You can buy a pretty well written book from Bosch there will explain, in detail every system and subsytem in the VE pump. It will NOT give you enough detail for rebuilds or repairs, but it's good reading just the same and has good drawings and/or photos. They have an online book catalog at: http://www.boschtechinfo.com/
The book that relates to the Isuzu VE type pumps is - Distributor Type Diesel Fuel Injection Pumps - Bosch No. 1 987 722 144, ISBN No. 3-934584-65-9, Costs $17 plus all the shipping and handling garbage. Bosch used to have a better manual specific to the VE pump, but it's out of print.
My explanations here are a bit simplistic - so don't get p*ssed off if I've left some details out.
As far as me describing all the adjustments to you - it's much easier to understand with photos and diagrams. But, as I said previously, many of the subsystems affect each other. So, making adjustments without a lot of experience is, more-or-less a crap shoot. On this pump as used in the Isuzu, there is not a lot of provision for seeing the results of your adjustments with common, generic, measuring devices. On many other distributor pumps, especially U.S. built Stanadynes/Roosamaster, you can affix a temporary plastic timing window on the pump, and adjust the timing advance with the engine running with an external trimmer screw. But, no such thing on the Bosch/Diesel Kiki pump. So, on the vehicle, it boils down to trial-and-error. If it starts better cold, or runs better hot or cold - then you did good. If it runs worse, smokes too much, starts hard, etc. - then you screwed up. If it runs good, but fuel mileage drops from 30 MPG down to 18 MPG - well, you screwed up again, etc. etc.
Here is a brief description of some of the systems within the pump. Keep in mind, that this is a rotary distributor pump. That means there is only one little pump that has to feed fuel to ALL the cylinders - in this case four. So, there is a distributor section on the pump - and it distributes the fuel from the one pump to whatever cylinder needs it - in the direction or order of the firing order. Now, with a lot of industrial stuff, and some cars - e.g. Mercedes, and in-line pump is used. And in-line pump has a separate pump for every cylinder, so by nature, is more durable. With a four-cylinder engine, an in-line pump does only 1/4 the work of a rotary. So, the rotary as used in the P'UPs has a planned trouble-free life span of approx. 250,000 miles, whereas an in-line pump is rated for at least 1,000,000 miles. There are. of course, many exception either way.
Back to the VE rotary as used in the P'UP or Chevette. Here are a few sub-systems that all work together. inside the VE injection pump. It is designed for a maximum power rating of 25 KW per cylinder.
Transfer pump - it uses vanes and draws fuel from the fuel tank with suction. This setup can be a problem at times simply because, if a fuel line starts leaking, it will suck IN air, instead of leaking OUT fuel. So, you cannot see the leak. Many times, the injection pump gets blamed when there is nothing wrong with it. One remedy is to add a small electric fuel pump. This transfer pump sends fuel on to the little high-pressure piston pump than can generate pressures over 5000 PSI for indirect injection engines and 18,000 PSI for direct injection setups. The transfer pressure gets higher as engine and pump RPM increases. As this pressure increases, in a range from 40 - 175 PSI, the fuel timing advances by means of a hydraulic piston held back with spring pressure. So, timing advance can be altered via fuel pressure or spring pressure.
Distributor head and rotor - this is the section of the pump that has ports that line up and distribute fuel to the proper cylinder in the proper sequence - with the Isuzu 2.2 it is 1-3-4-2. No adjustments, and no repair - it relies on close fitting parts - and if it gets worn - it must be replace. It is the most expensive part of the pump. It is also very dependent of good lubrication and will not tolerate any water, or temperature "shocks" - like getting hit with cold water when hot.
Axial piston pump - this is the single pump that is fed by the transfer pump, and creates the high pressures needed to open the fuel injectors. It is, more-or-less non repairable, it can only ge replaced. The amount of fuel it delivers can be changed by altering the length of its travel - i.e its stroke.
Mechanical Governor - it uses flyweights and springs and keeps the engine at the desired RPM up to the preset limit. The VE pump is designed to govern to a speed of a maximum of 4800 engine RPM. The pump runs at half engine speed. The Isuzu P'UP 2.2 engine is designed to run at a max. of 5100 RPM which is 2550 RPM at the injection pump.
Fuel timing Advance - works by transfer pump against a hydraulic piston. The pressure varies with RPM, 40 to 175 PSI. At 1000 pump RPM (2000 engine RPM), timing movement in the pump is 1.6 to 2.8 mm. At 1500 RPM it is 4.1 to 4.7 mms, and at 2175 RPM it is 7 to 7.9 mm. Transfer pump pressures at the given RPMs are - 1000 RPMs = 3.8 - 4.4, 1500 RPM = 5.2 - 5.6. and at 2175 RPM = 6.6 - 7.2 (these pressure figures are kilograms per centimeters squared).
Aneroid Compensator - monitors air density due to altitude and changes fuel delivery to compensate. Turbochargers use to do sort of the same thing and - in the old days - were usually called "altitude compensators." A turbo makes up for less air by forcing more volume into the combustion chamber instead of changing fuel delivery. Also, many gas engines used to be available with "high altitude pistons" for use in areas of high elevations. The problem with them - was and is - what happens if the machine gets moved to a lower area - and then has too high a compression ratio.
Cold Start Device - allows for extra fuel delivery for easier cold-starting. Many older diesels - especially with farm and industrial equipment, had a separate cold-start device which was sometimes, just a hand-pump that squirted raw fuel into the engine air-intake for cold starting. Some other systems have a hand-primer and a heater element - so fuel actually lights when the hand-pump is used, and a sort of "flame thrower" results. My John Deere 2010 and my Allis Chalmers HD6 both still have hand primers for cold stating. Most US built gas engines, before 1986, with carburetors, did the same thing via an accelerator pump. Every time you stomped down on the gas pedal - a charge or squirt of raw fuel was shot into the engine's intake.
Electric Shut-off Solenoid - shuts off the fuel to the engine turns off - and does so with an electric solenoid. Some other vehicles use a manual pull-lever to shut off fuel - or in some cases - the lever activates an air-choke (like in some older Mercedes). Many Detroit Diesel engines have both. The primary shut-off turns the fuel off, and then there is an additional "emergency" shut-off that closes off the air intake.
Fuel Injection Nozzle - it is set to open, when new, at around 1500 PSI in the Isuzu 2.2 or 1.8. Some diesels run at well over 3000 PSI. The pressure is determined by a spring with either shims or a screw-adjuster. The main idea is - that no fuel can run through the injector until the pressure setting is met - and it then releases almost instantly - with all the fuel charge running though tiny little holes - or the pintle opening - thus creating a fine atomized mist of fuel. Naturally, the higher the pressure - and the more "instant" it occurs, the more atomized the charge - which leads to better combustion.
With a used injection with wear, the pressure settles to a lower level. Also, since the parts are worn, the firing is not quite so "instant" anymore - so atomization is not as good as a new injector. Sometimes, when attempts are made to raise the pressure on a used injector to new specs - it works worse instead of better. That is because of the wear - and with increased pressure - it dribbles or leaks more before during a slower injection process - instead of doing so instantly. I have run engines on dynos with close-to-worn-out injectors - often running pressures 300 PSI low. I then ran the same engines with brand new injectors. Usually there was NO measurable difference in power as long as the older injectors were not plugged. I suspect there was some drop in fuel efficiency, but not much. Cold starting was also barely affected. From what I've seen - when you know injectors are getting very worn - they are probably due to fail somewhere down the road - usually by sticking wide open - or by getting plugged. So, as preventitive maintenance, it's a good idea to check them every 100K - 150K miles. Sometimes it's hard to figure what to do. I just pulled all the injectors out of my 94 Ford F250 with a turbo 7.3 litre diesel. It has 220,000 miles on it and was starting and running fine. Well - the injectors were running 200 - 300 PSI low, but other than that were working fine. I installed new ones - but hated to do it. I bought eight new injectors at $36 each - and now - the truck runs and starts absolutley the same and fuel mileage is unchanged. But . . . I plan on taking a couple of long cross-country trips and I don't want to be fixing things out on the road. In regard to the Isuzus and some other small diesels. I pulled the injectors out of my wife's 91 diesel Volkswagne Jetta with 190K miles on it. They checked perfect - and being cheap - I just put them back in. I also pulled the injectors out of my 81 Chevette with the Isuzu 1.8 and 260K miles and they also checked out fine - and - I put them back in. My 87 Suburban with a 6.2 diesel and Bosch injectors, to the converse, developed a very bad engine knock at 80,000 miles. Ends up it was one bad injector - but the other seven checked perfect. I replaced them anyway - I had gotten a pretty good buy on new injectors at $18 each.