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Old 20 April 2009, 04:23   #1
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Mercury 90hp problem

Hi Everyone

I am new to this forum and to RIBs. I have just taken over as boats officer for my BSAC branch and have had a steep learing curve finding out as much as I can about RIBs and have found this site particularly useful.

However, I now have a problem that I can't sort out. At the weekend one of our RIBs with a 90hp Mercury two stroke decided to malfunction. It starts no problem, idles fine and accelerates up to about 6 knots but then when you accelerate further nothing happens. It doesn't misfire it just doesn't go any faster. I have checked the plugs, wiring and fuel and everything seems ok.

If anyone has any ideas what the problem might be I would greatly appreciate it.

Cheers

Ian

PS The engine is around about 10 years old and has run perfectly up until now.
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Old 20 April 2009, 06:52   #2
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Is it an early Optimax? If so, it sounds like the engine management is stopping the engine from higher rpm as self protection. Do you have enough oil in the tank??

Would recommend someone plugging in a laptop with diagnostics and seeing what the fault is

Regards
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Old 20 April 2009, 07:06   #3
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Ybod,

Firstly, Welcome to Ribnet!

What RPM does it "top out" at? I assume you have checked all the throttle cables etc that it is going open throttle on the engine and not just at the lever? And the classic - tank vent is open? On a similar theme, if it's a dive club have you managed to swap the fuel line with one from another boat? my old Suz 30 ran fine with small bore hose, then got a 50 and it wouldn't run moe than 3/4 throtle without wheezing until I upped the fuel hose to 3/8 diameter....

Numerous things could be causing that - a 3 cyl 2 stroke will still run with one cylinder down and sound vaguley OK. So, a few tests for you:

1) Run it "full throttle" for a few mins, pull the plugs out, - are any of them more oiled than the others? Come back for more diagnosois if that's the case- otherwise this post will get very long!
2) as you accelerate, "poke" the choke - does it roar into life? - if so your idle mix is too lean (but I'd be surprised on a mult carb engine if that spontaneously changed).
3) Cghacklk your "wot stop screw" - has it shaken loose? (I had that happen on my spark advance stop with nasty consequences)
4) if none of the above then you'll need to start looking at things like the fuel pump diaphragm.

I'm sure there wil lbe a load of other suggestions along soon too!

Edit: 250 got in before I fifnished typing, but does have a good call - if it's a modern machine with engne management it's possibly rev. limiting, In which case pull & WD40 all the sensor & injector connectors - the engine management can't telll if it's a duff sensor or a duff connection!

Cheers.
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Old 20 April 2009, 07:09   #4
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Quote:
Originally Posted by 250kts View Post
Is it an early Optimax? If so, it sounds like the engine management is stopping the engine from higher rpm as self protection. Do you have enough oil in the tank??

Would recommend someone plugging in a laptop with diagnostics and seeing what the fault is

Regards
Hi There

Yes, the two stroke oil is full and I've been told that it doesn't have a rpm limiter on it. Someone mentioned that it may have something to do with the throttle cables - but not too sure how to check this.

Regards
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Old 20 April 2009, 07:27   #5
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Easy!

Take the lid off the engine, and get someone to open the throttle. Follw the mechanism through and somewhere on the engine it will move a lever on the side of the carburettors. That lever should move through approx 90 degrees. Check for slack in that mechanism if not.

Alternatively if you are feeling more adventurous, if you take the lid off the air box at the front you should be able to see straight down the carbs. you'll see the valves open as the lever is pushed forwards. they should go parallel to the bore (i.e fully open)

Some of the earlier machines had a vertical shaft that was operated by a cam on a slider at the top of the block. If you have one of them, just take the front cover off the air box - it's far easier to see in there!


Other option - it may be the control box has gone slack. Most of the cams etc in there have plastic bushing which will wear eventually. My old Yam had a perceptive increase in throttle response by spending about 8 on a pile of small plastic donuts! Having said that remorte boxes are usually notoriously difficult to get open wiothout damage, so I'd have a look at the engine end first.
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Old 20 April 2009, 15:27   #6
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if you would like to give me a ring tommorrow on 01869 326411
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Old 20 April 2009, 15:29   #7
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sorry i will require the obm serial number as well
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Old 02 May 2009, 06:47   #8
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Quote:
Originally Posted by ybod View Post
Hi Everyone

I am new to this forum and to RIBs. I have just taken over as boats officer for my BSAC branch and have had a steep learing curve finding out as much as I can about RIBs and have found this site particularly useful.

However, I now have a problem that I can't sort out. At the weekend one of our RIBs with a 90hp Mercury two stroke decided to malfunction. It starts no problem, idles fine and accelerates up to about 6 knots but then when you accelerate further nothing happens. It doesn't misfire it just doesn't go any faster. I have checked the plugs, wiring and fuel and everything seems ok.

If anyone has any ideas what the problem might be I would greatly appreciate it.

Cheers

Ian

PS The engine is around about 10 years old and has run perfectly up until now.
Just noticed this thread, This symtom can also be caused by a small bar that links the throttle gear mechanim to the advance and retard of the engine, as you throttle on it advances the spark. If the small plastic recepticle for the ends of the bar have broken or slipped out, the engine will perform exactly as you have described.
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