Pilot Air Jets

OK, so the other night, I pulled the carb tops on the 550, and I sprayed the pilot air jets with carb cleaner - or at least I think I did... Can someone please give me a description of all the different little holes in the carb body just underneath the diaphragm? The reason I'm asking is that I sprayed through the brass screw-in jets (1 in each carb), assuming this was the pilot air jet. While I was under there, I also attempted to shove the red tube down the long hole and spray, but I pretty much got an instant reversion of carb cleaner in my face. So (perhaps stupidly) I stopped trying to spray that hole and instead focused on the jet looking things. In retrospect, I'm sure I should've probably kept at it until the long tube holes were clear, but I don't know for sure. Before spraying, I bottomed the pilot screws, as per Rick's instructions, but would bottoming the screws block those other passages? Are the passages blocked by any other normal means, like when the float valve is closed, or the enrichment valve is opened or closed? I'm thinking that if they're supposed to be always open, then this bike will probably run right tonight after I pull the tops and clean them again!

 

The two jets under the diaphragm are the Main Air Jet and the Pilot Air Jet with the Main Air Jet being the larger of the two and more towards the center of the carb (at least on my bike).

 

So, the one that looks like a fuel jet that screws in is actually the main air jet, and not the pilot air jet? There is no apparent "jet" in the long tube hole. Is this the pilot air jet? If so, then obviously I need to get back in there! Does that one get blocked by bottoming out the pilot screw? If not, then what is the purpose of bottoming the pilot screw when you do this?

 

Okay - not sure I follow but I'll run you through all the brass bits.

Looking down on the carb from the top, underneath the rubber diaphragm are the two air jets I mentioned in the first post. They are both off the center of the carb. Still looking down, at the bottom in the center of the carb is the emulsion tube.

Flip the carb over with the float bowl off and the two jets visible are the fuel jets. The Main Fuel jet is center and the pilot fuel jet is off center. A picture says a thousand words and should explain this better than I can write it.

 

Air jets pic here.

http://xjbikes.com/Forums/viewtopic/t=4 ... t=pic.html

Other pics here.

http://xjbikes.com/Forums/viewtopic/t=4 ... t=pic.html

Hopefully, this puts images to the words.

 

Thanks for the links, but my XJ550 carbs just don't look like those. I'll take some pics tonight and see if I can figure out how to post them tomorrow.

 

How about this one?

 

Yep, that's more like it! So, what's the long hole that goes down from below the diaphragm, that doesn't have a visible jet in it?

 

Hmmmmm, are you referring to the needle jet? From the top looking down you will only see a hole, from the bottom however the main jet is screwed into it.

The needle jet is a brass cylinder filled with 18 or 20 small dia holes on the side of it. It helps to atomize the fuel as it's drawn up and into the air body of the carburetor. Those holes need to be clean!

To remove this jet you have to unscrew the main jet from the bottom and then push the needle jet up toward where the diaphragm sits. Sometimes they get stuck in place and need a gentle nudge to remove them.

 

I don't ~think~ it's the needle jet that I'm looking at. The needle jet would be the hole that the needle in the center of the big round piston goes into - down at the bottom of the main carburetor bore. The hole I'm wondering about is up in the top end, at a similar location to the pilot air jet.

When I had the pilot screws bottomed, spraying through the pilot air jet came out at the top side of the main bore, just aft of the butterflies. Is this the only place where it should be coming through?

 

Sorry vinco, I can't help you on that one, I have the Hitachi carbs on my bike.

Hopefully someone else will come along and be able to help out.

Here's a couple of links with pictures of some Mikuni carbs that may help....

http://www.xjbikes.com/Forums/viewtopic/t=3633.html
http://www.xjbikes.com/Forums/viewtopic/t=3690.html

 

I think I may know what you are talking about. There are 3 holes in and around the areas of the air jets. One is the main jet and one is the pilot jet as we discussed earlier. The third hole is not a jet hole!

On some models of the carbs, there was a brass plate that covered the air jets and was screwed down using a tiny screw and the third hole. If you are missing the plate (sounds like you are) then don't worry about it. You need to be careful not to try to put the jets into the wrong positions. Off hand, I don't know if all three holes are the same thread but I think it's pretty hard to screw in a jet to the screw hole.

I hope this fixes you up. I've seen some pics of the carbs with and without the plate here on the forum but couldn't tell you where.

 

Well, I tore the carb tops back down last night, and I took some pics. The hole I'm concerned about is the one that's on the right side, as you're looking from back to front on the carbs, toward the front. The pilot air jet is on the left side, in a roughly equivalent position. Now, let's see if I can figure out how to attach a pic...

Nope, I have to figure out how to resize it first...

 

OK, here's another attempt... Might work.

The hole in question is the one at the top right of the upper diaphragm bowl.

 

Looked at the wrong part of the carb. What did you use to clean them?

 

I used much carb cleaner spray - so much, in fact, that last night I had to pull the spark plugs, because the engine was hydolocked! Oh, well - at least I shouldn't have carbon buildup problems on my pistons or heads Oil change today.

 

Well, over the weekend, I pulled the carb tops again and doused the pilot air jets with carb cleaner again, changed the oil, and reinstalled the spark plugs after removing to relieve the hydrolock. Then I hit the start button, and whaddya know? It ran! I could even regulate it with the throttle. This is great! Well, not exactly... Once I got my composure enough to actually look at things, I remembered that I hadn't opened the pilot screws at all, and that it was running strictly on the enrichment circuit. So, what the heck - I opened them all to 2.5 turns out, and now it's right back to the same old antics. It fires immediatelywhen I hit the starter button, and it races to 4000 RPM quickly, before dying after about 2 seconds. It won't hit at all without the enrichment valves being wide open. After the race-up-and-die routine, it will not really "catch" again until after I let it sit for half an hour or so. It will, however, catch just enough to idle for a few seconds at 1K or less, but never give any throttle response and never get any higher. (This occurs if I do not let it sit for half an hour between attempts.) I closed the pilot screws back to 2 turns out, with seemingly no effect.

Suggestions? Comments? Ideas?