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gas leak when running only

  • Thread starter Thread starter VashonGS750E
  • Start date Start date
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VashonGS750E

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I have a 1980 GS 750E with a small gas drip coming out of what I think is a carb breathing tube, black, right lower side of bike next to the rt foot peg, but inside the frame area. It is a small drip that only happens when the bike is running, but I'm worried about a gas leak. The other thing I was wondering after reading another post is my carb / air filter tube connection is hard to keep in place. The tube rubber that connects the carbs to the air filter chamber has hardened and the ring clamps dont work to well. SOO I was thinking maybe I have an air leak which is causing my carbs to over compensate with too rich a mixture & too much gas is coming through the carbs ?
Anyway, I'm new to this web site, a returnee to the motorcycle scene after a 25 year layaway, so I apologise for errors in postings etc... I love my new old bike and am looking forward to this summer.
This is a great web site!
Scott,
80' 750 GS with 7,500 miles.
 
Welcome!

The gas will only leak when the bike is on because the petcock only let's it flow when the bike is running (the petcock turns on the fuel when it gets vacuum pressure from the no. 2 carb).

I'm no pro on this, but your float may be sticking in one of your carbs. Try tapping the carb float chamber with something. A carb rebuild may be in order.

For the rubber parts, that is a common problem with these bikes though I think that will only effect your mixture, make your bike run lean, and not cause a fuel leak. Try ordering new rubber boots. You can get them for about seven to ten dollars each. I order from www.bikebadit.com . Super fast shipping.
 
There is an O Ring around the float valve that has probably hardened up in a compressed position and is allowing fuel to seep past the outside of the float valves. Any bike that is as old as these should have all the O Rings Replaced in the carbs. You can get them from a member here www.cycleorings.com and are dirt cheap. As mentioned above float valves may also be sticking. Rubber connectors on each side of the carbs and into the air box must seal tight. Replace them for a good seal. That's an excellent bike you are working on there.
 
Used to take the ferry over to Vashon and run around on my old Honda 750F when I lived in Seattle 25 years ago. Cool place, at least back then.

Your air leak if you have one, shouldn't be causing gas to come out of your breather tube. Those connecting boots can be replaced and probably should be if they are too hard. Air leaks will lean out your engine too much, perhaps leading to overheating, etc.
 
Thanks everyone for the replies. Looks like I have a bit of work but not too bad. The bike is in great shape so its well worth it. I'll have to dig into the filter and carbs a bit.

As for Vashon MARK yeah it ( like so many other places in Western Wa. ) has changed a lot in the 25 years I've been over here! We may be moving this summer and I look forward to freedom from ferrys again...K2 is gone !!!
Thanks again for everyones input.
 
Is the fluid coming out of the tube attached to the airbox? Are you sure its gas and not water? My 550 will drip a little water until it is fully warmed up. It's just vapor which comes back from the crankcase breather tube and condenses and settles at the bottom of the airbox. This is normal.

To find out if you have leaking carbs, try setting your petcock in the PRI (prime) position and observe while the bike is not running. If you notice it dripping then you need to service the carbs (new float valve o-rings most likely). This is assuming your petcock is operating correctly.

It sounds like the rubbers which attach the carbs to the airbox may need to be replaced but note that it is very tricky to get these to fit snug on the carbs. Getting them to fit is a "3-handed" job. You need someone to push the airbox forward (while airbox is loose, not fastened to frame) while at the same time someone else screws down the hose clamps. I can sometimes get it to fit by using a long flat screw driver to pull the rubber over the carb with one hand while tightening the clamp with the other. I would try to get someone to help before ordering new rubbers.
 
to DimitriT.

I'll confirm where the gas is coming from but I do know its gas, not water vapor.

OK heres the other thing about this Canadian GS750 I got last fall. The gas petcock was replaced with a non-on off fuel valve according to the prior owner. IE no shut off, no reserve etc... I wonder if that should be replace with original type also while I'm at it?
I agree with your last paragraph and will get someone to help connect the air tubes, or replace them. They seem a bit short anyway for the connection.
Scott
 
I think you may have two problems here which are related. First off, you need a properly functioning petcock. Either one which automatically turns on and off using the intake vacuum from carb #2 or one with a manual shutoff (in which case don't forget to put a plug on the vacuum nozzle on carb #2).

I would look for a vacuum operated petcock as it will be one less thing to worry about (people will give their own opinions but how many times have people run the carbs dry because they forgot to turn on the petcock?)

Getting a new petcock may fix the problem altogether but if it is still happening, you will need to take the carbs off and replace the o-rings which seal the float needle valves. See the "carb cleaning" link on the main page for details. I wouldn't go through the whole process of cleaning the carbs (separating them from the rack will require a re-synching using mercury sticks) but just remove the bowls, take the floats off and pull the needle valve. You can get replacement o-rings from a Forum member called Robert Barr (http://www.cycleorings.com).
 
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