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Old 13 August 2007, 10:05   #1
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Help needed with Yamaha 40hp

I have just recently bought an old yamaha 40hp which is probably late 70's or early eighties. Took it out for the first time yesterday and soon realised it has a problem in that when I tried to accelerate in gear it cuts out as if maybe starved of fuel or wont power up under load ?

The engine starts ok and pumps plenty of water then once it heats up for a few mins or so it idles ok. It revs freely and highly when in neutral and powers the boat ok away from a standstill and I had it out running for over an hour but just wont go any higher than a few mph without cutting out?

Any ideas or help would be appreciated.
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Old 13 August 2007, 10:42   #2
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Take the carbs apart and make sure there isn't a blockage.
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Old 13 August 2007, 10:57   #3
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thanks Tim I thought it might be carb related but as iam a newbie I dont understand why it would rev highly and sound ok when in neutral but then when in gear it cuts out if you try to take the revs up - is it that the engine will be demanding more fuel when in gear? Talking of fuel having just read another thread about fuel line priming bulb positioning I think I may have put the fuel line round the wrong way as I had the priming bulb up at the engine next to the short bit of hose and the longer length connected to the tank - should that make any difference in supplying the fuel?
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Old 13 August 2007, 11:31   #4
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No, that shouldn't be an issue. The bulb should be fairly transparent once the engine starts sucking fuel.

jky
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Old 13 August 2007, 11:33   #5
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As long as the arrow on the primer bulb is pointing in the direction of the intended fuel flow ...
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Old 13 August 2007, 13:13   #6
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Iam pretty sure that the bulb has got "can" written on it with the arrow pointing towards the can so it probably is around the wrong way as it should be pointing in the direction of the engine - I just mounted it that way as it was handier to use but I'll change it round the other way to see if it makes any difference but I really dont think that this is the issue as it looks as though it will run all day long with the way its set up just now.

Iam planning to run it in a bucket tomorrow to see if it still cuts out when in gear and increasing the revs - just a bit worried it might be poor compression in the engine that wont let it rev when the engine is under load or hopefully it may be something simpler like a fuel problem. Still cant get my head round why it is reving highly when in neutral but not when in gear?
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Old 13 August 2007, 17:53   #7
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Try asking on here:

http://www.themarinedoctor.com/cgi-bin/YaBB.pl/YaBB.pl

but my two-penneth is either a timing advance problem or dirty carbs.

Regards,

Orve
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Old 14 August 2007, 06:44   #8
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Fuel thoughts

I would second the "reverse the hose" test as a first off - the arrows on these bulbs usually point in the direction of flow. If it's more convenient to have the bulb at the engine end then you can turn it round easily (I assume two jubiulee clips are holding the hose onto the fir trees) - remember at idle it will be using next to no fuel, and so the ball valves in the primer will probably not seat hard (or at all), which may explain why it will run OK at idle.

Yes, no load open throttle will need less fuel than when under load, and I'll bet you probably don't hold it at high RPM for long when you do that due to the antisocial nose, also most engines of that age limit throttle opening to about half when not in gear, so chances are you wonl't have built up enough vaccuum in the line to stop it by the time you are deafened by it and throttle back!

When you say it dies, does it "fade out" or does it just stop? Assuming a "fade", and you have no success with the reversed hose, a good test as to whether you are running lean or not is give the choke a quick poke as it starts to die (I assume it's remote control and you have a choke button, if not a quick "out-in" of the choke lever will have the same effect)- one of two things will happen- if it takes off like a scalded cat, your mixture is slightly too lean, if it coughs & dies you are too rich. A mommentary hesitation and it's probably OK. Engines are remarkably fussy about the air - fuel mix, both too lean or rich will kill it. Increase load (i.e accelerate in gear) and the "perfect balance" you had before will suddenly not be enough. No idea about to the carbs on that engine, but there will likely be an idle mix adjuster somewhere in there, which will probablky affect things up to around the 1500/ 2000 rpm region.....

I guess you have an SR4 - with the water ballast, the engine load to get up on the plane is quite phenomenal, and what is needed to push it along in displacement mode or even on the plane is nothing compared to what it needs to empty the ballast & lift the hull up to plane (try driving one with a 25 on the transom & you'll see what I mean!) Mine suffered a similar problem - I got round it by poking the choke on accel for a day before I got RSI of my thumb and adjusted things) But yes, the fuel consumption at load is a LOT more than at low / no load for a given RPM. (Think of your car with and without the trailer / roof rack)

Hope this helps. Not sure where you are in the Clyde, but I may be out this weekend if I can be of any assistance. Problem is at the moment I don't know whether I'll be free Sat or Sun yet.
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Old 14 August 2007, 11:14   #9
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Cheers for that guys and thanks for the info 9D280 – your quite right the engine does fade then cut out when trying to accelerate to anything above a crawl – in fact when it started to cut out I could pull back off the acceleration and the engine would keep running and recompose itself but every time I tried to get back on the acceleration the same would happen so I am beginning to think it might be getting starved of fuel.

I never thought to try and pull the choke open when it was about to die but will give it a go and change the fuel line round the correct way as I didn’t realise there was a valve inside there – just thought it was an empty priming bulb just used to get the fuel up to the engine – I didn’t get a chance to get it into a bucket today but will definitely get it started tomorrow – again you are right in saying I didn’t rev it for long or very highly when in neutral as it was well noisy and I remember reading probably on here that its not a good idea to over rev an outboard when in neutral.

It is an old sr4 ive got but it doesnt have the displacement hull – I cant tell if it was originally one as its been regelcoated but I don’t think it ever was as there is no trace of it – just a transom plug and no holes up the front of the hull. Although the boat felt stable enough in the water and looked as though it would handle well even though I didn’t get past a crawl!

Iam actually on the southside of Glasgow – just haven’t got round to updating my info. I towed the boat for 2 hours up to loch awe on Saturday as was expecting to get a good run out on the boat and get in some long awaited fishing as well. Obviously it didn’t turn out that way because of the engine problem – 4 hours of towing for a frustrating hour out on the water at a snails pace – also managed to destroy a nearly new prop as well so as you can imagine it wasn’t quite the experience I was hoping for!

Any help or advice you could give 9D280 would be much appreciated and I’ll let you know how I get on tomorrow but I think for the next test trip it will need to be closer to home – any ideas on where would be a good slip to use with a normal estate car not too far away- maybe port Glasgow /Greenock?

Saturday was a bit of a sketch but spare a thought for the guy I met who also was out for the first time – his engine didn’t start at all, his new waders leaked then managed to snap his hull with a ratchet strap doing it up too tight whilst securing it to the trailer!
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Old 14 August 2007, 11:52   #10
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Assuming your priming ball is oriented correctly, you might also try giving the ball a few squeezes while accelerating (assuming you can reach it), which will compensate for a weak fuel pump.

jky
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