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Discussion Starter · #1 ·
So my carbs are leaking out the little nipples at the bottom badly with the fuel on, If it was a bad float i wouldn't think it would leak out of these. Should I just do a complete rebuild?
 

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The carb bowls have an overflow standpipe. When the fuel level gets too high it flows into the standpipe and out through those nipples. The drain screw by the little nipples is used to drain all the fuel from the bowls. A float valve that doesn't seal will allow the fuel level to rise and drain. The nipples typically have hoses connected to them which route the fuel ahead of the rear tire, near the battery. This keeps it from leaking fuel onto the engine.

Another possibility is that the standpipes are broken or cracked. If you polish them with steel wool or fine sandpaper cracks will show up as black lines. These can be soldered (very carefully).

The bowl drain screws can also leak but usually that happens when they have been removed and not properly installed.
 

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Discussion Starter · #4 ·
Well its about 20 outside so im thinking maybe a seal went. I have a few spare carbs i can check out and throw on but i retuning is gonna be a pain
 

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Welcome to the joys of this model ownership, i hope you have strong hair.

Read up on the matter ( these carbs not hair ) before you "go in".

It is possible to do some good cleaning with the carbs still on the bike, like float chamber removal ( subject to how much the screws have been butchered ) and parts therein and under, and, the vacuum chamber and its pistons etc..

Various carb fuel ways can be got to as well as with a spray cleaner etc..

Be carefull about the slow running mixture screws, sometime a small rubber seal etc. can be misplaced and cause problems in re fitting the screw along with its spring and tiny metal washer, so be warned.

If yours has the side diaphragms under a 2 screwed alloy ( or steel ) cover, tread carefully.

Remember a flooding carb can fill a cylinder with fuel.......

You know of the very useful parts diagrams over on cmsnl ?

If you have a pair of the same carbs, examine them and understand what goes on before you tackle the ones on the bike. Go and turn the fuel off now.
 

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I wouldn't recommend making a decision until you disassemble the carbs and identify the problem. Especially if the current carbs were working well prior to the leak. You may only need to blow out the lines and clean the float valve seats. Possibly replace the float valves (use only genuine Honda needles). If the standpipes are cracked/broken you could replace the whole bowl with ones from those spare carbs. All are easier than installing a used set of carbs and starting from scratch.
 

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Discussion Starter · #8 ·
They were functioning but they were seeping gas out of the seals (Wet but not leaking), I assume I'd have to do a whole retune and sync one I get them back on anyway so that's why I want to use the spare ones.

A 1980 CM400T carb is the same as the 1981 A isn't it ?
 

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No, the manual trans carbs are not the same as those for the auto trans bikes:

https://www.hondatwins.net/forums/6...1654-carb-information-years-jetting-more.html

You can however use the floats, float valves, and float bowls to fix the leaks in your A model carbs.

Just fixing the leaks will not cause a need for any other work on your carbs. It should still run just like it did. One exception might be if it was previously set up with incorrect float heights (if you have adjustable floats).
 

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Discussion Starter · #10 ·
Carb work seems complicated to me but i'll remove and inspect. What do the standpipes look like and what else should I be looking at. Do i need to fill the floats with fluid or anything to check them? I'm a noob
 

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Discussion Starter · #12 ·
So the piece that lets out fuel when I open those screws.

Seems weird that both would crack at the same time but thats winter

could overtightening the screws wreck a seal? or I didn't drain the gas once I parked it last so could that have gunked up the floats?
 

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No, the stand pipe allows fuel out when it rises to the top where you see the hole in the top of the pipe. The drain screws open a port in the bottom of the bowl. They both exit thru the nipple on the bottom of the bowl.
 

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No, the standpipe is the brass tube in the middle that drains excess fuel when the floats or float valves allow too much fuel to enter the bowl. The drain screw opens a passage in the bottom of the bowl which drains all the fuel out.

The plastic floats on these bikes are pretty much not a problem (unless you drop it on the shop floor and step on it). There is no adjustment. Common problem comes from installing aftermarket re-build kits with float valves that aren't quite the proper length but even that shouldn't cause the problems you're having.

I think it will be helpful when you take the bowl off one of your spare carbs and actually look at what we're talking about.
 

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Discussion Starter · #16 ·
I looked at the spare ones, I see the pipe and the screw, I figure a good test would be to fill that little bowl with water and see if it leaks, that will at least tell me if it's a crack/ bad seal or if it's a true overfilling of gas
 

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Is there a reason gas would not work? I don't know where cvjmg is but Acetone is like 12$ a liter here, gas is only $1.30/L
 

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Discussion Starter · #19 ·
Mainly just didnt want to spill gas everywhere but why not. Ill try tomorrow
 
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