Yamaha F4A Outboard

[lexi]

Funny thing happening with this engine. Engine running all day, then died. Ah! the usual culprit of fuel connection fallen off, so carb is empty. Connect up and pump primer bulb. Carb floods out of top plate and wont start. Nearly in to base so used the electric for last mile.
Stripped carb at home, expecting dirt under float needle. Cleaned carb and blew out, checking float height etc. All back together and on the stand. Fill carb with bulb from remote tank and still overflowing!
The float needle seals to air pressure when you lift float to seat needle. It has no groove and the seat is shiny. The float is good as I left floating overnight in a can of petrol.Switch the fuel to integral tank. A few pulls on an empty carb, it starts and runs with no flooding. The integral tank feeds the fuel pump and when you pull start, that feeds the carb until the engine fuel pump keeps it pressured. So the float and needle are working, mechanical fuel pump would flood carb. It is only the primer bulb that is flooding it. I will take out on Lomond tomorrow and try. Maybe if I just squeeze bulb a few times and not fill carb right up? I have another identical engine and never done this. I can swop parts to confirm stuff before buying anything. Ideas welcome.

 

[penfold]

Is the float height set right? Small carbs are disproportionately sensitive to this. I'd replace the needle valve as a matter of course, small amounts of wear can cause mis-seating.

 

[lexi]

I will swop with the good needle on other engine first. I don't mind replacing parts if they are faulty They are £17 those needle jets. A t/stat for it is £38 quid and just a little thing. The float is set at 19mm as per manual. Needle holds 7psi on a Mityvac when you gently lift the float to take the spring pressure on needle

I have no running issues when going. I wonder if air being pumped in through primer bulb has any effect on this? The connection at end of tank hose sometimes drips and it is a pattern one. I have a genuine Yamaha one I will try from another boat. I normally pump the bulb till solid, then pull start.
The carb is a Nikki.

 

[penfold]

There's a limited number of things that will cause the symptom described; float perforation, wrong float height and damaged/worn float valve.

It has no groove and the seat is shiny.

Is the seat not part of the valve? If it's an integral part of the carb then it might unfortunately be time for a new carb, I thought only cheapnese knock-offs had rubbish value engineering like that. I'd be tempted to test the carb with the bowl off and run fuel through to see if the valve seals reliably when the float is raised.

 

[lexi]

The seat is a brass bush fitting, seated tightly in the alloy tube. It has the oriface for needle The seat is not listed on Yamaha parts list. Similar seats are on other Nikki carbs though and can be replaced and removed. I may try and replicate with the bowl off and carb sitting on a small glass. Then operate priming bulb.
Remember, the float is sealing when run and pressured from fuel pump. There is no overflow then and not a smell of fuel at all. I was also thinking of a sticky spring in the needle as a posssible.
For years, Yamaha carbs were all Mikuni. Nikki carbs are in Briggs and Stratton etc

 

[dobbslc]

I'm saddened to here Yamaha are not using Mikuni carbs anymore.
As sheriff Buford T Justice once said "what the hell is the world coming too?"

 

[lexi]

Just to update. This engine has been fine. I pump the bulb till I hear the fuel go into carb.
Then I stop. Engine then starts and runs all day with no flooding.