Nbr 2 carb overflow troubleshooting

[AndyH]

When I bought the 500K1 cafe racer several months back, it came with an annoying leak from the overflow tube on carb #2.
It did this whether the petcock was on or off, which had me a tad perplexed.  Anyway, I eventually tracked half of the problem to hardened fibre washers on the M6 screws fixing the petcock to the tank.

Having solved this, I proceeded with my restoration (see here. )

I'm now pretty close to completion and took the bike for a ride or two without drama a few days back.

Mindful of the other half of the problem (#2 o'flow leakage with petcock on) I left the petcock on for a few days with a small container under the overflow tubes of all 4 carbs. During this time I thought I'd do a quick clear tube check of the carbs.  1,3 and 4 were all 2-3mm below the bowl face but 2 was ~6mm below.  At this stage there was no sign of leakage and I left the clear tube connected to #2 overnight.  Level the same, and still no leakage.  I removed the clear tube setup and replaced the plug.

This morning, I turned the petcock off for a while, came in later and turned it back on.  Within an hour or so, ~15ml of fuel had drained from the #2 bowl! 

Because this leak is NOT a regular thing, I'm a bit puzzled as to what the cause may be.
I'm thinking either an overflow tube crack, or a problem with the float needle (I'm leaning towards the latter).

I have no details of any work that was done to the carbs prior to my purchase, but suspect very little as the drain plugs were full of hard gunge (an Australian term  ) which I subsequently cleaned out, and the o-rings were crap (which I replaced)!

Regardless of the above, I reckon I'll probably have to pull the carbs and give them a damn good going over!!

What say you, oh masters of the carb overflow sagas and solutions? 

[deltarider]

Before doing what you suggested, I'd do this first:
1. Tapping against the floatchamber with the stub end of a screwdriver
2. Remove drain screw of carb #2 and with container underneath open and close the petcock a fuel times and see if you can flush out any debris.
3. Add a fuel additive to the gas in your tank like Tunap, Forté, Bardahl.
If all this doesn't help, remove floatchamber #2 and clean or polish floatneedle, seat and swivelpin with a very little toothpaste or whatever.

[matthewmosse]

I had the same on my cb500, rather repeatedly. Redex fuel additive seemed to cure it for a month or 3. A tap on the edge of the float chamber sometimes sorted it, redex better track record on that bike.

[JamesH]

Oddjob has nailed this one. Systematic is the name of the game. But from what you describe my money is on a cracked overflow tube...

[AndyH]

Thanks guys.  Agree 100% on the systematic approach!
I'll swap bowls over and see what happens, then take it from there.

[deltarider]

Yeah... but know that these additives can do miracles. I'd start with the easiest.

[AndyH]

Swapped bowls, #2 still leaks.

Before doing what you suggested, I'd do this first:
1. Tapping against the floatchamber with the stub end of a screwdriver
2. Remove drain screw of carb #2 and with container underneath open and close the petcock a fuel times and see if you can flush out any debris.
3. Add a fuel additive to the gas in your tank like Tunap, Forté, Bardahl.
If all this doesn't help, remove floatchamber #2 and clean or polish floatneedle, seat and swivelpin with a very little toothpaste or whatever.

Did 1 and 2 before swapping bowls DR.  Interestingly, there were a few tiny black specks in the drained fuel.  Several drain sequences did NOT result in clear fuel...   This debris didn't originate from the tank because I drained #1 a couple of times with clean fuel coming out.

While the bowls were off I checked the slow and main jets: they were 40 and 100 respectively but definitely aftermarket.

As the leak occurs only when petcock left on for a long period, I'll continue to ride till my SOHC restorer mate returns from NW WA (Western Australia that is!  ) in a couple of weeks and then we'll pull the carbs and give them a good cleanout etc.

[MarkCR750]

Another possibility is that the number 2 float is too heavy and isn't floating correctly, I had this on one of my carbs,  the float had become porous and was weighed down with fuel, just enough that sometimes it wouldn't push hard enough against the float needle for it to maintain its seal, I noticed when I had the bank of carbs inverted on the bench with the bowls off to check the float heights, one was sitting a couple of mm lower than the other 3, I moved it to the next carb and it sat lower on that one, then I saw a minute pin prick of fuel weeping out of the float, I replaced it with a good second hand one and the issue was fixed.
Might be an idea to swap floats from 1 to 2 and vice Verda to see if the problem follows the float.

[K2-K6]

Have you checked that the tank cap vent is working and clear so that it can't build any pressure when left without use.

[SteveD CB500K0]

I've had a float fill with fuel too.
It's a bugger to find as they only do it when in situ.


Sent from my iPhone using Tapatalk

[paulbaker1954]

Had exactly same issue caused by use of aftermarket float valves. Changed to OEM one and problem solved

You could try swapping float valves over between carbs to see if the problem moves first but I would just change the valve.

Fiddly but can be done with carbs in situ

[AndyH]

Had exactly same issue caused by use of aftermarket float valves. Changed to OEM one and problem solved

You could try swapping float valves over between carbs to see if the problem moves first but I would just change the valve.

Fiddly but can be done with carbs in situ

Thanks for the info Paul.  I'm currently following thru on earlier suggestions in isolating/identifying the cause, but leaning towards a float valve problem (carb #2 still overflowing)..  It appears that the jets/float valve etc.are not OEM from earlier inspection. 

I have a full (4 carb ) set of non-OEM replacement jets/valves/o-rings but a tad reticent in changing out what I already have. 

[deltarider]

My initial thought was the float needle, the OE ones were rubber tipped IIRC, replacements if not original don't have this and as a result don't seal as well

None of the CB500s and CB550s had rubber tipped floatneedles with the exception of K3 and K4 models.