mercury 4 hp running problems

[boatybouy]

hi guys, i'm new to this so be gentle with me.<br />i've got a twin cylinder mercury 4.0 hp made in belgium (probably mid - late 80's.<br />i've de-gunked the fuel lines and inline filter and gas flows freely upto the carb.<br />now the crunch, it starts on choke straight away and runs but as soon as you put the choke in it cuts out. if you twist the throttle it accelerates but as soon as you drop the revs it dies. it sounds like fuel starvation to me. but i'm new to mercs. i've stipped the carb and cleaned it all out with carb cleaner and tried adjusting the float hieght but it's made no difference to it. i don't have a manual so i don't know what the float hieght is supposed to be. any help/ advice would be greatly appreciated.

 

[jaybirdtx]

Re: mercury 4 hp running problems

Try sqeezing the primer bulb when the engine starts to die. If it keeps running when doing this, the fuel pump is probably bad. Also check for the anti-siphon valve located on the tank where the hose connects. If its there, take a punch or screwdriver and knock it out completely. A lot of times when a fuel pump gets weak, it can't overcome the spring pressure of the anti-siphon valve. The float height, with the carb. upside down, should be about level with the carb. flange. Initial idle screw setting is approx. 1 1/2 turns out.

 

[boatybouy]

Re: mercury 4 hp running problems

hi, thanks for the reply,slight problem-<br />tank is mounted on top of engine (no remote facility).fuel is gravity fed from tank under engine to fuel tap then up to carb.<br />all pipes have been cleaned out, and fuel flows no problem. engine starts on choke but once choke is pushed in engine will not maintain revs unless you rev it up.

 

[Laddies]

Re: mercury 4 hp running problems

The knat engines were cursed with fuel problems, at one time there was a fuel pump kit available for the it helped some as it made the fuel pressure more uniform. That use to be the fix for your problem, don't even know if the kit is available any more. There was a reason the knats were not around long

 

[Paul Moir]

Re: mercury 4 hp running problems

Ditto what laddies said. The kit is NLA.<br /><br />Make absolutely certain that the passages behind the idle mix control knob are clear. OkieBoatMech has the initial adjustment correct. Here's my notes on the Gnat's carb adjustments:<br /><br />Carburetor<br /> Float level adjust 5/64 to 7/64 above casting rim when turned upside down.<br /><br /> Idle mix - seat and turn 1 ½ turns out, set motor for ~1000 rpms, richen till runs bad, then <br /> turn in until runs good. Slow to idle and readjust if necessary. Accelerate from idle. If it<br /> lean hesitates, then richen.<br /><br /><br />The carb is a real peice of work. We find we have to regularly wiggle the rich/lean control to get it running happy. Inline filter helps a lot, but it still has this problem.