1981 Mercury 70hp 2 stroke - problem

[ross_larter]

Hi All,<br /><br />I have a 1981 Mercury 70hp 2 stroke with a power problem:<br />This is what i have tested.<br />- Out of gear It rev's fine.<br />- In gear with the prop half out of the water and the water intakes under water , it revs ok and throws water up in the air.<br />- It has been compression checked OK.<br />- The carbs have been cleaned, and the fuel lines checked.<br />- the 3 plugs seem to be giving a spark.<br />- the pee hole is peeing OK.<br />- the throtel is opening fully.<br /><br />The problem is that when it is trimed under the water the revs drop and it only pushes the boat at about 5mph (It will do this for as long as you want won't stall).<br /><br />any ideas ?<br /><br />Cheers<br />Ross

 

[graddy]

Re: 1981 Mercury 70hp 2 stroke - problem

Sound like the idle exhaust ports could be clogged up with carbon. The P/H would have to removed to inspect. Just a thought. Or you could try removeing the lower unit and run the engine up to temp, shut off. Spray powertune into the carbs while running and then let the moter sit for 15 minutes. Start the engine back up and there should be alot of soot comeing out the exhast.

 

[jim dozier]

Re: 1981 Mercury 70hp 2 stroke - problem

Check to see if its running on all cylinders.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke - problem

Hi,<br /><br />I have pulled one spark lead off at a time and the engine does change note on all cylinders. so i think all 3 cylinders are firing ok.<br /><br />Cheers<br />ross

 

[mellowyellow]

Re: 1981 Mercury 70hp 2 stroke - problem

spun prop hub maybe?

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke - problem

> spun prop hub maybe?<br /><br />The rev's drop when the engine gets under load in the water, i though the revs would stay high if the prop was slipping, or am i talking about the wrong thing?

 

[mellowyellow]

Re: 1981 Mercury 70hp 2 stroke - problem

you're prob. right... to be sure, mark the prop/hub and see if mark moves at all. exaust is a good guess otherwise.<br />good luck,<br />M.Y.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke - problem

Can i spray carbon remover through the engine without removing the lower unit to clear the carbon build.

 

[ledgefinder]

Re: 1981 Mercury 70hp 2 stroke - problem

It does sound like power on two out of three, & I betcha it's either spark or fuel. They'll start & rev fine that way, you really only notice when it has to move the boat. <br /><br />What's the history - did this start suddenly or have you worked on it for a while? Is the power (what there is of it) smooth, or does the engine cough/surge? <br /><br />Need to check fuel, spark & compression in more detail. Spark: Are you sure it's firing all three when you're underway? You could try grounding one wire at a time & taking the boat for a quick spin. Fuel: Try squeezing the bulb when underway (checks fuel pump). Those motors have 2 carbs feeding three cylinders, so little hard to isolate carb feed, but maybe try grounding a top or bottom spark plug. Also see what the plugs look like. Is at least one of them firing well (dry, light tan "deer-colored" insulator)?

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke - problem

The power (what there is of it) is smooth , squeezing the bulb when underway makes no difference, i'll check the plugs next time i run it.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke - problem

PS<br />It started suddenly (4 weeks ago), I was going along at 30mph and it just bogged down to 5mph for (30 secs) then when back up to 30mph (for 2 mins) then back to 5mph and has been running at 5mph for the last 2 times i have taken the boat out on the water.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke - problem

PPS<br />Would it make such a hugh difference to speed if only 1 cylinder is not firing under load or could 2 not be firing ? What to people think about the carbon build up in the exhaust ports , is this a common problem ?

 

[ledgefinder]

Re: 1981 Mercury 70hp 2 stroke - problem

What you describe is very typical of one cylinder losing ignition, but really don't know anything until you try the running tests.<br /><br />1 out of 3 can make that big difference, you bet. Besides losing the pop from one cylinder, now also the motor can't get up into it's powerband. You've got a pretty highly-tuned motor there (70hp from 50 cubic inches).

 

[AndyL]

Re: 1981 Mercury 70hp 2 stroke - problem

I had a similar problem with the same engine whereby it would suddenly lose power but somehow keep running. Finally traced to a leaky fuel needle on the upper carb which would without reason allow huge amounts of fuel into the motor and choke it. When you have the fault, remove the covers to the carbs so that you can look into the carb throats and see if you can see neat fuel pouring in. Do not run the engine long like this as will overlean the mixture and could cause damage. Just guessing but may save you the hassle of pulling the powerhead.

 

[FlyBoyMark]

Re: 1981 Mercury 70hp 2 stroke - problem

Since it only boggs when you trim down...how about checking very closely for pinched or streched fuel line?

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke - problem

If i have it up on the trailer with the water intakes under the water and the prop just out of the water and full trim down it rev's fine so i don't think it is pinching the fuel line.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke - problem

Just pulled the spark plugs and the bottom one is all fouled up with oil and has lots of build up on it, looks like it would short it's self out, these plugs are only 2 hours old. I bet if I clean the plug up the boat would run fine for 2 more minutes then go back to the old problem.<br /><br />engine is mercury 70 hp 2/s: 925314<br />plugs are NGK buhw/2622<br />Are these the correct plugs.

 

[mellowyellow]

Re: 1981 Mercury 70hp 2 stroke - problem

not getting spark on that cylider perhaps?<br />start her up and pull the plug wires one at a<br />time. if there is no drop in RPM's when you<br />pull a wire, that cylinder is dead. I don't think<br />it's firing on all cyliders...<br />good luck,<br />M.Y.

 

[ross_larter]

Re: 1981 Mercury 70hp 2 stroke - problem

I'm guess i am not getting a spark on the cylinder but i am worried that the plug fouled up so quick when it was brand new.

 

[ledgefinder]

Re: 1981 Mercury 70hp 2 stroke - problem

OK I think you have it now.<br /><br />Can you tell if bottom plug is firing at all? Without seeing the plug, sounds like you've got no or weak firing on bottom cylinder. Does the 1981 have a distributor and one coil, or instead a switchbox and 3 separate coils? Also, how's compression? Do the other two plugs look good?<br /><br />Assuming you've got good compression (or at least 80 psi on #3) and the two other plugs look good... If you have 3 coils, you can swap coils and see if the problem "moves". If you just have the one, could try swapping plug/plug wire. If you do have a distributor, not sure how much trouble it is to replace cap.<br /><br />By the way, in my humble opinion having just the water intakes under water is not low enough to get the pump to work all the time. To be on the safe side, you want the water to be a good 4-5 inches above the caviation plate. The pump is a couple inches above the breakline between the lower unit & midsection, maybe 3 inches above the cavitation plate. As you know, the water intakes are a ways below the plate. So, you're starting it up on air unless you have some way to get the water up to the pump. Maybe the pump pulls the air through & suctions water pretty quick, maybe not. All I know is I burned up two pumps when I used to figure water up to the cavitation plate was deep enough - haven't burned any since I started running the tank deeper.