10-04-2024, 04:59 PM
(This post was last modified: 10-04-2024, 05:00 PM by John Cornforth.)
Hi Martin
Lots of possibilities, and without seeing the car that makes diagnosis difficult but here goes...
The SU usually has a fast idle mechanism linked to the choke control, so pulling out the choke knob a little bit gives you just fast idle, then pulling more gives you fast idle plus a lowered main jet for enrichment. Check this mechanism isn't cracking open the throttle and raising idling revs.
If the throttle is genuinely nearly closed, high idle revs are a sign of uncontrolled air entering somewhere in the inlet manifolding downstream of the throttle plate e.g. manifold to head gasket, carb to manifold gasket. If the SU has a vacuum port (for vacuum advance distributor) this should be blanked off. Sometimes squirting WD-40 onto a suspect joint will cause a change in engine note and aid diagnosis.
Don't fret about exact float level, SUs are tolerant of this as the "suction" on the jet is quite significant.
If the car won't accelerate on the road when the throttle is opened, either the mixture is excessively weak (spitting back and jerking) or you have a failing condenser giving a weak spark. The weak spark isn't enough when combustion pressures increase, and you get misfiring.
If you have an auto advance distributor set the static timing to TDC to begin with. Check that the distributor weights are being completely closed by the internal springs at zero/low revs or the timing will be all over the place - If you have a Lucas DK4A they are quite sensitive to accumulated wear in the various pivot points.
Lots of possibilities, and without seeing the car that makes diagnosis difficult but here goes...
The SU usually has a fast idle mechanism linked to the choke control, so pulling out the choke knob a little bit gives you just fast idle, then pulling more gives you fast idle plus a lowered main jet for enrichment. Check this mechanism isn't cracking open the throttle and raising idling revs.
If the throttle is genuinely nearly closed, high idle revs are a sign of uncontrolled air entering somewhere in the inlet manifolding downstream of the throttle plate e.g. manifold to head gasket, carb to manifold gasket. If the SU has a vacuum port (for vacuum advance distributor) this should be blanked off. Sometimes squirting WD-40 onto a suspect joint will cause a change in engine note and aid diagnosis.
Don't fret about exact float level, SUs are tolerant of this as the "suction" on the jet is quite significant.
If the car won't accelerate on the road when the throttle is opened, either the mixture is excessively weak (spitting back and jerking) or you have a failing condenser giving a weak spark. The weak spark isn't enough when combustion pressures increase, and you get misfiring.
If you have an auto advance distributor set the static timing to TDC to begin with. Check that the distributor weights are being completely closed by the internal springs at zero/low revs or the timing will be all over the place - If you have a Lucas DK4A they are quite sensitive to accumulated wear in the various pivot points.