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SU float sinking

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ekamm

ekamm

Jedi Warrior
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Drove my tr3a yesterday and when I stopped I had a significant gas leak from under the hood. Open the bonnet and the overflow on one of the carbs is running a steady stream of fuel. So when I open the bowl up the float isn't floating. At first no noticeable hole, but finally find a crack i the bowl. The only thing different I did was to add an lead substitute and octane booster. Could that have caused the problem or is it just 50 years old?
 
I haven't seen the floats on a TR3. Those are H-series carbs aren't they? Are the floats plastic or brass?

I've seen old brass floats (on Solex carbs) open up hairline cracks and sink. I have carefully dried these out and applied the thinnest amount of solder over the crack to re-seal it.

On fuel tank floats (plastic ones) I've found the crack/hole in the float and used a fine tip soldering iron to melt the plastic back together. (After draining/drying the float of course).
 
I don't think any thing you added did it. It just happens. Either fix it or buy a new one. I would say get new ones for both if plastic.
 
It's SU H6 and the float is brass. It's half full of gas and trying to get it to drain out but not having much luck. I have identified one place where there's a hair crack that is leaking out but barely. and other potential cracks are visible and don't appear to be leaking...yet.
 
Just old age, IMO. TRF has new ones listed under P/N SUWZX1303.

One way to dry them out is to heat gently with a hair dryer in a well ventilated area. Keep going for a minute after you can no longer feel fuel sloshing about inside, and then let cool thoroughly before trying to seal the crack.

Once you think you've got it sealed, you can test it by putting the (cool) float into a pan of hot water. The heat will make the air inside the float expand, and you'll see a stream of bubbles if there is a leak. If it does leak, you'll need to dry it again.

Good luck. Only time I've tried to fix an SU float, the solder between the halves came apart before I could get the new solder to stick. Likely I didn't have the surface clean enough, or maybe I needed a bigger iron (to get one part to working temperature before the whole float got hot enough to melt solder).
 
Would there be a problem drilling the smallest hole in it to drain and then soldering it as well?
 
IMO, it's time for new ones. 50 years of vibration have work hardened the brass. You may be able to fix the existing cracks, but new ones are likely to form soon. I don't think you can heat it hot enough to anneal it without wrecking it.

I haven't had this on the TR3, but I had the same thing happen to my '65 BMW cycle, some years back. Attempts to repair the sunken float were fruitless. New ones were "cheap," by comparison.
 
ekamm said:
Would there be a problem drilling the smallest hole in it to drain and then soldering it as well?
Click to expand...

No problem at all. I've done that on the Solex floats AND in lawn mower carb floats. Just keep the hole size to a minimum so the solder can easily close the hole back up.
 
I have heard of several leaky floats recently and it was attributed to the ethanol added to the gasoline that is attacking the tin or the lead in the solder. So it may not be caused by 50 years of vibration. It may be caused by a few years of having used ethanol in the gasoline.

The TR guys who race TRs were writing about developing new plastic floats. Are the new ones sold by TRF made of plastic or are they brass as the original ones. I'm only asking so I can be informed, not to be critical of TRA and VTR judges who will never remove the top of a float bowl to check.
 
Don Elliott said:
Are the new ones sold by TRF made of plastic or are they brass as the original ones.
Click to expand...
Don't know, but I would guess they are still brass. Generally, if TRF introduces an 'improved' part, they give it a different part number and mention the difference.

Never heard of ethanol attacking solder; in fact I'm pretty sure it was the solvent in the rosin flux we used to use on electronic components. But anything is possible, I suppose.
 
After closer examination and a little heat it appears that there are at least 2 or 3 cracks that are leaking. So I guess that it's time for new ones.
 
I know ammonia will cause stress corrosion cracking in brass and brass alloys, but dont know what in a fuel system would produce ammonia, uless you cleaned carb parts in ammonia at one time??? or used windex???

Hondo
 
No but I have used Berryman's B-12 carb cleaner.
 
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