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John Deere Tractors Discussion Board

Re: John Deer 4020 timing

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Posted by jdemaris on August 23, 2013 at 05:59:20 from (

In Reply to: John Deer 4020 timing posted by Robert Gregory on August 22, 2013 at 19:37:23:

One example of timing specs on a 4020 fuel injection pump. JDB633AL2721 model.

Pump marks are aligned on an non-running engine which puts actual injection time (at the pump) at around 2 degrees BTDC (that's engine degrees). Once running at 800 engine RPM - the timing mark on the pump advances one degree (pump degree) and that equals two engine degrees.

Once the engine reaches 1600 RPM (800 pump RPM) the timing mark advances 5 degree which equals 10 engine degrees.

Once the engine reaches 2400 RPM (1200 pump RPM), the timing mark on the pump advances 8 degrees which equals 16 degrees of engine timing advance.

Note that unlike a gas engine with spark that travels at the speed of light - a diesel has "lag" since liquid fuel must travel inside a pipe. So injection at the pump happens earlier then injection inside the combustion chamber.

The idea of blindly setting the static pump timing at 10 degrees makes little sense to me. If the pump advance was working at is should - that would give a total advance of 26 degrees BTDC! Now - if the pump was bad and the advance was no longer working - then bumping up the static timing by 10 degrees would make the engine run a little worse at low speed and and better at high speed.

Keep in mind that the pump runs half of engine speed and all pump degree are only half of engine degrees.


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