STRANGE FUEL LEAK on LEFT BS38 78-79

ANLAF

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Fellers, any ideas on this leak?

I turn on the fuels and the left carb starts to drip fuel from the UNI filter. Took filter off and see the fuel is coming out of the the pilot inlet (as named in the Carb Guide).

This bank of carbs is 78-79 BS38s, and the pilot hole is the lower left of the five intakes you can see on the back of the carbs.

Things I have tried:

1. checked all screws correctly tighted.
2. checked float bowl gaskets are in order.
3. cleaned with carb cleaner.
4. checked floats.
5. checked the floats correctly adjusted.
6. tried swapping good floats from second bank of carbs
7. checked the float needle pin is working properly.
8. tried using pins from second bank of carbs.
9. checked float needle seated properly.
10. checked the slide needle operates properly.
11. swapped around the float bowls.

I have repeated these things many times, tried variations, interchanged, but still leaks - HELP.

In the Carb Guide it mentions a potential float system defect but this is in pre-'78 carbs where the float chamber is vented at the bottom of the float bowl through a brass pipe seated in the float bowl, which opens above the correct fuel level. The 78-79 carbs do not have this vent.

I have installed my other bank of carbs and they work fine, but they are for my other XS650 so I am desperate to fix this problem.

Thanks

ANLAF
 
TwoMany

My 76-77 BS38s have the brass outlet at the base of the bowl, these 78 types don't.

So, I'm down to double-checking the bowl vents from the bowl itself, and the pilot and main vents from the outlet to the filter.

There must be something blocking-up and creating a vacuum pulling up fuel through the pilot vent. Pity I don't have compressed air to work with.

ANLAF
 
Is this happening while the bike is leaning way over on the side stand?
You never mentioned checking the float valve. I think you might merely have a float valve that isn't closing.
EDIT: okay, you did check the float "pins" and seats -- this probably isn't much help, but still sounds like a float valve. Is the bike leaning way over?
 
the fuel is coming out of the the pilot inlet (as named in the Carb Guide).
This bank of carbs is 78-79 BS38s, and the pilot hole is the lower left of the five intakes you can see on the back of the carbs.
On BS38s the lower left of the 5 intakes is the Main air jet, not the pilot air jet.

There must be something blocking-up and creating a vacuum pulling up fuel through the pilot vent. Pity I don't have compressed air to work with.
You mention creating a vacuum, so the leak only occurs when the bike is running?
If the main air jet itself is not plugged, there is pretty much nothing else in the main jet air circuit that can get plugged. You don't need compressed air to test it (or anything else on the carbs), just a can of carb cleaner. The main air jet is the tiny fixed brass orifice that is pressed into the opening in the intake bell. Spray cleaner through the Main air jet (the lower left of the 5 intakes), and watch it come out the bottom of the body.

If the main air jet circuit is blocked, then you won't have gas leaking out of the main air jet.
 
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Your carbs are over-filling, simple as that. Since the '78-'79 carbs have no overflow nipple on the bowl bottoms, the excess fuel comes out the air jets or up through the needle jet. Your float needles and/or seats are not sealing or a float is bad or set wrong. I assume you have no centerstand so the bike is healed over to the left on the sidestand. The problem could be in either carb. If the right one is at fault, the excess gas is transferring to the left carb through the crossover tube. The left carb is getting overfilled and leaking, making it appear to be at fault, but it may not be.
 
DogBunny and 5twins

The bike is way over left on the side-stand. I did consider that and on the last two attempts (when I had tkaen carbs off and refitted) I sat on the bike before I turned on the fuel. After about 20 seconds the lower left small vent (thanks for the correction - the main jet vent) becomes damp then out comes the fuel. This test shows it is something on the left carb then - the connecting tube between the carbs is higher than the float bowl, but of course it is there for a reason so fuel must be able to pass even on level ground. Anyway I swapped the fuel bowls around - fuel still there from the leftn nothing from the right.

So, coming through the main jet suggests fuel getting through where it shouldn't, so, Question what is the relationship between the end of the float needle and the main jet? is that a touch or are they supposed to create a seal when the bowl is screwed in place?

Thanks for the info re compressed air. I am well-armed with carb cleaner.

ANLAF
 
5twins

You mentioned float needles or seats not sealing, and I have swapped the bowls around. This suggests if I swap around the float needles the problem may leap from left carb to right. As for the floats themselves I have two pairs, and I have swapped them round again - and I know both pairs are functioning because i have tested them in my 76-77 carbs over the past week.

I have a strong feeling the needle jet is incorrectly seated or there is a fault in the seating between needle jet and main jet.

ANLAF
 
Normally over-filling float bowls will leak from the top of the needle jet. But if the bike is leaning way over, then the main air jet orifice becomes the low point, and it leaks from there.

With that much lean, the float valve needle can become cocked in its seat and/or the needle doesn't receive enough upward pressure from the floats to seat properly.

I would put a piece of two-by-four (a wooden board here in the U.S.) under the kick stand to get the bike more upright, and if you continue to have leaking I would re-check the float valves.

Regarding an incorrectly seated needle jet, if you dunked the carbs without removing the needle jet, then you may have destroyed the needle jet o-ring. But then the leaking would come from the needle jet, not the air jet. I think the problem is with the float valves.
 
DogBunny

Thanks for that. I have two pairs of float valves (I called them the pins, didn't I) and swapped them around. The valves themselves look in good shape, and two are working perfectly well in the other carbs, so I wonder if the seating could be damaged or blocked in anyway. It will only take a little bit of stray stuff to allow a gap for liquid to pass through. I see where you are coming from. I will look there.

ANLAF
 
Look wherever you like, it probably won't help. I hate to say it but you're a mechanical moron. I've watched you fuck up pretty much everything you touch on your bike. Yes, you eventually fix it, but you cause the problems in the first place most of the time. Then there's that other thing - YOU SHOUT OUT ALL YOUR POSTS LIKE THE WORLD IS ENDING. It isn't, just the life of your bike is, because you're killing it.

Not good dude. I'm going to sign off on helping you. Good luck with the bike. Others will step up and help for awhile but I'm sure they'll come to the same conclusion eventually too.

But please don't stop posting, it's very entertaining, lol.
 
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