SU carburettor question

[Tintin]

My 1971 4.2 is fitted with SU HS8's and as far as I can tell these are original. I have the air filter box off at the moment and on checking the fall of the pistons found a variation between them. With the engine off and cold, raising the pistons by hand is easy at first until the resistance of the dash pot oil against the damper kicks in and from here, greater force is needed to raised the pistons as you would expect. However, one piston rises about 17mm before resistance but the other rises only about 5mm before resistance. I swapped the dampers between carbs and the variation follows the dampers. The dash pot oil has been topped up and otherwise the pistons seem to be free to move.

Does the variation before resistance matter?

Robbie

[3.8etypephil]

It will have some effect upon the mixture, thus power and acceleration, but unless it is spitting back through the carburettors (weak), or puffing out black smoke (rich) on acceleration, it is nothing to get excited about. The manifold design ensures that both carburettors serve all cylinders, and as the carburettors wear, additional air will be drawn in via the throttle spindles, neatly compensated for by the extra fuel drawn by the worn jets and needles. Even an overhaul kit is unlikely to facilitate perfect balance between the two.

[Andy P]

Hi Robbie,
That's a good question and I don't have an answer for you I'm afraid,but that would annoy me just because they are not behaving in the same way and I would have to try and get the carbs operation as close as possible but that's just me being fussy .(Or as my wife say's ANNOYING!!)
Anyway on to the main reason of my reply,when I used an S.U. on my Harley Davidson(Only used one carb so didn't come across your issue)I found this company most useful and helpful if I needed any information or parts.
https://burlen.co.uk/
I had a quick look in the fuel section of the XJ Series Saloons Forum and couldn't find a reference to them so I thought I'd post a link to them.
Could be useful for future reference .
Enjoy your cars.
Andy

[Tintin]

When the throttle is opened wide the pistons will rise quickly before the oil has its damping effect and so the mixture between the two carbs will be unbalanced until both find the resistance of the oil. We take a lot of time getting carburettors "balanced", and having an inherent variation between the two seems to defeat the object of the exercise!

I have now had both dampers out, cleaned them in brake cleaner and inspected the one way valve ends carefully and checked that all the washers and rings were free to move - blowed if I could see any difference between them, so refitted them. After refitting the match was better but not as good as I think it should be.

I suspect the problem is in the one way valve system in the damper stopping the oil settling back down to the bottom of the tube, so that when the piston rises it does so against an airlock at first rather than oil.

Replacement dampers are only a few pounds and Burlen are only just down the road, so I think I'll pop over and have a chat with them.

In the meantime I have a disaster with one of the rear brake calipers and it looks like I'll have to take the sub frame out to change it; not a job I'm looking forward to!

Thanks both for your input.

Robbie

[Tintin]

Well, it was very useful to talk the issue through with Burlen, and as a result I bought 2 new dampers. These are a different design to the originals and when fitted give much improved and perfectly matched operation of the damping effect to the pistons (when raised by hand).

It will be a while before I start the engine and test the modification on the road, but I am hopeful that it may bring benefits!

Robbie

[mike020150]

Hi Robbie,
Dependent upon which diff casing is fitted (there are several), if you are lucky, it may be possible to replace the damaged calliper without removing the sub frame. Basically, you stand a better chance with the later diff casings. I have worked on series 1's and 2's where it was possible to extract both callipers and others where, frustratingly it only proved possible to remove one of them. From memory, on the awkward diffs, it's the offside calliper which just won't come out. Since you will still have a fair degree of stripping to do once the sub frame is out of the car, it does no harm (just a little swearing) to attempt removal in situ first. When working at home, removing smaller items is usually easier and safer than manoeuvring the whole heavy sub frame out of the car. Mike.

[Tintin]

Thanks Mike - that's very interesting. The initial reason for removing the sub frame was a sheared N/S caliper bleed nipple (I tried penetrating oil, heat, and hammering but to no avail) but there are a couple of other jobs to be tackled as well - particularly a leak from the O/S output shaft seal. I have spoken with Hardy Engineering in Leatherhead who rebuilt my Volvo diff and did the job very well and they also do classics. I am coming round to the idea of taking the Daimler up there. The bleed nipple issue has only brought the inevitable job forward - and removal of the sub frame also gives better access for a general overhaul of the back end. Unless I can get the air out of the master cylinder somehow else (I've just had the servo out to deal with brake fluid damage to the bulkhead and changed the reservoir and low pressure hoses) I'll have to have it trailered up there.

Life would be so tedious without the challenges of old cars!

Robbie