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SU Carbs – A Cautionary Tale

So there I was, getting ready to take my Morgan (’67 +4, TR-4A engine) about an hour north of me for an autocross event this past Sunday. It was early Saturday afternoon, and I was starting to put stuff together for the trip. Chairs, a tent for some shade, my helmet, etc. I was feeling pretty good, overall.

The last thing on my list for the day was to fuel up the car. At about 4pm or so, I headed out to my nearest filling station, about five miles away. As I was moving into the turn bay lane to enter the station, I caught a strong whiff of raw gasoline. When I stopped at the pump, I got out and opened the bonnet to take a look-see. The top of the float chamber for the front carburetor was soaked in fuel, and fuel was dripping down onto the generator. Given the summer heat, the proximity of the headers, and the electrical activity inside a generator armature, I’m lucky as hell it didn’t catch fire.

Initially, I assumed it was a split fuel line, so I loosened the clamp and took a look at the hose. There were some clamp impressions in the hose, but I couldn’t see a split. To be safe, I nipped a half-inch or so off of the end of the hose and clamped it back onto the inlet pipe. I started the car, and fuel immediately spewed back onto the top of the float chamber cover.

My next thought was that there was a failure of the inlet pipe-to-cover joint. Off came the hose again, and I gave the copper inlet tube a wiggle, but it felt solid. Maybe a crack in the aluminum casting? Nope. A gasket failure? No again. Since I had the cover off to look at the gasket, I turned it over to take a look at the needle valve. Therein lay my problem.

Something caused the needle to get hung up, and it wasn’t seating properly. With the engine running, the mechanical fuel pump overfilled the float chamber and forced fuel out of the tiny vent hole under the inlet pipe. The same thing would have happened if the float was compromised and allowed fuel into the float.

I worked the needle valve in and out a few times, and blew it out as best I could while standing next to the pump. I put it back together, and this time it didn't leak. Once I got home, I took it apart again and blew it out thoroughly and examined the needle for any signs of deterioration or wear. There were none. I also did not see any clear evidence of debris or sediment that may have contributed to a poor seat/seal. It hasn’t leaked a drop since, but I did put a fire extinguisher on the passenger’s seat when I drove up to the autocross.

What is particularly alarming to me is that there was no clear reason for the failure to seal properly, which means it could happen again at any time. I started researching the issue and came across several anecdotal stories of guys that lost their cars to fires that started from that exact cause. SU even made a change, adding a small overflow pipe that allows any overflow to be routed to a safer location, away from the headers and generator.

I sent an email summarizing the issue to some very experienced Morgan guys, and didn’t get a clear solution for the best practice for adding an overflow pipe to that teeny little vent hole. One recommendation was to call Joe Curto at British Superior Vintage Carburetor Parts. I called Joe, and will be sending him the float bowl covers. He’s got a solution to add the overflow pipe that involves a jig he developed to drill and tap the vent hole, and a threaded fitting for the pipe. He also said that he’s seen the problem happen from time to time and that “we didn’t design it, but we do have to live with it.”

To summarize…If you have SU HS6 carbs with no provision for an overflow pipe for the float chamber vent, you might think about making certain that your needle, seat, and float assembly are in perfect condition, and that there is no possibility of debris causing a poor seal. You might also consider sending the lids to Joe to have overflow pipes installed, and installing an overflow catch cup of some kind.

I would have been devastated to see my father’s car burn to the ground over something that stupid.

Leaking float needle valve would have been my first thoughts. Would suggest putting fresh needle and seat in. Maybe order two and have a spare set.



Looks like part #6 on above.

Have you thought of Grose jets. Fitted to all my cars with SU carbs - no problems.

Sir P

I fitted new stayup floats & needle valves with the Viton tips which are ethanol resistant after my classic Mini started suffering leaks from the same vent hole, it's been perfect since

https://sucarb.co.uk/hs-stayup-float-kit-hs2-hs4-hs6-small-bore-needle-valve.html

From a dull memory of bringing old cars back to life a sharp rap with a spanner on the side of the float bowl would often stop the overflow. I also noted over the years that the design of the needle altered to have a rubber tip which i imagine may have been an improvement on sealing. Not sure how well the rubber tip might operate with bio fuels or indeed the floats, given I think I read somewhere that Ferrari may be supplying brass floats for their older cars..

Just as Richard suggests a rebuild kit would seem to be the way to go for carbs that may not have had the benefit of new gaskets, needle valves fitted in years..

http://www.southerncarbs.co.uk/page_1822386.html

Thanks for the salutary reminder. My SUs are H4 with a short pipe overflows. The rear one is carefully aimed onto the exhaust, I guess a slight mod is required!
[img]https://tm-img.com/image/zoAKj[/img]

Be thankful it hasn"t caught fire and destroyed itself in the last 65 years!

There's one positive there. It wouldn't have been mine if it had!
I don't mean I don't want it. It is, for me, an interesting 'toy'.

The reason I posted my story isn't due to the cause of the overflow. There are several causes of an overflow, ranging from a worn needle to a leaking float to a failed fuel pressure regulator in a car with an electric fuel pump. There are enough causes that an overflow is a certainty at some point or another, and a prudent driver should take steps to mitigate the risk. I'm more concerned that other HS6 owners are aware of the risks involved with an overflow, and the non-straightforward nature of the addition of an overflow pipe to the aluminum casting particular to the HS6. Once a float chamber overflow occurs, it would be best if the excess fuel is routed away from exhaust components and sources of an electrical arc. It seems to me that to route the overflows of both carbs into a small catch cup would be best.

The nerd in me is working out the size of the catch cup, placement of a sight glass, cup drainage, and maybe even a float switch tied to an alarm of some kind.

I have twin H4s, so it is a bit easier to deal with the potential overflow. I will replace the short overflows with longer ones which will deposit away from the exhaust,or dynamo (yes I have one of those) Item 32 here