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Yachtwork, Marine Engineer
Category: Boat
Satisfied Customers: 2992
Experience:  MEC3 and RYA Yachtmaster Ocean
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It hesitates between 3000-4000 rpm, tbh there’s far too much

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It hesitates between 3000-4000 rpm
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: tbh there’s far too much to talk about or to be written on a message
JA: Is there anything else the Boat Mechanic should know before I connect you? Rest assured that they'll be able to help you.
Customer: if I could speak to him on a call that would be great

I've been working hard to find a Professional to assist you with your question, but sometimes finding the right Professional can take a little longer than expected.

I wonder whether you're ok with continuing to wait for an answer. If you are, please let me know and I will continue my search. If not, feel free to let me know and I will cancel this question for you.

Thank you!
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Customer: replied 7 days ago.
Outboard is a 2006 mariner 90hp 2stroke, just been serviced I’ve cleaned and rebuilt all carburetors, fuel pump has been rebuilt too. I’ve recently fitted a water separator with filter. Motor runs fine up to 3000-4000 rpm range then it stutters in between that range. It feels like a fuelling problem but unsure how to test this.

Dear customer, do you have power loss or just vibration?

Dear customer, your problem is very characteristic of a problem in the engine mounts, you probably need to replace them.
Customer: replied 7 days ago.
Power loss, it won’t go past 4000rpm sits at 24 knots at 4K
Customer: replied 7 days ago.
Engine mounts ? Can you explain a little more please
Dear customer, from what you tell me there is a lot of vibration in the engine and power drop, correct? The engine is on a base on which it is mounted, if you are worn or loose, vibrations and loss of power will be generated, if you want you can tell me what other symptoms you have presented.
Customer: replied 7 days ago.
I never said anything about vibration it is only power loss.

Dear customer, thank you for clarifying this, is your boat petrol or diesel? Or could you tell me your model

Customer: replied 7 days ago.
Petrol 2006 mariner 90hp 2 stroke

Dear customer thank you for clarifying this, in the boats the main cause of loss of power are clogged filters, so my main recommendation is that you should perform a cleaning of them especially the air filter, the constant moisture also generates the spark plugs wear faster, so there may be problems in the spark which produces choking sensation at high rpm.

Customer: replied 7 days ago.
This outboard has no air filter, fuel filter and spark plugs have all been changed.
I’m sorry but I have no confidence in you, you don’t seem to know anything about this outboard.

Dear client, I will leave the consultation for another expert to deal with if you feel more comfortable with it.

Hello, welcome. Who do I have the pleasure of speaking to? My name is***** am here to help. I have been in the marine industry for 25+ years and have been a licensed Marine Engineer since 2009. I like to think outside the box and was part of the team that set the UIM Round The World Speedboat Record. I would like to be your requested tech when you have boat issues. I will be happy to help you.

I know that can be frustrating. Hopefully, we can get you back on the water quickly.

You are back in the outboard section. I saw this question yesterday on the board but did not answer because of the lack of detail. I have never seen that type of response above. That is strange. Kind of like a bot was answering or something. Anyway, you have a human now. Only you were asking for a phone call, I don't normally take phone calls.

So we have a 2006 Mariner 90hp that has a flat spot between 3-4K?

There is a lot of text to read through I am trying to make sense of. If this outboard came into our shop we would first put it on a test tank. That is a spark fuel outboard tank and then we would take the boat out and test. The idea is we want to start narrowing down the possibilities. That test will tell us if this is an off-engine or in-engine issue. That gives us direction in our troubleshooting.

Customer: replied 7 days ago.
Hi Scott, your speaking with Stuart.
I have a 2006 mariner 90hp 2 stroke outboard, I’m having trouble getting full power out of it, it seems to be ok up to around 3000-4000 rpm then it seems to hold back. It feels like a fuel problem. Carbs have been rebuilt, motor has had a full service.
Could this be something as simple as adjusting the air fuel ratio.
Sometimes it will cut out at idle when you put it in forward or reverse unless you give it a bit of throttle.

Another test we want to do is start the engine, don't rev up or anything, just start and leave the RPMs as low as possible, then go straight to the spark plugs and pull one spark plug at a time and we should hear the RPMs drop out a bit. You'll get the sound difference. If we find a spark plug pull that is not as strong as the others then we have a weak or dead cylinder to track down. This tells us if we have a single-cylinder issue. That can make a flat spot in the throttle response because a missing cylinder then causes the engine to have a flat spot in power output that is RPM dependent vs prop load and we can get "behind the power" in engineering jargon.

Hello Stuart- It's good to meet you. I am sorry about that bot. I have never seen that before.

Does the engine kick in again after 4k or is that all you can get?

Customer: replied 7 days ago.
That’s ok, it won’t go past 4K
Compression on cylinders are 128,128,123

Ok, then we really need to do the two tests I listed. We want to run the engine on a test tank, and we want to do the cylinder spark plug pull test. Both will give us direction. Also, we want to pay particular attention to the fuel bulb when running at full output.

Another test we can try to rig up is a clear piece of fuel hose in the fuel system where you can get it. That will tell us if we have a stream of bubbles entering the fuel system somewhere. That will do the same thing, limit our top RPMs and the engine will just sit there not putting out.

That compression is really good for a two-stroke.

Is the 123 the bottom cylinder? That would be normal. The bottom cylinder normally starts to drop compression first, but those numbers are fine.

Customer: replied 7 days ago.
I’ll do these tests tomorrow , yes it is the bottom cylinder.
That gives me something to go on.
I’ll let you know how I get on tomorrow.
Thank you

So what else could it be? Reed valves? It does not sound like that. One carb that is not putting out the clean fuel-air mix. Maybe. You might pull all three spark plugs and examine them. They should all look chocolate brown. If we have a different one we can think about why that is. It sounds like we are not producing the hp we need at the top end and that is almost always not enough push on a single-cylinder to add to the total hp or a fuel restriction that also might be air getting into the fuel system from something as simple as a bad O ring in one of the quick disconnect ends that is feeding a stream of air bubbles. Happens all the time.

One more thing if you are testing tomorrow is when the engine limits out try gently squeezing the fuel bulb and holding pressure for a while, like a minute, just gentle positive pressure, and if we have an air leak that will force the fuel into the carbs and if the engine changes tone in any way we are onto the issue.

Customer: replied 7 days ago.
Hopefully it is something as simple as that

Ok Stuart, let me know what you find. You might mark me as a favorite and that way you can request me. I am in New Zealand so we are just starting our day at the shop and you are most likely at the end of your day. But when you are ready if you just open a new question, request me I'll help you and hopefully, we'll get through the system to get in touch again.

And I would say 80% of the time it is something as simple as what we are seeing. Once we have a test that gives us the direction we'll quickly find the fault and you'll be back on the water. That is how it normally works anyway.

I did my best to answer with the information I had. I hope it helped with the project.

You might want to mark me as a favorite Expert and then on your next question you can request me and we can pick up where we left off.

I hope I was able to answer your question. If you would like to ask me another question in the future, you can add me as a favorite Expert. You'll have the option to do that on your "My Questions" page if you choose to rate our interaction or you can request me by name if you wish. My name is ***** ***** I go byCustomeron the site.


Yachtwork, Marine Engineer
Category: Boat
Satisfied Customers: 2992
Experience: MEC3 and RYA Yachtmaster Ocean
Yachtwork and other Boat Specialists are ready to help you
Customer: replied 7 days ago.
That’s no problem at all, thanks for your help

You bet. We are here for you. Just let me know what you get on the tests and we'll get your engine working again.