Is this the fuel pump???

Mercury 1990 40hp oil injection.4 cylinder twin carbs

I have been having a lot of the same problems ie been going round in circles for the last month.
so i need someone with a clear head to refresh me and give me some straight info.

when under load engine stalls when put in gear and revved.

Thanks to laddies ive stripped and cleaned carbs ,
ive also put new fuel line on powerhead etc.
new inline fuel filter.
as said befor spark and comp tests good.


my trouble is that on muffs engine will idle but plugs have been found to be oily but i put that down to the idle mixture not being set up at the moment.
But on occasions the lower two cylinders dont seem to be firing .( removing spark leads dont make any difference.
if i squeeze primer bulb engine picks up and also if i spray fuel oil mix into bottom carb it also picks up .Spray it in the top one and it stalls?
so presumably no fuel to lower two cylinders.
Would i be wright in thinking that this is due to oily spark plugs and they are grounding out????.
but top two plugs are oiled up too.and cylinders seem to be running.

I have adjustment on top carb idle screw but nothing when bottom carb idle is turned.
so my question is what could be the prob if ____

1 ,i have good compresion.
2 ,good spark even when unlugged the leads ground out on the side of engine and jump nearly half inch.
3,carbs are clean thanks to laddies expert help.
4 fuel lines are clear and so is filter /
5, Tank is vented well

Is this a fuel pump diaphram prob , has anyone seen these symtoms before. if so i would be GRATEFULL for any info as i seem to be going around in circles and am getting VERY dizzy.

Re: Is this the fuel pump???

It does sound like a fuel pump issue. Because your plugs are oily it seems as though there is not enough fuel getting mixed with the oil.
I would put a rebuild kit in your fuel pump (not expensive or difficult) and then try it out. I bet your issues will be fixed.

Re: Is this the fuel pump???

But why does it idle with top two cylinders if bottom two not getting any fuel???

Re: Is this the fuel pump???

Ok ive ordered a new diaphram kit and primer bulb to try and eliminate everything.Will fit tommorrow.
I have used a strobe light to analize spray pattern in carbs, and what i see is in the top carb i can see fuel coming out of jet and spraying (albut not a fine spray) in to engine.
But bottom carb no fuel coming from jet untill you revv engine and then i can only see fuel coming to the top of the jet but not spraying into engine.
Is this fuel pump ???

Re: Is this the fuel pump???

Fitted new pump dia and primerbulb still no joy..gutted!!!
another carb clean will have to solve the problem as there is nowt else to fix???

Re: Is this the fuel pump???

how is your 1990 40hp a 4 cylinder? my 1990 75hp is a three cylinder. hopefully that is a typo.

Re: Is this the fuel pump???

no typo as said 4 cylinder 1990 40hp

Re: Is this the fuel pump???

Here?s a suggestion which may or may not be helpful. While motor is running look closely at lower carb throat and see if fuel vapor is being forced out of the carb (opposite the way air should flow in). If you see vapor or moisture (from fuel) on or around the throat of the carb you may have a broken or displaced reed in the lower reed block. I had this problem on my 50hp 4 cylinder Merc. Similar symptoms. Turned out to be a broken reed locating pin which allowed the reed to shift position on the reed block. This left a constant opening in the reed valve of about an eight of an inch. On piston downstroke fuel/air mixture was forced back out the carb rather than up into the combustion chamber. This problem could affect #3 & #4 since they share a carb and crankcase pressure/vacuum. Choking helped the motor run better since it effectively sealed off the carb throat thereby forcing fuel/air mixture into the combustion chamber. Spraying fuel into lower carb would also help because a cylinder isn?t receiving a full charge of fuel/air with a bad reed. Spraying fuel into upper carb kills it because #?s 1 & 2 are getting too rich.

I don?t believe it?s the fuel pump because the same pump feeds both carbs through one line, correct?

Re: Is this the fuel pump???

Thankyou BR I will check tommorrow.
What did you do to yours, did you fix it was it hard to do???

Re: Is this the fuel pump???

I did repair mine myself with help from a factory manual. Unfortunately getting to the reed blocks requires removal of the powerhead. This sounds more daunting than it actually is. I consider myself an average / somewhat above average mechanic and I was able to do the entire job myself. You?ll need to take your time and pay attention when disassembling. Be methodical, keeping assemblies together as much as possible. Once the powerhead is off, you?ll need to remove the crankcase cover to expose the crankshaft and reed blocks but do not pull the pistons out of the cylinders or you?ll need ring compressors to put it all back together. There?s enough play in the pistons and cylinder walls to pull the crank out enough to work on the reed blocks and do what you need to do. Check all the reeds not just those you suspect are problems. You?ll probably need the special Merc tool to pull your flywheel if it?s internally threaded like most. I used one from my local Merc dealer for a small fee. Other things you?ll need for this job are gaskets, (powerhead / exhaust plate, carbs.) gasket remover and flange sealant to clean up and then re-seal the crankcase cover, possibly new reeds if cracked or broken, reed locating pins if missing or broken, and loctite for reed mounting screws. Center main and end cap bearings should probably be checked for wear. Replace if necessary since you?re there. A torque wrench and knowledge of tightening sequences and torque specs would also be very helpful when re-assembling.

It is a bit of a job so you?ll want to double check the diagnosis before diving right in. Another tell-tale symptom is that the motor will run great in a higher RPM range and WOT while at the same time have trouble idling, particularly while in gear.

By the time I decided to do the job I was so frustrated with the motor that I just had to see what was going on inside to make it run so lousy. Like you, I hit all the obvious things first but to no avail. She?d idle fine on the muffs and in the water (on fast idle) but as soon as I tried to put her in gear she?d die immediately unless you slammed it down and gave her full throttle hoping she?d hang on. The repair was successful and she?s running like new again. Great low speed idle in and out of gear and smooth at WOT. Hope you can have the same luck. I would be conservative and allow 8-12 hours for the entire job assuming you?ve got everything you?ll need at the outset. Good Luck!