Leaky Carb - What to do Next

[Jeff in W.C.]

​The bike 1988 R100 RT with Bing CV 357/358 carbs

So, the right carb was leaking when petcocks on and not running. I last rebuilt the carbs about 20 years ago, so I thought it was time to do it again. Purchased rebuild kit with O-rings and gaskets, floats, float needles and diaphragms. I went through them cleaning them up and installed all the new goodies. Followed Boxer 2 Valves advice on setting the float height. Right carb still leaked. OK, adjusted float, but it still leaked. So, I took a really good look at the float needle seat and it looked like there was some left-over degraded rubber from the old needle that I didn't notice before. So, I got a pencil with a good eraser and cleaned up the needle seat. Looked good, reinstalled the float and float needle, readjusted the level. But, it leaked. Adjusted the float one more time, but it still leaks.

I have no idea what do next. I do have a second set of carbs and I could rebuild the right one for use, but that won't tell me what's going on with the carb. Anything else I should try first?

[barryh]

First I would try to identify the source of the leak. With the bowl off and holding the float up gently by hand does the float valve leak ?
If it doesn't another possibility is there have been reports of leakage due to a fractured overflow tube so that's worth a close look.

[Rob Frankham]

First I would try to identify the source of the leak. With the bowl off and holding the float up gently by hand does the float valve leak ?
If it doesn't another possibility is there have been reports of leakage due to a fractured overflow tube so that's worth a close look.

The other point, of course, is whether the leakage is coming from the carb at all. It's not always easy to tell. One recurrent 'Gotcha' is to spend hours chasing a leaking carb then finding that the leakage is coming from a split hose or a loose fit where it joins with the carb. I would certainly check the condition of the hoses... in fact, if they are 20 years old (or more) I'd probably replace them anyway. Suitable 7mm ethanol resistant hose is readily available on Ebay or Amazon (etc. etc.). I just got some the other week, enough for at least 3 airheads, for just over a tenner (UK money), having experienced a very similar issue with my RT which I'm gradually re-commissioning as it has been laid up for a couple of years.

Rob

[melville]

Do you have the Viton-tipped float needles in those carbs? They seal great, until they don't. I know you have only E10 gas available where you life, and that will even take out Viton over time.

[Jeff in W.C.]

The fuel lines are about 4-5 years old. I plugged up the right side and it didn't leak. So, it's not the fuel lines. I took the bowl off and wedged something between the float and the bowl retaining clip. No leaks. So, I'm sure it's the float needle not seating well either due to poor float adjustment, bad float needle bad float or bad seat. I spent more time cleaning up the float needle seat with a pencil eraser. While I can hold it up to shut off--even at what appears to be level, it leaks when the bowl is attached. The more I've taken things apart and put it back together along with my cleaning and adjustments. It's leaking quicker now. I'm wondering if I should order a new float and needle, or swap them first with the ones in the left carb (they seem to be working properly). Through trial and error, I guess I could narrow it down where the culprit is.

I've run out of time for this weekend and I'm on a trip (a motorcycle one) next weekend. So, it looks like it will be a while before I can circle back to this.

Mel, the float needle is new (I just rebuilt the carbs with new gaskets, o-rings, floats, float needles, etc.).

[Jeff in W.C.]

So, I had a little bit of time, so I finally found my stock of older carb floats taken out when previously renewed my carbs in the past. I found one that weighed in at 12 grams, which is in the middle of the range that Snowbum indicated. Snowbum indicated new floats are about 10 grams and they get too heavy around 13.5 grams. I swapped the floats and got the same result--an overflow around 20 minutes. I'm thinking it's either a less-than-ideal float needle or less-than-ideal float needle seat. Renewing the float needle seat is beyond my expertise, so I would end up sending it to Bing for a lot of $$$$$$$$$$. But, before that, I'm going to order a new float needle just in case that's the issue, since the float needle is MUCH cheaper than having Bing replace the float needle seat.

[SteveD]

I seem to remember Jeff @ME 109 replaced the seat years back and described the process somewhere? Can't find it in here anywhere though.

[Kurt in S.A.]

Maybe this by me 109?

https://forum.boxerworks.com/viewtopic. ... 678#p24678

Kurt

[barryh]

.... so I finally found my stock of older carb floats taken out when previously renewed my carbs in the past. I found one that weighed in at 12 grams, which is in the middle of the range that Snowbum indicated. Snowbum indicated new floats are about 10 grams and they get too heavy around 13.5 grams.

Just for the record 10 grams is not right for CV Bing floats, Snowbum must have been thinking of carbs other than the CV's. Last new floats I bought were a very accurately measured 12.5 grams and in my experience they are not sinkers until well past 15 grams
so your 12 gram floats are fine and not part of the problem.

[carott]

As mentioned by Barry earlier, check that the bowl itself is not leaking.
The brass overflow tubes can become split from freezing or not seal well to the bowl.
Fill the bowl with fuel on the workbench and check for leaks.
Apologies if you have already checked this.
Brett