Lost Revs in motor

[monkey76]

Hey Ausfish Members,
I am away on hols dodging the wind and my 40 hp merc is playing up. The problem is as follows:
It started losing revs when I last took it out, full throtle was about 80% it is now around 70%. I have changed the fuel, double checked the oil and put new spark plugs in the motor. It revs at 100% in nuetral but as soon as I place it in gear the motor does not rev anywhere it can and as a result the tinnie only just gets on ther plane. Any advice on what it could be would be appreciated, it is still under warranty however should I drive it home on new years day with this problem??

[russ81]

Had to read the title twice thought you might have lost member Revs57 over the back;D;D;D;D;D;D


Russ

[FNQCairns]

If it's 4 stroke you probably could, they are naturally very resistant to problems like this but if 2 stroke you may cause more drama. But how far is home and would it be the boat you are driving, could you troll the trip instead??

cheers fnq

[monkey76]

Thankds for the reply fnq, it is a two stroke it is about a 20 minute trip on the plane however I could troll it will just take a few hours which is no real concern I would rather do this than cause damage, do you have any idea what could be the porblem??

[Buddy Mania]

Losing compression perhaps?

[buggerit]

Maybe a silly thought or suggestion but does it have a fuel filter . Maybe just not enough fuel getting through under load .

Buggerit

[FNQCairns]

No good ideas apart from the typical guesses, if it behaves as normal right up until you need higher speed power it sounds like the main fuel circuit has a partial blockage, if it is then very bad to try and demand it to work in this range. Fuel delivery at full throttle is the sum of all circuits so if you do not ask for the engine to deliver on the main circuit they are pretty bullet proof.

By contrast it could be an electrical problem also as they cam mimic same and everything else also:(.

But really it's your call, if it behaves well at medium trolling speeds and some above you would be pretty safe from doing any lean condition damage as a result of a slow trip back as the timing advance only really starts at around 1/3 throttle and the main circuit is hardly working.

If it is a mehanical failure like you spat some rings then all bets are off, it is pretty uncommon, although not out of the question.

Not much help sorry, too many variables hope I helped somewhere for you to make a choice.

cheers fnq

[monkey76]

Thanks for the help I have checked the fuel filter i am begining to think fnq could be right with electrical problem oh well looks like a slow troll home not so bad may even get a feed!!! I just hope the problem is covered under warranty

[Splash]

Monkey,

What has changed since you last went out (and all OK) before this trip on the boat ?

THink hard.

Splash

[monkey76]

Hey Splash, the only thing that has changed is I filled up before I left I have since checked the fuel and oil it all seemed ok, I imediately re-fuelled the tank and changed the spark plugs i also tripple checked the oil mix which is as per mercury specs. Previous to this the engine had been running extremely well so I am puzzled. I have noticed the altinator makes a whizzing noise now which I previously had not heard (maybe I am looking and listenbing to hard) I have checked the prop it is solid and seems to be fine, a guy where I am thinks it could be electrical.

[Splash]

MMMMM.

Has your boat been stored in a differnt location recently (i.e. outside in elements) compared to before?

I know you have refulled but have u used a differnt petrol station to whre u normally fill up?

I have had similar problems to you recently (2 stroke 110 HP Johno). I have snce discovered dirty fuel, loose fittings on fuel & VRO lines (causing potnetial air leaks) and that my 1989 fuel pump has most likely never been replaced in 18 years.. :-(. My limiter valve on crank case is also an outdated part that needs to be replaced. I would also bet you loose RPM at low throttle and disappears at high throttle. Try it.

Here is a response I received a few days from a Legend on 'MarineEngine.Com' after posting my concerns..

"If You have the VRO pump still installed try the following:
Start the engine and let it run at idle.
Every 15 - 20 second there should be a 'click' from the pump.
If the interval is longer, the reason is probably in the 'air-motor' side: Low crank pressure, defective pulse limiter valve, defective membrane/valve in the air motor of the pump.If engine stops and pumping the bulb helps, this is a further indication.
If the clicking is more frequent, the problem is more likely in the fuel section of the pump: Leaking membrane, leaking valves, air-leakage at sucktion side (fuel lines).
To totally eliminate the boat system and connectors, hook the fuel hose with bulb directly on the pump and into a seperate cannister with fuel (2%).

The pulse limiter should allow blowing and sucking both ways. However if You get a crank case combustion, the shock should close the valve. You may for the test try to run without it, but if the engine 'sneeze', You run the risk of blowing the air membrane."

"The purpose of the limiter is to prevent the air motor membrane to blow to pieces in case of crank case combustion which is what people refer to as 'sneezing'. Technically it is what happends if combustion keeps on until intake ports opends and then travel down into the crank case. It is a leaf valve type that is normally opend, but gets closed at high pressure shocks."

Take heed of this advice.

Also, if you do a VRO container (and not use premix fuel), inspect your oil quality contained in this container. If sediment lingers at bottom of container, it needs to be cleaned out ASAP.

Good Luck!

Splash

[BM]

Monkey,

I'd be looking to a dropped cylinder potentially either by loss of compression or electrical issue.

When you say it doesn't reach full throttle do you mean it gets going and then starts to surge (starvation) and come good again then starve? or does it simply refuse to rev out?

Fuel system issues usually manifest themselves at higher rpms as a gradual surging up and down in rpm.

Electrical failures (intermittent) typically manifest themselves as rapid increases or decreases in rpm.

Revving when unloaded (in neutral) is not a good test of anything really, Although an engine that has lost a cylinder will usually rev well when unloaded and then falter when under load.

So a comp test and a spark test would be the first place I would begin looking.

Cheers

[Splash]

Thanks BM.

What specific breakdown in electronics could affect loss of compression?

Splash

[Chris J]

maybe something to do with the timeing gear.. when in gear it will put the motor timeing about 5% adv there might be a prob with that.. i dont now for sure but from what you have been saying so far sounds like that..

some electronics can breakdown under load..

[outsiderskip]

could have a faulty plug to

pete

[dogsbody]

Had a similar problem happen to me, check the linkages to make sure nothing has come adrift or stopping the linkages reaching full throttle.

Is the vent open on the fuel tank?


Dave.

[Owen]

I had a similar problem on my 50 Merc.
It would start perfectly, run fine up to 2,000 rpm then die.
Rev to whatever you like in idle but as soon as you stick it in the water it'd croak
After many hours doing all the usual fuel, air spark checks it ended up being one of the coils breaking down under load.

The early mercs had TERRIBLE electrics.
I ended up replacing my stator, coils and ignition box at various times.
If you have an older model and it has a black stator then look forward to it failing at some point.

Edit: Just noticed you said it's still under warranty, so the electrics will be the updated versions.... Could still be a lead or coil breaking down though.