Yamaha 50hp stutters

[mike uk]

My 2003 Yamaha 50hp (carb model) has started to be unreliable when accelerating quickly or when doing a "three point turn" using firm revs forward then in reverse.

Tickover and steady speed even at high revs are fine.

I have begun to suspect the accelerator pump ie the diaphragm that is actuated by a short rod connected to the throttle linkage. I understand the idea of this is that it injects a squirt of fuel into each of the 4 carbs to meet sudden demands.

So as a test I have disconnected one of the tubes from the carb body and operated the throttle briskly with the motor running in neutral. No squirt of fuel!

Am I right to expect a squirt of fuel like this? In which case I guess I'm right in thinking the diaphragm unit will need to be replaced - doesn't look too tough a job.

Any thought would be much appreciated.
Regards
Mike

[Tomfoolery]

That's how the acceleration pump worked on the carbs I used to work on, but that was the stone age (Carter single bbl anyone?). I don't know the fuel systems of today, or even yesterday, if that's what carbs are now.

All I can do is confirm that's how acceleration pumps worked.

[Catigale]

Mike...you need to have the shop manual to get this deep into a carb, imho. Look at it this way - the cost of the manual is recovered in the first 30 minutes of shop labour at current rates...

[windypatrick]

I'll chime in, I recently rebuilt the Johnson 50 hp carbs. Piece 'o'cake ! Never done a carb before, very simple little things and indeed aided by having the manual (essential for me). They were fairly clean but it did help.

[Ormonddude]

I am assuming its a 2cycle engine? I have just done a total rebuild on a 96 Yamaha C40 - there is no push rod on the fuel pump 1 cylinders compression pressure or back pressure is used to actuate a diaphragm that pulls fuel through 2 valves a inlet and a outlet the carbs are very easy to rebuild I used the Microfiche Diagram on Boats.net for anything confusing like some tiny O rings that go on the main float valve installation. If your engine uses a carb overflow fuel distribution system you may not get fuel from the fuel line also I would change it to individual lines because the carb overflow usually starves a cylinder at the top and is 1 of those (WHY) I want to ask engineers (answer we saved $2 in fuel line)

[Crikey]

Short of a shop manual, I can't say if there's accelerator pumps on outboard carburetors? I don't think there's a similar loading, as in an automotive equivalent, to justify one. The experience you describe could result from a retarded ignition timing or worn out mechanical distributor advance.

R.

[mike uk]

Thanks guys

The motor is a 2004 model 4 separate carbs, 4 stroke - more or less the last of the non EFI models. There's no mecahanical distributor for the spark - it's all electronic.

The fuel pump has 2 outlet tubes each of which splits just before the carb assembly with T joints so there one tube going into each carb. There is also another T joint taking fuel into the accelerator pump. From the accelerator pump there are 4 small tubes going into the side of the body of the carbs.

I've got the SELOC workshop manual (which is where I've got my basic info from). It mentions the accelerator pump in the "strip down" section but it strangely doesn't mention it in the troubleshooting and diagnostic testing section.

So the idea of disconnecting one of the tubes to see if fuel is being pumped under rapid throttle action was my own but I'm not sure if the fuel would come out of the tube under such conditions (or maybe because it's not got the carb suction pulling as well as the pump pumping, that is why there's no squirt of fuel??)

Any suggestions (apart from buying a new one to see what happens?)

Cheers
Mike

[Ormonddude]

OK I have looked at all 3 models of your year of production 2004 - There is no acceleration pump - unless you mean Fuel Pump? because you do have that and a filter- have you inspected the Filters? a slow take off would be caused by a dirty filter. Please use this schematic and try to clarify after you clean the filter \
http://www.boats.net/parts/search/Yamah ... parts.html

[mike uk]

The model is FT50CETL - it's not on Boats.net - maybe not sold in the States?

Here is a Yamaha website with the details

http://www.yamaha.co.za/site/1630/view/ft50cetl.html

Mike

[Ormonddude]

I cant get a look at schematic anywhere so I have to go with generic answers (I'm sorry) check any filters and any connections for blockage or air leak the fact that it only acts up when accelerating tells me its having trouble on the big draw but can handle a slow steady draw other things to look for is water in the fuel and any breather valve blockage

[Seapup]

Has it sat at all...A month or two?

My gut would strongly say a simple case of dirty carbs. Thats exactly what the honda 4 strokes do if you let gas sit in the carbs. Will still run at basically any RPM if you are very slow on the throttle, but bog or die of you give it gas quickly. Gas gumming up the carbs can happen quite quickly.

[Carioca]

I had to synch the 4 carbs with a vacuum gauge to get a smooth result.

http://www.carbtune.co.uk/carbdtls.html