Weird happenings when cold


Ex. Club Member
Since fitting a 1.3 into my K11 I have noticed it runs a bit weird when cold.
There seems to be a really bad flat spot at certain revs (don't have a Rev counter fitted) which is most noticeable at 40mph in 4th. I put my foot down and absolutely no acceleration. It's fine once I have a few miles under my belt and the engine is warm tho.
I presume its a cold map/fuelling thing but is this normal behavour?
It's a standard 1.3 but with a less restrictive exhaust mani, bigger airbox and a 1mm head skim?