Honda Twins banner
1 - 7 of 7 Posts

·
Registered
Joined
·
63 Posts
Discussion Starter · #1 ·
A couple of weeks ago, we had a pretty big cold snap on a day I took out my ‘80 CM400T. When I went to take it back home, fuel was pouring out of the overflow tubes from the carbs. It would happen off and on after that, and now it almost always happens.

I tore apart the carbs today and could not spot anything wrong with them. The float seems to be seated nicely, it moves freely and has no water. The rest of it also moved easily and didn’t show any particular signs of something awry. When I took it out again tonight, it worked great on the way to my destination, but on the way back it began leaking again.

Any thoughts on how to start diagnosing? At least last time when it was overflowing, the bike just didn’t run: I at least knew what had to be done.
 

·
Premium Member
Joined
·
10,203 Posts
Carb overflow can happen when the float valves fail to seal but then I would expect more of a drip than a steady flow. Could be intermittent but unlikely to happen on both carbs at the same time.

Grunge in the fuel system can get caught in the float valve preventing it from closing. Seems improbable it would happen on both carbs at the same time but, if a few chunks came loose I suppose it could happen.

Personally, I would start by removing the floats/valves and blowing compressed air through the fuel lines to clean them out.

Overflow standpipes can crack or break off but that would be constant, not intermittent.

Some prankster could be opening the drain screws.:evil:
 

·
Registered
Joined
·
63 Posts
Discussion Starter · #3 ·
Good point on compressed air. And in my rush to post before I fell asleep last night, I made it seem like it’s both carbs. It’s only the right one and it is pouring out like it’s the drain screw, but I’ve open and closed those a bunch; plus it doesn’t always start happening (i.e. last night I rode it twice with no issues and on the third ride, it started). When I rebuilt these a few months ago, this carb wasn’t the one that was gunked up (not saying it wouldn’t be now though), so I was a bit surprised to see it pouring out so much.

Tear down and compressed air again, I suppose!
 

·
Registered
Joined
·
805 Posts
I would play around with the float on the leaky side once they're apart and see if the needle sticks in the bore as you move the float up and down at different angles etc. Then follow the rebuild guide solution if they stick.

These carbs can be frustrating, I've had mine out 3 times so far, things keep popping up that were obscured by broken things before. Also the carb boots gave up and split. I'm sure it'll be worth it once they're 100% fixed.
 

·
Registered
Joined
·
63 Posts
Discussion Starter · #5 ·
Yeah that makes sense, and I'll definitely do the rebuild again. I had also planned on replacing the jets soon too.

I'm just confused because of how much leakage is happening. If it were a bit more drip/drip I'd have thought something was up with how I cleaned them, but it's a steady stream as though the drain screw was opened (though at times it holds, so it wouldn't be that. Plus I've checked). Though maybe this is a "normal" error that happens when something get stuck?

I visually inspected everything, tore it all down and didn't see any sticking points or things obviously wrong. I hope I'll get some time this weekend with some compressed air :)
 

·
Registered
Joined
·
815 Posts
It is possible to clean out the fuel delivery channels without having to remove the carbs from the engine, which is a real joy, as you know.

I replace the float chamber screws with allen bolts these days, making float chamber cover removal real easy.

The floats can then be removed with the carbs in situe and compressed air etc. used from the fuel stub and needle valve holes.

The needle valve holes can even be cleaned with cotton buds and solvent, or a fine metal polish. Removal of the polish etc. is easy again with solvent, fuel and compressed air. Even aerosol carb cleaner.

You did separate the carbs when you cleaned them as crap can sit in the "corner" due to diameter difference of carb body hole and cross over feed pipe, and then be dislodged by fuel flow.

You did clean out the fuel tank, filler cap, fuel tap and filter ?

It is possible to fit an in line fuel filter between the tank and carbs, the small vw type being a good fit behind the side panel tank hole.

Different fuel taps will also fit the tank stub, ones with a cleanable filter screen and sediment trap.

I have had no problems with fuel flow using the small vw type fuel filters at all, except when they are "spent", ie blocked.

I thought I had seen a "magnetic" fuel filter to trap all that "rust mud" that is in steel tanks, the lazy way I suppose.

PS as a matter of interest ( site completion ?? ) , what's the record for the number of times you have removed, cleaned and then refitted these carbs, to cure a flooding issue ?
 

·
Registered
Joined
·
63 Posts
Discussion Starter · #7 ·
Good news folks, after the 6th rebuild, there's now no leaking. My assumption is that some stuff got stuck above the needle, but nothing visible was in there. Thanks for all your help.
 
1 - 7 of 7 Posts
Top