Bad carb leak after disassembling

tarrelbeer

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Hi people!

First time on the forum. After owning and working on a CM400T for the last years im now working on a 1982 CB750f project (EU).

While in the process of doing general inspection and maintenance using the FSM I cleaned out the bottom end of the carbs using carb cleaner and put in some new gaskets.

After installing the carbs on the bike and turning on the petcock fuel started spewing out of the following spot at Carb 2:

f1mkXD9


https://imgur.com/f1mkXD9

Was not able to find a similar issue on the forum but before disassembling again and possibly messing up more, I could very much use your opinions/advice since it is not obvious to me how the leak could have started and how to fix it.

This was not leaking before.

Thanks in advance!

EDIT: After pondering it some more, just realised that i've had to put a lot of pressure and pulling on the carbs to fit them back into the intakes, especially the sides, could this have caused this leak? Is there some kind of oring inside at the connection point of the fuel rail that can be leaking now?
 
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amc49

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There are o-rings on both ends of the T there. You could have cracked the braze joint there too but not likely.

Try it again, sometimes they leak until the o-ring gets exposed to solvent and swells back up to seal, wouldn't be the first time. I fill them up, turn off spigot and let sit 24 hrs. to check again if the old rings still used leak.
 
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tarrelbeer

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There are o-rings on both ends of the T there. You could have cracked the braze joint there too but not likely.

Try it again, sometimes they leak until the o-ring gets exposed to solvent and swells back up to seal, wouldn't be the first time. I fill them up and let sit 24 hrs. to check again if the old rings still used.

Holy cow, I just checked it again and its bone dry now with the bike idling! I guess stupid questions don't exist, were it not for your reply I would have been digging in the carbs again by now and probably breaking it more instead of fixing. I'll be careful to check it some more but all seems fine for now!
 

amc49

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I deal a lot with old o-rings, you learn a few things about how VOCs (fuel being one) can affect rubber parts.
 

George Mooyman

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I wish that would happen to mine. I had my carbs out and fitted new kits tested for petrol leaks on the bench before installing ran the bike and it was fine. And now number 2 carb is leaking out the drain plug. Now I got to RIP it all out again and I hate the tightness of that airbox.
 

Wez_

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I have a small set of hex keys that allow me to remove any bowl with the carbs still installed in the bike. Also stock air box. You need a pretty low seat or sit on a cylinder block. You'll need to a line of sight to get the hex key seated in the head of the bolts to loosen them, but it's worth the effort. Beats removing the carb bank. Sometimes you get lucky and just need to clean the bowl and or make sure everything is working as it should. Also ensure the bowl basket is aligned in the groove. The piece that rides in the bowl under the tang of the float (forgot it's name)needs to move effortlessly. Use a Q-tip and toothpaste to polish inside that chamber. Clean and reinstall.
 

amc49

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I've done the same with not removing airbox but the screws need to change to do it.

If bike stores in the cold then the fuel bowls can crack the overflow tube when condensation water in them ices up to crack the tube. The drain plug itself and the seal can corrode to do the same.
 

DetailGuy

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Sometimes the float seals leak, better to do a leak test, mount the carbs over a plastic box.

May sure the floats stop the fuel from dripping out...before you fix them on the bike...plenty of video on utube of leak down test, good luck. 🏍️
 

user8086

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Float balls are stuck.. you're going to have to pull the carbs off..you won't be able to get to the bowls with the carbs on.. and get yourself a hair dryer , don't use a heat gun... Warm all your boots up real good.... I have rebuilt many of these carburetors CV 42 and the 43... You do not have to take the airbox out but you better put it back in before you put your carbs back on.. and do all the linkage you can two throttle cables and your choke cable will make it a lot easier.. and even after you're there rebuilt it is not uncommon to give them a small tap with a hammer a tap not a hit.
 

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