SU HIF 6 carbs query

02GF74

New Member
terms I am using; all refer to HIF carb
========================

piston - alloy cylinder with skirt, at the base of whcih is the needle. It moves up and down in the vacuum chamber to meter the fuel.
piston rod - hardened steel stube fittied inside the piston; its base is sealed to hold oil for the damper unit.

The base of the piston in HIF carburettors has two transfer holes and a rectangular channel in the centre. This channel runs front to back and in the centre is the needle guide holding a spring loaded needle. The needle guide, held by a screw, sits inside the piston rod that has been pushed through the piston.

My query is about the location of the piston rod and the needle guide.

Should the piston rod be fitted flush with the base of the piston, with the base of the channel or some intermediate postion? I think it would make a difference since if the rod were flush with the base of the piston, then the channel is more or less blocked off otherwise air would be able to pass through the channel when the piston is in its lowest position.

Next how should the needle guide be fitted? Should it be fitted so it is flush with the base of the piston, with the base of the channel or some intermediate position? Clearly the position of the guide determines how far into the jet the needle is so will affect the mixture at all positions.

I have the Haynes SU manual which shows that the guide is flush with the base for HD and HS types but appears to be flush with the channel for HIF types.

Taking apart my HIF 6 carbs, fitted originally to a Rover P6 3500, one of them had the rod and hence the guide flush with the piston base (the correct postion I think) whereas the other was slightly away from the base by about 0.5 mm. This second carb has it idle adjust screw wound fully out to keep the idle speed low that would indicate the rod and guide are not in the correct poistions.

Incidentally I managed to get the piston rod to be flush with the base and fitted new needles and it idles much better at lower rpm.
 
Hi,

I've had many recent problems with HIF6 carbs - see 'V8 engine / poor running' topic, but they are now sorted.

A replacement carb was fitted to one side & is ribbed at the bottom of the piston, the other is totally flat & original to that engine (early high compression).

In both cases the guide etc is completely flush with the flat base of the piston, not the ridge. The car runs very well now, after a thorough tuning & balancing including mixture adjustment. My problem was poor fuel flow through one of the carbs which has now been cleaned & sorted.

Hope you sort it out soon - I was starting to get withdrawl symptoms!

Phil Robson - Hull & E Yorks RO
 
Back
Top