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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 16209
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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Hello, I have a 2 stroke 40HP yamaha outboard. 1993 model

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I have a 2 stroke 40HP yamaha outboard. 1993 model 40PLRR. 2 cyl.
The engine starts and idles fine. Under throttle is applied the engine bogs and will not pick up. I can put it in gear and get around but only at slightly more that an idle. I have tried new spark plugs, new gas, less that a 100:1 oil mix. I have also checked compession in both cylinders, and all is good.
Stange thing is that if I mess with the trottle a bit by moving quickly from a idle to full throttle I can sometimes get the boat to give good power and get on plane, this will only happen after 20 - 30 attempts.
Thanks for any help, Matt

Hi Matt, and welcome to Justanswer. My name is XXXXX XXXXX I will be helping you today. If you need clarification on something please post back here.


It does sound like you have a case of clogged carbs. But before you jump into tearing the carbs apart, you should still go through the motions.


If you brought this engine to me here is what I would do and the order I would do things in.


1. Compression. Your results should be about 100psi or better, all cylinders should be within 10-15% of eachother. You did not say what you got for a reading, but that is what it should be.


2. 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.

If the engine runs good when cold, but cuts out when hot. What I would do is run the motor until it starts to fail, then check spark again. I will either have good consistant spark or I wont.

I would also check and set if necessary the base and advance ignition timing.


I would 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.

Or, Get a small outboard tank (like a 3 gallon) and fill it up with known new fresh gas and try it.


Lastly, if compression and spark and fuel quality check out, then the problem is going to be in the carbs. Carb jobs are very common on marine engines. They make up 80+% of all my work. It takes a piece of dirt the size of a piece of sand in there to gum up the works. If the carbs were cleaned in the past and cleaned well, but hooked back up to a dirty fuel system, they will clog again pretty much right away. When you do these fuel system jobs you go through the whole fuel system, from tank, to pump, to carb. If I do a fuel system job for somebody, and they do not have one, I install a fuel water seperating filter kit to try to keep these problems from happening again. The filters in the engine are equivelent to filtering fuel through panty hose, the external filter kits filter down to about 10 microns, and they will seperate water, they work much better.


That is my method to my madness, it has served me very well. Its nothing new, an old timer taught me to troubleshoot in that order, and thats the order that has always worked for me.


Also, the troubleshooting method above will help you find 99.9% of the most common problems in an hour or 2. There is always a chance you have an oddball problem, but that would fall into the last .1%.


Post back here with any additional questions or concerns my friend.

Good luck!



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Customer: replied 6 years ago.

Hello Jason,

Thanks for the reply. I suspect the fuel problem is correct. I did put a new carb kit in last year. I did not stabilize or drain the fuel over the winter (my bad). I will take the carbs aparts again and soak them in carb cleaner. Assuming this fixes the problem, I did have another issue with the engine that plagued me last year. Although the engine would rev and provide power to get on plane it did so in a very slow manner (not snappy). This is a 40 HP engine on a 350 pound boat it should pop out of the water. (12 pilot Someone told me to gap my spark plugs, even the new ones. Is this really going to make a difference? I got new plugs last year and the boats did seem snappyer for the next few times out, them back to sluggish performance.


Hey Matt,

Plug gap isn't going to effect it like that, so you can leave those alone. They should be gapped right around .035. If they are gapped to much, not only does the ignition system have to work harder to make them fire, the potential for misfire is there. If the engine ignition misfires, the engine will usually shake pretty bad.


You are correct in that the engine should pop the boat right out of the water. If your engine is reving up slowly, as in not making power. I would suspect that your carbs are not properly synced, and the timing is not properly set and advancing the way it is supposed to. That is something you are going to need a timing light for.

The other thing to look at is to make sure it has the correct proppeller. But if the engine was working fine at one point with the existing prop, then that is not an issure.

Jason and other Boat Specialists are ready to help you
Customer: replied 6 years ago.

OK Jason,

Any idea on what size prop is ideal for this engine/boat. I can't recall the size of my prop but I will check. I mix my fuel now with 100:1 (1 OZ of yama lube to roughly 1 gallon of fuel) if I put 4 gallons of fuel I put 5oz of oil. Someone also told me once they could tell by the sound of the engine that it was running rich. My ear is not tunned that way to make out the difference. My email is [email protected]. I am going to accept this Answer for now. Thanks for you help. Reply back please if you have any further advice on the above questions.


Hi Matt,

First off just to let you know, we can have no contact outside of this page. Its a Justanswer rule. Because of that, they anonymize your email and I can not even see it.


I really don't know the prop size for that boat. It would be something chosen by the boat manufactorer. The best thing to do would be to shoot Rigid boats an email and tell them what engine you have, they will tell you what prop should be on it.


You do not need add any oil to the fuel. The engine has oil injection and takes care of that for you. It does not hurt anything per se, but it will foul out sparkplugs quickly running that extra oil.


I do think your problem is a combination of dirty carbs and timing sync. It is a bit of a process to do it all, way to complicated for me to even try to explain it to you. If you plan on keeping this engine a while, you really should order a copy of the factory service manual for it. I would skip the aftermarket seloc and clymer manuals, often times they are just full of wrong information.


Keep me posted. If you still need help you can post back here at any time.


Customer: replied 6 years ago.
Ok there is no oil injected with fuel on the engine or separate oil resevoir. So mixing is a must. I have manual and will look into the timing stuff.


That motor does show as being oil injected. If it is not, that means that somebody removed it.

Also, if there is not oil injection on the engine, then you are doing your premix way to light, and could really be damaging the engine. The oil ratio on that engine should be 50 to 1. Which is 2.56 ounces of oil to every gallon of gas. Do not run the engine until you get the premix mixture correct. This could really be part of your overall problem. You are essentially runing the engine now with less than 1/2 of the lubricating oil that it needs!


Also, are you sure that either your model number is XXXXX? In your initial question you stated the engine is a 2 cylinder. According to the parts diagram, a 40PLRR, that should be a 3 cylinder.

Customer: replied 6 years ago.
Well the C40PLRR is a 1993 2cylinder for sure. I rechecked the manufacturer plate on engine. I do have the SELOC manual and it specifies 100:1 oil. I was initially thinking that was thin. When I first got the engine I ran 50:1 and that is when someone said it sound rich. So I checked SELOC manual and I have been at 100:1 since. You did mention that after market manual to be incorrect so I have been checking web for other sources. 50:1 does sound more standard.

You have to love how Yamaha labels there engines.


In your initial question you stated the engine was a 40PLRR, which is in fact a 3 cylinder with oil injection.


Now you added the C, and the model number becomes C40PLRR, which makes it a different engine. A C40PLRR is a 2 cylinder premix only engine, no oil injection.


Either way, that SELOC manual is incorrect. You do want to mix the oil at a 50-1 ratio. Any less and you will burn up the engine. As I said earlier, those clymer and seloc manuals are often just wrong! You should grab a factory manual.