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73 R75/5 Carb leaking from tiny hole near bowl gasket

shanman
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Joined: Wed Apr 08, 2009 11:23 pm

Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by shanman »

Its a good suggestion. The leak started randomly without having taken the carb apart whatsoever. I replaced o rings gaskets and float needle only because I thought that would help solve the leak. It's a strange one. I appreciate everybody's thoughts we'll figure it out!

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drpetemurray
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Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by drpetemurray »

Snowbum notes a BMW Bulletin specific for the R 75/ 5 stating the following.
Fuel Level: .767"-.787" or 19-20 mm.From the bottom center of the float bowl.

The OP 16mm fuel measurement suggests a low level in the bowl. I would also wait about 20 seconds with both petcocks open and at least 2 gallons of fuel in the tank when checking the bowl volume. A fuel level in the bowl of 16mm is far from the carb body and the gas is not seeping from the bowl.
I suspect the fuel is rising through the float choke reservoir cylinder and a faulty cork-fitted gasket is the cause of the leak.
When you drop the float bowl do try to observe immediately the fuel level in the choke cylinder, that level is elevated compared to the bowl chamber.
My /5 floats are set at 20mm. Good Luck.
BTW BMW shop manual suggests float level is set with the carb inverted and the float tang touches the ball on the needle ( touches not compressing the ball). At this moment the float should be parallel to the carb body. I am not sure how high the fuel level in the bowl would measure with this method.
PeteM
Stroudsburg,Pa
73 R75/5 , 1014 RTW, IBA 359

shanman
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Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by shanman »

-drpetemurray, what is the float choke reservoir cylinder?

Another note is that the cloth braided fuel line entering this carburetor is saturated with gas I'm wondering if this is related to the leak. Could the fuel in the line even after petcock turned off be dropping into the carb even with float needle closed and leaking at or near where fuel line enters carb?

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schrader7032
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Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by schrader7032 »

Interesting suggesting that the fuel line could be leaking. Do you see a path from the fuel line input to the carb and see how it could be getting around to where you see it dripping? One way to tell where the leak starts is to ensure that the carb body is clean and dry, then blow some baby power onto things. Petcock closed, make sure you get the powder distributed so you can tell where the leak is coming from.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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drpetemurray
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Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by drpetemurray »

shanman wrote:
Tue Jan 17, 2023 2:05 pm
-drpetemurray, what is the float choke reservoir cylinder?


When viewing the float bowl from the inside. I am referring to the front of the bowl's unplugged side. Fuel is drawn into the choke mechanism through this cylinder. A poor gasket fit or seal will leak at this corner.
You mentioned you have a wet fuel line which is above this area in question.
PeteM
Stroudsburg,Pa
73 R75/5 , 1014 RTW, IBA 359

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srankin
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Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by srankin »

LOL, get rid of that cloth braided fuel line and install something better. I did thirty years ago and rarely have a leak. That stuff is in my opinion one of BMW's worst jokes. Fuel like of the proper diameter from an auto parts store for fuel injection lines lasts many times longer. Sorry at this time I forget the diameter line you would need. My friend at his shop buys a roll of it from one of the suppliers.

As for fuel leaking with the petcock off, it should not. There should be no fuel flow with the petcock off and in proper working order.

Did you have the petcocks off the tank for some reason? If so, perhaps they are not installed properly onto the tank. There is a gasket ring on them. Perhaps it is shot and leaking?

Kurt's idea is pretty good for determining where the leak is. It could be possible to leak from the tank petcock area and run down the braided line onto the carb and then the ground. St.
Owner of a 84, R80RT and 78, R100RS

shanman
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Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by shanman »

I'm going to try fuel line perhaps the simplest thing of all was the problem and I was trying to make it complicated! Thanks for all the advice folks.

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schrader7032
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Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by schrader7032 »

I believe the original fuel line is 6mm in internal diameter, but the general car stores will likely only have 1/4" which is bigger than 6mm. You may need to use small clamps to hold the line on.

I've never had any real issues with the original braided fuel lines. Never had any leaks to date.

...update... I had a short length of original fuel line and it does seem to measure 7mm ID or a shade under.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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srankin
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Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by srankin »

Look around various places including other motorcycle shops, the proper non branded line is out there, sorry I don't have a direct name of the product or source. The last time I needed fuel lines changed I bought a couple of feet from my friend at his shop.

Yeah, I hear a lot of people say they never have problems with the braided stuff, that may be fine with them, I hate the stuff, I got tired of of finding seeping fuel lines and in the miles I have on my bike replacing it every couple of years. The fuel line I use now has only been replaced because I have the tank on and off and the line stretched a bit and was not snug on the taps and tees. Was not leaking, just figure it was time to replace it before it did. St.
Owner of a 84, R80RT and 78, R100RS

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Randolph
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Re: 73 R75/5 Carb leaking from tiny hole near bowl gasket

Post by Randolph »

Original fuel line is 7mm. Petcock hose barb is 8mm.
I use Motion Pro 1/4" gray fuel line. Gray because it blends in with the engine color.

https://www.motionpro.com/product/12-0052

Ebay has a Motion Pro site also.

Also slip a washer over the petcock barb ahead of the fuel line so you can grab it and push the line off the barb instead of trying to pull it off. Chinese hand cuffs, eh? I use an M10 with 20mm OD washer. The 20mm sticks out far enough I can grab it with my fingers.

-Randy

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