1973 Mercury 850 mystery problem

[Hoosierfish]

my 1973 Mercury 850 has me about baffled. Been working on issue a while now it basically has no power above idle. It always dies when shifted into gear. After I quickly throttle up to keep it running it will idle ok but when I throttle up it bogs real bad and sometimes dies. After many adjustments the best I can get is the front of the boat up and never enough power to get it on plane. I did have an intermittent spark problem that I fixed with new switch box. I put on a new coil and plug wires. Changed spark plugs. And lastly rebuilt the fuel pump. Still the same issue bogging lack of power. has good strong spark on all four cylinders, 135 to 140 lbs compression on all cylinders. Looking for some fresh tips or ideas.

 

[Hoosierfish]

Also rebuilt both carbs

 

[Faztbullet]

Engine needs rebuilt as low crankcase pressure.

 

[Chris1956]

Check the spark plug wires for arcing to ground and resistance. They should have zero resistance.

 

[Hoosierfish]

I’ve installed new plug wires. Should I still check them and if so how do I do it? Thanks

 

[Hoosierfish]

“Engine needs rebuilt as low crankcase pressure.”

what would cause low crankcase pressure?

If if this is the issue would it be the powerhead although it has decent compression?

 

[jimmbo]

A common cause of low crankcase compression is leaky crankshaft seals, crankcase case gaskets/seals, transfer port covers, oil recirc line broken/cracked/missing, scuffed cylinders/pistons, worn rings(they do seal better with the higher compression in the cylinders), reed valves broken/bent.

 

[Faztbullet]

Inlines are bad about wear around the ports due to side thrust of piston. These are worn by the skirt of piston and when skirt is worn crankcase compression drops. Compression test just lets you know engine can support combustion.

 

[Chris1956]

You installed new OEM spark plug wires with the brass screw on terminals, or some other wires?

 

[Hoosierfish]

Plug wires are oem with screw on terminal

 

[Hoosierfish]

During trying to diagnose the problems, when we were rebuilding the fuel pump we noticed with the fuel pump out and cranking the engine there didn’t seem to be much pressure coming out of the hole where the pressure activated the diaphragm. It didn’t seem to me that it had enough pressure to even compress the spring with the plunger on it any at all. I have no experience to know how much pressure should come out there but when cranking you could barely feel it. Does this sound like a crank case pressure issue?

 

[Chris1956]

Does your motor have dual fuel pumps? Just a question.

If she starts and idles, I would expect crankcase compression to be adequate.

 

[Hoosierfish]

No just one fuel pump. It is a little hard to start first time I start it for the the day but starts right up after that with a little throttle. I really don’t want to spend any more money on it if it need a total rebuild I will just buy a newer one. I was thinking of looking into the timing a little deeper. We have set it according to the repair manual I bought and no change but it seems to me the advance on the distributor isn’t working as it should. Timing advance is about 5 degree before tdc when it starts to move the throttle arm and 21 degrees before tdc at wot. When I move the throttle from wide open to idle the distributor doesn’t always want to come back to the idle stop. This is what makes me believe it isn’t working just right and if it’s off some then maybe it isn’t advancing correctly. Timing was checked while cranking motor without fuel and moving throttle from idle to wide open.

 

[Chris1956]

Max spark advance of 21* BTDC sounds right. Idle pickup timing is 4-6*BTDC.

So move the throttle from all the way retarded, until the carbs are still closed, but ready to open. That timing should be 4-6* BTDC. If it is not, there are two bolts (5/16" wrench) on the midsection of the distributor, that when loosened will allow the distributor to rotate a bit to set the correct carb synchronization with the timing..

 

[racerone]

Tried opening the low speed mixture screws 1/4 turn at a time ??----Too lean on low speed results in bogging.

 

[Hoosierfish]

I will recheck the idle pickup timing. I tried opening up idle mixture screws 1/8 turn alternating top to bottom carbs until it got the best and started going back worse. (Almost 3 turns out on both carbs) when it was its best it still acted the same. Lack of power and sluggish.... it will idle most of the time and rev up what sounds to be perfect on muffs just not in lake under pressure