Normally this are old news but there are still people who think a bike is good when it looks nice - what definately not must be true.
I had so many Turbos in my hands (around 30 last 3 years) that I don't even bother to test the mounted chargers on the bikes I rebuild. They all get a complete overhauled charger mounted - regardless if mounted charger press full boost on the boostgauge or not.
1. old chargers are often worn due mileage, age or wrong treatment from preowners.
2. The stock chargers don't been build very precisely in the 80's and don't bring the entire performance a Z750 Turbo can provide. Most stock Turbos - even new ones in the 80's - bearly do 100HP on the dyno. But even with this chargers a Z750 Tu is pretty fast and most owners are not aware that their Turbo provides less performance she could provide with a good charger.
3. It's not just the performance in high revs. A well overhauled charger changes the entire bike experience and make 2 bikes out of a Z750 Tu - one wich is nicely usable even in the city becouse a good charger provides boost much earlier and responds MUCH faster. This brings more power in midrange and simply more fun and make the bike much more usable. In high revs you sit on a cannon bullett with a insane acceleration providing full and easy boost in no time so that you get in trouble with shifting fast enough...
I send my chargers to the US to a well known Turbo specialist who owns 2 Z750 Turbos
itself and makes a awesome job. Subjectively his chargers feels like 20HP more - without any tuning - just with 100% precision. Your original Turbo stays original becouse we only use and mount stock Hitachi HT10 chargers.
Other problem is the coating from the downpipes. You see often Turbos with ugly, rusty downpipespider - due the extreme heat even high temp colours collaps. Thats why I let my spiders ceramic coat becouse a nice restored bike with rusty and ugly downpipes isn't nice really. The ceramic coat is very close to stock appearance and leasts many, many years and looks like new even with high mileage.
And at least it is very important to clean the oilsump and mesh, overhaul the 2 oilpumps from the engine - one for the engine and a seperate one for the charger - with new seals to assure a proper oilflow.
All this costs serious money but any cent is worth to spent on this parts becouse you get a optimal Turbo with 100% performance.
Most Turbo charger overhaulers deny to work on a Hitachi HT10 becouse there are no parts available anymore and even some of the companies who overhaul HT10's fail to bring them back to 100%. I've ridden overhauled chargers from 5 different Turbo companies in different countries now and found my favourite overhauler in America. His chargers provide the full performance and more important - the full joy. So no Tu rebuild will left my shop with a freshly overhauled from this source becouse it's not enough to build nice looking Tu's - they have to ride and perform nicely as well.