HELP! - Rejetting and pilot jet problem, is this jet right?

[herennow]

Hi Folks,

Here is a pic of the new 20 pilot jets along with the 17.5's I plan to replace.  (later 3 jet carb)

Supposedly Mukini jets, they have the right "mukini quality" square marking, but the small lower emulsification tube is much narrower. This is a bigger jet so I would expect it to be set up to have more fuel in the emulsification tube which might be why the tube is smaller and has smaller holes (to let less air into the air/fuel mixture) but the fact that there is a ring on the old one might indicate it is a different jet? Those lines often mean something!

Can anyone remember if there was a difference? Does this look right.

Thanks!

[herennow]

OK, I think I've found part of the problem,

The rejetting wiki  states on two places that that "Pilots = N224.103 or VM28/486 (apparently they are the same)". When talking on the phone the supplier had no N224.103 so I asked for and ordered VM28/486.

Now that I'm googling a bit deeper it seems they are NOT interchangeable - I found this following info over on a suppliers site

"The Mikuni N224.103 pilot jet has a total length of 28mm with a 2.5mm outside diameter of the thin end tube with the bleed holes. This jet is also identified by the machined groove next to the size stamping. Be careful not to confuse this jet with the VM28/486 which has a 1/2 mm smaller diameter end tube."

I'm assuming these are not in fact interchangeable. Can anyone confirm? (or deny  )?

If so, it might be a good idea to update the wiki if anyone here is in charge of that.

http://wiki.gstwins.com/index.php?n=Upgrades.Rejetting

[herennow]

Hi Folks, I've done a deep dive on the mukini jets to try to get the answer to this.

From what I understand, the VM28/486 (with the thinner emulsion tube) is used on carbs with limited space that have a smaller air chamber around the vacuum side of the pilot jet. In this case the N224.103 jet (with thicker emulsion tube) will not even fit and one will damage the body by screwing it all the way home.

Conversely, carbs designed for N224.103 jets (like ours) will have a larger opening and will be able to accept either pilot.

Jets of the same number will flow the same amount of fuel with the same vacuum applied, hower the mixing/emulsifying of the air+gas may be less efficient with a VM28/486 pilot in a N224.103 spec carb. Whether this will have any noticeable effect is not clear but doubtful.

I sucked it to see what it tastes like and she runs fine with the VM28/486 #20 pilot so, for a single data point, it seems the N224.103 and VM28/486 jets are interchangeable. However, I might still get a pair of N224.103's just to be on the safe side.
Cheers
HnN

[Endopotential]

"Sucked it" as actually running the bike with it I hope?

I can't imagine mouth to mouth with a carb jet is precise enough.  Plus last time I tried, unleaded 89 tastes awful 

[herennow]

Hehe, yes, sucked meant testing on bike.

Checked the plugs this evening - they look a little better but still look quite lean....  (although my bum tells me things are much better).

As well as getting the N224.103 pilot jets, I might go for a bump on the midrange jets to 62.5 to see what happens.
Cheers

[herennow]

Hi Folks,

Having some time to waste I bumped my mid mains up to 62.5 and my mains up to 137.5  (against Buddha's recommendation), because well - more is more, right?

Nope, he was right, less is more, it definitely felt softer with the 137.5 and 62.5s (2004, K&N drop in and stock everything else).

Will change back over the weekend. Or maybe I'll go back one step at a time. Mains, test and then mid-mains.

Dealing with a problem on another bike I've been told (as has been stated here) that reading plugs on the porcelain is not easy with modern fuel. However, it can still be read on the base plug ring. This worked quite well for me. My base ring was quite sooty with the larger jets. See pics below.


Interestingly bike lost power terribly at around 6000 RPM but that went away when I cleaned all connectors (especially those related to coils/ignition) and installed new plugs. The spade connectors to the coils were quite dodgy/loose.....

[mr72]

Base ring on my plugs is always black regardless of jetting. With today's fuels, reading plugs is iffy. IMHO.

Sent from my SHIELD Tablet K1 using Tapatalk

[herennow]

Interesting Update -the major flat spot at 6000 did not go away but it is much reduced.

I changed the mains back to 135 and went for a ride - seemed great - then as my elevation went from 1000 feet to around 3000 feet it came back, but less strongly. Then as I dropped down in altitude again it went away.

Seems to indicate that I am borderline rich with the 62.5 mains (as Buddha told me .....) and that at altitude it gets richer and bogs down.

Buddha, does it make sense that a too large mid-main could do that. I know carb troubles should be throttle related rather than RPM, but with aggressive throttle use up hills this is a consistent stumble between 6 and 7 k and its fine after 7 k - pulls really well.

With less aggressive throttle the stumble is still there but less pronounced.

I hate jetting carbs, I'm just glad I've got a known good setting to go back to.... Hopefully.

[The Buddha]

I don't think a mid main increase is ever necessary unless you're doing K&N and pipe, and even so, I don't see how it can be better than needle and 1-2 washers.
Also Less isn't more either. Correct is more, and thanks for all the experiments.
Further those pilots being interchangeable and whatever - it may have got some weird outsourcing differences, I'd be shocked if you get the other part number and find the jet is like your original 17.5. Its likely the same crap you have in a different baggie with a different printed part #.

Cool.
Buddha.

[herennow]

Hey Buddha,

I did get the new "proper" N224.103 part recently, and it is the same as the original (thicker emulsion tube). See picture below. The two outer jets are the same two as the old jets in the original photo (the bottom two). In this case, at least, I can confirm the info "from google" that the N224.103 has the large emulsion tube and the VM28/486 the smaller.

But, there always is one isn't there.... the holes on the side of the emulsion tube in the new part are 3% smaller than the originals. Probably no practical difference but interesting nonetheless....

Never know what one's going to get in those baggies

[herennow]

HEHE, "interesting" response when I told the supplied that the emulsion tube holes were slightly different to stock. Seemed ok but interesting that they are different. One would assume the 8 side holes would be identical in a specific jet part number, as opposed to the jet sizes that do obviously change under any specific part number.

Don't read to closely is it gets a bit circular fast. However, the Jets do seem to work fine....

Hello,
I must point out that this product is manufactured according to the Japanese carburetor sample, so the data is not wrong.
I also believe that the diameter of the atomizing orifice you are talking about is different, but all the atomizing orifices of your N224.103 you mentioned are only one diameter size, which is obviously not the fact.
Actually, carburetor manufacturers will make corresponding optimizations according to the products sold in different regions, and atomizing holes will also make corresponding changes. Because of the air and humidity in many areas, or the temperature is not the same. For example, the frigid climate and the tropical climate are different, so they will make relative changes.
In fact, the slight change of the atomization hole does not affect the use of the product. Generally, it can be adjusted by using the air-fuel ratio screw, or by changing the size of the jets, which does not affect the use.
But if you insist that all N224.103's atomization holes should be the same, I will respect your ideas, but at the same time my experience and the actual cases I encountered, I can say for sure that not all Atomization holes are the same.
If you think that my product is a problem, I can refund it to you. I am very happy that we can explore this issue rationally.

[herennow]

OK changed the mains back to 135 and now have 20/60/135.

Funny how the mind can jump to conclusions. The stumble at 6k to 7 k RPM was still there. However, this time I did what I should have done right at the beginning and marked my throttle to identify exactly at what throttle opening the problem occurred.

I realised that altitude had nothing to do with it, it was the fact that I was opening the throttle more while climbing, and that the engine was working harder going uphill. If I limited my throttle to at most just over 3/4, there was no dead spot while accelerating from 5k up to 8k. If I went WOT there were bad stumbles at 6 to 7 K. This was worse uphill but still noticeable on the flats. This happens in different gears (and hence different speeds)
I'm guessing that the main jet at 135 is too rich for my setup on my particular bike (stock exhaust + K&N drop in). But it's strange that it should manifest only at one place on the rev range when at WOT and not over more of the range. The pulls like a champ from 7k up.

I'm wondering what happens at 6K that affects this???

[mr72]

what happens at 6K, and under that kind of load, is that vacuum increases enough to pull the slide far enough to open the main jet.

I think it's more likely it's lean than rich. Usually the bike will run great when rich down to probably 12:1 or even richer especially under high load/WOT... some FI cars map down to 10:1 or so at high revs and WOT without any kind of damage other than continued running this way will increase carbon buildup and shorten the life of the catalytic converter ... but it's unlikely your bike is going to have that problem.

I think under WOT/high load/>6K RPM running as rich as 10:1 is probably close to ideal as long as at part throttle or less load you get back closer to 13:1-14:1 and very light load you get close to 14.7:1.

Not that any of that matters except to quantify why I think it's more likely too lean than rich. What was Buddha's recommended jetting for a K&N drop in filter? Could be needle shimming will get the job done better than jetting up, since the needle shim will mostly affect how quickly it enriches under given vacuum (load) conditions.

[herennow]

Hi Josh, thanks for helping me think through this.

The reason why I think it's rich is that by dialling back the jets from 62.5/137.5 down to 60/135, the problem was reduced but not completely eliminated.

Also I did not have this with the old jets (57.5/127.5 iirc).

If I go WOT at 4k it's fine until it gets to 6k and then it stumbles a few times between 6 and7k. After that pulls well.

My plan of attack is to try 132.5 and see what happens and if still there to try the 127.5 to get a reset/ baseline test condition.

I'm feeling a bit lost as I don't have an understanding of how the mid-main mystery jets work and their potential impact on this. are they just a constant feed into the throat varying only by venturi vacuum, are they fed through the needle somehow? How do they impact over the range of throttle movement....?  Who knows.

Cheers
HnN

[mr72]

Hi Josh, thanks for helping me think through this.

The reason why I think it's rich is that by dialling back the jets from 62.5/137.5 down to 60/135, the problem was reduced but not completely eliminated.

Also I did not have this with the old jets (57.5/127.5 iirc).

If I go WOT at 4k it's fine until it gets to 6k and then it stumbles a few times between 6 and7k. After that pulls well.

My plan of attack is to try 132.5 and see what happens and if still there to try the 127.5 to get a reset/ baseline test condition.

BTW IDK at all about the 3-circuit carbs but what this sounds like to me is maybe it is too rich at that point due to excessive overlap between mid and main jet, or in other words your main jet is opening too early.

Reasons for this would be diaphragms are worn out (too flexible), return spring under the slide is missing or bent or whatever, or if you shimmed the needle, maybe un-shim it and see how that goes.

But IDK what the right jetting with the K&N drop in filter is. Try it with a paper filter and see if that helps. The thing is with the K&N you might be lowering the air velocity under all conditions and you definitely change the behavior of the carbs and how the circuits interact. It's a finicky system. There's no performance gain with the K&N, it increases engine wear and makes it potentially difficult to tune, so I would say the easiest thing is just to switch back to the paper filter.

[herennow]

"hmmmm", says stroking beard, "Insert, what insert?"

My bike came with the K&N filter so I never researched it. I had no idea there was such a thing and don't know if it has it or not. looking forward to finding out this evening. Major "doh" moment if I've been jetting while aiming at a moving target...

FYI, Re the three-stage CD carbs, Haynes manual says this:


"There are a number of hybrid instruments found on
motorcycles. One of these is part fixed-jet and part CD in func-
tion. At tickover, metering is controlled by a normal pilot circuit,
independent of the main carburettor and automatic in opera-
tion, as in most instruments. As the butterfly valve is opened,
more air is admitted, and fuel is drawn from the primary main
jet located on the engine side of the valve. Further opening of
the butterfly causes the secondary main jet to be brought into
action, this being located centrally in the instrument as in con-
ventional slide or CD types. Up to this point (idle to low speed)
the carburettor has functioned as a fixed jet instrument, but
from here on, the vacuum induced begins to act on the suction
chamber, and the instrument functions as a CD type. This
additional sophistication is designed to give an ever smoother
transition between the idle and low speed stages."

Someone kicked an old 70's Honda manual my way where they also use prim/sec main jets on a CD carb:

They say this:

"Primary and Secondary Main Fuel Circuits:

The carburetor illustrated in Fig. 35 & 36 divides
the main jet system into primary and secondary
circuits. The purpose of this design is to provide
a smoother transition from low speed to inter-
mediate systems.

As the throttle valve starts t0 open and induction
port vacuum extends upstream, fuel is discharged
from the primary main jet nozzle before the
secondary circuit becomes operative (Fig. 35j.

As the throttle valve opens farther, fuel is dis-
charged from the secondary main jet nozzle
(needle jet), and the vacuum piston begins t0
rise (Fig. 36).

Fuel is delivered from both primary and secondary
circuits throughout the operating range of the
vacuum piston. Fuel mixture adjustment can be
made by replacing the main jets with jets of
larger or smaller diameter."

Hope this sheds a little light on the working of our carbs.

Based on this, and if the restrictor is not present,  I might first go back down in size on the mid-main (primary main) just for the hell of it and then the main (secondary-main) after that if no difference.  If the restrictor is present, taking it out might sort out all my problems - but that's a long shot as it would be way too easy.....

I wonder if the jetting chart would look something like this for our carbs (blue added in by me):

[The Buddha]

If I limited my throttle to at most just over 3/4, there was no dead spot while accelerating from 5k up to 8k. If I went WOT there were bad stumbles at 6 to 7 K.

This is a dead giveaway sign of a slide rising too fast. Plug one hole with a 4-40 nylon screw and cut flush (absolutely not at all recommended) or train your wrist ( )
Cool.
Buddha.

[herennow]

Hi all, just checked - no restrictor ring on the filter..... Damn!

Buddha, in the test runs I did, I would go WOT sometimes at 4 K and sometimes at 5 K.  In both cases the bike would run fine until it reaches the magic 6k rpm mark and then the stumbles would start until around 7K. If the slide was rising too fast would I not get this problem at, or close to, the time I go WOT, rather than at a later specific RPM?

Note stumble was more pronounced when the engine was working harder (going uphill).

Thanks!

[The Buddha]

I've had it do that with the stock filter. Putting a pipe on it will get it to the disaster zone. And maybe even a stock pipe is close.
Cool.
Buddha.

[herennow]

Thanks again for the input folks. Mr 72, I think you might have something after all. I went for a ride and repeatably found the situation where *slightly* closing the throttle gives me noticeably more power than when WOT.

In classic slide carbs, which I am much more comfortable with, this means that the main is too small  (running lean up top), does this also hold for CV tye carbs?

As an aside, I checked float levels again (just to make sure). I checked before I restarted jetting, but now fuel level was way too high again. Adjusted them back but can only think that maybe I bumped the floats while changing jets that caused this....  Be careful out there folks!
Cheers
HnN