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Yamaha 1978 750 Special Fuel Leaking Problem

I recently bought a Yamaha 1978 750 Special motorcycle. It ran fine on the way home, and I then put it into our garage for winter storage. After a few months, it smelled like Gas in our garage. We found that there is gas very slowly leaking onto the ground from the black box underneath the gas tank. All we need to do for now is stop it from leaking; we can fix the problem by taking it to the shop in the summertime. There are what appear to be two fuel shutoff valves on each side of the tank with three positions: On, Res, and Pri. We took the hose off of the carburetor, and changed the setting to all 3 options with the other end of the hose pointed downward to drain into a gas tank... But nothing came out! I don't believe the bike has frozen, and the gas is very fluid in the tank (there's a lot I can see in there). Can anyone tell us how to fix this or explain what's going on? I'm not very good with motorcycles.

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These type of fuel shut offs do not have a manual off position. They are automatically operated by engine vaccuum when working properly. They have malfunctioned and allowed fuel to overflow into the airfilter box(the black box) untill repair time use a long piece of hose to connect the large outlet of each shutoff to eachother or plug the end of two short pieces. The carbs have a drain screw on the very bottom of each facing left , right or straight down. you could also drain the carbs however once the fuel supply is cut to them they should not leak any further. Leave the carb fuel hoses from the tank disconnected. You should drain the tank by siphon and the residual gas that is already in the air box can be drained from the hose on the bottom of the box. This is a mess but once you get all this drained it should stop leaking

Posted on Jan 16, 2013

  • alanwadenels
    alanwadenels Jan 16, 2013

    This helps a lot! Thankyou!!

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6ya6ya
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Posted on Jan 02, 2017

ckoira57
  • 267 Answers

SOURCE: Motorcycle set in storage for three years. Today

http://www.cmsnl.com/yamaha-xv750bc-virago-1991_model9775/partslist/
this site has exploded view diagrams and part names of all sections of the bike.

http://www.yamahaownershandbook.com.au/index.php
this site has various Yamaha manuals for free download.

Your throttle cable is likely to be rusted. It can be saved with a liberal spray of WD-40 into the cable shroud and left to run down the cable inner. You have to remove this cable and hang up from one end.
Fuel residue is likely to interfere with the smooth operation of the carburettors. Remove the fuel line from tank, and remove drain plugs from carby's. Spray continously some Carburetor Cleaner (avail. all auto accessory stores) down the fuel hose till it comes out the base of carby's. Allow to evaporate and repeat. A full tank cleaning fluid is also available from most auto shops. Thoroughly check all rubber components, wheels and brakes before getting back on the road. Cheers

Posted on May 22, 2010

Testimonial: "Thanks for the quick response. I have checked the cables and they operate smoothly and have no rust. I also sprayed carburetor cleaner inside carburetor several times through drain hole but shaft connected to throttle cable still does not turn. I think I now need to remove the gas tank to get at the linkage and perhaps clean the inside of carburetors more throughly. But how to I remove the darn gas tank!! I have examined every thing I can think of but cannot figure it out. Any idea here would be deeply appreciate."

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