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Ask Dominick Your Own Question
Dominick, Marine Mechanic
Category: Boat
Satisfied Customers: 1009
Experience:  25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified
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I have a 1999 Johnson 90 HP v4 that has always run great. However,

Customer Question

I have a 1999 Johnson 90 HP v4 that has always run great. However, at the end of last season the engine would "hesitate" or bog down when pulling a tube -- it seemed worse when I would zig zag the boat to make a fun ride for the tubers. This season it seemed to do this anytime we were running at high RPMs when under load, and now it does it at most speeds other than idling (it will idle, or even slightly over idle, smoothly when under load).
When NOT under load, with the gears disengaged, the engine will rev to full rpms with no problems.
When under load (meaning anytime it is in gear) at full throttle, the engine seems to go to full rpms, then down to about half, then full again -- but it does it all in just a couple of seconds, repeating high-low over and over again, so the engine feels like it is stuttering constantly. As noted, it exhibits this behavior at most rpms other than low rpms (idling or slightly higher).
I have replaced the water separator and the engine's internal fuel filter. No leaks in fuel line that I can see. I emptied the internal tank and refilled it with new marine fuel. No change. Spark plugs were replaced. Battery is new. I even tried doing the hokey-pokey.
I then put an external tank on with new marine gas and it did the same thing. Putting on the external tank takes out of the equation the water separator and all fuel lines except 1) the bulb and 2) the last 18" of fuel line that is "hard wired" to the engine (not a quick connect). The engine's internal fuel filter was full of gas and spotlessly clean (since I replaced it last season after the problem first started).
The carbs were disassembled and cleaned by the dealer about 2 years ago, when I started having problems exceeding 4000 rpms. They found no problems and said the engine was fine "in the tank", but under load it still won't exceed about 4000 rpms.
The fuel bulb does not lose pressure, and pumping it or manually choking the engine does not improve the stuttering either.
It starts great and idles smoothly.
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.
Hello. My name is***** souds like a ignition issue. A failing ignition component can drop 1 or more cylinders then come back. Most of the time this is a coil or power pack issue. Now to determine which you will have to use a timing light to check the spark when the engine is acting up. Connect the light to one cylinder run the boat watching the flash of the timing light. The light should be flashing steady with no dead spots. If no misfire go to the next cylinder do the same test. If you find a misfire swap the cylinders coil with a known good firing coil one you have already test and tests good. Post back with findings.

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