A7 running issues

[Upperfield]

I have an intermittent running problem with my A7 Ulster Rep which has happened twice now, ruining the day and possibly stranding me.
 
The car runs a (virtually new) 1.25” HS2 SU with blue spring and No6 needle, the original cam-driven fuel pump (properly rebuild by an expert [not me] with an ethanol-proof diaphragm) and an original (virtually new too) BOSCH 009 with the Accuspark electronic kit fitted (with the white insulation paste underneath it) and the blue insulator to manage the advance.  I should say that the carb sits quite high and points towards the front of the car (to keep it away from the heat of the Ulster-type exhaust manifold) – I measured it and the throttle spindle is approx 6” above the level of the top of the cylinder head for clearance etc and the fuel tank is in the scuttle.
 
At the Centenary last year it ran well on the couple of hundred miles journey to get there, but on the second (very hot) day on a challenging run it started dying away would ‘pull’ only with the choke pulled out.  Thinking it was a fuelling issue, I adjusted/checked the mixture setting (not sure that much change was actually made) and replaced the oil in the damper (it seemed too ‘difficult’ to push up) the car ran reasonably well again – but I’m not sure that anything I did actually contributed to that.
 
The car has ran well since.  However, I was doing a PCT Trial last Saturday and the problem reappeared and I had to use the choke to get up any hill on the trial and also to get up any hill on the way home.
 
I have since stripped and rebuilt the carb – all looks fine, stripped and rebuilt the pump and all looks fine.  I haven’t had time to run the car yet but I bet that it will be fine.
 
On both days when there were problems, I was going up/down steep hills at relatively lower revs, the weather was fairly hot at the Centenary but a lot cooler last Saturday.
 
It is either a fuelling problem or ignition (which I assume is either ok or not, full stop – I have a spare Accuspark unit available but am not convinced it is the issue).
 
I’m now of the mind to swop to an SU fuel pump.  This Is based on two observations (1) my last A7 with SU Carb and pump, BOSCH etc always pulled like a train and (2) this carb is much physically higher in the engine bay than the A7 pump was ever designed to supply and so on lower revs it struggles to keep the float bowl full resulting in hesitation and needing the jet lowered to get sufficient fuel to pull.
 
Grateful for your thoughts and suggestions – am I right in thinking that an SU Pump is the solution?
 
Thanks.
 
Hugh

[Chris KC]

I can't answer your question Hugh, except to say that I run an Ulster rep with an HS2 and an SU pump (and a fuel filter) and never experience fuelling problems. They are both pretty much "trouble-free" units. The only issue you might get is that the points on the SU pump tend to stick after long periods standing idle, in this case various levels of tapping & prodding usually free them up again.

The oil in your SU dashpot should be thin - SAE 20 from memory.

Could it be fuel vaporisation?

[John Mason]

You mention going up and down hills when it plays up. Could it be the fuel level in the float chamber is not quite right and when not on a flat road the level will change either going up and down on a steep hill.

John Mason.

[Upperfield]

Thanks Chris, I’m running the SAE20 that Burlen sell for it.  Will go for an electronic version if I buy one. Thanks again. Hugh.

---

Thanks John, I will check that ASAP too.  Hugh.

---

Chris, on the fuel vaporisation, I’ll look at it again, the first time it happened it was a quite warm day ok but last Saturday wasn’t.

I’ll have to think about the carb trumpet though as it is completely under the bonnet, an area that does get hot ok. I have a plate under the carb to catch leakage though and that should shield the float chamber.

Have added a photo for info.

[Chris KC]

I must say I've never seen a carb arranged like that! Most open to the offside where they draw cooler air from the bonnet louvres(-ish). It appears the price of using that glamorous trumpet is that the carb is sat in the hottest place possible. That's not to say it can't work, just outside my experience.

[Upperfield]

It’s round that way to get that plastic end of the jet body away from the heat of the exhaust manifold to save me worrying about it melting and resultant fire. Will put trunking in though to get cooler (more dense too) air in which will help with vaporisation too.

But it runs v well 99.9% of the time. What would cause the intermittent problem?

Thanks again.

[Tony Griffiths]

This is a (very) long shot but - take the car for a run. Stop and check the inlet manifold and the area of the intake on the carb next to it. Is it covered in cold, condensed water? If so, it's carb icing.
My Chummy has an SU and suffered that problem - only cured by wrapping copper wire around the exhaust manifold and the inlet manifold.

[Malcolm Parker]

The fuel pipe from the crankcase mounted SU fuel pump to the carb looks to take a tortuous route round the front of the engine cylinder block.   My Ulster rep has an SU down draught carb mounted high up in the engine compartment.   It is fed by an SU elactric fuel pump mounted on the scuttle.   The carb and fuel pipe keep cool and I have never had any problems,  My old Ulster rep used the mechanical SU pump like yours.  I found that when I was revving it hard the pump could not deliver sufficient fuel.    

[Upperfield]

Thanks Tony, I will check that tomorrow evening and report back.

Hugh

---

Thanks Malcolm. The fuel pipe certainly ‘does the rounds’ ok, I will check tomorrow if it remains cool or gets warm.

Overall, the ‘arrangement’ of our two carbs is very similar and your previous experience supports my thoughts on an SU pump.

So current thinking lean towards it being a fuel, rather than an ignition, related problem?

Thanks again all. 

Hugh

[John Cornforth]

Hi Hugh

I wouldn't worry about the height of the carb above the mechanical pump, from memory the delivery pressure is more than 1 PSI which corresponds to a head of more than 3 feet of petrol. Worth checking that the unions are tight and not drawing in air on the input (tank) side.

The angle of your carb is effectively the reverse of a transverse setup like a mini, and with the float bowl to the side there should be little or no effect from facing up or down hill. The standard fuel level in the SU is 3/8 inch below the top of the metering jet and is not especially critical, as long as it doesn't sit high enough to cause leakage when switched off and parked. The "suction" on the jet when running is many times more than the amount required to raise the level by 3/8 inch to the top of the jet and allow discharge. Setting fuel level is normally done indirectly by measuring the float valve clearance, as it's easier than taking out the metering needle and peering down the metering jet.

If pulling the choke out makes things better, it suggests to me that the mixture is leaning out. This could be caused by the liquid fuel feed to the metering jet being replaced by fuel with a lot of micro bubbles of fuel vapour, the "froth" being less dense than normal. I think the combination of heat from a hard working engine and reduced airflow from lower speed could be making things just a bit too hot in the float chamber. Given the thermal mass of the float chamber, I would estimate that is takes a few minutes to heat up and cool down.

You don't say what fuel you are using, but you could try E5 rather than E10 in case the ethanol content is the culprit. The Esso brand is supposedly actually ethanol free (E0), but it is hard to tell. Unfortunately, moden fuel is blended for use in systems which continuously circulate it back to the tank to cool it, and I suspect it contains a greater percentage of lower boiling point components.

You might like to try measuring the temperature of the float bowl with one of those IR thermometers when and if things are playing up. A water soaked flannel would be one way of cooling the bowl down fairly quickly as a diagnosis.