SU woes

[Hugh Barnes]

Ok, not Austin 7 related, but I know there are people here who are well versed in the ways of the SU carburettor and I'm hoping someone may be able to help.

As well as Sevens, I have a 1932 Riley Nine fitted with twin 1" OM SUs running GG needles. OMs do not require and never had a spring fitted in the dashpot to temper the behaviour of the piston. Over the winter, I built a new engine for it, after running in, had it set up on a rolling road and embarked on a 2000 mile tour of Scotland. Apart from a mag crying 'enough' in the damp and me changing it for the spare, I had trouble free trip. I've used the car a bit since I returned home at the end of June. The other evening, I went out in it and, all of a sudden, it has developed a fault whereby the engine wont rev. When changing up through the gears, the engine dies when accelerating in the new gear (with your foot hard on the throttle) and will also show the same symptoms when the throttle is blipped to change down.

I have had the inlet manifold off and replaced the gasket as there was a sign of a leak. No change. I have had the filters out of the carbs and the pump and cleaned them, I have checked the flow of fuel from the pump. I have also fitted new plugs and changed the mag as part of my trouble shooting. All to no avail.

A pal came the other day who used to be a motor mechanic and he did a test whereby he removed the caps from the dash pots, dropped a screwdriver down each one, applied downward pressure with his hand and asked me to blip the throttle. The fault immediately disappeared. So, problem is caused by the pistons rising too quickly, basically letting too much air in, weakening the mixture and the engine dies.

The questions for my learned friends are as follows:

What could have suddenly changed to cause the symptoms? and

What might the solution be? I have fitted a couple of lightweight springs today, that has made a small improvement, but not enough. But the carbs are not designed to have springs anyway, so am I just curing the symptom and masking the fault?

Your thoughts, no matter how random or obscure would be welcomed...

Hugh

[Alan]

Assuming you've checked that the pistons rise and fall ok, the symptoms suggest a weak mixture to me. This can only happen if something has changed. I'd suggest looking at fuel level, jet position and needle position to see if any of them have slipped from where they should be.
However, it may not be the carb. How are your cylinder compressions? And are you sure there are no air leaks?

[Special Bits]

I agree with Alan, something is causing the mixture to go weak by the sounds of things.

SU carbs are not prone to blockages, but giving the internals a blow through with an air line is worth a try.

Check that the float hasn't got a puncture and is therefore floating low, shake it, to see if there is fuel in it.

Re set the float level.

Use a damp rag to wrap around each inlet joint in turn to check for leaks, with the engine running, probably a two person job.

As with anything like this, do one thing at a time, so you know which one fixed it, or not, as the case may be.

Stuart

ps, the float low would cause it to be rich, not weak....

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Does it run and rev properly with a bit of choke?

[Chris KC]

Seems highly unlikely twin carbs both developed the same fault simultaneously?

It sounds just like what happened when the accelerator air control valve (which controls air / fuel ratio) went on my Micra. Any popping or spitting?

[Special Bits]

OH!! Just had a thought.....

Have you put some fresh fuel in???

Despite the protestations from some that fuel doesn't go off, my experience is that some modern stuff does, and remarkably quickly!

[Speedex750]

OH!! Just had a thought.....

Have you put some fresh fuel in???

Despite the protestations from some that fuel doesn't go off, my experience is that some modern stuff does, and remarkably quickly!

Hi Hugh,

I've always fancied a Riley.........

You seem to have pointed the finger at the carbs but is there any other evidence the mixture is at fault like white plugs? All the symptoms you describe could come from ignition side weaknesses as well? And SU's are very simple things.

An air leak is usually just a problem on one carb making tickover uneven but once the revs pick up the in leakage is swamped by regular mixture and it usually runs OK. Are you getting enough fuel through, is your pump making pressure? I would hope that vapourisation isn't a problem at this time of year but blow through your fuel lines both ways (mark one mouth, I know it doesn't taste very nice). If you've a fuel filter, change it.

It's hard to envisage an engine that did run OK, suddenly not. I've heard about duff unleaded but I've found that once the engine is running it seems to work OK even with very old fuel (4 years old with my AH). Try priming the fuel chambers with fresh, or run the engine with a plumbed in can of fresh, to test that theory? Are sure you haven't got sticky valves or low compression that do fit better into a scenario of an engine standing for a while?

Random thoughts, hope of interest.

Dave

[Hugh Barnes]

Thanks all for the input. I am getting a bit fed up with this. The symptoms all point to fuel weakness but...

To address the points raised:

The pistons rise and fall easily in the carbs.
The plugs seem a little black, but I put this down to the way the engine is runnning poorly. The actual ignition point on the plugs seem about the right colour.
Fuel level looks ok as to needle and jets. But to be honest, once those sorts of things are set on SUs, it's hard to see how they could change.
The carbs have been apart and are clean. All the filters (in the pump and the float chambers) are clean.
The floats are good.
The pump is good and delivers plenty of fuel.
All fuel lines run freely.
The engine ticks over perfectly, the fault only manifests itself on fast increase of revs.
I haven't checked the compressions but the engine had a complete rebuild over the winter with new liners and pistons and has done about 2,500 miles. I suppose I should do that..
Fuel is the last tankful I purchased in July but the engine ran perfectly about a week before the fault appeared. I could try fresh petrol.
To eliminate them, I have swapped the magneto and fitted new plugs.
It does spit back through the carbs when 'faulting' but that is the normal behaviour of a weak fuel issue...
The carbs are mounted on an ali inlet manifold which is in turn mounted on the block. I have remounted the manifold with a new gasket and some gasket silicone.
I have checked there is no internal failing of the inlet manifild allowing air to get in, but for completeness, I should perhaps try another one to eliminate that possibilty.

I will agree, it does feel like an air leak to me, but I buggered if I can find one..

The only thing that can resolve the fault is the test with the 2 screwdrivers down the top of the dash pots preventing the pistons rising too quickly. The engine revs away like a good'un when I do that. Yesterdays test was to fit 2, admittedly unidentified but matching springs to the pistons but they didn't make mcu difference. But as I say, the carbs shouldn't have springs as designed, so that would just be masking the fault rather than resolving it...

Thanks for the ideas. I will continue my efforts and provide an update...

[austin]

I always run on "super" petrol in the old cars now, the "regular" stuff just seems to go off after about two weeks and is useless and would run the way you describe.

Had similar on a different car this year. (HIF44 carb on A Series engine).

It had a blocked breather pipe that made a big difference as it ran perfectly at low revs but wouldn't do anything else.

Changing the dashpot oil to correct spec fixed the issue completely.

[Austin in the Shed]

Is your fuel pump working properly ?

[Hugh Barnes]

Yep, as described, pump behaves flawlessly... OM carbs are far less sophisticted that later series models and don't have such things as breather pipes (I think). To be clear, when I open the throttle, the engine will die completely - almost as if one had run out of petrol or someone had turned the ignition off in error...

Today task is to track down a replacement manifold to eliminate that...