron style carbs (needle). But pwk/tm/tmx/vm, nope.
The needle (metering rod) in lectrons controls everything from idle on up thru WFO. The powerjet just adds more fuel and even that is adjustable. Tuning Lectrons is nothing like any carb with jets.
thats what i thought. i had a set years ago but cant speak with any recent experience.
So I've had my P0 NJs in twice and had the same idle issue... Both times that was the only change in the carbs. I'll press on and play with the pilots.
In theory, there should be little to no flow through the needle jet at idle, but as soon as the throttle is even cracked open there's a depression across the needle jet and it will flow - even if it's not much.
I suspect that in this case, the slides may be high enough at idle to allow for some fuel flow from the needle jet and maybe enough to upset idle mixture enough to have an impact.
Coupled with a magneto ignition I think running a bit of a higher idle makes sense. Either way with a little luck I should have some more info by tonight.
only will affect if the slide is too high. feel free to solder up an old jet and put in in there, then set idle circuit. then take main jet out, put in correct one. will 100% be the same if everthing is in good order.
know how i know? ive had a solid clogged main jet before from a piece of rubber coming out of a fuel line cut. idled great. minute it got throttle, one cyl would cut out.
thats just how these carbs work, period. even the PJ34's i have are like this. and those have the weirdest idle circuit, ever.
And you can take out the main jet and still get it to idle and up to about 1/4 throttle or more. And many of us know that form finding main jets loose in the float bowl bottom.
I totally believe you guys, I'm just trying to figure out why I can't get an idle where before I could get it to idle atleast somewhat. Spooner pinged me and have some direction as well :notworthy:
I've got the leanest NJ I'm willing to try in right now needle on second clip. I'll report back how she's running later hopefully. I've got a few other things I've been ruminating on as well.
look closely at float level
:umm: I might have a bad coil. I could not get a spark after kicking it a dozen times... I'll have to check connections, etc first.
I can't find any values for the Hpi coil, but it looks sorta good. Looking at the stator it seems like I should have continuity between the blue and white wires? Anybody know?
should be ~1ohm or under
But probably not open? :bang: that's not a real question...
Now I'm wondering if this thing has been bad for a while. I'm hoping this isn't like Marks bad iridiums...
im pretty sure my issue was user error. (rotor slipping).
:cheers:
Stator replaced. Starts easy now, the 5-6k stumble is gone! Like I'm pretty confident I finally got it now! I not got in a little 20mile test ride but it did not do its stupid stumble before the powerband as hard as I could try. I did have to go up on the mains because it seemed to cut out at wide open a tiny bit with the 240s.
I'm still not able to get a decent idle. It's either flat just off closed throttle at 2 turns out or it's unable to idle at 1.5 turns. Could I have simply worn out the part where the slide touches the adjuster?
Either way it's so good to have the old girl back and running. :gentleman:
My flow bench data-test post, more to come
cliff
Jet needle clip position influence on vm34
That's an interesting chart, but a little hard for me to read the details to work out exactly what we are looking at.
New eyes might fix that but in the meantime are the two lines top and bottom clip positions with all other variables held constant?
And is the chart a measure of flow through the needle jet or total fuel flow into the carb or something else?
Tease, total flow of fuel (via air) on the main.
Clip positions are the only viable.
Mind you, this is an analogy of the real liquid fuel.
Just to see effect of circuits at various mass air/throttle positions.
cliff
Are these calculated or measured fuel flow? Cool stuff
Measured with mass air flow meter
c