Carburettors - Part 2

In Part 1 the focus was on fitting new air filters.  The reason is to get as much air into the engine as possible.

With that objective achieved by upgrading the filters, the next step is the mixture.  Once again, there are mixed opinions on whether this is necessary.  Certainly, the mixture will need re-setting, but another school of opinion is the need to change the needles, too.  The needles are carefully profiled in such a way that the mixture can be controlled quite precisely across the rev range.  I think it was Peter Burgess that states in his book that there are more fuelling points on an SU than in a fuel injection system.  However, the carbs are quite sensitive to having the right needle as Bryn and I found out one evening messing with his carbs trying different needles.  A skilled tuning expert can probably profile the needles to suit individual engines.  

However, for my part, I prefer to go with the guidance given by Moss and others that for a Stage 2 head with K&N filters, then a set of AAA needles is about right for both HS and HIF.  In the photo above you can see the slimmer profile of the AAA needle at the top against the standard needles fitted in the HIF carbs.

To swap the needles, you will need to remove the dash pots and the will mean undoing the three screws holding the pots down to lift out the pistons.  Take care when removing the dash pots, there is a big spring underneath, not a powerful one but nevertheless it is there. 

It's quite possible that the piston and dash pots will be quite dirty and possibly sticky from years of use.  Under no circumstances should you attempt to clean this with any form of abrasive.  Take a bit of carb cleaner (or even some petrol) and rub it in to remove the deposits.  The piston should slide smoothly inside the dash pot.  If it does not, then it needs more cleaning.  Also, try and avoid mixing the parts.  The piston will take a wear in its dashpot and its best to keep it that way.

Removing the needles for is fairly straight forward, it's usually a screw on the side of the piston.  Many of the pistons will have self-centring needles, but earlier HS pistons may need the needle centring (good luck!). 

The needle will slide out complete with bush and spring once the screw is slackened and as Haynes always say, refitting is the reverse of dismantling.

Avoid damaging the needles or bending them etc while fitting for obvious reasons.  One problem of the self centring needles is that they put some pressure on the side of the jet, so be aware that it may be necessary at some point to rebuild the units and replace the jets.

Once refitted to the carbs, then you will need to check the mixtures and idle speeds again.

HIF Improvements

One area where the HIF changed from the HS was the fitment of a wax-stat that was meant to make minor adjustments to the jet so that as the air got warmer, thinner and less dense it would compensate.  The wax-stats can give problems later on, especially if the car is stood idling in traffic for long periods, resulting in rough running.

Another change was the fitment of "poppet valves" on the butterfly spindles.  This was designed to reduce the amount of fuel settling on the walls of the inlet manifold when the throttles were snapped closed as you took your foot off the pedal.  As the throttles opened again, the unburnt fuel would result in poor emissions.  To counter this the poppet valves would briefly open as the throttles closed allowing a small amount of air through to prevent fuel condensing.

However, over time, the valves become worn, and can affect the setting up of the carbs, making it difficult to get a decent idle speed.  In addition, the valves were quite sizeable and thus caused a major restriction in the air flow through the carb.

Therefore when rebuilding the HIF carbs use throttle plates form the HS carbs.  This immediately reduces the restriction, and makes setting the carbs a lot easier as I found.  More importantly, the immediate and noticeable benefit I found was that I had much better engine braking when coming off the throttle.  If you are not rebuilding your carbs, then it is usually recommended that you solder the valve closed.

Another difference from the earlier cars is the fitment of the vacuum advance on the balance tube of the inlet manifold.  I am not sure of the exact reasons for this but the earlier cars had the advance pipe coming off the rear carb just on the throttle spindle.  I disconnected the vacuum advance after I read in Peter Burgess' book that the inlet manifold at idle creates too much advance, and this explained the pinking I always got at low speed despite retarding it.  It's academic now I have the EDIS, anyway.

HS Improvements

The major one here is the use of dust seals on the spindles to minimise wear as were used on the HIFs.

General other improvements

For serious airflow benefits, the throttle spindle can be filed down to reduce the restrictions and smooth the airflow even further.  The piston base can also be gently radius-ed to improve airflow, too.

I shall return to this page as and when I can get more photos and carry out further testing myself.

And I leave you with a look at some serious induction noise and airflow issues...