Mariner/Mercury 3.5hp 4 stroke wont rev

[stellapoint]

I've just bought an immaculate barely used 3.5hp Mariner and the vendor said it started but wouldn't rev.
I assumed it was just a blocked main jet but when I stripped the carb it was as clean as a whistle inside.
All the jets and orifices clear.
Float and needle Good.
Choke and throttle flaps working.
New fuel freely flowing to carb.
No intake blockages
It starts easily enough but then nothing happens as you open the throttle, I've tried manually too but nothing.
I did a compression test as I'm running out of ideas and it was fine.
Anything please I could try next?

 

[B27]

It keeps running OK at the same speed when you open the throttle?
Or does it splutter?

Don't assume it's put together right.
Blocked exhaust?

 

[VicS]

I don't believe the throttle valve can be opening.

 

[stellapoint]

I'm with both of you but alas it does appear to be working exactly as it should. I've done it manually and with the twist throttle.
I wondered about a blocked exhaust but there's plenty of exhaust coming out into the water.

 

[stellapoint]

It keeps running OK at the same speed when you open the throttle?
Or does it splutter?

Don't assume it's put together right.
Blocked exhaust?

When you open the throttle there is the tiniest increase in revs but basically nothing happens. You can see the cable moving the throttle flap

 

[Snowgoose-1]

Could try a teaspoon full fuel in the plug hole to see what happens.
Borrow a remote tank and squeeze the bulb might help to find out whether fuel or air problem.

 

[VicS]

When you open the throttle there is the tiniest increase in revs but basically nothing happens. You can see the cable moving the throttle flap

But you cannot actually see the flap opening can you?
I am sure if the flap was opening the engine would either rev or die depending on the state of the fuel side of things.
I would remove the carb from the manifold so that I could see the flap and check whether or not the it opens when the twist grip is turned..

 

[stellapoint]

But you cannot actually see the flap opening can you?
I am sure if the flap was opening the engine would either rev or die depending on the state of the fuel side of things.
I would remove the carb from the manifold so that I could see the flap and check whether or not the it opens when the twist grip is turned..

When the carb is disconnected the flap moves when I move the actuator arm.
I'm making the assumption that when the carb is connected the flap will still move when rhe arm does.
I've tried it with and without the cable.

 

[VicS]

When the carb is disconnected the flap moves when I move the actuator arm.
I'm making the assumption that when the carb is connected the flap will still move when rhe arm does.
I've tried it with and without the cable.

And will open fully?
If so then I have no idea what the problem is.

 

[John_P]

Old fuel ?
Air vent blocked ?
Deformed or damaged fuel hose ?
Weak spark ?
There is a fuel filter in the fuel tap check it is clean as this is often over looked.

 

[Stemar]

I had an obscure fault with a 2T Tohatsu. It turned out to be a poor connection on the coil's earth wire - might be worth checking

 

[stellapoint]

Old fuel ?
Air vent blocked ?
Deformed or damaged fuel hose ?
Weak spark ?
There is a fuel filter in the fuel tap check it is clean as this is often over looked.

Thanks John, brand new fuel, cap removed and still no better, good flow from hose at carburettor bowl ( but thanks for the filter tip- I wasn't aware of that.
Spark looks strong

 

[stellapoint]

I had an obscure fault with a 2T Tohatsu. It turned out to be a poor connection on the coil's earth wire - might be worth checking

Thanks Stemar, I'm all ears and I'll have a look although the motor looks like new so I'd be surprised