Needle Jet Question

[Czakky]

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?

[teazer]

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.

[Czakky]

Thanks Teazer, I looked at needle profiles and my head starts to hurt trying to figure it out...

[teazer]

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. 

[Czakky]

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...

[1976RD400C]

Here's the charts from the Mikuni manual that shows how the needle jet and the needle clip positions change things

[edgefinder]

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

[edgefinder]

Primary needle jets with the hood up?

[Czakky]

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...

[teazer]

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.

[Czakky]

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....

[edgefinder]

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.

[teazer]

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.

[Czakky]

That simplifies things a lot. I get it now...