Sierra 18-99391 Timer Base for Johnson/Evinrude V6 Outboards
Sierra 18-99391 Timer Base for Johnson/Evinrude V6 Outboards — Product description
If your loop charged Johnson or Evinrude V6 is starting hard or missing up top, the timer base under the flywheel is a prime suspect. This Sierra 18-99391 is the direct replacement for the 185 to 250 HP loopers built from 1988 to 2001, and it keeps Quick Start and S.L.O.W. working just like the factory intended.
The timer base is one of those parts that does its job silently for years, and then one day your big Johnson or Evinrude V6 starts hard, misses up top, or just won't hold timing. This is the Sierra 18-99391 replacement timer base, and it's built specifically for the 6-cylinder loop charged blocks that ran from 1988 through 2001. If you've traced an ignition fault back to the timer base under the flywheel, this is the part that puts spark timing back where the engine expects it.
Fitment here is specific, so let's be clear about it. This unit covers the 185, 200, 225, and 250 HP loop charged Johnson and Evinrude outboards built between 1988 and 2001. These are the V6 loopers, not the older cross-flow blocks. The two families use different ignition layouts, so before you order, confirm your powerhead is a loop charged design. Get that right and this is a direct fit under the flywheel.
It's also set up for engines running Quick Start and S.L.O.W. Quick Start bumps the timing on a cold start so the motor lights off fast and settles into a clean idle. S.L.O.W., the Speed Limiting Overheat Warning system, pulls timing back and caps rpm if the powerhead runs hot, which is what saves you from cooking a piston when a water pump gives up. This timer base keeps both of those factory functions working the way they should.
Sierra has been supplying ignition and engine parts to boaters and OEM builders for decades, and their gear is engineered to meet or exceed the original part it replaces. That matters on a high-horsepower two-stroke. The potting and connectors are built for the heat, vibration, and salt that live inside an outboard cowling. You're not gambling on a bargain module that quits halfway through the season.
On the install, this isn't a five-minute plug-and-play. The timer base sits under the flywheel, so you'll pull the flywheel and the stator to get to it. Disconnect the negative battery cable first, pull the power pack, then unplug and swap the old base. While you're in there, take the extra ten minutes to clean or replace any corroded pins on the 4-pin and 6-pin harness connectors. A green, crusty connector will cook a fresh module in weeks, and it's the number one reason a good part gets blamed for a bad install.
Timing is the other thing to get right. These engines want their base timing set precisely, and a couple of degrees off will show up as hard starting down low and a miss up around 3000 rpm. Set it to spec with the flywheel key seated correctly, especially on the later 1998 to 2001 250 HP motors that can be fussy about sync. If your Quick Start advance isn't engaging, check the trigger magnet gap before you blame the base. Too wide and it won't pull the advance in.
Real-world life on these varies with where you run. Saltwater boats tend to get three to four seasons before the potting starts to show cracks, while freshwater rigs often run them far longer. Either way, when you do a timer base, it's smart to look at the matching ignition coils and trigger at the same time so you're not back in the flywheel again in a month. A full ignition refresh on a 225 or 250 is cheap insurance compared to a tow.
Bottom line, if you own a 1988 to 2001 loop charged Johnson or Evinrude in the 185 to 250 HP range and you've got an ignition gremlin, this is the correct Sierra timer base for it. Confirm the loop charged fitment, do the connectors while you're in there, and set your timing carefully. Do that and you'll be back on the water with a motor that starts right and pulls clean.
Key Features
- Fits Johnson/Evinrude 185, 200, 225, and 250 HP loop charged V6 outboards built 1988-2001, a direct fit under the flywheel
- Supports Quick Start and S.L.O.W., keeping factory cold-start advance and overheat rpm protection working
- Sierra engineering meets or exceeds the original timer base it replaces, built for high-horsepower two-stroke demands
- Marine-grade potting and connectors handle the heat, vibration, and salt inside an outboard cowling
- Restores correct spark timing to the 6-cylinder ignition mounted under the flywheel
- Direct OE-style replacement, no modification needed for the listed loop charged powerheads
Why Buy from NVN Marine
- Authorized Sierra Marine reseller, full manufacturer warranty
- NMEA member and ABYC certified, advice from real boat techs
- Same-day shipping before 3 PM ET on in-stock items
- NY headquarters and Fort Lauderdale flagship retail store
Technical specifications
| Title | Sierra 18-99391 Timer Base for Johnson/Evinrude Outboards |
|---|---|
| Brand | Sierra Marine |
| Manufacturer Part Number | 18-99391 |
| UPC | 736983851291 |
| Part Type | Ignition Timer Base |
| Fits Engines | Johnson/Evinrude Outboard |
| Engine Configuration | 6-Cylinder Loop Charged V6 |
| Horsepower Range | 185, 200, 225, 250 HP |
| Model Years | 1988-2001 |
| Compatible Systems | Quick Start and S.L.O.W. |
| Cross Reference | CDI 133-3533 |
| Application | Outboard Ignition System |
Frequently asked questions
What does a timer base actually do?
It sets when each cylinder fires by triggering the ignition as the flywheel turns. On these loop charged V6 outboards it lives under the flywheel and feeds timing signals to the power pack. When it fails you'll see hard starting, a miss up top, or timing that won't hold.
Which engines does the Sierra 18-99391 fit?
The 185, 200, 225, and 250 HP loop charged Johnson and Evinrude outboards built from 1988 through 2001. These are the 6-cylinder loopers, not the older cross-flow blocks.
How do I know if my engine is loop charged?
The loop charged V6 blocks and the older cross-flow V6s use different ignition layouts and pinouts. Check your model and year against the 1988-2001 loop charged range before ordering. If you're unsure, we can help you confirm from your engine model number.
What are Quick Start and S.L.O.W.?
Quick Start advances timing on a cold start so the motor lights off quickly and idles clean. S.L.O.W. is the Speed Limiting Overheat Warning system that pulls timing and caps rpm if the powerhead overheats, which protects the engine if the water pump fails. This timer base keeps both working.
Is this a direct replacement for the original part?
Yes. Sierra builds it to meet or exceed the original equipment timer base it replaces, and it's a direct fit for the listed loop charged engines. See the specifications for the factory cross-reference number.
Should I replace anything else at the same time?
It's smart to inspect the matching ignition coils and trigger while the flywheel is off, and to clean or replace any corroded pins on the 4-pin and 6-pin harness connectors. A corroded connector is the most common reason a fresh module fails early.
Is it hard to install?
It's a real job, not a plug-in. The base sits under the flywheel, so you'll pull the flywheel and stator to reach it. Disconnect the negative battery cable, remove the power pack, then swap the base. Many boaters do it themselves with the right puller and a service manual.
What timing should I set after installing it?
Set base timing to your engine's factory spec with the flywheel key seated correctly. A couple of degrees off shows up as hard starting down low and a miss around 3000 rpm. If Quick Start won't engage, check the trigger magnet gap before blaming the base.
How long will it last?
It depends on where you run. Saltwater boats often get three to four seasons before the potting shows wear, while freshwater rigs frequently run them much longer. Cleaning the harness connectors at install helps it reach its full life.
