SU carb problem - Triumph Bonneville

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Mon 1 May 2006 21:38
SU carb problem - Triumph Bonneville - Bonneville
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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Mon 1 May 2006 21:53
SU carb problem - Peter D
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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Mon 1 May 2006 22:01
SU carb problem - fordprefect
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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Mon 1 May 2006 23:48
SU carb problem - Triumph Bonneville - Screwloose
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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Tue 2 May 2006 14:06
SU carb problem - Triumph Bonneville - Armitage Shanks {p}
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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Tue 2 May 2006 14:28
SU carb problem - Triumph Bonneville - Armitage Shanks {p}
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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Tue 2 May 2006 14:35
SU carb problem - Triumph Bonneville - martint123
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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Mon 20 Apr 2020 13:42
SU carb problem - Triumph Bonneville - P Burden

I bought a T140 (1979) with the same SU conversion. (FZX1367 HIF6 (1 3/4") carb).

I ended up buying parts directly from 'Burlens' to replace the float valve & gaskets etc... the Needle Jet is NLA for the T140...

I had to get a new float which happily enough is fitted to early Land Rovers. Look on Fleabay first as its usually cheaper then 'Burlens' (mine had a leak on the welded float seam and leaked fuel out of the overflow similar to yours).

Buy a 6mm thick SU carb insulating gasket to go between the carb & head... it'll stop the fuel from boiling. (available on Fleabay).

Bit late after 14 years in the reply but hey.... :-)

If you look for 'T140e Recovery' on 'www.thetriumphforum.com' my rebuild is on there.

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Thu 23 Apr 2020 02:06
SU carb problem - Triumph Bonneville - edlithgow

I miss the SU.

So "deightfully simple" (as above) that even I can understand them.

This does not apply to my current 80's Aisin, or all the emissions gubbins that used to hang off it

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Thu 23 Apr 2020 12:14
SU carb problem - Triumph Bonneville - Railroad.

SU's and Strombergs. Carburettors really don't get much simpler.

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Thu 23 Apr 2020 14:04
SU carb problem - Triumph Bonneville - bathtub tom

SUs didn't have that damn diaphragm. I used to carry a spare.

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Thu 23 Apr 2020 22:39
SU carb problem - Triumph Bonneville - Big John

SUs didn't have that damn diaphragm. I used to carry a spare.

Changed many a Stromberg diaphragm - likewise carried spares! Well it was for a VX 4/90 twin carb where somehow they always seemed to fail in quick succession.

Memories - if you were about to run out of fuel one carb ran out 30 secs before the other. Proved useful a couple of times giving you a few extra seconds to pull over safely on two cylinders - used to happen a lot was skint in those days and the VX4/90 liked a drink!

On other cars I always loved the SU carb, usually very reliable but float valves were an issue if cars hadn't been used for a while - as were the SU electric fuel pumps fitted to some cars.

Edited by Big John on 23/04/2020 at 22:40

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Fri 24 Apr 2020 09:40
SU carb problem - Triumph Bonneville - bathtub tom

SU electric fuel pump

Could often be induced to work for a while with a tap from a spanner. I had one fail inside a compound at Woburn Safari park. No way was I going to get out!

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Tue 28 Apr 2020 21:25
SU carb problem - Triumph Bonneville - Big John

SU electric fuel pump

Could often be induced to work for a while with a tap from a spanner. I had one fail inside a compound at Woburn Safari park. No way was I going to get out!

Yup - when the one on my Morris Minor was playing up I could provoke it back into life by kicking the top of the passenger footwell. I tried cleaning/sanding points etc but it really needed a new one fitting to become reliable - which it was thereafter.

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Mon 11 May 2020 17:42
SU carb problem - Triumph Bonneville - Andy 1955

True, when I had a 74 Sunbeam Rapier spent loads of time trying to sort it twin Strombergs. Finally 25p new diaphragm . Always carried spares after that. Also people forget to check the oil level in the damper for SU's and Strombergs. First point of call.

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