Are these the wrong Mikuni pilot jets for XJ900?

So, still messing with the '83 XJ900 carbs. I got them all cleaned up and replaced the seals. Throttle shaft screws were tons of fun, but much less stressful thanks to the 3D printed body protector I designed here. I ended up having to use an extractor on the first one, but the rest came out ok once I got the hang of it.

However, after I got them all back together, I was looking at jets (I figure I'll probably have to at least drop the mains a bit since I live and ride primarily at altitudes above 6000 feet') and noticed there are two different styles of pilot jet: drilled, and not drilled.

I know for a fact that the previous owner had replaced pilot jets because 1: They were the only shiny new parts in the carbs and 2: I found one of them still stuck on the end of an easy-out in the tool kit. I'm sure that's why they had the completely wrong carb in #1 also - probably screwed up the carb body getting the pilot out.

The jets they installed were the drilled type. Like this:



However, as near as I can tell researching information here, that style is only used on older XS versions of the Mikuni carbs. I don't think they were used in the XJ900 version.

I couldn't see a hole in the pilot tube as shown here (image 6) which is supposedly the indicator of whether you need the drilled ones. The threads are much deeper too.



Did the PO not only destroy a carb trying to replace pilots but also use the wrong replacements? And if they are wrong, will they cause a problem if they are in a carb without the drilled hole from the main?

If I remember right, I think the one stuck in the end of the easy out might have been a non-drilled type, but unfortunately, I threw it away and didn't think to take a picture or note the style since I wasn't aware there were different styles.

Thanks.

 

Why yes, yes he did! Congrats to him for a job done as poorly as possible.....it takes a lot of effort and skill to muck things up this greatly.

Pilot fuel jets with drilled bleed holes are for early Mikuni carbs (mainly used on XS models) where the pilot fuel jets drew their fuel supply thru the main fuel jets rather than drawing fuel from the bowl itself (the same deep well where the pilot fuel jet lives was still present, but it was blocked off by a screw or a rubber plug). The main fuel jet passage had a horizontally drilled passageway from it over to the pilot fuel jet well and that's how fuel got from the bowl to the pilot fuel jet (there's probably a very good reason that Mikuni used this method, but it escapes my little mind).

Oh, here it is:

A: The pilot jet meters fuel from the float bowl into 4 tiny fuel feed holes in the carb throat that feed the engine at small throttle openings. Easrly XS1100 pilot jets are an air bleed type and allow air to mix with the gas before entering the carb throat (where it mixes with more air). The reason to mix it with air first is to help atomize it but more importantly it dilutes the fuel with air so it is easier to meter precisely where it enters the airflow in the carb throat. Larger pilot jet means richer mixture at idle and first 1/4 throttle, smaller pilot jet gives leaner idle and first 1/4 throttle. On 1980-'81 XS1100 models, which have bowl-fed pilot fuel jets, the pilot circuit stays active over more of the throttle range than other models.


https://xs11.club/forum/maintenance/carburetors/867198-carb-identifier-guide-take-2

The two pictures midway down the page shows carb bodies both with (right-hand carb body) and without (LH body) the drilled passage between the main and pilot jet fuel well. Note that on thr right-side carb, that the pilot fuel jet well has threads where a block-off screw/gasket was used to isolate the pilot fuel jet from the fuel in the carb bowl, and all of the pilot jet fuel supply is thru the drilled passageway from the main jet well.

 

Thanks for confirming Len. That was the article I saw that led me to believe these were wrong. Just wanted to make sure I wasn't misinterpreting. That makes sense now why on the XS version the threads near the end are so much larger that the threads on the jet. I didn't realize there was a plug that went over it.

Every time I think I'm at the point this thing is ready to run I find another issue.

Funny thing is the guy I bought it from had a garage full of other bikes in various states of disrepair. Wondering now if those are all sitting there due to botched jobs too...

I can look on the bright side...probably wouldn't have gotten it so cheap or even had it go up for sale at all if he hadn't exceeded his ability to figure out what he screwed up.

 

@co.dirtbiker here is a thought....this is the perfect chance to try out a set of the Murray's Carbs and see how they do on the big bike. They aren't cheap but I bet they are not any more expensive than finding a good used set of factory carbs for your bike. Especially if you could find a set used. Was there not a set for sale here just a couple weeks ago? And the rest of us want to know but are too cheap to try them.

 

@Fuller56 that would be interesting, but alas way out of budget. I don't see one listed for the 900 either. Just the 750. Although my 750 still has a bit of a flat spot...hmm...

There couldn't possibly be much left wrong on these carbs. The carbs themselves are in great shape. The only real issue was that the PO had tried to swap just one of them for a BS36 instead of a BS35. And kept the BS35 slide. And had essentially no o-ring on the float valve. And messed up the alignment. And put in the wrong pilots. But other than that!

If I was starting from scratch, maybe, but I've already invested probably $250 to get these back up to spec and it's just a few more $ for a new set of pilots.