Myths and truths about pilot jets

[GLoweVA]

Alright, after reading a thread about a guy having issues about swapping jets around to get his bike tuned right and reading about all of you rant about which pilot jets to use, I thought it was time to share a little of what I've learned about pilot jets.

Originally, our bikes came with the 6 holed pilot jets. Or as some call them the ‘bleed type’.

If you look up the stock number for a replacement jet
For the SG, H, SH you get: 4G0-14142-42-A0 JET,PILOT #42.5 (available)
For the F, SF, G, LG, LH you get: 256-14142-42-A0 JET,PILOT #42.5 (not available, superseded to the 4G0)
http://xs1100.com/partinfo.php?myid=5974
(Because the 78E has stock 45 not 42.5 I'm not going to talk about them.)
So in other words, Mother Yammie has only the 4G0 ones available for the 79-81 bikes.

Here's a pic of the 4G0 jet in the package:


Here you can see that it is indeed a true Mikuni:


So do these ‘non-bleed’ ones work? YES, because the air in the pilot circuit mixes downstream/after the metering orifice!
(I know this blows your mind, but it’s true!)
To test this, I put one each in a bank of my G carbs and sprayed carb cleaner into the air jet and you can see the spray coming out is metered by the orifice. Granted, this way shows the fluid flowing the opposite the way the fuel flows, but it’s obvious that the ‘air bleed’ holes have no effect.

With the stock 6 hole:


With the new 'no hole':


I did this same test with a set of ’79 carbs that TC lent me and the results were the same!
So it doesn’t matter whether it’s the early or late carbs, the design is the same.
So why did they put 6 holed ones in? Your guess is as good as mine.
Maybe that's what was available at the time. (since they used those in the 650's?)

NOW, the jets you get in the other (non Mikuni) kits are not the same!
They are 8 holed:

You may think these are VM style and not BS style:


They look like they might work because the metering orifice is at the end like the BS style: (the allen wrench was used to gauge the depth of the orifice)


ALAS, you can see that the orifice is BIGGER. Even thou it's listed as a 42.5, it is way too big:


That is why those 8 holed ones won’t work. They’ll make you run too rich.
So in conclusion, the key is to get genuine Mikuni and to make sure they are the correct size.

I hope this helps some understand a little more about their carbs.

[GLoweVA]

I made a visual aid to help with this concept:

[mack]

Interesting

So by using these pilots without the air bleed holes would mean that it would nearly be impossible to have a lean condition unless your floats were set very very low?

[TADracer]

Yep, exactly. The hole in the end is the critical thing as all the flow goes through that notwithstanding whether air is mixed in before or after the jet, it all still passes through that hole. Now if the air were mixed in before the pilot jet in the BS-34 then the small hole would cause a lean condition but since it happens after the jet the mixture is right because the side holes do not come into play.

Reason why it is important to run the real thing is that Mikuni measures their jet sizes by CCs of fuel flow per minute under a specific test pressure. Other jet manufacturers might measure under a different parameter so one guy's 42.5 can be completely different than another guy's 42.5. Since the pilot jet is the main controller in the throttle opening range most riding occurs, tuning it is more critical than tuning the main or needle jet.

Now, what I did find is that those after market jets make a wonderful restrictor in vacuum line to help stop timing plate bounce when I switched to Keihin carbs and drilled them for a vacuum advance tap.

[GLoweVA]

So by using these pilots without the air bleed holes would mean that it would nearly be impossible to have a lean condition unless your floats were set very very low?

Mack, It's not the non-bleed ones, but the non mikuni's that people have problems with running too rich.

Now, what I did find is that those after market jets make a wonderful restrictor in vacuum line to help stop timing plate bounce when I switched to Keihin carbs and drilled them for a vacuum advance tap.

WOW, so there is a use I can find for those 4 crappy non-Mikuni pilot jets that came with the kits.

[D0wn5h1ft]

Oh man!

OH MAN is this awesome. I have been trying to figure out why I keep getting lean coughing back through my carbs FOREVER. I've cleaned the crap out of them and tried stepping up from 42.5 (stock) to 45 to fix it but 42.5 is too lean and 45 is too rich. HOWEVER, the 45 are those damn 8 hole ones you were talking about which would explain why I'm so rich when I swap them in.

I looked at that opening you were talking about and it's HUGE compared to the stock ones I have in even though it's only a half step bigger. Time to take them back to my yamma-steal and tell them to give me the right ones.

Every day I jump on here I realize more and more how little I know.

Thanks!

[natemoen]

Now I want to see a jet with side holes and the end hole blocked be used in the same test. If that isn't done then you can't really say for certain that there isn't some mix from the side holes. Who says it isn't there to cause turbulence and cavitation in the fuel.

[GLoweVA]

Now I want to see a jet with side holes and the end hole blocked be used in the same test. If that isn't done then you can't really say for certain that there isn't some mix from the side holes. Who says it isn't there to cause turbulence and cavitation in the fuel.

Alright Nate. I've got the carbs of my SG right now in prep for the engine swap, so this weekend I'll do just that. I'll put some saran wrap or thin plastic over the tip and reinstall and shoot the Carb cleaner thru again.

[natemoen]

I would think saran would melt and just break with carb cleaner. I was thinking about a sacrifice jet and use rtv or something.

[GLoweVA]

I would think saran would melt and just break with carb cleaner. I was thinking about a sacrifice jet and use rtv or something.

Good idea. Though I don't have any to sacrifice, I may use a tip of a toothpick to block the orifice, then I can later pull/extract the wooden toothpick to reinstall the jet.

[Bonz]

My though exactly... What effect do the holes have on the raw fuel being tumbled around, possibly more ready to accept being mixed with air. I.E. atomization.

[trbig]

Now hold each type of jet in the open, with your fingers on just the threads, spray into the opening to shoot the carb cleaner the correct direction, take a couple pics and tell us there's no difference as carb spray shoots out of those holes on the sides as well as through the orifice.

When you shoot something out backwards where the opening (Which would then be the exit) is the same on both, you'll obviously get the same results. Fuel (Or carb cleaner) going in the opposite (And correct) direction with different things past the opening are not going to act the same.

So in your hypothesis, it would be fine to mix and match holes/no holes on the carbs as long as they were Mikuni and the same size, right?

[Bonz]

Unless the holes tumble/agitate so the fuel more readily mixes with the air when it gets out of the hole.

[SFerinTEXAS]

Anyone can see on a bare carb body where every passage goes. There are none to the area in the middle of the jet.

[mack]

This is good

As you know I'm a big proponent of setting the carbs levels with tubes verses just setting the float heights to a specific measurement. I've been working on all the year carbs with depth guages and have the rich setting figures out but but was unsure of what caused the lean condition. I knew it had to happen within the 4mm range from the bottom of the mixing chamber but all I could come up with was the level of the first hole at the top of the pilot jet. Once the fuel drops below the first hole the ability of the carb to siphon fuel starts to diminish. By the time the second hole is exposed the carb/cylinder is in full lean starvation as the pilot circuit can no longer siphon fuel up to the three little holes in the top of the barrel. I was never sure of my hypothisis until this post. Right in front of the idle mixing screw is a manifold of sorts where all three circuits meet. I believe that is where the vapourization takes place just before it is injected into the barrel.