I have an embarrassing problem.. (with my Mariner 15hp) help please

[rogerds]

First post - please be gentle and forgive any forum etiquette breaches !
Mariner 15hp Bigfoot 2005/6 - low hours
Me? - fairly mechanical - cars / bikes but new to outboards

End of last season started running uneven/ rough / stalling on idle - made manoeuvering (spelling ?)in a crowded marina difficult and very embarrassing !
so... routine engine service (New plugs, oil changes, fuel filter etc) no change
so.. assume its something to do with fuel
Drain & clean tank & new petrol
New hose & bulb
Strip carb, clean, ultrasonic jets
Still the same
But.... if I rev engine a little and put hand over air intake till engine almost stalls and then release, this seems to 'cure' the problem for a while but then its back again
(engine runs fine a higher revs)
so... is there still dirt in the system??
Clean carb again - still the same
as a last resort tried some Seafoam through the carb - still the same !
Please...
Hoping the collective wisdom of the forum may help me on this - want to get the boat back in the water but only when i can trust the motor
Any suggestions - even the most obvious! that i may have overlooked?
Thanks for your time
roger

 

[rogerds]

ps.....havern't considered the mixture screw (still has brass cover on).... should i???

 

[Texasmark]

Later model engines sold in the US have to meet strict emission standards and the old custom of having a movable low and high speed mixture control no longer exists......jets are fixed and usually capped with a brass screw or in some cases sealed access to the removal access by wax or epoxy.

Since you have been in your carbs, if you disassembled the entire carb that removable hardware will allow, cleaned and have blown the parts out with 100# compressed air.....stick the hose in a hole and air comes out somewhere on the casting you may not have this problem....but it won't hurt you to do it anyway: At your engine's age it's time for engine rubber to be deteriorating in the form of very small pieces of black rubber showing up here and there requiring you to change out all your internal fuel lines too....if you still have your old fuel filter, cut it open and see what's in it.

Also the diaphragm in your fuel pump may need changing too as it's rubber....get the whole fuel pump kit and change the valves too. If a valve is leaking or the diaphragm is ruptured, it may not move fast enough at slow rpms to suck adequate fuel in it's current condition as demonstrated by your hands over the carb test.......another check of the fuel pump diaphragm being faulty is to squeeze the bulb at low speeds and see if that solves the problem.

Last, your float may not be set right in the carb. Can you squeeze the bulb with the engine off and get it to firm up good and solid? If it doesn't get hard but when you put reasonable pressure on the bulb and it gives way, you are leaking fuel some where.....have any fuel in the engine compartment? How does it start....act like it's flooded? Fuel floating on the water around the engine when starting????? Carb needle valve not seating properly in addition to possible float position problem. Possible float to carb housing interference not letting float function properly at low speeds and high fuel levels in carb bowl.

 

[rogerds]

texasmark - you're a star ! - thanks for coming back to me
Boat is back in the water and engine has been run for 3-4 hours (boat is a 5 /6 knot 'plodder' ! ) and is fine when pushing the boat along but....
still have irregular idle with stalling at slow(er) speed
There was once a small leak of fuel from the fuel 'pump' ? on the near side of the carb thats activated when the throttle is opened - but i can't see that affecting the slow running
Your suggestion about the fuel pump itself sounds eminently sensible and possible - will get round to doing that as soon as i can and report back
Sincere thanks for your help
cheers
roger