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Old 27 July 2005, 05:25   #1
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*HELP* Yamaha 55HP 2 Stroke *HELP*

Hello, recently started using a Flatacraft Force 4 with a 55HP Yamaha Outboard motor on it. I'm having two quite major problems and if anyone could shed some light or give me some advice it would be much appreciated.

1) The first problem is with Fuel, I found out that the boat should do about 4 hours at full tilt on a 20 gallon tank. On Sunday I somehow ran it dry in 1h 30mins, not even at full tilt! This is what made me think I had a bit of a problem. Should I just buy a new tank and fuel-line (with the little pump bit on it)?

2)This might be happening because of the last problem but I don't know enough about it to know. When the boats is running at around half throttle the boat runs fine but when I get it up to full throttle it loses power and drops right back down again. When I run it in safe (don't remember what its called when you pull the throttle lever out to the side) and go full throttle it runs fine and holds the revs for as long as I like.

Any ideas?

Thanks

J
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Old 27 July 2005, 06:52   #2
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Not sure about problem number one but its a 2-stroke!

Problem number 2 sounds like it has difficulty reving under load. First thing I would do is bite the bullet and purchase a clymer manual for your engine make sure you get the right year! Im not sure what year your engine is but heres an example: Amazon, yamaha clymer manuals

It sounds very much like a fueling problem along the lines of a blocked jet in the carb. Im guessing being a Yamy is a 3 cylinder triple carb 55HP hence the recommendation of a manual. I would carefully strip down each of the carbs and blow through them with compressed air and check the floats are not getting stuck etc.

good luck
regards,

Alex
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Old 27 July 2005, 09:53   #3
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Two suggestions here:

1. Check fuel pump on engine. Could be split/holed diaphragm. Not expensive to replace either. If fuel pump is defective it can supply fuel at lower revs and overfuel at higher revs causing both the problems you mention.

2. Get it to a dealer for a tune-up and service
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Old 27 July 2005, 10:00   #4
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Quote:
Originally Posted by plainjames
I found out that the boat should do about 4 hours at full tilt on a 20 gallon tank. On Sunday I somehow ran it dry in 1h 30mins, not even at full tilt!
At the risk of asking a silly question, is it definitely a 20 gallon tank, and not a 20 litre tank?

Twenty gallons would be unusually large on a 4 metre RIB. Portable tanks are generally 20 litres . . .

John
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Old 27 July 2005, 10:30   #5
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For a start, don't EVER rev the tits off an outboard in neutral-they will rev fully even when not firing on one or more cylinders as there's no load. You'll end up with the engine running away and then bang, no engine.

It sounds like you've got some crap in your fuel system or your fuel pump aint working properly. DO NOT run it at full throttle like this-if you're not getting enough fuel to a cylinder(even on a VRO motor) you're not getting any lubrication to that cylinder either. Byebye engine.Incidentally, it will use more fuel if it's running like this as the cylinders that are firing are heavily overloaded-though I do agree that you've probably got a 20 litre tank not 20 gallon.

Check your fuel tank for crap, strip/check your fuel pump(s?) and strip/clean out your carbs with proper carb cleaner-blowing it through every hole.
Check your spark plugs to see if there's any marked difference between them and see if they are wet or dry.

I can't emphasise enough that if you run it like this you WILL trash it.
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Old 28 July 2005, 03:33   #6
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Thanks

Thanks for all your tips and advice, will have to wait until I get back to the boat to check the size of the tank as you might all be right about that. I stripped and cleaned the carbs last week and they were all clear. Have checked the diaphram aswell! We have had it into a boat shoop twice and both times they say that it is fine and both times it is still not working thats why I came on here to see if anyone had any ideas. I've already learnt alot thanks. Is the best bet now to take it to a Yamaha dealer?

J
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Old 28 July 2005, 04:40   #7
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If its a 20 ltr tank, then you aren't far off the mark. I have a (healthy) Yam 50 motor on my rib - and its normal consumption is a full 25ltr tank for a half days reasonable use.

You said that when you give it full throttle under load the revs just dies off completely ?? - or does it struggle to get to high revs, but not die down completely ?? If it struggles I'd definitely suspect some kind of fuel problem - but if it dies completely back to low revs - then you might want to check your ignition system - on the yam (maybe on other obs too...) the ignition timing is advanced by a linkeage to the throttle as the throttle is opened.....


I'd also check the fuel filter and fuel lines & connectors... the fuel connectors (at least on my engine) can sometimes draw air.....

If you want me to scan some pages from the Yamaha workshop manual let me know (QUICK - I'm going on holidays at the weekend...) - there are soem teenagers lying round the house at the moment that could be put to good use!

BTW - I'm not sure about that boat shop you're using...
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Old 28 July 2005, 05:01   #8
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Hello James,

if the carbs are clean and fitted well, with new gaskets (?) then they should be fine.
A diaphragm can be a harder problem to diagnose: I've had ones that looked fine to the naked eye but had tiny pin holes that were almost impossible to see.
Strange, but not unusual for a shop to say that the engine was fine when it obviously isn't. Especially true if they don't know much about Yamaha engines.

When you take the sprak plugs out do they all look the same?
What condition are they in? are they wet and oily or dry and grey/black?

I'd definitely go with what Nos4r2 said: never rev an engine thats not in gear: quickest way to end up with a very expensive anchor!

You could also try another forum (NB, not in competition to this one, more like a last resort, honest officer!) and I'd recommend iboats.com.
Although it's an american site it has masses of useful information and some great people to help and give advice. I guess if they can't help then something is bizzarely wrong! Of course you have to allow for the time difference: most of them will be sound asleep right now! They usually give very good help but get really sniffy if you just say "motor not working right". They prefer engine make, model, serial number and as much detail as you can give.

I'm sure this is one of those easy fixed problems, just a bit of a difficult one to trace. I'll bet once the problem is found it will run like a sweetie!

If you've got a local Yam dealer why not nip in and pick their brains?
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Old 28 July 2005, 11:46   #9
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I'm not sure about the boat shop either, think the guys are really taking the piss out off me or they're just useless or lazy, it's been in twice!

If you could scan in a few pages tonight that would be great as I also go away tomorrow but if it's too much hassle don't bother.

Should the spark plug be wet and oily or dry and grey/black?

Unfortuatly don't have time to check the other forum but will do it as soon as I get back from the west coast, thanks for all your help.

J
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Old 29 July 2005, 05:25   #10
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just got this now James - `I'll get some scanning done in the afternoon and send you the results. Can you send me your e-mail addressin a pm - I'll need it to attach the pages in an e-mail....
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