SU woes

[Special Bits]

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...

If the fuel is from July I would try that first! I use outboard motors a lot and they are very temperamental on old fuel, anything over a month old can cause problems. As said above super seems to last better and some of the standard stuff can go off in a couple of weeks. I always use super in the 7 now as it doesn't get much use.

[Speedex750]

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...

Hi Hugh,

It stills feels like ignition to me?

However if you think it's mixture related (odd with black plugs) - it does sound like how an unchoked cold engine responds with flatness / coughing / spitting? That may also fit in with the cooler weather at this time of year compared to June. How does it run on choke with the choke levers pulling the jets out? Or alternatively you could wind the mixture nuts down 3 or 4 flats and see if it runs like that and then back to normal when warmed up?

SU pumps can run OK on an open end but stall against pressure, something subtle to do with the spring behind the diaphragm. But you'll be able to tell if the pump is clicking or see the kick on the ammeter when it does.

Dave

[Erich]

As I recall, the OM had a weighted piston. I have read that the spring and piston from a 1 inch H1 will fit the 617T OM and improve acceleration. But that doesn't sound like the issues it ran well before.

Erich

[nrzam]

Take this with a large pinch of salt as I am new to all this! Wondering if your ignition isn't advancing?

Ian

[Hugh Barnes]

Yes Erich. I think that's how SUs developed - early versions had heavy pistons and no spring (my pistons both weigh exactly 251gr) - later versions were light pistons and a spring. I think that probably indicates a cost saving rationale.

The ignition on this model of Riley Ian, is a magneto and has a hand advance/retard like earlier Austin 7s. I run the car fully advanced but thats where it develops maximum power, as established when setting it up on a rolling road.

I have tracked down another twin carb inlet manifold and will collect that tomorrow and fit it to see it that makes any difference. I've got to start eliminating things, even if I think its unlikely to be the culprit. I'm also going to try and prove there hasn't been any failure of the gaskets between the carbs and manifold...

---

Oh, and the pump is ok - it clicks away quite happily and, as you suggest Dave, you can see it register on the ammeter...

I'll try some new fuel as well, but the fact that when it does run, it runs perfectly, I think that less likely, but it's another thing to try and eliminate...

thanks all...

[Ian Williams]

Hugh, I agree with others that it sounds like weekness at some point on the needle profile, remember that the wrong profile can be both week at one point and rich at another. As was suggested earlier have you experimented with the choke out a bit, this will lower the jets and effectively richen the mixture throughout the range. I wonder if the suggestion about the time of year may have something to do with it, your car I assume has no thermostat so may be running cooler at this time of year?
Therefore if the mixture was set during summer months with a completely hot engine, and was set up with the O2 ratio border line lean around low to mid piston stroke this could possibly give symptoms described when the engine is not getting up to full operating temperature. If having the choke out improves things try partially blocking the radiator and getting the engine fully warmed up, see it that alters anything. Otherwise I have to agree that a sudden air leak is likely, but that would show as different plug readings on the cylinders affected, you have twin carbs so not all cylinders can be affected by an air leak.

[GK5268]

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:

It does spit back through the carbs when 'faulting' but that is the normal behaviour of a weak fuel issue...

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

Taken the above point from your post, spitting through the carb could indicate that this is a coil problem, the coil is breaking down under load, simple enough to swap the coil over for a known good one, it is the one thing you have not mentioned. Good luck!

[Colin Swinbourne]

I think it was mentioned that the car was on a mag.

However, mags have a condenser. Could this be causing problems?

I am having exactly the same problem with my 1929 Rover - I have this on a similar SU with a coil running through a mag as the distributor. it was running fine until recently (when I had leaks from the jet gland- now repaired) It now starts easily, I can open the throttle ok, and will even drive ok (for a short distance) until it gets warm- then exactly the same symptoms!

My current thoughts are that the condenser could be causing problems?

Good luck.

[John Cornforth]

Hello Hugh.  I run an SU on my Pearl of the early unweighted piston variety, and it has always been very reliable.  It seems to me that you have an advantage with a twin-carb setup in that it is extremely unlikey for both carbs to develop a fault simultaneously.  This leaves other "mechanical" items which are common to both, like the fuel supply upstream and the manifolds downstream.  However, in my experience sudden changes are more often the preserve of electrical items.  It's entirely possible for an ignition system to have just enough spark voltage to function at low revs, but as soon as combustion pressure rises, up goes the demand for voltage and any shortcoming is exposed as a misfire.  Richening the mixture artificially can make it easier to fire, but it's only masking the problem. I have also experienced the effect (especially in cold weather when the engine never gets really hot) of a downward spiral, where a misfire results in fouled spark plugs which in turn are more likely to misfire.  The black deposits can be "burnt off" out of the engine with a blowlamp but this isn't always completely successful and a new set might be easier.  Best of luck sorting this out.

[Hugh Barnes]

I've tried to eliminate ignition problems by swapping out the mag and fitting new plugs. The mag has recently been at my 'mag man' and I stood by him as he had it on his test rig and ran it up to 6000 rpm without missing a beat - both at room temp and then after raising to 50c in an oven, so I can't believe its anything electrical. I have to continue down the fuel/carb route...

Pulling the choke out does improve things but lowering the jets 3 or 4 flats may improve things but why should I suddenly have to do that?I'm the sort of chap that wants to understand 'why' before I start making changes.. And there should be no sudden reason to drop the jet 3 flats...

The night the fault showed itself, I drove 10 miles (albeit with some difficulty) to the local Riley group (and 10 miles back) so the engine certainly gor to operational temperature but there was no change to the fault behaviour... And when I've been exploring the problem, I've got it up to operational temp in the garage...

I'm going to change the manifold tomorrow as my next step...