SU carb float needles

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BlackIP2
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SU carb float needles

Post by BlackIP2 » Fri May 31, 2019 6:07 pm

After an aborted ride out yesterday due to a non-starter and petrol pouring out the bottom of the engine, I reckon I need to replace my SU float valves and maybe check the petrol tap diaphragm too.
I'm a bit puzzled though as to why it suddenly happened, because I pushed the bike out of the garage, started it to make sure it was working, which it did, and then switched off and went inside to get ready. When I came back out all kitted up, that's when it happened. It looked like the petrol was coming out of the joint between the exhaust and silencer, so I can only imagine it was going straight through the carbs, into the combustion chamber and out of the exhaust port as I was spinning the motor. I disconnected the fuel pipe at the tap, whipped out the plugs and let it stand for an hour after which it fired up fine.
Could it be that the brief start up might have somehow interrupted the vacuum on the petrol tap and left it in the open position?
Back to the float valves then. I've seen in some previous posts that it's best to use viton-tipped ones, available from Burlen, but their website shows two sizes - 0.070" and 0.096".
Does anyone know which ones are the right size for air-cooled IP2 carbs?
Or is it easier to just get the carb rebuild kit from Andover and do a complete refurb? However, my bike (usually) starts fine, tickover is regular, though the choke doesn't seem to do anything, so I'm mindful of "if it ain't broke don't fix it" but it looks like the floats need sorting anyway.
Views and advice welcome.
Cheers
Mark

johnbirchjar
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Re: SU carb float needles

Post by johnbirchjar » Fri May 31, 2019 11:28 pm

I used veron tiped ones recently,I will check my records tomorrow and report back with part number,J.B.

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Richard Negus
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Re: SU carb float needles

Post by Richard Negus » Sat Jun 01, 2019 8:52 am

Hi Mark,
It sounds like you have two, possibly three, problems.
.the fuel tap should stop fuel supply to the carbs when the engine stops. There are overhaul kits available but my experience says replace the complete tap as the seating surface in the tap body may be corroded, something a new diaphragm/plunger may not fix.
.for the carb/carbs to flood and fill the engine, the float needles are leaking and should be replaced. They come as a set of viton-tipped needle and new .070" seat under Burlen part number WZX1100 (I think). The kit also includes the square-section rubber ring to seal the float chamber cover. You'll probably have to re-set the float height to 0,5 - 1.0 mm.
.and while you've got the carbs apart, replace the O-rings in the cold-start mechanism

Just a thought - do you have a 'Prime' position on your fuel tap? This was fitted to early IP2's but later ones had a different outer plate that stopped the prime position being used. 'Prime' position bypasses the vacuum tap such that the fuel is always on. With dodgy float needles, that's not a good thing.
R.
Slower slower, no faster faster - the joys of old fogey mode to hooligan mode at the flick of a switch

johnbirchjar
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Re: SU carb float needles

Post by johnbirchjar » Sat Jun 01, 2019 3:47 pm

Hi Mark,the info I have(according to the invoice from Burlen) is that WZX1100 was replaced by VZX1100,and like Richard says,replace ALL "O" rings while you have the carbs off,hope this helps,ride safe,J.B.

BlackIP2
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Re: SU carb float needles

Post by BlackIP2 » Sat Jun 01, 2019 5:57 pm

Thanks Richard and John.
Carb parts have been ordered from Burlen.
I actually have a Yamaha tank on my IP2, which I have only recently changed along with the bodywork. The fuel tap does have a prime position, but I only use the normal 'on' setting. I cleaned the tap and checked it before using the tank, but as I didn't have the problem with the IP2 tank, I guess the tap could still be faulty, so a rebuild kit had been ordered for that too. However, I am now also looking at changing the tap for a more simpler on-off job, which should guarantee that element of the problem not happening again.
Many thanks for the advice. Always appreciated.

johnbirchjar
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Re: SU carb float needles

Post by johnbirchjar » Sat Jun 01, 2019 11:06 pm

Some time back I altered the fuel delivery system on my Classic by doing away with the "Vac" system and having a constant flow and just a "ON/OFF" tap and reserve,(no "Diaframe " being eaten by p**y ethanol evil)J.B.

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