Mercury 115 burns way too rich

[frozenokie]

1986 Mercury 115 Inline 6 burns way too rich. I put in new carb kits in each carb and used my shop manual to reset the needle valves/floats. Before I did the carb kits, I couldn't even keep it at an idle. Now, it idles for a few seconds until it loads up with too much fuel. Also bogs just a little out of the hole unless you "feather" the throttle then runs awesome until it loads up after about 10 minutes at WOT, but not in between WOT and idle. Are the floats set wrong (too high or low)?



Also - new fuel pump, fuel bulb, and plugs.

In addition - I'm only getting 4400 RPM with the correct prop (13x19) per the prop shop technician. Could the rich running cause that too?

 

[Chris1956]

Re: Mercury 115 burns way too rich

It does sound like you have some carb issues. The only adjustment is the idle mixture. These are a bit tricky to get correct, but if it runs rich at speed higher than idle, you have other issues.

If you pump the primer bulb, do the carbs leak? if not, the floats are likely OK. you might check the fuel pump diaghram. If it leaks, raw fuel will enter the crankcase and load the motor up at above idle speed. You might find #6 spark plug is completely fouled.

The best way to set the idle mixture screws is to start at 1-1/2 turns out from lightly seated, and adjust by ear and tach to get highest RPM at idle, with smootest running. The idle mixture is now likely too lean and motor will stumble when you try to accelerate. If so, open top carb mixture screw 1/8 turn and try again. Repeat with mid carb screw, and finally botom carb idle mix screw. Stop as soon as you have achieved reasonable acceleration.

 

[frozenokie]

Re: Mercury 115 burns way too rich

The carbs did leak after priming. So, I pulled the carbs again and reset the floats and also found that all the new needles and seats were sticking. I reinstalled the old ones (which didn't show signs of excessive wear) and they didn't stick so I left them in. Then I reset all float measurements to the shop manual. The fuel pump does not show any signs of leaking and it stays dry after running on the lake.

The most confusing thing is that I can only achieve 4400 rpm and I am using a 13x19 prop that the prop tech said would be right. I should be around 5200 rpm. Am I missing something??

I stumbled across this possible issue also. When I turn the crankshaft by hand clockwise, it makes a slight creak, but ever so slightly. Kind of like opening a door that needs oil on the hinge. I could also "feel" the noise through the crankshaft flywheel if you know what I mean. This is a rebuilt motor back in Dec. of '07. So I'm wondering if they didn't get the crankshaft shimmed properly, therefore possibly causing low rpm at WOT and rich burning of fuel. Is this a possibility?

 

[AMD Rules]

Re: Mercury 115 burns way too rich

............... The fuel pump does not show any signs of leaking and it stays dry after running on the lake....

The fuel pumps have a rubber diaphram inside that uses crankcase pressure pulsations to pump the fuel. I think Chris was suggesting that you might want to check the condition of that diaphram and check valves. Even a pinhole can cause the syptoms he described. They're quite easy to rebuild with a new kit, and that might be the source of your low rpm's at wot as well.

Next time your on the water at wot, pump the fuel bulb and see if the rpm's pick up. Sign of a weak fuel pump.

Check compression lately? Was that motor broken in after the rebuild, per typical procedure of double-oil, varying throttle etc?

Good luck.

 

[frozenokie]

Re: Mercury 115 burns way too rich

Yes, I put a new fuel pump kit in last month. No difference in rpm when squeezing the bulb. I retrieved some instructions on breaking in Mercurys from their website and followed them to the letter. First two hours of mostly idling and maybe a little jump up on plane then back down, varying throttle positions and didn't get it up to WOT until after five hours of operation. It now has roughly twelve hours on it.

I did find that after the fuel bowls are full by priming with the bulb (bulb gets hard) that I can continue to squeeze more fuel through by applying more pressure and the bulb very slowly collapses as it lets very small amounts of fuel to pass through. I thought that the bulb should be "rock" hard and you should not be able to squeeze any more fuel through it at all after primed. Am I wrong?

 

[j_martin]

Re: Mercury 115 burns way too rich

Yes, I put a new fuel pump kit in last month. No difference in rpm when squeezing the bulb. I retrieved some instructions on breaking in Mercurys from their website and followed them to the letter. First two hours of mostly idling and maybe a little jump up on plane then back down, varying throttle positions and didn't get it up to WOT until after five hours of operation. It now has roughly twelve hours on it.

I did find that after the fuel bowls are full by priming with the bulb (bulb gets hard) that I can continue to squeeze more fuel through by applying more pressure and the bulb very slowly collapses as it lets very small amounts of fuel to pass through. I thought that the bulb should be "rock" hard and you should not be able to squeeze any more fuel through it at all after primed. Am I wrong?

7 or 8 lbs fuel pressure can overpower the needles. You can easily get 12 lbs on a primer bulb with a popeye grip on it. Normal running fuel pressure is about 2 to 5 lbs.

The squeak is the water pump. That's a good sound.

hope it helps
John

 

[frozenokie]

Re: Mercury 115 burns way too rich

Hey thanks J_martin. I almost started to cringe when I noticed that squeaking after all the hard work and hours I've put into this old Laser.

I'll be testing her again tomorrow. I'll update yall on this thread when I return. BTW - I did take the fuel pump apart again just to inspect the contents and found no problems. We'll see how it goes this time.

Thanks for all who replied.

 

[jimg984]

Re: Mercury 115 burns way too rich

low rpm could be oversized prop i run a 17 pitch on my1978 115 hp
alot depends on how heavy boat you got, mine is a older fiberglass trihull quite heavy

 

[WANTONSOUPGUY]

Re: Mercury 115 burns way too rich

Take a look at the choke...maybe the electric switch is stuck open and is dumping fuel in...

 

[Yepblaze]

Re: Mercury 115 burns way too rich

You say you addressed the carbs but it failed to remedy, and may have been worse.

Then replaced the new parts with perhaps problamatic used parts.

If you have bad needle/seats, or other reason for the carbs to be in need of attention, fix it.

Just because a problem is addressed once twice or more, it aint fixed till it's fixed.

Yes, I speak from experience(s).

Make sure the new seats have the gasket washer under them.

PS I always buy my parts at the dealer and get Quality Mercury Parts that "most always" are good.

If someone swapped lower units with a different gear ratio, you may need a lot less pitch.

It's not impossible for an ignition to be acting up, or with typical crumbling wiring so common on some of these older motors, to have weird electrical issues start popping up, like perhaps the choke activating on it's own, I suppose.