Usage:
3406B (7FB) Truck Engines
Description of Change: An orifice screen assembly is now used in the fuel injection pump group on 3406B Truck Engines. The orifice screen assembly restricts oil flow to the timing advance mechanism (drive group). The restricted oil flow increases the time required to obtain full advance during acceleration and reduces engine and total vehicle noise levels.
The new orifice screen assembly (Illustration 1) is located in a single tapped hole in the end of the fuel pump camshaft (Illustration 2).
Illustration 1. 7C2568 Orifice Screen Assembly.
Keep the following points in mind with regard to this new feature:
Illustration 2. Location of orifice screen assembly for the automatic timing advance unit.
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- 1. When timing advance operation is checked with the 6V3100 or 8T5300 Engine Timing Indicator Groups, allow additional time (at least 30 seconds) at each rpm test point.
- 2. Acceptable operation of the timing advance unit (ability to obtain full advance, for example) is more sensitive to any defect that causes increased oil leakage from the advance unit or increased restriction of the oil supply. Check for loose bolts, worn or damaged seals, or a plugged orifice screen if full advance is not obtained.
- 1. When timing advance operation is checked with the 6V3100 or 8T5300 Engine Timing Indicator Groups, allow additional time (at least 30 seconds) at each rpm test point.
Adaptable To: The new 7C2568 Orifice Screen Assembly is effective with fuel injection pump Serial Number 66120 (approximately 7FB55250). It is not adaptable to earlier models.