Merc 90 loss of power

neil.mccrirrick · 12 March 2011, 12:11

Hi All,

At the end of last season my outboard, (merc 90), lost power at cruising or high revs. Not at first, after about 15 to 20 mins playing around at different speeds etc. If I throttled back sometimes it picked up, other times it didn't, but if I pumped the bulb, or selected neutral and let the engine rev on the idler all was good again for a while.

It felt like it was either down to it not venting properly, at first so I checked that, and switched between tanks, (I have two connected by a three way switch). Same problem.

So I changed the fuel filter and tried that, same problem.

The weather got too cold and so I left it as it starts no problem on the flush muffs, so I know I need to be on the water to test it.

Since then I've re routed the fuel line, (it did look like it may have been under some pressure the way it was, and today went to change the plugs. The top plug was really loose, loose enough to come undone without the socket, so my question is do you think that could have been causing the power drop off?

I've taken the airfilter off, and all looks clean, clearly the next step is to strip the carbs to check for debris, but I don't want to go that far if you think this may be enough for another water test, now the weather is hopefully getting warmer.

I would just drop it in the water, but its a 90 minute tug to get to the nearest water etc etc

aquatec · 12 March 2011, 12:40

Hi,
You do not say which Mercury 90 you have or the age. So, lets suggest its is a two stroke 3 cylinder model. It looks as if your problem is fuel related. I would suggest you change the fuel pump diaphragms. It comes as a kit with diaphragms, gaskets and non return valve discs. If you have the early type discs made of rubber they get weak and soft over the years and get sucked through the housing, therefore not locking off the fuel and allowing the fuel to run back. This makes the pump run overtime to keep up any pressure. The new discs are clear plstic and impervious to fuel.
I would also look at the fuel primer bulb. If it is a genuine Quicksilver item it will have two non return valves. Remove it and try to blow through it from the outlet side (there is an arrow showing flow direction) back through. If you can blow through it change it for a new one. Aftermarket ones mostly only have one non return valve, not as good. get a Genuine Quicksilver one.
Lastly, I would remove the carb float bowl drain plugs and prime some fuel through. This should clear any dirt or water from the bowls. Put some rag below the carbs to catch the fuel. You need fresh fuel also. Do not use fuel from last season.
Water test the boat as it needs to be under load.
Let us know how you get on.
Regards,
Aquatec

neil.mccrirrick · 12 March 2011, 12:53

All great advice, and well guessed, it is a 3 cyl model and I think 2001 if memory serves, based on what you've said I think the fuel pump diaphrams are defo a good bet

Thanks

Searider · 12 March 2011, 14:35

Worth changing the primer bulb before diving into the carbs.

Had all sorts of problems with Tempo ones.

Is yours a genuine one?

DJS · 12 March 2011, 17:31

I had a similar problem with mine (same year and model of engine as yours) just after I bought it and on the first run out! After a thorough inspection I discovered it was drawing air at a compression joint. Hence worth checking every connection!

Nos4r2 · 12 March 2011, 18:17

Hold your tank above the engine when it starts playing up if you can and see if it suddenly improves. If it does, it's fuel pump for definite.

neil.mccrirrick · 14 March 2011, 21:52

The primer bulb is a quicksilver one, so there's a good thing

Ordered the fuel pump service kit, so hopefully it will be here before the weekend so I can fit it and get out Sunday to test.

Moving the spare fuel tank will be awkward to do Nos, but if the kit is not here for the weekend I'll do it to try it and prove the fault, thanks for the top tip.

All being well I'll post next week with the problem resolved

Thanks all as ever

Neil

neil.mccrirrick · 19 March 2011, 12:32

Ah well, fuel pump service kit has not arrived, but I have separated the fuel pump, (I was hoping the postie would turn up with the kit mid job, but he's just been and all he brought was bills

).

The discs are plastic and look ok. I was hoping to see a load of dirt, or some holes, but sadly not. I'll re assemble and take to the water tomorrow, you never know something I have done may fix this, and if not it moves me on.

I will see if I can use the spare fuel tank as Nos described, the problem here is I don't think I've any spare fuel line to do it easily, which means trying to detach the main fuel line, (not an easy job as the main tank is crammed, and I do mean crammed, under the centre console, obviously I can get it out, but it involves a lot of swearing

Update next week.

neil.mccrirrick · 21 March 2011, 13:56

No update on this yet I'm sorry to say. We attempted to launch at Knott End, Fleetwood on Sunday. The sea was a little choppy coming over the slip, so we launched off the side, off me new rollercoaster trailer.

Went in absolutely fine and all was good, until a wave picked the rib up just as it was broadside to the trailer.

For those that know rollercoasters, there is a pin in the bar for the extensions off the trailer that is there to hold the lightboard. Well I can only wish I'd noticed it earlier and covered it in foam or even took them off as I don't use them, but one of them did this, see pictures.

An inch higher and it would have been the rubbing stake, 10 seconds loger and it would have been the handle, both of which would have just been a warning for the future, ah well.

Repair is set for Thursday evening, and hopefully back out next weekend to test.

uncle al · 21 March 2011, 17:19

your problem reads like a classic fuel pump diaphrams, which is usually cured fitting a diaphram kit,the diaphrams dont have to be punctured to be defective just stretched.I know the slip at knott end by the coast guard station i have launched some coast guard boats from there for sea trials after servicing them, many moons ago

neil.mccrirrick · 22 March 2011, 11:46

Service kit turned up in the post last night. I'll strip the fuel pump again and change all the innards fingers crossed it's all good.

Thanks for all the advice

kwil · 22 March 2011, 12:02

Sorry to here about your problems. You are not the first to put a tear in your boat at that slip with those style trailer board fittings and you will probably not be the last.
I hope this weekend goes better.

Keith

neil.mccrirrick · 02 April 2011, 20:19

Been out today knott end to Piel island approx 10 miles in a moderate sea no problems at all, (fuel pump serviced). Stopped fir a pint, on the journey back I switched to my reserve tank to use some of the fuel and the same problem occurred, switched back to the main tank same problem loss of power and then stalls deffo fuel.

When I went to pump the bulb no fuel at all and it was difficult to pump, squeezed ok but really slow to inflate to allow another squeeze.
Both tanks are vented deffo all I can think now is there is something wrong with the switch between the tanks, I'll strip it but i can't make sense of how I can travel for 40 mins or so all good and then restart and struggle again, weird.

Any thoughts welcome i have checked all joints and pumped as much fuel thru out if the water via the bulb as I could can't see a leak anywhere

Perplexed

C-NUMB · 02 April 2011, 23:42

How about just borrow or buy a new portable tank whit new bulb/lines. If that works OK for a few hours then You know for sure its not the engine or the connections there?

neil.mccrirrick · 05 April 2011, 20:00

Ok, I'll deffo rig the spare tank so it can be run separately, different fuel line and bulb. In the meantime though I've separated all the components.

The way I'm set up is:

From the outboard one fuel line to a three way switch. From the switch one line to the main fuel tank, one to the spare.

Its all dis connected now. If I blow through the main line to the switch, I can blow into the main tank, only, in the off position can't blow, and in the other position I can blow into the spare tank.

Blowing down the fuel line from the spare tank, no obstructions, all good.

At the end of the line between the outboard and the switch, obviously there is the bulb, and that is fine, I can only blow one way, and its the right way, took it off to do this. But I also have this at the end, see pictures.

With the grey part off I can blow through the line, but there is a lot of resistance, connected I can't at all, as I guess its supposed to work.

So question is, could this be the problem, and before I re connect all back up, is it better to leave this out of the system and simply connect the fuel line directly to the bulb?

Nos4r2 · 06 April 2011, 00:20

Get rid of that horrible plastic fuel connector-they are bloody awful- and either replace it with the cast alloy version or connect directly to the bulb.It wouldn't surprise me if it wasn't that causing an intermittent air leak.

DJS · 06 April 2011, 09:29

Neil I agree with Nos4r2 and as per my previous post; connections were my issues and know doubt with perseverance you will crack it!!

Meeko · 06 April 2011, 17:25

Sounds like classic fuel starvation.
I've had this as well both on a Johnson 115VRO and a Merc 90. It was drawing air so check your hoses for perishing and air tight connections at the tank, bulb,engine connection,etc etc as that was part of my problem.Both boats have fixed 80 lt tanks. The Merc was worse.
The other thing that was also a factor on the Merc was the throttles were not synchronised and the slow running jets were all to cock,not worn, but out of sync. Also found water in the float bowls so that gave erratic running and the Merc engine was a pig to start. Spent all last early season faffing about and went back to basics and hey presto, now working and no probs at all at WOT. Also as an after thought make sure that there is no air entering that is not metered.Yep carb gaskets but I suspect elsewhere before taken the carbs off.
Meeko

Meeko · 06 April 2011, 17:33

Just seen the photos of your fuel line. Are they plastic and if so they harden in petrol as you know and then leak air like a collander. I would be very very sus of these connections and change them to something more simple. I've had experience of these connectors and they are problematical. O rings and non return valves!!!!!!!!!
Get some good quality rubber fuel line.It doesn't harden,can be crimped with Tie Wraps and seals air tight. Just because you don't get a fuel leak doesn't imply its air tight and remember your pump relies on suction as well as pressure.
Hope it helps
Meeko

matt h · 07 April 2011, 08:26

agree with Nos, change the plastic ones to alloy version, also check out you fuel lines, they can collapse inside and will only alloy very limited fuel through, just blowing down the pipe will not highlight an issue inside.

Just a thought, (without reading it all) in the fuel tank perfectly clean inside and the pickup is unobstructed?