01-07-2020, 12:02 AM
(This post was last modified: 01-07-2020, 12:03 AM by Tony Griffiths.)
When I bought the car from David he explained about misfiring on hot days and how it would vary with altitude and temperature. As I'd always run my previous vintage Seven with a standard updraught manifold, and never had a moment's trouble, this was somewhat puzzling. When I examined the neat arrangement of a side-draught SU and 4-branch exhaust (complete with a number of beautifully shaped aluminium shields to deflect heat) that David had cleverly engineered, I noticed that there was no hot-spot - and wondered if this might lead to ice forming in the manifold. Indeed, this proved to be the case for, after a run with the temperature at a mild 15C - and the car running normally - I discovered the manifold covered in condensation and ice cold. A subsequent journey in much hotter weather brought about the misfiring problem with even more condensation and an extended area of cold on the manifold. As a crude fix, I annealed some copper wire and wrapped it around two of the four-branch exhaust pipes and over the inlet manifold, as in the picture. The result was an instant fix - though I'll have to conjure up something that looks a little less inelegant. This solution is probably only relevant with an unheated inlet manifold when used on the road where the throttle opening varies; on a racer with full throttle all the time, it probably would not be necessary. Remember: in a carburettor, it's winter all the time...