[Chadow]

A brief update!
Having finally got some spare time, I changed the rear tyres to a deepre profile - actually some ols Scalextric tyres from a Lotus 72 and they fitted!  It also solved the problem of the exhaust holder scraping the track.  The next issue was grip, so the higher profile tyres gave me room below the chassis to add a bit of lead. Sadly, my spring gauge broke a couple of years ago, so the weight is completely unscientific, but I found that a piece about 1/3rd the size of the motor works perfectly on Scalex F1 cars such as the Lotus 72 and Tyrrells. Thickness wise, the lead i use is old flashing and  is probably about 60 thou.

So - on my track (painted Scalextric Sport) using Truspeed controllers in analogue the lap times are:

OOB - 14.2 secs
Scalex tyres added - 8.2 secs (haven't got anything better - yet)
Weight added - 5.8 secs

Average lap time in digital mode with existing early 70s cars - 5.0 secs.

Problem solved! (Apart from getting a chip inside it)