Needle Jet Question

[rodneya]

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.

[m in sc]

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'

[edgefinder]

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?

[Czakky]

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?

[edgefinder]

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

[Czakky]

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!

[m in sc]

i think chucks std practice was to cut them to 2mm but i cant remember 100%.

[teazer]

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.

[Czakky]

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.

[m in sc]

FYI, JET TUBE AND NEEDLE HAVE ZERO TO DO WITH IDLE CIRCUIT.  (SORRY CAPS.)

[teazer]

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.

[Czakky]

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!

[edgefinder]

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.

[m in sc]

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:





lifted from here:
https://www.mikunioz.com/tuning-tips-and-manual-downloads/?v=7516fd43adaa

[Jspooner]

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.