Unsolvable Carb Issue: Video

Bike: xj900

 

Bad float, float pin, inlet needle, needle seat or o-ring on the seat. If one of the float posts has been repaired the pin could be out of alignment, or the tube on the float is slightly pinched causing it to bind on the pin at random times. Start swapping possible parts between carbs to see if the problem "follows" the swapped parts.........start with the needle and seat (swap it with the #1 carb, for instance), and see if the leak "moves" of stays at the #2 carb. If it stays at #2, then swap the float and pin, see what happens.

 

Not to sound condescending but I've done everything you suggested multiple times...and then some. I swapped everything with every carb. Put brand new pins, floats, seats, you name it, I've done it. Problem persists ONLY in Carb #2.

I'm starting to wonder if fuel is somehow bypassing going into the float bowl and making its way directly into the overflow tube.

 

Don't know of any direct path from the fuel passages to the overflow port. Only other thing I can think of is a fracture (hairline of other) or flaw in the fuel inlet valve seat bore in the #2 carb body that isn't allowing the seat o-ring to seal.

I would also.....just for curiosity's sake.....put a clear tube on that carb and just watch what it does.

Also, are you sure that when you put the petcock on prime that it's actually flowing fuel? That's easy enuf to check.

If you want to get fancy you could find yourself a small glass bowl and substitute that for the fuel bowl on that #2 carb and watch whet happens in real time. You'd need an extra set of hands. And a fire extinguisher handy......

If it truly does not EVER happen in stop-and-go driving, then that's a clue of some type (like Sherlock's dog that didn't bark....), perhaps the additional vibration or fuel movement in the bowl from that type of riding jiggles something that is seizing up.

 

I've tried testing the o-ring seal by leaving the carb bowl off and keeping the float up with a rubber band, leaving the petcock on PRI over night to see if there was a leak. No dice.

I'm not quite sure what you mean by putting a clear tube on the carb. Are you referring to the overflow tube?

The PRI function of the petcock is indeed working. I tested it today actually.

I've done the makeshift clear bowl test. Everything works perfectly when the carbs are on my worktable. Put them on the bike, drive on the freeway, leak city.

Your hairline fracture theory is the only logical explanation I can think of.

 

Is there any other time that it NEVER leaks (besides on the workbench or in stop-and-go driving)? Engine hot? Engine cold?

 

When I first bought the bike 3 years ago it didn’t happen. It seems like these symptoms started happening after I broke the rack and ultrasonic cleaned the carbs. After that they just haven’t been the same. About 3 months ago I did take it on a 1700 mile journey after one of the MANY times I “fixed” the carbs and it did not leak but the mpg was horrible. Like, 25-30mpg. I was feeling confident last week thinking the problem permanently disappeared and went on another trip only for the issue to return and this time the bike started bogging throughout the entire journey.

 

To answer your question it only DOESN’t leak when the bike just sits there. That’s all I can tell you.

 

Does the spark plug on the #2 cylinder get fouled/black?

 

Yes

 

All the overflow leaks I have experienced have been bad o-rings on the float valve seat (my XJ550 and my FZ750). You say you have tested these and the test does look sound but I'd still
be inclined to renew. They are I think an imperial (not metric size) althoughI have found some (metric) that work.

 

thanks. I've decided to just switch out the carb body. Whatever the reason nobody can figure it out so this is the logical next step.

 

Guys. Please watch this vid. I may be onto something.

 

I’m seeing a “this video is private” error

 

Same

 

Sorry about that. This should work.

 

3 years ago I did an ultrasonic carb clean and things just haven’t been the same since. I have now just realized that one of my mistakes was switching out the emulsion tubes with a generic one from some generic kit. I’ve learned a l lot since then and realized that this is crucial to have the exact correct ones in there. I managed to find what I think are the originals, however, when I look at the numbers on them it says #318 Y-1. The factory manual says #318 Y-0. Is there a difference? I can’t possibly think where else these emulsion tubes would come from in my box of spare parts. I had an xj550 for a minute and currently own a Seca 650. I also worked on an ‘86 FJ1200.

 

If it is like my xj700, the # 2 carb is hooked to the vacuum line to the petcock. Perhaps the vacuum is preventing air from reentering the #2 carb and causing the drain tube to siphon the gas out of the carb. Disconnect the vacuum line and see if it still continues to drain.

 

550 emulsion tubes are style 342, Hitachi carbs use a completely different type emulsion tube that won't physically fit into a Mikuni carb, Style 318 tubes are used in XJ650 Turbo (size O-8), XJ700-X and XJ750-X (size Y-2), and XJ900's (size Y-0).......never had a Style 342, size Y-1 tube (and can't find any info on it), but logically, it should exist. FJ1100 and most FJ1200's use a Style 378 size Y-2 tube. All that being said, an incorrect emulsion tube wouldn't cause a leak as you're experiencing. Still think it has something to do with the fuel inlet valve, float issue, or a casting defect, but.........who knows. Very curious, and thus very frustrating.

Just spit-balling here: sometimes the stampings are rather faint and/or degraded.......could it be that the "1" is really just the left vertical element of the number "0" (which on stampings tends to be "square" rather than with rounded lines)?

Mikuni main needle jet sizes (which run from -0 to -9, regardless of emulsion tube "style") are as follows:

For "O" size needles, they start out at O-0 size with a 2.600mm orifice ID, and then increment by 0.005mm per size step (so an O-1 needle has an ID of 2.605, the O-2 is 2.610, the O-3 is 2.615, etc.)

The "Y" size needles start out at size Y-0 with an ID of 3.100mm, and again, increments by 0.005mm per size step. So a Y-1 size would be 3.105mm, and the Y-2 checks in a 3.110mm, etc.

You might be saying to yourself that these sizes are mighty darned close to each other, since 0.005mm is about 0.0.0001968504" (and that difference is way too small to be seen visually).......and you'd be right. Mikuni was more of a race-carb manufacturer (as opposed to Hitachi) and thus there is (or was) an incredible range of "tuning" parts available, so that racers could experiment and get things "just right" for their bike/situation. And of course this leads to the observation that your bike is probably not going to perform significantly different if you use Y-1 emulsion tubes instead of the stock Y-0 size tubes (as long as all carbs have the same size tubes, of course!). If you start straying 2, 3, 4+ sizes away from stock, well then of course, presto-bingo, you're a "tuner" with all the joys and heartaches that come from such a profession......

 

Ha! The vacuum was disconnected at the time of this video.

 

I'm just going to have to fiddle with it some more. Thanks Chacal.

 

May I ask another question. I removed the carbs and it seems oil made it as far back to the airbox. What are possible reasons for this happening. I checked the valves and all were in spec except cylinder #4 where the exhaust was at 0.23mm and the intake was at 0.16mm.

I should mention that I tipped over the bike about two weeks ago. The bike was on it's side for about 5 minutes. When this happened oil started leaking out of the bike. I'm wondering if it made it's way up to the carbs. Seems odd.

 

The crankcase vents to the air box through a rubber hose. The oil from your engine likely traveled through this hose to your air box when you tipped it. No big deal in my book

 

Not sure of the internal passage routing on that carb, but a siphon connection between the main jet and the drain via the air jet emulsion tube passages seems most likely. I suppose it could siphon through the enrichment circuit also. Almost has to be an internal crack or defect I think? Very strange. Very thorough tests there also.

After it is done siphoning, how much fuel is still in the bowl if you open the drain?

 

Sure looks like a Y-0 in the picture you posted?

I am guessing since the 900 has overflow tubes then you won't contaminate the oil with gas. However, since thinned overfilled oil also has a way of getting blown out the breather to the air box make sure your oil is not overfilled and contaminated with gas. Hopefully, it was just the tip over and that would fix itself

So knowing what you know about blowing into the vent tubes for the carbs I would say no way should the vent hose be routed tied to the frame with the opening exposed to turbulent air under the tank. It's not hard to imagine duplicating what your experiment is doing while driving down the road, especially at highway speeds. I read one post where those vents should drape over the airbox, but hopefully @hogfiddles will chime in and give you the exact routing

Back when hogfiddles was still learning:
https://xjbikes.com/forums/threads/help-with-900-carb-hoses.31026/

 

Ugh. Never checked.

 

Looking closer at the pic I think you're right. It does say Y-O.

About the placement of the vent tubes. I think you are also right. I always thought it was pretty guarded under the tank as far as wind flow but in the back of my mind I always questioned the placement of those tubes. As mentioned in the link you shared, there is no information regarding where those hoses should be routed. I just assumed they were in the right place. When I bought the bike from the PO he was the original owner and this was how he had the vent tubes routed. Why should I question? It's interesting because when the PO sold me the bike he didn't mention the slipping starter clutch (which I since fixed thanks to Jetfixer) and I can only wonder if he was also having this leaking issue as well leading him to sell the bike for pretty cheap in my opinion. He did tell me he was selling cheap because it was going to take a lot to get the bike road worthy but never gave me the specifics. Man, it's interesting the abuse and mishandling these old magnificent machines endured only to find it just takes the simplest things to get them going. If this fixes my issue Rooster, it will be over 2 years of dealing with it. thanks for noticing that. I will get the carbs back on asap with the CORRECT emulsion tubes. This bike might finally be fun to ride.

 

Very interesting stuff. Thanks Chacal. I think you're right about the "1" being just the left vertical element of the number 0. Since putting in the generic emulsion tubes I got from some kit I've had horrible mpg. 25-30mpg average. Bike runs fine but the plugs have always been super sooty and something just didn't feel right about the bike. Looking forward to putting back on the originals.

 

Sorry for my rookie reply (above). I didn’t completely read your post.
Sounds like I have a lot to learn from this discussion

 

I have had some where the overflow hoses tucked down between the airbox boots, and others where theyre longer and go over the front top of the airbox.

personally I find that tucking down between the airbox boots fits/looks/would flow better.

 

Another issue SOLVED! I through the hoses over the air box. It almost seemed like there was a little space there for the vent tubes to slide into. In addition I installed the original emulsion tubes not to mention changed out the fuse box to blade style and got those fuel levels perfect in the bowls and now WOW! This is one amazing bike! It just feels tight. I put some new Bridgestones on there as well. It doesn't bog anymore when I open the throttle at highspeeds. I love this bike.

I've had this issue for almost 3 years. THREE YEARS! I almost sold the bike because of it. And I tried everything. EVERYTHING! People were telling me to try all sorts of things. Some people even scolded me and told me and to just give up and replace the carb body. But I knew there was something peculiar. I went to all the different forums but xjbikes is the one that solved it. Thank you everyone. Special shout out to Rooster. You really have an eye. Happy riding everyone.

 

I don’t think the tube placement was the issue.... I think the emulsion tubes would more likely be it.

that said, I’ll hafta check overflow tubes place thin my 900’s..... I may be thinking of the 700 tubes.

I’ve been down most of the summer since before April With a bad sciatica issue, and have just been able to get working on things about 2 weeks ago.

 

Sorry to hear that. I had bad sciatica once and it took about two months of daily strengthening exercises to relieve the pain. I now do them pretty regularly and I've been about 98% pain free ever since.
I can't quite see how the wrong emulsion tubes would cause pressure to build up inside the carb body forcing fuel out of the overflow tubes. I kind of wish I performed the experiment of just repositioning the vent hoses before I changed the emulsion tubes but nevertheless because I performed the experiment of blowing air down the vent tube and I was getting a similar overflow issue, I can only think that with the opening of the vent tube facing the front of the bike air was being force down the vent tube and not allowing the carb to vent properly. It also explains why this only happened when I was on the freeway as oppose to stop and go traffic.