carb damper oil

[max-it-out]

Dismantling the Stroms to have a look at various things and noticed the damper oil looked a bit dirty , so emptied it out - it looked quite thin oil . The only oil I had in stock was 5-40 , so used this to re-fill. Will this make any difference ?
Also , reading about US emissions regulations , it seems that the Stroms fitted in `68 were designed to produce quite a lean mixture . Would it be possible to use fine grade abrasive to thin the needles slightly to give a better A/F ratio ?

[Gfhug]

Mark, for SUs it is always advisable to buy new needles rather than try to modify the profile of existing needles. Who knows what profile you’ll create and will they all be the same?
Hope this might help you

Geoff

[johnetype]

Also , reading about US emissions regulations , it seems that the Stroms fitted in `68 were designed to produce quite a lean mixture . Would it be possible to use fine grade abrasive to thin the needles slightly to give a better A/F ratio ?

To what avail? Lean is a relative term and no carburettor produces the perfect mixture across all conditions, it's just that engines running too lean have more potential problems than running too rich so they are set to never be too lean resulting in parts of the performance envelop where they are too rich. Jaguar went to great lengths to select a carb and needle combination that would supply the correct mixture to run the engine efficiently and effectively. The emission regulations forced Jaguar to do that more effectively and SUs couldn't hack it so Strombergs were chosen. Unless your Strombergs have done mega miles they will continue to do that without benefiting from having the needles sandpapered. In any event as the needle and jet wear the effect will be to richen the mixture ever so slightly.

The oil in the damper only affects driving during acceleration where it serves to richen the mixture temporarily as you move from part throttle to WOT. From that it follows that thicker oil is best but realistically I doubt you'll be able to tell the difference between using 20/50 or 5/40 oil.

The biggest limitation on an XK emissions engine fitted with Strombergs is not the carbs but the inlet manifold and the timing. Once you have removed or disabled the secondary throttle assembly in the inlet manifold and replaced the distributor for one with vacuum advance and the UK standard timing curve you will approach performance close to that of the triple SU set-up and if it's still not good enough for you then that is the route to go next.

[max-it-out]

Good advice as always - thanks . Yes , the inlet manifold forces the A/F mixture to make a couple of 90 degree turns which probably reduces efficiency . I`m not sure about the reasons for this - maybe it`s to increase turbulence to give better mixing , or maybe increase the warming effect
When you think about all the features on the emissions version -

Twin ZS carbs
non-vac distrib.
retarded advance curve
strange manifold with warming diversion ducts

The above all contributing to a 20-30 BHP loss


It`s surprising they managed to sell any , although I suppose all the competitors had to cope with the same regulations .

[max-it-out]

Just a quick update for anyone contemplating doing the same - after a small amount of work on the needles using fine abrasive , a test drive showed better acceleration but examination of the plugs revealed moderate black sooty deposit which they didn`t have before , so probably a bit too rich .

Back to the standard needles I think ( B1AR ).

[tonyabacus]

The type of oil used will have an effect as its primary function is to damp out any vibration as the piston lifts and assist the spring in controlling the rate of lift as the throttle is opened. The smoother the lift the better. So in theory a thin oil allows the piston to lift quicker than a thicker oil as there is less resistance. Without oil the piston tends to rise in jerky movements noticeable in the throttle response with hesitation and flat spots.

Using a multigrade oil may vary the rate as in its basic concept it changes viscosity as it heats up. So as the engine warms up the oil in the damper can change and engine performance will change slightly too. It used to be the case that the manufacturer would specify a monograde oil and in some cases even a thin bicycle oil like Three-In-One would be recommended. The other issue is to keep the chambers filled as one carb may use oil at a different rate to the other, but not overfilled. In a regularly used car once a week would be a norm for checking.

In racing circles especially with SU's people have experimented with all sorts of fluids from the normal oils to gear oils to brake fluid and everything in between.

As for sandpaper on the needles it is not to be recommended, each needle is finely graduated along its length to reflect the characteristics of the particular engine requirements at a given throttle/timing setting. Old age is robbing me nowadays of memory but I seem to recall each SU needle had something like 20 graduations along its length and needles adjacent to each other on the needle chart could be quite different. If anyone has a needle chart perhaps they could post it here so others can understand why it can be so hard trying to find an alternative needle. So, in sanding a needle you have no way of knowing how and where to sand and that will affect performance and even less chance of producing two needles with the same characteristics.

Every engine is different because of manufacturing differences, tolerances, wear, heat and fuels used, so the only way of being certain that the engine and carburation is set up to suit its individual character and giving its true performance is on a rolling road. When you have spent hours, days or even weeks trying to get it right, money spent on a rolling road is not only money and time well spent, or saved depending on your viewpoint, but gives a certain satisfaction of knowing you are got it as good as it will ever be.

Here is an example of the graduations on an SU needle and what the different parts of the needle do which may help explain better why sanding a needle is not so easy to get right

https://th.bing.com/th/id/OIP.KL4Q8N5O7 ... =5&pid=1.7