Timer Base Johnson Evinrude 4 Cylinder | Sierra 18-99426

Sierra 18-99426 Timer Base | Johnson/Evinrude 65/70/75 HP 4-Cyl Cross Flow | 1973–1978 | Screw Terminal Power Pack

The Sierra #18-99426 is a replacement timer base for Johnson and Evinrude 4-cylinder Cross Flow outboard engines. Fits 65 HP, 70 HP, and 75 HP models from 1973 to 1978 using screw terminal power packs. Replaces BRP/Johnson/Evinrude 385433, 581300, and 581900; CDI 133-1900. Screw terminal power pack engines only — confirm your power pack connection type before ordering.

Part Details:

• Part Number: 18-99426 (Sierra)
• SKU: SIERRA-18-99426
• Manufacturer: Sierra
• Type: Timer Base — CDI Ignition System
• Fits: Johnson/Evinrude 65 HP, 70 HP, 75 HP 4-cylinder Cross Flow outboards
• Years: 1973–1978
• Power Pack Type: Screw terminal (not plug-in connector)
• Replaces: BRP/J&E 385433, 581300, 581900; CDI 133-1900
• Condition: New

Cross-Reference — Replaces:

• Johnson/Evinrude (BRP/OMC): 385433, 581300, 581900
• CDI Electronics: 133-1900

Additional Information

What a Timer Base Does — CDI Ignition Function:

The timer base (also called the stator plate or trigger plate in some references) is a core component of the CDI (Capacitor Discharge Ignition) system used on Johnson and Evinrude outboards of this era. It houses the trigger coils (also called charge coils or sensor coils) that detect crankshaft position and signal the power pack to fire the ignition at the correct timing interval for each cylinder. A failed timer base causes one or more of the following symptoms:

• No spark on one or more cylinders — the affected cylinder's trigger coil has failed
• Intermittent spark or misfiring on one or more cylinders, especially under load
• Engine runs on some cylinders but not others
• Engine cranks but will not start
• Spark present at idle but lost at higher RPM (weak trigger signal)

Timer base failure is a common cause of ignition problems on these engines and should be tested before replacing the power pack, as the two are frequently confused in diagnosis. A defective timer base will kill a new power pack if installed together without identifying the root cause.

Screw Terminal Power Pack — Critical Fitment Qualifier:

The Sierra 18-99426 is for Johnson/Evinrude 65/70/75 HP 4-cylinder Cross Flow engines using screw terminal power packs. Johnson and Evinrude produced different power pack variants for these engines — some using screw terminal connections and others using multi-pin plug connectors. The timer base wiring harness connection must match the power pack connection type. To confirm your power pack type, inspect the connection between the timer base leads and the power pack — screw terminal packs use individual wire leads secured with screws, not a molded plug connector. If your engine uses a plug-in power pack, a different timer base part number is required.

Cross Flow Engine — Identification:

The Johnson/Evinrude Cross Flow designation refers to the engine's loop-charging/cross-scavenging design used on these 65–75 HP 4-cylinder two-stroke outboards from this era. Confirm your engine is a 4-cylinder Cross Flow model (not a V4 or other configuration) from the engine serial number and model designation before ordering. The model year and HP designation alone are not sufficient for parts confirmation on these engines — use the engine model number from the plate on the engine block.

Diagnosis Before Replacement:

Before replacing the timer base, test the trigger coil resistance with a digital multimeter per the CDI Electronics or Sierra ignition diagnostic procedure for your engine. CDI Electronics publishes free ignition troubleshooting guides for Johnson/Evinrude outboards that cover timer base and power pack testing. Replacing a timer base without testing risks incorrect diagnosis — and at this price point, confirming the fault before ordering is strongly recommended.

Installation Notes:

• Confirm engine model, year, HP, and power pack connection type (screw terminal) before ordering
• Test timer base trigger coil resistance before replacement to confirm fault
• Inspect power pack and stator for faults at the same time — a defective power pack can destroy a new timer base
• Refer to the engine service manual for timer base installation torque specifications and timing procedure
• Check ignition timing after installation per the engine service manual

Frequently Asked Questions

1. What is a timer base and what does it do?
The timer base houses the trigger coils that detect crankshaft position and signal the power pack to fire the ignition for each cylinder at the correct time. A failed trigger coil in the timer base causes no spark or intermittent spark on the affected cylinder(s).

2. How do I know if my power pack uses screw terminals?
Inspect the connection between the timer base wiring leads and the power pack. Screw terminal packs use individual wires secured with screws at the power pack; plug-in packs use a molded multi-pin connector. If your engine uses a plug-in power pack, a different timer base part number is required — the 18-99426 will not be compatible.

3. What are the symptoms of a failed timer base?
No spark or intermittent spark on one or more cylinders, misfiring under load, engine runs on some cylinders but not others, or cranks but will not start. Spark loss at higher RPM with good idle spark is also a timer base symptom. Test trigger coil resistance per the CDI or Sierra diagnostic procedure before replacing.

4. Should I replace the power pack at the same time?
Not necessarily — test both components independently before replacing either. A defective power pack can damage a new timer base; a defective timer base can be misdiagnosed as a power pack failure. Identify the failed component by resistance testing before ordering replacement parts.

5. What years and HP does this timer base fit?
Johnson and Evinrude 65 HP, 70 HP, and 75 HP 4-cylinder Cross Flow outboards from 1973 to 1978 with screw terminal power packs. Confirm by engine model number and power pack connection type — HP and year alone are not sufficient for ignition parts confirmation on these engines.