SU carb blockage?

Technical advice Q&A

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Malc
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#1 SU carb blockage?

Post by Malc » Thu Jan 16, 2020 1:22 pm

My car won’t start! It has been laid up for a few years while I did some work on it but when I came to start it, it wouldn’t. I think I have traced to problem to the carbs in that there appears to be a blockage and they are not getting petrol. The pump is fine and the fuel line is fine and petrol is being pumped when it is disconnected at the carbs. However as soon as the fuel line is connected, the pump stops. If I empty the fuel filter bowl, it fills about a centimetre, then the pump stops. If I turn the engine over, the fuel filter bowl remains half empty. So I assume there must be a blockage at the carbs - I assume that some crud has got in, although I am surprised that causes a fuel shortage to all three carbs. Anyone got any other theories?

I assume the only solution is to remove the carbs and clean them, and it appears the ‘easiest’ way, (and I use the word advisedly), is to take them off with the manifold. However can anyone advise me if there is an easier way to clear a blockage, i.e. without having to take the carbs off - can they be cleaned in situ? Many thanks.
Malc

Series 1 2+2, 1968 Blue exterior, grey interior

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Robbiee
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#2 Re: SU carb blockage?

Post by Robbiee » Thu Jan 16, 2020 1:37 pm

Hi Malc
Simplest start is to remove the top of each float chamber and check for sticking needle valves that stop the fuel flowing in from the supply (usually as the chamber fills and a float lifts), they can stick
Give em a clean and maybe replace and try again
(No need to remove the carbs from the car to do this)
Robbi
Robbie 1962 3.8 Coupe OSG 1962 OTS ODG

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tinworm
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#3 Re: SU carb blockage?

Post by tinworm » Thu Jan 16, 2020 1:42 pm

I was typing a similar fix myself and was beaten to it by Robee
1968 E-type roadster, 1964 E-type fixed head 1995 Ferrari 355 1980 Ferrari 308 1987 V8 90 Landrover 1988 Bedford rascal van 1943 Ford GPW

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mgcjag
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#4 Re: SU carb blockage?

Post by mgcjag » Thu Jan 16, 2020 1:51 pm

Hi Malc...where the banjo bolt screws into each fuel bowl lid there is a small cone shape metal gauze filter inside each one....also re check your main glass filter ...most replacement filters work in a reverse direction meaning dirt dosnt collect visably in the glass bowl but hidden inside the filter element....Steve
Steve
1969 S2 2+2 & Building a C type replica

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christopher storey
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#5 Re: SU carb blockage?

Post by christopher storey » Thu Jan 16, 2020 2:09 pm

As Steve has said..... and, if water has collected in the bottom of the tank, it will have been pumped through to the filter element in the glass bowl, and experience has shown that this "blinds" it . I should take the filter element out altogether, and then see if the bowl fills properly

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malcolm
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#6 Re: SU carb blockage?

Post by malcolm » Thu Jan 16, 2020 3:01 pm

I'd go along with the needle valves in the float chambers. Mine stuck the other way so fuel kept flooding. Although maybe odd that all three should stick? An easy first check to make though.
Malcolm
I only fit in a 2+2, so got one!
1969 Series 2 2+2
2009 Jaguar XF-S

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Robbiee
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#7 Re: SU carb blockage?

Post by Robbiee » Thu Jan 16, 2020 5:00 pm

I agree but worth an easy quick check!
Robbie 1962 3.8 Coupe OSG 1962 OTS ODG

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Malc
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#8 Re: SU carb blockage?

Post by Malc » Fri Jan 17, 2020 11:14 am

Thanks everyone for your advice. I am away now for a week (Iceland! My partner said she wanted some winter sun - I’m not sure Iceland is what she meant...) so won’t be able to try it all out until my return but I shall let you know the outcome.
Malc

Series 1 2+2, 1968 Blue exterior, grey interior

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rfs1957
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#9 Re: SU carb blockage?

Post by rfs1957 » Fri Jan 17, 2020 5:10 pm

Make sure you know how the float bowls breathe (via specially-shaped washers under the banjos) before you do too much around the float-bowl cover area.

You might yet be looking at a pump problem ........ it sounds like it will move fuel as long as there is no resistance.

I've never had this happen to me but believe an electrically-dodgy fuel-pump might exhibit this laziness.

I would check the fuel pressure on the exit from the pump, if done intelligently and safely, i.e. with the addition of a long piece of petrol pipe, you will soon judge whether the pump can sustain a consistent jet against a finger that is partially blocking the end.

(Ed - now expect someone to remind you that you shouldn't smoke whilst doing this.)
Rory
3.8 OTS Cream 877393 Built May 28th 1962
1978 Mini Van
(plus bevel and belt single-cylinder Ducatis)

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cooper66
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#10 Re: SU carb blockage?

Post by cooper66 » Sun Jan 19, 2020 9:11 am

I had the same issue after the complete rebuild of the car. My pump was running but no petrol arrived to the filter, I took a seringue, plugged it on the pipe after the filter and vaccumed in order to have the pump properly feeded. If the petrol filter is not full it is a pump issue / fuel lines / filter issue not the carbs. One stupid remark but it happened to me, enough petrol in the tank ?

Then once your filter is full, if you still do not have any petrol to the carbs it is, as said previously, the float chamber needles most probably.
Pierre

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rfs1957
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#11 Re: SU carb blockage?

Post by rfs1957 » Mon Jan 20, 2020 2:46 pm

A correctly functioning SU pump does not require priming, it should work and produce a suck even when dry.

Otherwise, every time you ran out of petrol you would need to re-prime it.
Rory
3.8 OTS Cream 877393 Built May 28th 1962
1978 Mini Van
(plus bevel and belt single-cylinder Ducatis)

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