Too Fast Tickover with SU Carb.

[John P]

Hi,

I have a 1 1/4" SU Carb on my Ulsteroid and have never had a problem with the idle speed although it was a pain to start which I put down to the very small head of fuel caused by the scuttle tank being only just above the carb.   (It's mounted on an an angled adapter atop a downdraft manifold).

Recently I've had the Carb off and also fitted an electric fuel pump which has significantly improved starting.

Anyway since doing this the tickover is now a bit on the high side even with the adjusting screw wound right back.   I have also checked the screw which slightly opens the throttle when the choke is operated and it's clear of the cam.

I believe leaks in the intake can have this effect but I only removed the Carb from the adapter it all looks OK.

Before I strip it down would a high fuel level in the float chamber have this effect as with the electric pump there's obviously much greater pressure in the fuel line which I guess could affect the level.

Any suggestions would be appreciated.

John.

[Alan]

If it’s the normal HS2 type there is a breather tube connection which needs to be blanked off. Otherwise is almost certainly an air leak somewhere.

[Reckless Rat]

With the carb off the car, lift the dashpot and shine a torch through the venturi. You can then see whether the butterfly is seating properly. If you can see light then that may well be the cause of your high tickover. Badly seated butterfly or wear on the spindle shaft.

[jpsmit]

+1 the butterfly.

Since the carb was off, in addition to air, check that the throttle cable moves freely and returns completely.

[mk1-mark]

If you can't find any leaks around the manifold to head surfaces & you are sure that there aren't any leaks between carb & manifold, you have checked for an un bunged vac advance tube & the butterfly is seating satisfactorily then it is most likely to be wear in the throttle spindle.

Even if this looks OK, you can check for this by getting the car warmed up & ticking over them dropping a drip of engine oil on either side of the carb spindle where it enters the carb body.  If the revs drop you have leaky spindle bushes.

It is VERY, VERY common on S/H SU's.

[jpsmit]

You should be able to spray something like WD40 around the joints at idle and if the sound changes you have found your spot.