SU Carb advice please

[Baileysbus]

OK I realise this isn’t a motorhome related question, but someone here will know the answer.
SWMBO runs about in a 81 MGB GT and the SU carbs are giving her (and therefore me) grief. One of them is flooding and it’s not the floats. I’ve hauled off the dashpot and piston and notice that the needle has lateral movement on it like about 1/4 inch. The set screw on the side of the piston isn’t loose.
My distant recollection of the needles last time I saw one was that they didn’t have any play on them? Am I right or just imagining?
It’s an HIF SU carb I think ....... from one of the last B’s.
Any help appreciated to jog my failing memory!!!

Bob

 

[LesW]

On later SU carbs there was movement in the needle, but it sprung back to the centre.

Early ones as fitted to your MGB had fixed needles.

 

[ivorantony]

First check the float needle valves, the the tips are soft rubber that gets eaten by any ethanol in the fuel and then won’t seat properly,
Check also for any debris under the seat, check fuel pressure isn’t too high, and last is check height of float in float chamber and make sure it’s correct,

 

[tacr2man]

If its flooding then it has to be to do with float , there is a rubber nosed float valve that can cause probs when the rubber tip goes hard , perishes . Needle on dashpot will only cause mixture problems. you can see the float valve in the bottom left of pic in packet . HTSH
https://www.ebay.co.uk/itm/CRK116A-...430365&hash=item58dff6e758:g:7KAAAOSw4HNaT41m

This might give you a clue as to cause as well

 

[davejen]

£94 .!!!!
My last MG B didn't even cost that!!!!!
Cheers, Dave

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[Baileysbus]

On later SU carbs there was movement in the needle, but it sprung back to the centre.

Early ones as fitted to your MGB had fixed needles.

This ones definitely on the wonk then. There’s some spring in it but I can definitely rotate it with my finger on the point of the needle and it doesn’t sit centrally on release. I’d photo it but it’s pi$$ing down at the mo.

Thanks everyone....... I’ll keep digging!

 

[tonyidle]

This ones definitely on the wonk then. There’s some spring in it but I can definitely rotate it with my finger on the point of the needle and it doesn’t sit centrally on release. I’d photo it but it’s pi$$ing down at the mo.

Thanks everyone....... I’ll keep digging!

Later spring needles have a totally different method of mounting in the piston. They're designed to rub against the side of the jet. They cannot cause flooding.

 

[davejen]

Sounds like the float needle, as said ethanol in petrol causes damage to rubber etc. so I would definately check it first.
Cheers, Dave

 

[stevec]

Would second the float valve but also check that float isn't punctured and has fuel in it. It won't float so we'll if it has!

 

[dave newell]

By 81 the main needle was spring mounted and expected to rub on one side of the jet, this eventually leads to wear in the jet and on the needle causing a rich mixture but not flooding. Flooding can only be caused by either the float valve not sealing or the fuel pressure is too high. The latter is very unlikely unless the fuel pump has been replaced with a different spec unit. The float valves had "triton" tips but the brass seat would eventually wear until it no longer sealed. Threads could well need overhauling, they are quite simple devices and I would expect a repair kit to be readily available. Main jet is a standard part but the needle is specific to the application, in this case the '81 mgb.
I served my apprenticeship in a BL dealership from '77-'81.

D.

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[tonyl]

Burlen has a useful series of technical articles on its website and can supply various sets of parts to sort out issues with float valves, jets, etc. They also sell individual parts if you drill down deep enough on the website. For those with especially deep pockets, restoration services are also available. £300 + VAT per HIF.

There have been plenty of stories about the effects of modern fuel on seal material but it can also apparently affect the float. Metal floats are available to replace plastic items from that era. I would start by replacing floats and float valves on both carbs and go from there.

 

[Robert Barron]

Do you know for certain it is flooding or is it just running very very rich?

The reason I ask is that I have in the past found that the "O" ring which seals the choke mechanism to the body of the carburettor can split and this results in very rich running and it's impossible to to lean off the air fuel ratio using the adjusting screw.

Worth just removing the choke mechanism and checking this caused me a lot of grief and head scratching, hope it helps you.

Robert.

 

[Baileysbus]

Do you know for certain it is flooding or is it just running very very rich?

The reason I ask is that I have in the past found that the "O" ring which seals the choke mechanism to the body of the carburettor can split and this results in very rich running and it's impossible to to lean off the air fuel ratio using the adjusting screw.

Worth just removing the choke mechanism and checking this caused me a lot of grief and head scratching, hope it helps you.

Robert.

Hi Robert

Definitely flooding ........ pouring out more like I’ve still got to find dry weather to get near it ....... it was snowing here today and the car is outside! Might be a split fuel pipe going into the carb, there’s that much coming out. Hey ho. Just when I don’t need to be spending time under the bonnet

 

[mitzimad]

if you have petrol flooding out of the vent it definitely float or inlet valve

 

[Baileysbus]

if you have petrol flooding out of the vent it definitely float or inlet valve

Yep it’s looking like a strip down and fettle. Just what I need! A whack with a heavy screwdriver handle hasn’t worked

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[stevec]

It's the SU fuel pump you need to whack, not the carbs!

 

[Baileysbus]

It's the SU fuel pump you need to whack, not the carbs!

Not sure about that Steve. The pump works fine it’s the base of the front carb it’s pouring out of I recall that with the earlier SU’s, a subtle whack with a persuader would dislodge the float, but it’s not so easy with the HIF where the float is underneath in the body of the carb ..... Still to wet here to get in about it. So much for summer being here

 

[The Wino]

On my old mini based kit car it was a regular occurrence I would sometimes sit at the traffic lights and hear the fuel pump clicking away had to jump out quick lift then bonnet and tap the carb float chamber. If its just the front carb you could just look at that one first and see if the float is porous and not pressing up enough to close the valve. There is a bloke on Peterborough who rebuilds SU's we got one for my sons mini saw his workshops very impressive and I think for a twin carb set up he will also balance them on a rig to get the best out of them.....at a price