Help needed with Yamaha 40hp

OLDSR4 · 13 August 2007, 14:05

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.

Tim M · 13 August 2007, 14:42

Take the carbs apart and make sure there isn't a blockage.

OLDSR4 · 13 August 2007, 14:57

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?

jyasaki · 13 August 2007, 15:31

No, that shouldn't be an issue. The bulb should be fairly transparent once the engine starts sucking fuel.

jky

BumbleAbout · 13 August 2007, 15:33

As long as the arrow on the primer bulb is pointing in the direction of the intended fuel flow ...

OLDSR4 · 13 August 2007, 17:13

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?

David G · 13 August 2007, 21:53

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

9D280 · 14 August 2007, 10:44

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.

OLDSR4 · 14 August 2007, 15:14

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!

jyasaki · 14 August 2007, 15:52

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

springtide66 · 14 August 2007, 16:19

Id say it is a fuel problem under load, but you could try also changing the plugs. They can break down under load but still give a good spark when tested on the casing. For that matter so can the HT leads.........

Nos4r2 · 14 August 2007, 16:58

I'd put money on it being one or more of what's been said. Doesn't sound like anything major.

9D280 · 15 August 2007, 10:34

OldSR,

Yeah, the primer bulb has two small ball valves - one at each end- the one on the tank end shuts as you squeze it, forcing the fuel out the engine end, and when you let go the rubber returning to it's original shape creates a vacuum, pulls the engine end one shut, and sucks the fuel through the other from the tank to fill the vaccum. When the engine is running both sit open (or closed, if it's the wrong way round!). I never could figure out why they put the same type connector on each end of the hose, especially when most connectors you can buy "Inline" versions of both M & F versions..... But that's another discussion entirely!

My thoughts with the hull were just that as I was talking about engine load, the water ballast will increase the load until it empties. (think of the difference between solo and having 4 people on board).

As for launching, there's a Slip at Largs marina, which is around the £7 each way mark to use - but has the advantage of a handy pontoon to tie to while you fiddle with things, and all the usual facilities you would expect with a marina. Alternatively if you cross the Erskine bridge there's a nice wide concrete one at Helensburgh Peir, if you don't mind all the shopping trolleys getting tangled round your prop, but I think is free. Parking might involve a pay & display? Failing that there is another by the Blairvadach Outdoor centre, (carry on along the seafront past the marina and Rhu, however you'll need to be good at reversing a trailer as it is narrow. Also gets a bit slippy for my liking below about half tide. (I also have an estate car & launched there without issue a few weeks ago). That one is absolutely free to use.

With the choke trick, don't leave it out for long or you'll flood it whether it's running lean or not. If you're on tiller steer /mech choke put it back almost as soon as you pull it. (and be prepared for a kick - be careful it doesn't get puilled out your other hand- and make sure you got your kill cord attatched for obvious reasons - tho' you should be attatched to it anyway!

)

Still not sure of the weekend plans yet, other than it will involve the RIB!

OLDSR4 · 15 August 2007, 20:11

cheers for that mate - I might give largs a go but I think I'll need to get this engine problem sorted first before I head out for the next trip.

Got it up and running again today in a barrel at home - first thing I done was change the fuel line round the other way - it was definately round the wrong way as I felt the priming bulb go firm whereas it kind of stayed squashed in before! It reved highly in neutral and then I tried to rev it in gear but couldnt rev it high as almost all the water was thrown out by the prop. Decided to take the prop off and try again - it seemed to rev high in gear but as there was no prop there would have been little load.

Anyway I then took the hood off and moved the throttle on the carb by hand gradually and it seemed to work fine - But then I moved the throttle quickly and it started doing the same it did on the water - carb started spluttering and then cut out - I then tried pulling out the choke as you said and yes it did take off like a scalded cat and I could rev it quickly without it cutting out but as you say I didnt have the choke out for long as it started dieing again - probably getting flooded.

So basically when you rev it hard in gear it splutters and cuts out but then when you open the choke it revs high but only for a short period. Does this help shed any light on the problem?

9D280 · 16 August 2007, 09:17

Yeah, props are quite good at emptying buckets!

Sounds like exactly the same as I had - somewhere on the carb will be an idle mix adjuster. Unfortunately I have never seen inside the engine you have, so can't help other than i would guess it will be the screw with a spring behind it to keep it from vibrating out of adjustment. (anyone else out there know these engines?) You just need to fiddle with that until the mix is right. I would guess 1/4 - 1/2 turn anticlockwise, but that is a complete guess! There may be two screws with springs (well, my Suz has two) - one will probably be the idle stop on the throttle shaft, it's the other one you want to adjust.

One word of warning - if you're adjusting your idle in a bucket, remember when it's afloat the prop exhaust will probably be a little further under the water, creating a small but significant increase in back pressure. If you adjust it perfectly in the bucket, there's a chance it may just cough & die (if it even starts) when you launch, unless you have a big wheelie bin & can get the water surface roughly where it would be when afloat. Probably not so bad for you if the water ballast is sealed up, coz you'll float a bit higher than me at idle!

I suppose the good thing about Largs for this is that you can do the adjustments in the relative safety of the marina.

doggypaddle · 16 August 2007, 12:50

Its a long time since i worked on outboards so i may be wrong, but depending on the age of the engine, if you rev it up on the carb linkage....no ignition advance so it may well splutter anyway? if i recall correctly you get a fair bit of throttle opening only advances the timing, usually 1/8-1/4 before the cam plate touches the carby linkage.
i may be wrong...... clamshells had only just come out when i last worked on them!! but one thing is for sure, unless the engine is under load with a prop on a boat or a proper test prop in a tank/dyno you wont really know if its fixed.

9D280 · 17 August 2007, 12:48

D'oh! If I'd read your last post properly then I'd suggest just going & testing it - no engine runs particularly well with a reversed bulb...... I'll bet it now runs OK!

If not then yeah, doggypaddle does have a good call, it's pointless testing in "no load" conditions. Nobody has a dyno, (this is probably where i'll end up eating my words!) and I suspect a test prop isn't something you have lying around, so I would suggest a trip to Largs for this one (even if you never launch there again), because if it dies as soon as you leave the marina & open up, you can then pootle back in at idle (which is the sort of speed that the marina speed limit would want you at anyway!), tie onto a pontoon, tweak the adjuster in relative shelter and calm water, then head back out to try again. That's exactly how I spent an afternoon trying to balance three carbs & make mine run sweet.

I tried to get a pic of your carbs from the Yam website to figure out where the idle adjust is, but there's something in there killing my browser after the "welcome" page, so no help as to which screw to turn yet, but hopefully you won't need to!

Still not sure when I'll be available this weekend, but if all goes to plan I should be around for the Lifeboat trip on the 25th, and might leave the Lamlash leg out & go straight to Tighnabruach (otherwise it's going to be a stupidly early start for us) so might see you then? As I said, now you've turned the bulb round it will most probably run OK.

OLDSR4 · 17 August 2007, 22:13

Yip think you are right mate - only way to test is to get it back in the water! upon reflection I think you are spot on doggypaddle - when I reved it straight on the carb it did splutter and die but when I used the linkage on the outside of the engine it sounded fine.

Weather is supposed to be very poor this weekend but hopefully either get it down to largs one night through the week or have been looking at boatlaunch.co.uk and have found what appears to be 2 public slips which might be ok for a quick test trip - one at newark castle and one at inverkip - anyone got any experience of them and any idea at what times would be best to launch?

Nos4r2 · 17 August 2007, 23:53

DONT rev the nuts off it in neutral or without load-there's no rev limiter on them and you'll damage it.
Without load is no test anyway-it'll rev out even if it's running like a bag of crap.

Take it out and try it.

From memory I think they are Mikunis. 1.25 to 1.5 turns out is the idle screw setting if they are.

OLDSR4 · 12 September 2007, 11:20

Thanks to all for the help and advice and am pleased to say I got out at the weekend there and the engine ran without fault.

After my last post I took it out again with the fuel line round the correct way and it ran a little better but was still dieing when I increased the revs - I tried adjusting the carb but didnt make any difference so finally spat the dummy and took it into an outboard shop. The guys at the shop cleaned out the carb,fitted new spark plugs and ht caps - all very reasonably at just an hours labour plus parts - better still told me it had good compression and showed me it running with a test prop in the tank.

Have just got another couple of questions regarding prop and electrics but will start another thread as its not really relative to this problem.