Early 80s Mercury 70 HP bogs down with throttle

[RMAC18]

Mercury 70 HP
Serial # 6104809

I bought this a year ago and it will push an old fiberglass bass boat about 20 mph according to GPS speed. I was running it wide open and it just bogged down and died. I restarted the motor but now it bogs down whenever you push the throttle and it maxes out at 5 MPH and it is running rough. A few weeks prior I replaced the gas line and primer bulb. I also replaced all 3 spark plugs and the fuel filter. I emptied all gas and replaced with new and ran 5 gallons of Seafoam. It starts fine and idles fine and will run at slow speeds but bogs down when you push the throttle. Any ideas on how to trouble shoot this?

Thanks,

 

[jimmbo]

Did you adjust the Idle Mixture Screws? They might be set too Lean, Otherwise, it is likely dirt/gum/varnish in the carbs. To be cleaned properly, they need to come off and disassembled and cleaned with a Carb cleaner, not some very thin oil like SeaFoam. Did you you mess with any adjustments on the Throttle Linkage?



20mph? What is it pushing a Barge? What RPM was it attaining at WOT? Is it Running on all three Cylinders?

In fact, Start with a Compression Test? Then, does the spark jump a 3/8" gap? Then move on to the carbs

 

[RMAC18]

Thanks for the reply. It is Tidecraft 17’ fiberglass bass boat that seems heavy to me. I bought it a year ago and it’s my first boat and I’m trying to learn.

I did not mess mess with throttle linkage adjustment.

the RPM meter doesn’t work and never has since I bought it. The gas gauge didn’t work either but I fixed that. I’m not sure where the RPM meter connects. Wiring is a total mess under dashboard. I could rewire if I could tell where it connects.

sorry to be such an idiot but is there a good resource on how to do a compression test? I had a boat mechanic look at it when I bought it last year and at that time the compression was apparently good. I got the boat cheap and I’m willing to spend some money on it but I always want to learn how to maintain it myself as much as possible and go to mechanic for things beyond my control.

i did replace all 3 spark plugs but I don’t know how to tell if it’s running on all 3 cylinders. Also not sure how to determine if spark jumps gap.

i did just buy a service manual for it and it seems like a good idea to clean carbs properly,

i appreciate any help to get me over the initial learning curve. Thanks

 

[jimmbo]

"I bought it last year and at that time the compression was apparently good". Well, you should find out yourself if it is good.
There are lots of Videos on YouTube on doing compression tests.
A quick check to see if a cylinder is working is to pull a plug wire off when the engine is running. If it slows down/runs rough, then you know that cylinder was carrying its load. If little or no difference, then that cylinder is just going along for the ride.
Your engine, when right should be able to push an 17ft a bit faster than 20mph, 24- 25mph. However if the boat is water logged...

 

[RMAC18]

Compression test results:
cyl 1 - 10 PSI (yes 10, super low)
cyl 2 - 132 PSI
cyl 3 - 125 PSI

I tested each 4 times with a $30 Innova compression tester from Auto Zone. Results were pretty consistent. The only difference is that while testing cyl 1 the flywheel would turn continuously as long as I turned the key, whereas cyl 2 & 3 would turn for only a few seconds and then I’d have to turn key off and then on again.

i started motor on muffs and let idle in driveway toward up prior to test. When I removed spark plug on cyl 1 there was a clear liquid that ran out. I think it was water but hard to tell for sure.

is it fixable?

thanks

 

[jimmbo]

Did the water actually come out of the spark plug hole, or from the larger recessed area where the spark plug sits?

BTW, those engines will run fine on the hose, even with a weak cylinder. Once they are in the water, the weaknesses show up very clearly

 

[Faztbullet]

Rebuild or swap motor time.....

 

[RMAC18]

Did the water actually come out of the spark plug hole, or from the larger recessed area where the spark plug sits?

Maybe a tsp came out of the spark plug hole. It appeared clear and I tried to smell it but I couldn’t tell.

 

[jimmbo]

For a tsp of water to come out of the spark plug hole, the cylinder would have to be almost 1/2 full of water. Considering that the inlet and exhaust ports are lower, I find it difficult to believe there was that much water in there,

Edit: I guess there could be some water lying in the bottom of the cylinder, below the lowest of the ports, then when the compression stroke happens, the cylinder volume is reduced, and the water could rise high enough to drain, but the cylinder would be getting very close to Hydro-locking

 

[RMAC18]

For a tsp of water to come out of the spark plug hole, the cylinder would have to be almost 1/2 full of water. Considering that the inlet and exhaust ports are lower, I find it difficult to believe there was that much water in there,

Edit: I guess there could be some water lying in the bottom of the cylinder, below the lowest of the ports, then when the compression stroke happens, the cylinder volume is reduced, and the water could rise high enough to drain, but the cylinder would be getting very close to Hydro-locking

It may not have been water but there was a small amount of some liquid.

At this point what can be done. How difficult is it to rebuild this type of motor? Is there any possibility it’s just a gasket or some type of seal? Appreciate any advice on options moving forward.

 

[jimmbo]

Those engine often leak water around the sparkplug, when the waterjacket cover gasket slips. A lot of people think it is coming from the sparkplug hole. Inspect that area with the engine running

As for leaky gaskets, there is no Head Gasket to worry about, but sometimes the inside the exhaust cover there is a problem with the Exhaust Baffle Plate. That can leak and dump in thru the exhaust port.

How difficult is it to rebuild? Well a lot depends on you Mechanical Aptitude. I know there are some people on the board that will say they can do them Blindfolded while sleeping.... This engine like all the old style inline Mercs, has no removable cylinder head, so the entire Crankshaft, Bearings, Rods, and Pistons are removed/installed as one assembly. Special ring compressors are recommended when installing the Pistons. A Machine Shop that can do Blind Hole boring will also be needed

 

[Texasmark]

Ouch. #1 compression/fluid real cause to worry.