Strange surging vibration under load

[moggiegeek]

Ive got a 1098 moggie with 12G295 head, rebuilt engine, maniflow inlet /exhaust, standard cam. HIF 38. K+N filter, Electronic ignition, new distributer+ coil. It has adjustable spax all round and anti roll bar at the front, Minilites wheels and standard servo'd brakes. At tickover the engine is silky smooth, picks up when revved beautifully
At any speed above about 15mph I get an oscillating perceptible surging motion. it is still there but reduced under acceleration. It is persistent no matter what I have tried.... which include:
Balance wheels
Service engine, check for loose bolts, engine mountings, g'box mountings, air leaks, vaccuum advance, play on distributer, dashpot oil, sticking carb slide loose pulley damper suspension mounting points - pretty much all is in as new condition. new propshaft joints...
I dont know what to check next.
Might it be a carburetion problem? needle problem?
Could it be engine mountings too tight?
Could it be backlash in differential?
What else can you suggest?

[palacebear]

Had something similar-sounding with mine ... on over-run or in slow-moving traffic. Mine has standard 948 with H2 carb and no electronic ignition). Problem disappeared when I leaned-off the mixture a little bit.

[moggiegeek]

Many thanks for the suggestion about adjusting the mixture. All other suggestions very welcome