Vacuum advance?

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Hi, I'm back in a Fourtrak after a D22, Nissan Patrol & an L200, everything that was good about my old Fourtrak is still true but I'm not convinced the injector pump is advancing. 98 TDL so it's got the EGR & Emissions ECU. EGR is blanked. Starts fine hot or cold, boosts fine, great mpg (32 brim to brim) but feels a bit flat, I tried driving it with the vacuum pipe to the top of the injector pump off & it was no different. On full throttle its fine but part throttle it surges a bit. Bit smokey on start up & under power, doesn't use any oil or water. I suspect the injectors need doing, it's got that smell to it. Anyway...I'm not sure what I'm looking at on the vacuum circuit really, there's a feed from the high pressure side of the intake manifold with what I guess is a one way valve in (if you take it off it sucks before the turbo kicks then blows after) then a 'T' and a solenoid & a feed from the low pressure side of the intake. What does the solenoid do & what controls it? (Remember, this is a TDL so no vacuum 4x4 engagement; all on the lever). Any simple tests to check if the pump is advancing? I've removed the cover on the top of the pump & the diaphragm looks to be in good condition. I don't think this is a timing issue because nothing's been messed with for years, tappets are done too. The surging feels like something opening & closing but could just be my imagination. Thanks in advance.

OK, getting more into I see

OK, getting more into I see it's not a vacuum advance but a positive pressure actuator to add more fuel when the turbo is on boost (LDA). Still don't know what the solenoid does.