Mariner 70hp poor Acceleration

[naio83]

Hi to all, and thanks for accepting me on the forum. This is my first post.
I have a 1980 2T, 3 cylinder, 2 carb, 70hp Mariner on a 15 boat. Most of the time it was stored and with very little use.





At the beginning of this year I started to put it into value again to start using it since it was stored for the last 6 years.
- Compression checked of 150 on each cylinder.
- New Battery
- New gas tank
- New sparks
- New Fuel line
- Full Carbs rebuilt by local mechanic.
- New oil on the transmission.


The engine starts fine and runs smoothly with 1 or 2 persons on the boat, but If I try to go with 3 or 4 persons it loses power when it tries to reach the medium revs.
Especially if there are waves and the motor has to make an effort on putting the boat on plane. Sometimes, if the water is still and after a few attempts It makes it to the med/high revs and works fine for a while or even for the whole trip but the problem comes back the following day.
I tried to contact the mechanic several times and he said that he has checked the stator and the trigger already and that he doesn't know what else to check (it is not easy to find mechanics that want to get their hands dirty on those old motors where I live).
Things that I have considered and tried:
- Fuel pump / filter: The mech said it was ok.
- Water pump impeller : I test the water pressure, no overheating alarm.
- Double check on the fuel line.
- Squeeze the Primer ball when it happens.
- Check the gas venture.

I am adding a couple of videos with the boat with 3 persons onboard and trying to accelerate, if I keep on trying to accelerate the engine stops as in the second video:

Any help or piece of advice would be great!

Thanks in advance!!!

 

[racerone]

Not running on all cylinders.----Does spark jump a gap of at least 3/8" on the 3 leads ?

 

[naio83]

I thought that it was running fine with the 3 cylinders, there is no vivration, it sounds good, new plugs.
No problem with manual warmup lever either.
Thanks in advance!

 

[racerone]

It sounds good to you but it has no power ??-----Strange.

 

[naio83]

Thanks for your reply, What I mean is that when the engine is powering with 2 persons onboard and 3/4 persons onboard makes the same sound, the difference is that with 2 persons empowered to full throttle and with 3/4 persons happens as in the video.
Sometimes, after some tests it works fine.
But maybe something with the cylinder, I did not consider since it has good compression, new plugs, the engine does not move extrangely, etc.
I might try to inspect the plugs + spark + cables.
Again, thanks for your help!

 

[fastjeff]

I have a LOT of experience with these motors, and yours is doing the classic Merc Triple bog. The reason you get away with it with less load is that it's on the verge of bogging.

Try this: Open the idle mixture screws 1/2 turn OUT (CCW). That might do it. If not remove an inch or so of pitch from your prop.

Jeff

 

[naio83]

Thanks for your reply Jeff, I will try to give a look this weekend (the boat is in a marine and is limited what you can do there during weekend) and also double check de spark plugs as racerone suggested (not sure if I will be able to check the gap).
By any chance do you have a photo / diagram of where exactly are the screws and wicht to turn? Concerning the propeller pitch not sure If i will dare!
I will try if I can during this weekend.
Thanks in advance again for all your help.

 

[Texasmark]

The "Hole Shot" defined, is when you are just leaving the "no wake" area where you launched and want to start your journey. You firewall the throttle, the boat rises out of the water, bow (front of the boat) comes up as the boat starts gaining speed and then comes back down as speed increases....defined as being "on plane".

When you reach your desired speed, if less than the max capability of the engine/boat/load combination, you pull the throttle back as desired.

3-4 people in a 15' boat (style/type/functional type things not listed) with 70 HP including the gear that is normally carried plus any supportive equipment these folks have is quite a load. You gave no WOT (wide open throttle) performance numbers which help to resolve this problem.........you furnish these numbers and the pitch of your current prop.....would help!

My experience with 2 cycle/stroke low cubic inch engines is that they need RPMs to develop their HP. HP = (engine torque x RPM)/5252). Since the cubic inches aren't there...considering your power stroke is only half the length of the cylinder....since the exhaust ports start opening, you don't have the cubes to develop the torque so you have to supply RPMs to get where you are headed.

Your propeller is part the driving gear train and has a certain amount of "slip" (defined as the difference between a screw turning in a piece of wood and something movable) that subtracts from the ability of the engine to develop the needed prop thrust to propel the boat in the water.

In going from idle where the boat is "in" the water to "on plane" (water no longer touches the back of the transom.....the plate on which the engine is mounted) the resistance to the prop coming up to speed is much greater and the "slip" starts out very high and as the boat gets on plane and accelerates, is reduced because the "drag" of the boat hull against the water has significantly reduced.

Apparently the prop currently on your boat has a degree of pitch that allows the engine to meet the resistance needs and is able to develop power within its power band when lightly loaded. Adding the extra load mentioned puts the load curve above the power curve so the engine craters.

As "fastjeff" mentioned, opening the low speed carburetor screws, in unison, a little at a time...like some folks (Chris 1956 for one and I support that) a screwdriver tip width at a time going from the "hole shot" to "on plane" up to the 1/2 turn he mentioned lets you see if your engine is capable of developing its power curve above the load curve as is currently set up.

If not then as he mentioned, you have to do something to get it up there. Since you are dealing with a gear train, you need to change the gear ratio allowing for the drive RPMs to increase with respect to the load RPMs. You do this with the propeller's pitch....reducing in your case.

So, first off we need for you to get in the boat alone and firewall your throttle and let the boat develop maximum performance. At that point take the RPM and speed (GPS is most accurate) numbers and post them. I assume the gear ratio between the crankshaft speed and the prop shaft speed is 1.64:1 (engine : prop).

Then load up with another person and do it again....and another person (total 3) and do it again. Post the numbers on here and we will tell you what you need to do with your propeller's pitch and maybe is it the propeller or the engine's performance capability at fault. In reducing your propeller's pitch you stand the risk of over-revving when boating alone and your top speed will usually be reduced...expect that if prop pitch reduction is the answer and watch your RPMs. When they hit max, back off the throttle to prevent over-revving. I don't have data on your engine but for similar year models max RPM ratings are 5500 for your max recommended RPM.
---------------------

You said new oil in the "transmission".......assume that is the Lower Unit gearbox in-which the prop shaft is located. What you didn't say you had done and needs to be done without delay, is to replace the impeller (paddle) inside your water pump so that your engine cools properly. Other thing that may be a good idea is to replace the engine's fuel filter.....cutting the old one open to observe the volume of crud and type.

The lower unit section is where the gear box is located and is bolted to the "mid-section". Removing the lower unit you will see the water pump sitting on top encircling the drive shaft and the impeller, rubber paddle is under the cover.
---------------
Just an observation: I was fishing a 1000 acre lake (small so any boat movement is petty easy to monitor) and observed a 65 Merc of your engines design, on a fiberglass bass boat type craft of approximately 15', occupants were a woman and 2 men, all seemingly lighter weight individuals, with a boat full of gear. On several occasions I saw them moving from one spot to another. Every time they decided to move, their "hole shot" wasn't a "shot at all"......it took a long time for that boat to gain any speed...engine problem, prop problem, who knows.....

We'll be waiting for your numbers. Good luck!

 

[faztbullet]

I would pull and clean the high speed jets since it has been setting, partial clogging will cause bogging with increased load.

 

[naio83]

Thanks for your extremely complete information,

The loss of power with load (more than 2 persons on board) occurs exactly on what you have described as "In going from idle where the boat is "in" the water to "on plane""
I will try to answer some of the information from what I know:
- The boat is a really basic fiberglass. I don't have the exact weight but the complete set ( boat + engine) is put into the water with a machine that has a top capacity of 500 Kilograms, so it is for sure less than that.
- Regular gas with 1:50 mineral oil mixture.
- Yes, with the oil in the transmission I refer to the Lower Unit gearbox.
- About replacing the impeller in the water pump. I tried to find someone who can do it, but a couple of mechanics in the marina told me that it was pumping the water "OK", I think that is a lot of work to do for and old engine and they do not want lo lose time on it.
I know that is a basic "service" replacement, but still don´t get someone who is willing to do it and I don't have the tools, space or knowledge to do it unfortunately but I will try to find someone.
- Regarding the engine's fuel filter, I think that I can do it since from what I have seen it does not seem to be complicated. I have thought about the fuel pump also...


I will try to test all the cases. The boat does not have a Tach. I will have to get one. For speed, the only measure that I have is that with 2 persons on boat (170 Kilograms approx) the top speed is 60 Km/hs on WaterSpeed GPS.
I also may try opening the low speed carburetor screws to see if the engine can develop the power that it needs.

Thank you all again!

---------
Ignacio

 

[Quicksilver]

37 mph (60kph) with 375 lbs on board is not that bad with a 70 hp. We don't know whether the prop pitch is right or the motor is trimmed for max. speed. yet.

 

[Texasmark]

37 mph (60kph) with 375 lbs on board is not that bad with a 70 hp. We don't know whether the prop pitch is right or the motor is trimmed for max. speed. yet.

I certainly agree on this point. I had a 70 Johnson triple looper on a 15' Kingfisher pretty much flat bottom, lightweight, open fishing boat and I was tickled with 35 alone as I recall from the 1980s time line.

The impeller in the water pump could get to be a problem with it having been stored for long periods. Rather than remain pliable, so that the blades can expand and contract with the "Eccentric" design (to develop water pressure) of the water pump chamber, the rubber becomes hard and ceases to flex as the blades follow the contour of the pump housing.

This causes loss of pressure as the portion that was bent back the most in the compression area passes through the initial wide stage of the casting, the blades no longer expand with the expanding area and don't touch the walls accumulating water to compress and send to the engine block. In doing that folks report that pieces of the blade break off and wind up blocking water pump passages along with the lowered water pressure can cause the engine to overheat.

You do have a dilema in getting that problem resolved as you have indicated. So lets move on to the next best thing:

On Mercury engines that I have owned, mostly around the year 2000, the engines internal wiring harness used the tan colored wire as the over temp sensor wire. On these engines, next size larger than yours in block cubic inches, if you stand at the rear of the engine and look at the plate (water jacket cover) that encircles the spark plugs and covers the rear of the block, you will find a single (possibly tan) wire protruding from a small plate looking thing and a mounting screw.

This is the over temp sensor and on my engines running around 125-145*F thermostat opening temperature and 195F "Over Temp Warning" sensor activating [shorting out internally and applying ground (battery -) to that wire], the "over temp transducer" (horn) located either in the control box or strapped to the wiring harness adjacent to the control box, with 12volts applied to the other side of it from the ignition key being in any position but OFF, on either the purple wire or red with a purple stripe, activates.

So, if you got through that, find the wire embedded in the rear of the block. Follow it back to the electrical cable that connects the remote control to the engine connection. Somewhere between the two places you may find a junction or a Y connection where you can unplug a wire for access to the center conductor.

Assuming you found such, get a piece of scrap wire and connect one end to battery - or a shiny part of the engine block proper to get electrical conductivity (ground) and connect the other end of the wire to the available wiring harness connection that goes to the control box.

Go to the control box and turn the key from OFF to ON. You should hear the horn sound. If so, put everything back where it was your sensor is working. If not you won't know if and when your pump fails and your engine overheats which can cause severe damage.

While boating if you should hear the horn sound, you should shut off the engine immediately. If you are in a situation where you need the engine to get back to the dock, you will need to let the engine cool, allowing the OT sensor to open back up and "limp" back to the dock repeating the process. Don't run the engine with the horn blowing. Then you 'll have to figure out some course of action to get your water supply/pump back in working order.

Edit: Found my conversion chart: F-C approximations: 125F = 52C, 145F = 62C, 195F = 92C

 

[racerone]

You risk serious damage to this motor if impeller is old !-----You may not find an overheat warning sensor on this motor.----Just a few wrenches / sockets required to replace impeller.----Tool sets are not expensive these days.----Cheaper than 1 hour of shop time !

 

[Texasmark]

I have a LOT of experience with these motors, and yours is doing the classic Merc Triple bog. The reason you get away with it with less load is that it's on the verge of bogging.

Try this: Open the idle mixture screws 1/2 turn OUT (CCW). That might do it. If not remove an inch or so of pitch from your prop.

Jeff

I agree with this "triple bog" assessment. I'm not a person professionally educated in what I am about to say, but from my experiences: Back in the days of the OMC to name my familiarity, twin cylinder, large cubic inch, high torque running lower RPMs for the rated HP, Cross Flow engines, you could load them up and they would stay with you. Example was a 16' wooden boat, good beam, 2 good sized guys, a couple of 6 gallon portable fuel tanks, battery, gear including big cooler chest and live shrimp (in a water filled live well)....all talking about weight in the boat.....with a stock 25 hp Johnyrude and stock factory supplied prop, would get that rig up on plane and move out with it. Instruments were sparse at the time usually, just a speedometer if you had that so I have no idea as to specific operating conditions, just that they would do it.

When they upped the HP to 35, the engines just did it better.....butttt it did take fuel to do that and those engines loved their gas. BTDT This was before the days of the BIA and no such thing as an oil rating of Two Cycle - Watercraft 3rd iteration rating for your pre-mix oil What you had was....25:1 regular 4 cycle engine oil you put in your car's engine.....a quart per 6 gallons, not only a pint of use rated premium oil like TC-W3 of today....I liked Amalie brand Penn. oil....remebering their motto: Amalie, better than it has to be! Used it in my car and boat.

Anyway, along comes OMC with their loop charged triple in 1968-69 timeline...Evinrude named it the Triumph at 55 hp...triple because the Looping used reflected exhaust pulses from one cylinder to make the flat topped piston keep the fresh fuel charge in the cylinder until the piston moved past the exhaust ports to close them off.....since there was no dome on the top of the piston to do that like in old school cross-flow designs besides the hole in the piston and other changes the Looping process made in the fuel induction paths.

Well "I find" that as a result of the fuel efficiency you obtain with the Loop Design, you sacrifice low end torque and have to make up for it somehow. On heavy transom area Bass Boats, on their 150 HP V6 for one, they upped the cubic inches from 2000 cc to 2500 cc to get it.

Another way was to use a ported prop like the Mercury Laser of which I had one on my BB. The ports allow a prop pitched to run the top end (max MPH) to provide a fast "hole shot" by allowing exhaust gasses to dilute the water into which the blade has to turn allowing the RPMs to come up fast. Since HP (that twists the prop shaft allowing the prop to develop the thrust to push the boat) = (Torque x RPM)/5252, RPMs replace the lack of torque in the equation to get the shot.

So, when you overload these engines they crater and maybe it's because the exhaust reflection doesn't get back soon enough to the cylinder that is loading up to fire next so there is an inadequate fuel charge (it leaks out) resulting in an inadequate contribution to engine torque and escalating until the engine just caves in. Solution is usually back off propeller pitch until the timing is adequate and the engine can maintain its performance and accelerate. Just my guess.

 

[faztbullet]

You need to clean high speed jets as this will also cause bogg as carbs try to transition from idle to mid-high. Engine setting long periods will allow fuel in carb to evaporate and leave behind oil which causes gum in carb.