Is there an SU expert in the house?
I have a Triumph Bonneville motorcycle (1978) with an SU HIF6 carb conversion. Recently while out on a run (after 15 miles or so, engine nice and warm) it suddenly started cutting out on idle, and when I investigated further I saw fuel gushing out of the float chamber breather hole. Panic! I managed to get home safely by using the fuel tank tap to throttle the fuel entering the float chamber to match consumption, but now I have a problem to fix.
Anyone know what might cause this? My first thought is that the float has jammed 'open', but I'm not a carb person so I don't know what exactly would cause this.
I should mention that apart from cutting out at idle, the engine could be restarted and still ran smoothly at normal engine speeds. I should also mention that I've had the bike garaged for a few years, and I filled up with LRP (instead of leaded petrol) for the first time ever just before I took it out on this run - I mention this just in case LRP might make a difference to something inside the carb.
So my questions really are:
1. If I strip it down, what should I be looking for?
2. What else might be the problem?
Hoping someone can help....
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Worn out demand valve. You really nead a new male and female to cure this as the nylon male does not polish up well. Regards Peter
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One other possible cause would be a float that has filled with petrol and sunk. This should be obvious once you open up the float chamber.
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Bonneville
fordprefect has got it spot on. The HIF6 [ex Rover SD1?] was notorious for filling it's white plastic float with petrol and flooding. Have a look at the float and see if there's anything sloshing about inside.
These weren't as prone as other SU's to float needle-valve issues. They are so delightfully simple that it'll probably be pretty obvious where the problem lies. You'll need, at least, a new O-ring for the base as the original must be getting a bit brittle by now.
You can still get the parts for all SU's from Burlen Services. Remove the four screws holding the base; undo the [slightly hidden] slot-head horizontal brass float pivot-shaft on the side and there you are. Don't touch anything with a cross-slot on it.
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I have an SU specialist living 100 yards from me (East Midlands). He makes his living working on SUs and Strombergs - e mail me for details if you wish. He does parts and would refurb/rebuild carbs sent to him.
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Further to my last posting
Andrew Turner
Carburettor Specialist
01572 767665
www.su-carbs.co.uk
I have just spoken to him and he confirms that he will refurb existing carbs and can supply spares and overhaul kits.
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If it's none of the others, then if it's been sat for a bit (posting 1st of May, has it been laid up over winter?) a small speck of dirt can hold the float valve open enough to see petrol sloshing out.
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