Leaky Left Carburetor - R100RT

[nevada72]

Greetings all. My R100RT is coming along nicely. It is actually 100% rideable and enjoyable. There is just one thing - the left carb intermittently leaks (all over my boot!). I'll start the bike, let it warm up, head out - no leaks. But after awhile I'll smell gas, look down, and sure enough, it's leaking gas onto my boot. It does it for a few seconds and then stops. But farther down the road it will do it again. Maybe every 5-10 minutes.

I've replaced the float bowl gasket 3 times. I have somewhat adjusted the float level to see if that helps - it hasn't. I thought maybe the float was sticking so I played with that a little bit to see if that did anything - nope. I took the bowl off and tested the needle valve - it shuts off the gas when I lift the float and let's gas flow when I lower it. The carbs were recently rebuilt, but not, as it turns out, by someone who had a full knowledge of Bing carbs. He reversed my chock assemblies rendering the choke useless, which I discovered and fixed. That said, all the soft parts (float,diaphragm, gaskets, o-rings) and needle valve is new. I don't even know for sure where it's leaking from because it only happens when I'm riding. I'm assuming from the overflow, but I just don't know. The right carb is just fine - never leaks.

Any ideas?

 

[r1200rst]

overflow tube

Greetings all. My R100RT is coming along nicely. It is actually 100% rideable and enjoyable. There is just one thing - the left carb intermittently leaks (all over my boot!). I'll start the bike, let it warm up, head out - no leaks. But after awhile I'll smell gas, look down, and sure enough, it's leaking gas onto my boot. It does it for a few seconds and then stops. But farther down the road it will do it again. Maybe every 5-10 minutes.

I've replaced the float bowl gasket 3 times. I have somewhat adjusted the float level to see if that helps - it hasn't. I thought maybe the float was sticking so I played with that a little bit to see if that did anything - nope. I took the bowl off and tested the needle valve - it shuts off the gas when I lift the float and let's gas flow when I lower it. The carbs were recently rebuilt, but not, as it turns out, by someone who had a full knowledge of Bing carbs. He reversed my chock assemblies rendering the choke useless, which I discovered and fixed. That said, all the soft parts (float,diaphragm, gaskets, o-rings) and needle valve is new. I don't even know for sure where it's leaking from because it only happens when I'm riding. I'm assuming from the overflow, but I just don't know. The right carb is just fine - never leaks.

Any ideas?

I went nuts with a similar problem that turned out to be a split in the overflow tube. If you have compressed air you can fill the bowl with water and blow up the tube to look for bubbles. I was able to repair the split with simple solder. Hope this helps. Michael

 

[20774]

Consider Section 19 on this page:

http://bmwmotorcycletech.info/bingcv-2.htm

How are the carbs aligned...I've always tipped the top of the carb in slightly.

 

[beemerphile]

Now you know what a "Bing Boot" smells like.

With these, your boots continue to smell like leather...

 

[bikertucker]

Had a 90/6 that had a split overflow tube also. Was able to clean it up and soldered over the split.

Good Luck

 

[nevada72]

Thanks for the info gents. I'll take a look at that overflow tube.

And section 19 of Snowbum's info may be a clue as well. I'll replace the hoses that connect the carbs to the motor with the recommended part. The ones on there are old and cracked anyway.

 

[sterob]

A last resort may be to replace the seat itself. I did this a while ago and it fixed my problem.
You need a M7 tap , nut and bolt and make a simple puller to get the old one out.

 

[pglaves]

I suppose it is covered in Snowbum's write up but the very first thing to do is to verify that the setting of the float provides the proper fuel level in the bowl. If the fuel level is too high then fuel can literally slosh into the open end of that overflow tube and dribble out the bottom.

The easy initial step is to put the bike on the centerstand with the fuel petcocks "on". Then turn the fuel off and carefully remove each fuel bowl and measure to verify they are the same.

If they are not identical then you need to properly adjust the float(s) to get the correct fuel level in each bowl.

 

[nevada72]

I suppose it is covered in Snowbum's write up but the very first thing to do is to verify that the setting of the float provides the proper fuel level in the bowl. If the fuel level is too high then fuel can literally slosh into the open end of that overflow tube and dribble out the bottom.

The easy initial step is to put the bike on the centerstand with the fuel petcocks "on". Then turn the fuel off and carefully remove each fuel bowl and measure to verify they are the same.

If they are not identical then you need to properly adjust the float(s) to get the correct fuel level in each bowl.

This makes perfect sense, and not something I had thought of. Much appreciated!

 

[James.A]

That brass nozzle that the fuel hose slips on to can get loose in the carb body. Similarly, the fuel hose that slips on to it can weep. It's one of those "out of sight" kind of things.

 

[Anyname]

Following up on Paul's comments, it's also possible that one or both floats is starting to sink.

 

[khittner]

Following up on Paul's comments, it's also possible that one or both floats is starting to sink.

If your floats are the same brown color as a used cigarette filter, they're too heavy and ready for replacement.

 

[nevada72]

The floats were just replaced. I also have to wonder if the left one isn't catching on something.

 

[FELAW]

Leak

The floats were just replaced. I also have to wonder if the left one isn't catching on something.

If:
1) the floats are set to the right height.
2) the overflow isn't split or loose
3) the float needle seats when the bowl is off
4) you get a leak when the bowl is attached.

then it seems that you have the situation where the float works ok without the bowl and not ok with the bowl on.
Therefore, Make sure the float is properly adjusted on the pivot armand not hitting anything when the bowl is installed.
I had a leaky carb and had to trim the float bowl gasket about a 1/16" in one place because the float was getting stuck on it and not raising the needle enough to seat and shut off the fuel flow.

 

[nevada72]

I went to take a look at the carbs in earnest this afternoon. I did as PGlaves suggested and emptied the bowls, opened the petcocks, then shut them off. The amount in each bowl was identical. I also checked the float needle to see if it shut the fuel off when the float seam was level to the bottom of the carb. It did.

At that point I decided to take both carbs off to compare, and maybe even swap parts like float needles to see if the problem follows the part. I checked the bowl gasket to see if there was any overlap as mentioned by FELAW. The gasket looked very good with no overlap. However, when I was removing the left one I noticed the fuel line came off very easily. So I put the bowl back on and turned on the petcock to see if I could find a leak. Sure enough, when I moved the carb around it leaked out of the hose. I tried to put a hose clamp on, but due to the fuel line diameter (not BMW fuel line), the clamp will not fit between the hose and the carb body. So I did the best I could and got it over the top portion, but not all the way down.

I took the bike out for a quick spin to see if there was any improvement. There was in fact. It was dry for a good 5 miles, but let loose on me once.. It was definitely better, so I'll get new 1/4 inch fuel line from NAPA and see if that takes care of it. I'll report back tomorrow with good news I hope.

 

[Friedle]

1/4 inch fuel line is not correct. Order the correct 7 mm size fuel line from your dealer or one of the aftermarket BMW suppliers on this forum. You won't need clamps and your boots will stay dry. NAPA is not a BWW motorcycle parts dealer. I buy my tractor and 99 Jeep XJ parts from NAPA.

Friedle

 

[happy wanderer]

I went to take a look at the carbs in earnest this afternoon. I did as PGlaves suggested and emptied the bowls, opened the petcocks, then shut them off. The amount in each bowl was identical. I also checked the float needle to see if it shut the fuel off when the float seam was level to the bottom of the carb. It did.

At that point I decided to take both carbs off to compare, and maybe even swap parts like float needles to see if the problem follows the part. I checked the bowl gasket to see if there was any overlap as mentioned by FELAW. The gasket looked very good with no overlap. However, when I was removing the left one I noticed the fuel line came off very easily. So I put the bowl back on and turned on the petcock to see if I could find a leak. Sure enough, when I moved the carb around it leaked out of the hose. I tried to put a hose clamp on, but due to the fuel line diameter (not BMW fuel line), the clamp will not fit between the hose and the carb body. So I did the best I could and got it over the top portion, but not all the way down.

I took the bike out for a quick spin to see if there was any improvement. There was in fact. It was dry for a good 5 miles, but let loose on me once.. It was definitely better, so I'll get new 1/4 inch fuel line from NAPA and see if that takes care of it. I'll report back tomorrow with good news I hope.

Do yourself a favour and buy the right fuel line. BMWs are METRIC bikes and the fuel line they use is 7mm. Not 5/16" or 1/4" which is often found on BMW bikes because that is what the auto parts stores carry. Go to a VW shop and get some 7mm line or order some off Amazon or eBay. It won't leak!

 

[nickrides]

The old school VW cloth covered fuel line is the coooolest looking fits good too!

 

[nevada72]

I get what you're saying about the 7mm line and getting the BMW part was my first inclination. However, there are as many folks out there who swear the BMW line is of poor quality for today's ethanol fuel as there are who think it's the only way to go. In the end it wasn't the fuel line that is the issue.

I went for a somewhat longer ride yesterday, but not the 100 miles I was hoping to do. The leak actually got worse. At one point it was just pouring out continuously for miles. It was pouring out at the rate one would expect an open petcock would do - in other words - quite a bit. I pulled over hoping it would keep leaking but nope - as soon as I stop - it stops. Very frustrating. I would get back up to speed - about 55 MPH and 4000 RPM and it started again. So I got the idea to kill the motor, glide to a stop, and pull the float bowl. It was VERY full. Way higher than when I did the test in my garage. So obviously something is preventing that float needle form shutting off.

I'm back to pulling the carbs today and I'm going to swap parts (needle and float) and see if it follows the parts.

 

[pglaves]

It is now my carefully considered opinion that you are suffering from the discharge of crud from a fuel line (or maybe the tank). If it was truly a damaged needle or seat it would leak all or most of the time. Since it comes and goes it seems to me it is crud periodically preventing the needle from full closure.

Start by using all new fuel line, coupled with removing the fuel tank and petcocks and ensuring that there is not grit or crud in he bottom of the fuel tank.