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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 15866
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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Just replaced the starter on my 1998 Larson 18 Flyer. Boat

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Just replaced the starter on my 1998 Larson 18' Flyer. Boat was working well for a week after. Took my daughter water skiing for the first time (using this boat). Lost top RPM/Speed...and now at cruising speed (5mph or less) it is "sputtering". The engine does not sound as smooth as it did before. My top speed was anywhere between 4 to 5 RPM...Now I can just make 3rpm. What could be the problem?

Hello, my name and I will be helping you today.


I have some questions back to you.

1. Do you need to know where you closest shop/dealer is?

2. Are you trying to fix this yourself. And if you are, you will need quite a few tools, some of which that you probably will not have and have to go out and buy.

3. Are you just looking for information on what we do in the shop to troubleshoot issues like yours, where we have all of the tools. That way you can try 1 or 2 of the easy things first before calling the shop.

4. Something else??

Obviously I am not there to do this work for you, and I don't know what you have for tools or abilities. So how it is I can best help you here?


Also, what engine do you have? How old is the gas your trying to burn now? When was the last time the engine made full power as it should?


Customer: replied 4 years ago.

The engine is a 4.3 Mer Cruizer serial number OL010931. I am very new to boats. I am seeking your advice on the easy stuff. I have an appointment to bring it to a marina during the early part of next week...But I had a very bad experience with another Marina back in PA (basically, ripped me off!) so I want to make sure I am not being taken when they look into it (i.e. If it needed a spark plug...and they are trying to sell me on a new engine). I don't own the tools to do this myself. The boat was running fine (was on vacation at Lake Keuka NY) for the entire week after I replaced the starter. The day it happened (Sunday July 14th) it was working fine all day (was on the lake around 12 noon and the loss of top speed/rpm and hesitant crusing speed occured after our attempts to water ski--which was at 3pm). We are really strapped for cash...and I know my wife will not approve of a huge expediture if the repair is going to be extensive. I was hoping to have the boat back on the lake ASAP.



Now when you say sputtering, is it popping out of the carb, or backfiring at all?

Customer: replied 4 years ago.

at cruising speed (5mph or less) kind of slightly revs up and slows down on it's own (without me adjusting the throttle). Also...Since you mentioned backfire...It does not backfire..but for the last week it takes a while for the engine to shut off. It would just keep chugging along in neutra for about a min or so when I turn the boat off. This would not happen each time I turned the boat off...but more times than not. Sometimes this would be accompanied with some white smoke (checked my oil and it is fine) when it finally would turn off. Also would get some gas fumes when I turn the boat on.

Gotcha Jack,

All engines need 3 things to run properly. Compression, spark, and fuel. The problem with a lack of power situation is you have to do some troubleshooting to figure out which of the 3 the engine is missing, giving you that lack of power, as all 3 can do it. Just 1 or any combination of the 3. There is no rule that says there is only 1 thing wrong with the engine that you have to find.


I can also tell you from experience, about 75% of the time it is fuel related somehow. Wether it is bad gas, water in the gas, weak fuel pump, a clogged carb ect... 3/4's of the time it has something to do with fuel. However, you can't jump to that conclusion right off the bat, because if you do, things get missed.

If you brought this to me here is how I would go about troubleshooting it.


I would do some preliminary troubleshooting to figure out which system is causing the condition. And where I need to start with is engine compression and spark tests, and you will need these tools to accomplish this.


1. Compression test - Your results need to be 100 psi or better as a bare minimum. A like new engine would be 160+ psi across the board. Used but healthy engines are typically in the 120 to 140 psi range. And they all need to be reasonably close. If any cylinder comes in below that 100 psi magic number, or you have a huge difference between 2 cylinders. Then the engine is worn out, or has mechanical problems, or there is a bad head gasket. And you can stop your troubleshooting here, as there is nothing you can do to an engine with low compression to make it run better. And the compression issue has to be addressed.


2. Spark test. You have to check for spark. These are the testers we use in the industry.

But you can also get cheap spark checkers at most good autoparts stores.

You need to hook the spark tool up and set the gap to 7/16ths of an inch. The spark should easily jump that gap, be hot and blue in color, and be consistant with engine rpms. Weak looking yellow spark will not cut it. Weak yellow/orange looking spark can mean the cap and rotor are burnt out or the plug wires are bad. But it can also mean a weak ignition coil. What I do next is pop the distributor cap off and see if it looks pitted, corroded or burnt in there. And if it does, the engine gets a full tune up.


Also, we should check the quality of the fuel itself. Either pull a sample of it and put it in a clear jar overnight. In the morning, look for any kind of dirt/debris/water in the fuel, or any kind of seperation line. As contaminated or old fuel simply does not burn as it is supposed to. And people get bad gas all of the time.

Or, Get a small outboard tank (like a 3 to 5 gallon) complete with fuel line and primer bulb, fill it up with known new fresh gas and try it. This is the easy way to do it as it requires minimal tools. If the engine immediately runs better on the portable tank. You have bad gas.

If nlo change, pump the primer bulb over and over, if the engine picks up, the fuel pump is weak.



If you have gotten this far, and there is no fix yet. Then its safe at this point to pull the carb and open it up and see what it looks like in there. And then decide if you want to try the rebuild route, or just replace it. Not all carbs are rebuildable. They are made out of cheap cast pot metal, and over time will leakdown internally. What we say as a rule of thumb is any carb over 10 years old has a 50/50 chance of being rebuilt successfully.

That is the quick, easy and most efficient way to troubleshoot this. I don't know how much of this you can do on your own. And obviously I am not there to do this work for you, but I work for tips so I do want to make sure you are happy with me and the answers I give you here before doing the rating. If you have a further specific question on this, by all means feel free to fire away. If not, please let me know if the ratings box gives you any troubles.


Thanks in advance, and good luck!


Customer: replied 4 years ago.

Thank you Jason...Very helpful. Was hoping you could answer one last question (in very general terms). What would be (round about) the best case (lowest cost/time) price and the worst case scenario. If it is something like "bad spark plugs" my wife will not give me a hard time about fixing it...I paid 7k for the boat...If the carb etc.. is I looking at a total loss?



A tune up runs a couple of hundred bucks for parts and labor. Typically not to bad.


But, if you had water in the gas an you have to get it pumped out, that's typically by the hour + 5 bucks a gallon for disposal, and it makes a difference if you have 15 gallons or 50 gallons that's got to go.


Replacement carbs are 400 to 500


It's tough to say without troubleshooting it myself. And worst case scenario would be a fail on the compression test. That can mean the engine either needs a valve job, or is a total loss, but that's worse case scenario.


Sometimes it's just a 9 dollar fuel filter that's clogged up... because the previous owner never took care of the tune up items. But you have to go through the motions.




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