Quote:
Originally Posted by paddo
The ratio of shim thickness to change in release pressure is governed by the K value of the spring so it is independent of the ground state pressure of the injector without shims.
The spring K value (ratio of change of spring force : change of spring compression distance) will only change very slightly with age (unless corroded or severely overheated) so the ratio will stay essentially constant, provided springs look OK on inspection.
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I am not sure what is going on in the above paragraphs.
Even if the length of the Spring does not change over time the strength of the individual springs decreases and the strength varies from Spring to Spring even if they are compressed to the same length they will all have different pressures.
That happens even with new Springs but the difference will not be as much from new Spring to new Spring.
This means that after the cleaning and lapping and clearing again of the parts you assemble the Injectors. You Pop Test them and you write down the pop/opening pressures of them.
Pick an Injector and decide what pop/opening pressure you want to achieve and use the formula in the Manual to estimate the change in Shim thickness you want.
Take the Injector apart and replace the shim/s with the size you estimated.
You assemble an Injector with that Shim (write down how much of a Shim Thickness you changed). You Pop Test it again.
If it came out OK you are done. But, most often it is still going to be off.
But, now you can take the original pop pressure and your new pop pressure and the amount of Shim Thickness you changed and a make yourself a formula as to how much pressure change you got per thickness of shim change.
You rummage through your selection of Shims and try to find one with the thickness you calculated.
You take the Injector apart again and change the Shim/s thickness and repeat the process until you get the pop pressure you want or you give up and decide that will have to be close enough.
Then you move on to the next Injector and repeat.
It seams like there is always one Injector that you will not have the exact sized shim you want.
In a Fuel Injection Shop they will charge you at least 1/2 hour of their labor rate per Injector.
I person who is used to doing the job and has all the tools and equipment he needs and is doing a whole set of Injectors can do the completed rebuild 10-15 minutes per Injector.
(And they will try to balance them as close as they can without spending too much time on the set.)
Also if you have a lot of Injectors you can cut the time even more because it is easier to find a balanced set from a large selection of Injectors and the cleaning and lapping process moves faster per Injector.
It takes less time then that to do direct Injection Injectors that do not adjust the pressure with shims because there is an adjusting screw for that.