Aye Glyn, welcome to the esoteric world of the Thumper Club.
As for the "racing" engine saga query, it sounds as though there is weakening of the air/fuel mixture, though I may be wrong!
Blocked vent from the fuel tank due to corrosion and or blocked drain hose from the rear of the fuel tank to the front of the swing arm pivot.
Perished inlet rubbers between the carbs and the head. The SRX type with a pre-formed curve in them are now difficult to find. But, the XT inlet type do fit and are more readliy available, both as OME and pattern parts. However, they are straight, rather than curved and re-fitting the tank over the carbs can be a pain. A test is to spray WD40, or similar, over the inlet tracts whilst the engine is running and note any change in engine speed or bubbles on the surface of the rubber. The rubber may also have parted from the inner metal supports moulded into the inlet tracts, allowing air to leak between the support and the moulding.
Fowler's of Bristol for OME and eyewatering prices, or Yambits for pattern parts and slightly lower prices. Another possibility is the Japanese 'JAUCE' website, their equivilent to our eBay. However, be prepared to multiply the current price in JYen by approximately 3 times to cover carriage and taxes, both local and import. The quality of used parts is usually good and myself and Ian have both used the service for parts not available over here (UK) for SRX400 3VN monoshock machines.
Perforated diaphragm in the carb causing an air leak. Would suggest a carb refurbishment kit from link:
NRP_CarbsIan and the Steve's L and H may also be along to add their pennyworth.
I hope this helps as a starting point and suspect that after 50years of motorcycle maintanence you could contribute to our knowledge as well.
Good health, Bill
