Lumpy running

[davidsfirth]

I noticed something odd when out this weekend. On a motorway run all was good until I came to roadworks with a 30mph limit. As I decelerated the engine was running rough (sort of jerky) as the speed dropped off, then, when driving at 30 mph through the roadworks, it felt as if it was misfiring (the lumpy running/jerkiness you get), not all the time, but frequent. Out of the roadworks, car accelerated fine and ran fine at anything up to 70 mph ( I didn't go beyond that). The jerkiness was back once off the motorway and on side roads, but still pulling well in all gears, and running smooth from about 35 mph.

[svenedin]

Hard to guess what is causing this. One possibility is that the ignition timing is insufficiently advanced so that the car runs badly at low rpm. When the revs rise the distributor centrifugal advance operates and it runs better. If you have a strobe light it would be worth checking the ignition timing at idle and at higher rpm.

Another possibility is the fuel mixture is incorrect. Have a look at the colour of your spark plugs.

Stephen

[Bill_qaz]

Sounds like too rich so if you haven't adjusted anything and it has just started, make sure the choke is going fully off at the carburettor.

[oliver90owner]

Sounds like too rich so if you haven't adjusted anything and it has just started, make sure the choke is going fully off at the carburettor.

^^^^ This, or possibly a leaky fuel supply valve in the carburettor.

What does it do at idle? It should be even more evident at lower engine rpm.

[OGY985K]

Hot day? If so, Fuel vaporising at lower speeds is a possibility.