Carb flooding

[nigelmmoc]

Hi, 948 has a flooding carb, replaced rubber fuel pipe and cleaned dirt out of float bowl, no fuel in plastic float, float valve seems to be airtight on blowing through inlet with valve closed, refitted but sadly problem continues. The tag is AUC980 which seems to be an interchangeable A40 lid, lid itself marked AUD9203 as is float? Thinking to put fuel filter between pump and bowl to stop any particles getting through in case that is causing the flooding, brass pin holding float to lid does not seem to want to shift despite attempts at gentle persuasion, looking carefully at the pin its not entirely clear if this pin is meant to come out or not, according to Moss video it should come out easily, obviously need to get pin out to change float valve although not entirely clear that the valve has any issues. Wondering about the above issues and the best way forwards with these and or whether there might be a better float lid /overflow system that pipes xs fuel safely away, rather than onto float lid?

[philthehill]

It has to be either the needle valve or you have too much fuel line pressure.
Did you hold the valve shut when you blew through it? A good test to see if the valve is leaking is to remove the float chamber top completely from the carb turn it upside down so only the weight of the float is holding the valve tight, blow through it and if air passes the valve it is the valve. So replace. The pin is meant to come out so do not be afraid to give it a sharp tap. Rest the pin support on a vice and tap the pin out. See the Burlen web site http://sucarb.co.uk/technical-carburett ... ettings-hs for maintenance details.

[katy]

The pin only comes out one way. Look real close on the sides near the ends and on one end you'll see small flutes stamped into it. This is the end that comes out. The flutes are there to keep the pin in place and stop it from turning.

[nigelmmoc]

Thanks yes thought I could just see some sort of flutes as you say so if I go for the jet I will try the pin that way with a bit more oomph.

[nigelmmoc]

Thanks yes tried the float valve weight upside down and it stops air no problem. I had held the valve shut first time I tested.
After first clean of bowl taking out some dirt it seems like more dirt has got to the bowl after road test despite tank and line seeming in fair condition. Wondering if an inline filter may be needed? I understand about possible excess fuel pressure but its just the old pump which has always worked fine so cannot see why that would be the cause unless something odd is going on. Bit of a mystery?

[philthehill]

Katy's right about the float pin and it states clearly which way it should be removed in the maintenance procedures as per the Burlen web link I quoted above.
Have you run the tank low recently? If you have that could be a source of the dirt in the float chamber. An in line filter between the pump and the carb is no bad thing at least it keeps the dirt out of the float chamber. I would not run an engine without having at least an in line fuel filter fitted. Whilst fuel line pressure is not so important and less variable on a standard car with a stock fuel pump it is should be considered if you cannot find anything else wrong to cause the float valve to leak fuel past its seat. Whilst not essential a fuel pressure regulator does keep the fuel pressure at the float valve constant and can help MPG.

[bmcecosse]

Is this carb a recent fitment on your engine? Was it working ok - and now suddenly it's overflowing? A 948 would have had an H2 carb - with metal float etc..so it sounds like you have a more recent carb fitted - HS2 ?? Is the float bowl dead vertical?? There already is a filter in the fuel pump (assuming standard issue SU electric??) so no need for any additional filter.

[philthehill]

From the tag attached to the carb you quote AUC980. That carb an HS2 was originally fitted to an Austin A40 1960 -1961. So not original carb but the float chamber will be vertical.
Yes there is a gauze filter fitted to the underside of the SU pump but the extra inline filter will do no harm and will filter out finer particles of crud.

[bmcecosse]

And may lead to 'the vapours' on a hot day..... The SU carb can cope easily with crud - unlike a fiddly carb with tiny jets!!

[philthehill]

I can honestly say that I have never experienced problems (quote ' Vapours' un quote) with SU carbs caused by inline fuel filters when fitted to the various BMC vehicles fitted with the ubiquitous 'A' series engine I have owned and driven over the years but have experienced dirt in the carb.

[bmcecosse]

Me neither Phil - but others report it whenever the sun shines..........

[JOWETTJAVELIN]

Inline fuel filters are the work of the devil and should be avoided at all costs. I had real starting difficulties on a car with one fitted - until I threw it away.

[philthehill]

I cannot understand what the difficulties would be so please enlarge?
If the in line filter is fitted between the SU pump and float chamber any vapours generated would be pushed onwards to the float chamber and out of the float chamber vent by a fresh charge of cold fuel provided by the fuel pump as soon as you switch on the ignition. If it was unsuitably fitted i.e above the exhaust manifold before the SU pump then it may cause the pump to have difficulties priming.
I have a Filter King combined pressure regulator and filter assembly on my competition Minor fitted under the bonnet and after the pump; it does get very hot under the bonnet of that car but I have never had any problem with 'vapours'. I also had one installed on my split screen Minor fitted with a 998cc engine which used to travel between Hampshire and the west coast of Cumbria weekly and again NO problems. I have an inline filter installed on my motorbike and again no problem and I certainly would not ride without it being fitted mainly because of the small jets.
The only car that I have ever owned that had problems with 'vapours' was a Ford Cortina 1200 (the one with the round three section tail lights) which you used to have to leave for at least 20 minutes to cool down to be able to get it started again.

[bmcecosse]

If a filter is fitted on the pump inlet (as some do) - it seems to lead to 'the vapours' on some cars...... I agree a filter on the pump outlet should be ok, but is completely unnecessary since the pump filter will already have caught any nasties, and the SU can easily deal with most particles since it has a relatively huge jet area to allow any crap to be sucked through. One tip is to rev mightily -and hold a hand over the carb inlet - sucks away any nasties lurking in the float bowl.......

[nigelmmoc]

Hi thanks for all the replies, the carb was on the car and not recently fitted, it started overflowing suddenly and seems intermittent, I think its a hs2 type, bowl is vertical, guess I should have a look at the pump filter to see what is in there, we did have a recent overheating incident and I wonder if that may have had anything to do with the change in the carb's functionality?

[bmcecosse]

Doubt it! Have you removed the valve and fitted a new one?

[nigelmmoc]

valve on order although old one looks fine, might go for in line filter if valve does not solve it, will report back later, thanks to all

[JOWETTJAVELIN]

Phil - the problem was that, on a car with one such inline filter installed between the (mechanical) pump outlet and float bowl - and it was installed in the right direction - it took longer than is acceptable to fire in a morning, until I removed the silly thing and threw it in the bin, after which it started first turn of the key. I never had any problems hot-starting, though.

If one of these filters was necessary, the design engineers would have fitted one at the factory.

[philthehill]

jowettjavelin
Many thanks for your explanation.
Phil