70 hp 3-cyl. no top end power

[nagel88]

I’m trying to troubleshoot my ‘77 7-hp 3-cyl Merc. My gut feeling is that I’m just not firing on all cylinders. For background, this motor is new to me and runs fine on muffs. The biggest issue is a lack of power and stalling if I try to push it beyond 1800 rpm. I’ve gone through the fuel system from tank to carbs and had my passenger pump the primer bulb when it started to falter - no change.
On one trial on the water I turned back toward the dock because it ran so badly, and suddenly it came to life and took off like a rocket. I thought it was simply some fouled plugs, but next trip out it ran poorly again even with fresh plugs.
It’s not much better at idle. If I get the speed down below 1000 rpm it will usually stall when going into gear unless I can ram the stick forward fast enough to kick it into gear and crank open the throttles in an instant. Scary at a busy dock!
I’ve gone through the tests on the ignition components per the Seloc manual. The stator readings seemed “irregular” (too little resistance) and the meter danced around when I jiggled the harness. When I pulled the flywheel and got the stator on the bench, the readings were right on the money. I want to be fairly certain before commiting to buying a new stator. I’ve seen figures for a voltage test on the stator, but despite diligent searching, I’ve not found the procedure to do this test, (which wires? motor cranking or running?) I’ve already thrown a lot of money at this boat. Can anyone help?

 

[05GlastronSX]

Re: 70 hp 3-cyl. no top end power

i think your assumption is correct....I am not good with wiring so i cant help you there but i believe that there is a problem with spark.

 

[sport15]

Re: 70 hp 3-cyl. no top end power

If jiggling the harness on the engine causes flaky meter readings while one the engine, look at the connections on the motor and the condition of the harness, check for rotten wire, corroded conncetions etc...

 

[nagel88]

Re: 70 hp 3-cyl. no top end power

Thanks for the replies. I've looked over all the connections as I made my tests. All I can think to do now is split the protective sheath on the stator harness and look at the wiring inside. From other posts I've learned that it's common to find crumbling insulation on wires. If I find nothing, I may want to go ahead and spring for a new stator.
I've also seen posts about bad rectifiers causing problems. Any opinions on that?

 

[deejaycee_2000]

Re: 70 hp 3-cyl. no top end power

Have you done a compression test?

 

[nagel88]

Re: 70 hp 3-cyl. no top end power

I hope this update catches someone's attention. I finally threw in the towel and took my boat to a local shop. I was assured the problem was fuel and I agreed to a thorough carb rebuild (at a fairly hefty price). When I picked it up I was told it ran at full power on the "dyno."
Well, I took it out the next morning. The first thing I noticed was my tach no longer worked. It seemed to idle well enough and I took it out and began easing the throttle forward. Like before it sped up a little and then began to bog down and finally would stall if I pushed harder. In other words, it runs exactly like it did before. It won't run above 1600-1800 RPM.
It goes without saying I'm taking the boat back to the shop. I don't know what his dyno is, other than a tank of water, but he's clearly not putting the motor under a load and seeing the problem. Assuming he has done a thorough job on the carbs, and hasn't just scammed me, where do I go from here?
My suspicion was leaning toward ignition. I hesitate to tell the guy how to do his job. After all, if I was so smart I would've fixed it myself, right? Any suggestions on how to resolve this would be greatly appreciated. I'd really like to get this boat going while it's still summer!

 

[jaymasta]

Re: 70 hp 3-cyl. no top end power

I hope this update catches someone's attention. I finally threw in the towel and took my boat to a local shop. I was assured the problem was fuel and I agreed to a thorough carb rebuild (at a fairly hefty price). When I picked it up I was told it ran at full power on the "dyno."
Well, I took it out the next morning. The first thing I noticed was my tach no longer worked. It seemed to idle well enough and I took it out and began easing the throttle forward. Like before it sped up a little and then began to bog down and finally would stall if I pushed harder. In other words, it runs exactly like it did before. It won't run above 1600-1800 RPM.
It goes without saying I'm taking the boat back to the shop. I don't know what his dyno is, other than a tank of water, but he's clearly not putting the motor under a load and seeing the problem. Assuming he has done a thorough job on the carbs, and hasn't just scammed me, where do I go from here?
My suspicion was leaning toward ignition. I hesitate to tell the guy how to do his job. After all, if I was so smart I would've fixed it myself, right? Any suggestions on how to resolve this would be greatly appreciated. I'd really like to get this boat going while it's still summer!

compression test for sure....maybe you have a dead cylinder or something, have you checked the resistance of each wire, and also checked with a spark tester to see if each one is sparking strong?

 

[Kevin W]

Re: 70 hp 3-cyl. no top end power

Check for water in the gas.
i could be wrong but its easy fix.
The boat shop may have ben using there gas.
once you introduced your gas again you may have contaminated gas.
my 70hp had a very simmilar problem.
i believe the main jets pull fuel from the very bottom of the carb and the idle circuit pulls from a bit higher i think the water was getting pulled in at around 2k rpm.

Take the covers off and remove the main jet.(the lower brass bolt.
Catch all the gas that comes out in a clear cup or white cup.
look at the bottom of the cup for water.
mine had a small amount there.
To fix it i drained both carbs, gas line, pump and tank.
added fresh mixed gas with seafoam additive.
i also added a water separating filter inline before the engine.
i have not had a hiccup since.

 

[nagel88]

Re: 70 hp 3-cyl. no top end power

Well, the boat is back at the shop. I just received a call this morning and now I'm told I'm losing #2 cylinder because of a bad switchbox. It's supposed to be ready this afternoon, so I'll see whether the guy knows his stuff or is just gambling with my money.
As far as gas goes, the carbs have been drained a number of times, although not since I got it back from the shop, and I've never seen evidence of water. That is a good tip, though. I've run into that many times on my lawnmower after it sat all winter. Thanks.

 

[nagel88]

Re: 70 hp 3-cyl. no top end power

After reading posts, I want to add a thought and see if anyone can comment on my reasoning. The main problem has always been the motor will die if I try to push past about 1600 RPM.
If I'm only dropping one cylinder as the mechanic says, wouldn't the motor simply start missing and run rough rather that completely die? And if the problem is in the ignition, would not the high-speed winding of the stator be the more likely problem, rather than the switchbox, considering the symptom?

 

[Kevin W]

Re: 70 hp 3-cyl. no top end power

I am not a marine mechanic but i can't picture a switchbox working at low RPM's and dropping at sligtly higher

 

[nagel88]

Re: 70 hp 3-cyl. no top end power

I got another call from the mechanic. He says the switchbox fixed the problem and the motor runs fine on the "dyno" (tank of water). He also tells me that my tach quit working because the rectifier was bad. After replacing that, he says it still doesn't work because the stator is bad.
I can think of a lot of things to say right now, none of which are fit for polite company.
I'm going to order a stator and put it on myself, which may be something I should have done in the first place.
If I pick up the boat with the new switchbox, and it still doesn't run above 1700 RPM, I'd say that pretty much pins the problem on the stator.
The thing I don't know is how I get any satisfaction from the shop that took large wads of my money and never fixed the original problem.
Is there anyplace on this forum where members can report on shops where they've had either good or bad service?

 

[srussell28]

Re: 70 hp 3-cyl. no top end power

Please let me know the outcome, I've been having the same issue on a 85 50 hp mariner. I have replaced the switchbox, stator, rectifier, silonod, plugs, wires. Ohmed out the stator, trigger both are fine. New gas tanks, bulb, rebuilt carbs twice. I'm stuck. thanks

 

[Kevin W]

Re: 70 hp 3-cyl. no top end power

another issue that may be there is E10 gas.
10% Ethanol based gas is new this season in my area and in alot of other areas.
engines new to E10 gas can have trouble with cleansing.
Ethanol cleans out your fuel system into your carb.
i had cleaned my carbs Very Very clean 3 times b4 all the varnish from somewhere stopped cloging up my float.
When the float clogged the carb overflowed and choked the engine dead in about 30 seconds or less at half throttle up.
It did restart and run at low rpm.
I am guessing the increased fuel pressure dumped gas in there quickly.

Like i mentioned B4 I am not a marine mechanic but i am not new to engines and repair of them.

 

[nagel88]

Re: 70 hp 3-cyl. no top end power

Well! I'm happy to report the mechanic actually knew what he was doing (or else he was a hell of a good guesser). He put on a new switchbox and the problem is solved! I took the boat out right away and it took throttle and got up some good speed. Yesterday I had it out for about 5 full hours and it never missed a beat. d
I still need a stator as the charging part of mine is now dead, and it will neither charge the battery nor give me a reading on the tach. I have one in the mail and will put it on myself.
Once I have the tach working I can have a better idea of how to get the boat dialed in as I'm a it wary or over-revving it.
I hope my ordeal will be helpful to anyone else out there with mysterious motor ailments. And thank you all for the replies and suggestions.

 

[Kevin W]

Re: 70 hp 3-cyl. no top end power

I am glad that i was wrong.
unfortunately sometimes mechanics have to guess on some things.
the symtoms are so close to others problems that he may not had a choice.
Good luck and hopefully all your problems are over now.

 

[barry soxman]

Re: 70 hp 3-cyl. no top end power

[ I am having a problem with my 1978 70 hp merc that sounds similar to yours. I bought it as is for a 16.5' sylan fiberglass bowrider. I first had no power and very poor runability and I was afraid it was just too small for the boat. After some investigation I found my fuel pump was bad. After rebuilding the fuel pump, It appeared to run good but the boat would not plane -it just plowed water at ~12mph. I susspected the prop and changed from a 14p to 12p --better but with 3 adults it still woundn't plane-just plow to ~14mph. With just my son and I on board it will acually plane and go ~26mph. I do not have a tac but the motor doesn't sound like it out of range for rpms. After some checking I found a formula for predicting boat speed and with the weight estimates I had it suggested a speed of ~31-32mph. I worked the numbers around and the formula said a 48hp motor would give me the speed I was getting. Thats when I read about your problem and looked over my motor a little better. I fould the orange wire removed from the switch box and connected with the blue wire. The post for the orange wire appears to be dead. I bought a repair manual on CD from E-bay but it is worthless. I t has no wire diagrams-none! If you or anyone could spare a copy of the wiring diagram so I can find were the orange wire goes and if it being paired up with the blue wire would have anything to do with the motors lack of power I wiould be very thankful. It idles smooth and appears to run good when it can rev up. I have power tilt with 1 cylinder leaking slightly so although it wouldn't hold position completely I was able to move it to help with bringing the nose down.

My e-mail is bsox56@verizon.net

thanks,
Barry