Funny that, everything you read about the Square Four, rabbits on about the rear two cylinders over-heating and yet the pistons look absolutely fine on yours no score marks or anything, you can get or change your Solex to a Performance Solex, by re-jetting &/ or by changing the choke size 19, 22 & 24mm being available and the best people to have a word with is possibly the Square 4 people or the Imp Club, changing to a SU or any other could be a question of space... although there ia a picturte of a different carb on a racing Square 4 side-car outfit! by Stan Nightingale.
Tuning Solex carburetters for performance / by R.C. Pack and Charles Wheeler. - Brentford (51 York Rd, Brentford, Middx) : SpeedSport Motobooks, 1974. - [1],127p. : ill. ; 22cm.
ISBN: 0851130690 (Pbk)
British Solex carburetters. - [Brentford] : [Speedsport Motobooks], [1976]. - 218p. : ill., port. ; 21cm.
ISBN: 0903192705 (Pbk)
http://www.imps4ever.info/tech/carbs.htmlSolex carb's are not as poor as people are led to believe & can give good economy, A performance Solex, according to literature available, was a match for other more exotic carb' set ups.
If you have a copy of Great British Bikes from the 50s, there's a road-test, the limiting factor for the Square 4, apart from over-heating from the rear two cylinders a myth?, was its weight, rigid & plunger suspension limitations, the Cruciform induction tract, according to Legend most Square 4s were bought by overseas fans & then a lot were snapped up & exported to the States in the 60s and 70s, I should think they are a tad rarer than a unit Triumph, but each to their own!
Also you could fit maybe, the four pipe head, power outputs varied from model to model and are somewhere in the region from twin pipe 34.5bhp 4G model of 1951, price £260/7s 997cc (65x75mm), dry sump, weight 412lbs, wheelbase 56", Gear ratios, 12:1, 7:7, 5:7, 4:5, to low-mid 40s bhp, last attempt 1953, produced up to I think 1958, but all models could if coaxed exceed just about 100mph, if it got there rather more sedately than a Triumph twin!
Surely, this was one of the first super-bikes, described in 1951 as pulling 8 mph in top gear and as having vivid acceleration, Bob Currie i would imagine took some shifting, not being known for his light-weight!
1949 Ariel Square 4 shed 100lb from its predecessors!
4 pipe mark 2 came into being in 1953!
Cheers
John
JBW