1300 Surging at lower speeds

[Ralph]

I have a 1300 Dolomite, 1979 and it has a slight surging when at lower speeds around town, hardly noticeable to passengers but I can feel it. It has one of the later SU HS4E carburettors with the by pass valve in the throttle butterfly. I have heard about this valve being soldered up and wondered if doing this would get rid of the surging, or if not what is the purpose of this valve anyway, I assume it is something to do with emissions.
Ralph

[Carledo]

The poppet valve in the butterfly is supposed to help with potential cutting out when returning to idle, the idea being that engine vacuum on a closed throttle at revs will suck it open and increase the air a little so it drops to idle a little more slowly. I suppose it IS indirectly related to emissions, since the tune required to get post 75 models thruogh even the lax emission test that applies to them, (4.5% CO and 1200ppm HC) does often produce a slight stumble when returning to idle after revving as the revs drop momentarily BELOW idle, then recover.

The main symptom of a completely duff one is an absolute refusal to idle at less than 1200rpm but partial failure can result in other symptoms like irregular idle or "hunting", which you may experience when tickling along at very low revs, as well as when the car is stationary.

In my experience,soldering them up doesn't do any harm and it's one less thing to go wrong! An experienced tuner can usually tune out the "stumble", or at least minimize it.

Steve