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Old 07-11-2005, 07:17 PM   #1
Airhead OP
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Mikuni BST40 experts?

The carb on my 2000 LC4 is leaking gas through the drain hose. I checked the drain plug screw, and it is fully closed (I even removed it to see if there were any damaged threads, etc...it seemed to be fine). The gas leak is quite slow with the petcock turned off...maybe one drip every 5 minutes or so. With the petcock turned on, however, the thing pretty much pees gas.

My question is, is the drain plug screw bad? Or does the drain hose have a secondary circuit that will also drain, say, a bad needle/seat even when the plug is fully closed? I don't want to go replacing the needle if the plug screw is the real culprit.

One key point...if I run the bike with the petcock closed until it stalls, it does not leak any gas...but if I then open the drain plug screw, gas will empty from the bowl. This leads me to believe that the plug works correctly, and that something else is going on.

Any opinions? Creeper? Meat?

BTW, anyone know the part number for the bleeder plug screw? The PO munged up the phillips head flats, probably trying to fix the gas leak by overtightening the drain plug screw. KTM's microfiche doesn't even show the screw as a part...
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Old 07-11-2005, 07:49 PM   #2
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Quote:
Originally Posted by Airhead
The carb on my 2000 LC4 is leaking gas through the drain hose. I checked the drain plug screw, and it is fully closed (I even removed it to see if there were any damaged threads, etc...it seemed to be fine). The gas leak is quite slow with the petcock turned off...maybe one drip every 5 minutes or so. With the petcock turned on, however, the thing pretty much pees gas.

My question is, is the drain plug screw bad? Or does the drain hose have a secondary circuit that will also drain, say, a bad needle/seat even when the plug is fully closed? I don't want to go replacing the needle if the plug screw is the real culprit.

One key point...if I run the bike with the petcock closed until it stalls, it does not leak any gas...but if I then open the drain plug screw, gas will empty from the bowl. This leads me to believe that the plug works correctly, and that something else is going on.

Any opinions? Creeper? Meat?

BTW, anyone know the part number for the bleeder plug screw? The PO munged up the phillips head flats, probably trying to fix the gas leak by overtightening the drain plug screw. KTM's microfiche doesn't even show the screw as a part...
Geeeezzz. Next time I take something (like the carb) off my bike and take it appart... I should really take pictures.
I can't remember if there is a standpipe for an overflow in the bowl or not. If not, you probably have a damaged screw seat (soft zinc alloy you know) or a porosity, or crack in the drain screw seat area.

Sorry...not much help. Maybe someone has a BST on the bench and can pop the bowl off to take a look.
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Old 07-11-2005, 07:58 PM   #3
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Sounds like your float level is set too high. The screw drains the bottom of the bowl, but there is a standpipe that discharges "downstream" of the screw "shut off/bowl drain."
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Old 07-11-2005, 08:07 PM   #4
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Quote:
Originally Posted by Mack
but there is a standpipe that discharges "downstream" of the screw "shut off/bowl drain."
I knew that was the brain cell that died recently.

What the hell would I do without my off-site, auto-run, sub-memory slash hell-of-a-sweet-guy, lovingly known as Mack? More of this no doubt.
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Old 07-11-2005, 08:09 PM   #5
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Quote:
Originally Posted by creeper
Geeeezzz. Next time I take something (like the carb) off my bike and take it appart... I should really take pictures.
I can't remember if there is a standpipe for an overflow in the bowl or not. If not, you probably have a damaged screw seat (soft zinc alloy you know) or a porosity, or crack in the drain screw seat area.

Sorry...not much help. Maybe someone has a BST on the bench and can pop the bowl off to take a look.
I got one here. I took it apart. The overflow has a ~ 1-1/2" pipe. At the bottom is the drain screw which drains fuel from the bottom.
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Old 07-11-2005, 08:45 PM   #6
Mack
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Quote:
Originally Posted by creeper
I knew that was the brain cell that died recently.

What the hell would I do without my off-site, auto-run, sub-memory slash hell-of-a-sweet-guy, lovingly known as Mack? More of this no doubt.
Awww, Creeper







Sorry musta been the Prednisone talking......

I'm back to normal now, so its off to the Renazco dungeon for you.
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Old 07-11-2005, 09:30 PM   #7
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Quote:
Originally Posted by Mack
Awww, Creeper







Sorry musta been the Prednisone talking......

I'm back to normal now, so its off to the Renazco dungeon for you.
Not for a week or so...
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Old 07-12-2005, 05:57 AM   #8
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Quote:
Originally Posted by Mack
off to the Renazco dungeon for you.


where is that big palooka? he takes a pot shot at me then flees!

maybe he's making seats for all those poor buggas that have to ride standing up for months!
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Old 07-12-2005, 06:17 AM   #9
Airhead OP
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Thanks guys! I figured there was some kind of overflow drain that would bypass the screw...the symptoms didn't make sense otherwise. I'm replacing the needle/seat and checking the float level as soon as the parts come in...sounds like it will probably fix the problem!
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Old 07-12-2005, 06:20 AM   #10
Airhead OP
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BTW...can you take the bowl off (easily) without taking the carb off the bike? Doesn't seem like you can...

And, when you check the oil level on bikes with a dipstick instead of a sight glass, do you do it with the dipstick screwed in, or unscrewed?

Obviously, I didn't get an owner's manual with the bike, and the only one I can find online os for the '03, which has a sight glass.
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Old 07-12-2005, 07:12 AM   #11
meat popsicle
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Quote:
Originally Posted by Airhead
BTW...can you take the bowl off (easily) without taking the carb off the bike? Doesn't seem like you can...

And, when you check the oil level on bikes with a dipstick instead of a sight glass, do you do it with the dipstick screwed in, or unscrewed?

Obviously, I didn't get an owner's manual with the bike, and the only one I can find online os for the '03, which has a sight glass.
since having the dipstick unscrewed might lead to inconsistencies from person to person, i am betting its screwed.

I don't remember anyone saying if you can or can't get the bowl off that way. Have a tool that will get the fasteners? might just try it or someone will speak up. There is a few carb removal tricks around here somewhere that should be in the Index Thread... hope someone finds them!
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Old 07-12-2005, 08:01 AM   #12
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Quote:
Originally Posted by meat popsicle
since having the dipstick unscrewed might lead to inconsistencies from person to person, i am betting its screwed.

I don't remember anyone saying if you can or can't get the bowl off that way. Have a tool that will get the fasteners? might just try it or someone will speak up. There is a few carb removal tricks around here somewhere that should be in the Index Thread... hope someone finds them!
I removed the bowl with the carb on. I had to use a 1/4" ratchet and bit. The second time I tried to do that though the screws were siezed and I had to pull the carb and fricken cut them off with the dremel. I replaced them with stainless allen heads and spring washer + flat washer and install them only to snug tightness.

You don't really want to use spring washers directly against the aluminum as it will chew it up. Maybe you can use two spring washers, as I think that's how mine was originally.
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Old 07-12-2005, 11:50 AM   #13
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You can get the bowl off without too much hassle. I use one of those right angle screwdriver things that have a flat tip on on side and philips on the other. Only problem I had was replacing the idle jet. Thought it was in tight but found out not tight enough when I rolled off the throttle 1/2 mile from home and the bike said "BBBBBbbbbbbbrrrrrrghh... puh!" and quit. Long half mile push. I ended up using a driver from mini screwdriver set and now the jet's in there real good.

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Old 06-05-2009, 06:22 AM   #14
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I know it's an old thread, but it is one that I searched to try and find a cure for my overflowing carb.

The bike was parked for about 8 months and when I started it, it mysteriously suddenly pissed fuel from the overflow, as long as the motor was running.

I stripped and checked for sticky floats, checked the float-needle for wear and lastly checked that the float level was set right.

Re-assembled the lot and still the same problem. Then I took it all out again and this time blew into the fuel line-in and manually opened and closed the needle. The problem was clear to see, the whole needle-seat moved. So the needle seated properly and shut the incoming fuel off, however the seat allowed fuel to bypass it. The seat seats with a little o-ring, which had become hard and allowed the seat to move in it's recess.

Here's some pictures to show what I mean.











Lastly, I changed those crap screws that keeps the bowl on to stainless allen heads. Just another step to nirvana.



Maybe this helps someone sometime.
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Old 06-05-2009, 06:41 AM   #15
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Quote:
Originally Posted by metaljockey
+1. That's the o-ring that i replaced on mine when I had the leaking fuel problem. Fixed it right away. I think I went to an american sized o-ring as well, which was just a tiny bit larger and it seems to be doing well.
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