Pilot Jet Epiphany

[GLoweVA]

Today, while I was pulling the engine out of the SG, the mailman delivered these:



Note the part number is 4G0-14142-42-A0 which is what was listed for the SG. Here's a close up, it looks to have some kinda symbol (Mikuni?) stamped in it right next to the 42.5.



NO HOLES!!! WTF! So, this got me thinking about those pilot jets I got from Partsnmore months ago, they had 8 holes. I thought all our pilots were supposed to have just 6.



So, this got me thinking and doing some research. I even looked a thread from 8 years ago: http://www.xs11.com/forum/showthread.php?t=8782
First thing I had to wrap my mind around is the crossover and if the pilot towers are plugged and how this played into the pilot jet holes.

My SG doesn't have the crossovers (and no plugs), but my G does have the crossover (and it has the plugs).
So looking at the parts list again, I've come to the conclusion that having the crossover or not, determines which of the 2 different 42.5 pilots you get.
256-14142-42-A0 for carbs on LH LG G SF F
4G0-14142-42-A0 for carbs on H SH SG

The 256-14142-42-A0 have 3 holes as seen in this pic from ebay:



So, I took another look at the drawing of the carb flow circuit and thought about how the fuel flows into the pilot circuit.



Since the pilot jet has a seat that seals the top of the jet to the carb body, then the only passage for the fuel to get thru is the metered orfice at the top and it is what is sized (42.5).
The holes on the side is where the fuel would come from if the tower had the plug in it. If there's no crossover, then the fuel would come thru the bottom (no plug).
The pilot fuel doesn't mix with the air (from the air pilot jet) until after it gets a little higher in the tower, so the holes in the sides don't affect the mixture.

For those carbs that have the crossover, then the pilot fuel comes thru the main jet, but because the pilot jet hole is so small, the amount of pilot fuel is small compared to the size of the main jet. This also applies to the amount of holes in the pilot jet. If the metered orfice is so small (42.5) then the amount of holes (6 or 8) shouldn't matter and if you don't have the crossover, then it doesn't matter if there's any holes, because the fuel's not coming from that direction, it comes straight from the bottom.

I hope this clears things up a bit for a lot of us.
Basically:
- If you've got no crossover, then any 42.5 jet will work. (regardless of the number of holes) cause all the fuel is coming thru the bottom hole anyway.
- If you do have the crossover and the plugs, then your pilot jets need holes, but it shouldn't matter how many. (6 or 8) It's the small 42.5 hole in the top that meters the amount of fuel.

Disclaimer: this is all written about the BS type jets which is what all our XS11's have. Note the 78's have a different size pilot jet, so the theory should be the same, but the size is different.

Okay, now that this is done, I can update my thread on Scarlet's resurrection.

[natemoen]

Explain then why people who have had their '80 & '81 bike since new and they have the 6 hole pilot jets?

Part numbers for a jets usually refer to just the general style of jet, not the specific holes in the jet. If they had a different part number for every different jet it would get to burdensome.

[DGXSER]

Let me complicate your theory a little George, and perhaps point out a flaw in that diagram. Look above the pilot jet and you see blue air bubbles in the mixture, recall we have a main air jet in the inlet bell, and a pilot air jet in the inlet bell. Now, the holes in the emulsion tube are to let air into the mixture for the main jet mixture. So, guess what the holes in the pilot jet do.

Also, notice where the crossover passage ties into the pilot jet passage. It is below the threaded area of the pilot jet. This is accurate from what I have seen in carb bodies. So, how are the holes in the pilot jet controlling the fuel flow from the crossover?

As for the part nos, recall that this is a subassembly that Yamaha bought from Mikuni. And from 79-80 and the later 80-81 bodies were different parts from Mikuni. So they gave new part nos to everything. That is my theory anyway.

What I do know for absolute certain is that I have worked on three different XSives bikes where they could not get the thing to run right or run at all. The bike ran perfectly once we switched the pilot jets from the no holes N151 style or the eight hole VM style to the 6 hole style. No other changes made.

[jetmechmarty]

I bought genuine Mikuni jets from Jets R Us. I didn't pay any attention to the holes. I have no idea what I put in there other than the size and the fact that they're Mikuni. The bike runs great. The fuel economy is terrible. (28 mpg)

[skids]

That drawing on the left side is one that I did long ago. The pilot jets were added and I can tell you this is wrong. The holes are air bleeds and whether or not you have the crossover, the fuel enters the pilot jets from the bottom.

[GLoweVA]

Well, thanks fellas. It was just a theory and as DG put it, he's right, the jet does go further into the tower so that the crossover does feed the jet from the bottom.

So the question now is, why does mother Yammy sell this particular no holed pilot jet for the SG?
Or maybe like Nate says, the part numbers just refer to the size of the jet and not to the number of holes.

I thought I was on to something, but it looks like I'm back to square one again.