1978 Mercury 700 70hp outboard issue-little power in forward gear

[bashunov456]

Hey guys, I am looking for help with my Mercury Outboard. I picked up a 15.5 footer boat with this outboard on it. Took her out on the water the day I bought it and everything was working flawlessly until we shut off the engine near a dock. When we tried to start it up again it was not having it, just cranked and cranked but did not run(my initial guess is that it was flooded). We tried it again and again with breaks but couldn't get it to start running and walked back to the cars. We drove to the boat about 30 min later and it fired right up and we drove it back to the launch to pick it up. At that time is when I noticed that almost full throttle was just barely above crawl speed. I did not mess with it much at the time since we only needed it to go slow since it was dark and I was not risking anything. When I got it back home I ran it with muffs and notices nothing coming out of the pisser tube so I pulled the lower unit and replaced impeller/water pump gaskets and put it back up(the impeller was totally shot- the boat was sitting for over a year before I bought it). At first I accidentally had the gearbox in neutral but the boat in reverse so I pulled it back down and lined them both up to neutral and put back and hit the water.

When I put her in reverse the whole engine was moving up and down and going crazy so I had to give it barely any throttle in reverse. The boat ran and started good every time we tried but for whatever reason full throttle in forward is just above crawl speed (guessing about 10mph) and will not go any faster. Also, even while hot I would have to use the idle assist to get not to stall when putting in forward than drop the idle assist once its moving. What could be wrong? I am pretty sure I lined the shift splines up correctly since the prop free spins in neutral so I am guessing the problem is not related to me pulling the lower unit but maybe something to do with what happened that day. Could something of over-heated while we were trying to crank it over on the dock the 1st day? I checked the throttle cables and they seem to be moving correctly. I did not test spark yet but I pulled the plugs and the bottom 2 (3 total) looks to be a little fouled since they have a little black oil on the ends of them.

Any feedback would be appreciated!

thanks

 

[schematic]

sounds like you lost spark to one cylinder.....Use the trouble-shooting procedures at CDI Electronics. (likely a switch box issue)

 

[sutor623]

First off, do you have any experience with outboards? Just curious.

That is NOT GOOD that you had a blown water pump. The issue here is that we don't know if you blew it while you were running hard, or got lucky enough that it happened just outside of the dock at an idle.

The very first thing that I would do is check your compression and make sure you didn't overheat your powerhead. After that, do a spark test and see if all cylinders are firing.

In regards to the going crazy in reverse, you may not have set the reverse lock pin in correctly while installing the lower unit(this is only for NON-power trim motors). If this happens, when you go to put the motor in reverse it will hop around like crazy and sometimes come completely out of the water. You will not notice an issue in forward. If you have power trim than there is another issue, possibly with the gearcase. Have you changed the lower unit oil? If not drain it out, look for water and metal chunks.

Can you give us a serial number? Let us know how compression and spark test go.

 

[bashunov456]

Hey guys, thanks for the replies. I did change the lower unit oil after I swapped the impeller. It was black and smelt really bad but no water seemed to be in there. On a side note there was greyish bubbly oil/water in the lower unit when I pulled it but none of this when I drained the oil, I think the seals are shot around the prop which I will have to replace over the winter since there is a minor leak coming from around the prop (seems to be black oil, not the new gear oil I put in there?).

This is my first boat, no experience with outboards, just working on cars and motorcycles. This is a 1978 Mercury 700 70hp-serial# 4977880. This unit does not have power trip so I think you're right about the reverse pin not being aligned correctly.

I rented a compression tool from autozone and did the test in neutral (not wide open throttle like it says to) and the engine was not warmed at all and got about 75 PSI in top 2 cylinders and 80/85psi in the bottom (this is a 3 cylinder). I have yet to do the spark test but I pulled the plugs and noticed a little amount of black oilish liquid on the bottom 2 spark plugs and top was totally dry.

Someone else mentioned that the carbs could be clogged which is causing my throttle issue, I was going to rebuild them over the winter but was going to try putting seafoam in a fresh tank of gas and seeing if that helps a little.

What do you think about my situation? I am willing to spend 500 or so to buy parts and go through the motor over the winter to get her up to par, I already ordered the service manual (manufacturer one, not Clymer). The engine sounds good and fires right up no problem...

 

[sutor623]

Compression is a bit low up top, but should still run better than it does, so I am not going to attribute your issue to compression. I would check spark just to rule it out. Get a spark tester and crank the engine over (with all spark plug leads off the plugs) and see if you get a nice blue spark on all cylinders. If you do, Id say it is a fuel issue.

Sea foam is great to clean up old fuel/varnish deposits, but honestly, if you rule out spark you may just want to do a fuel system overhaul for safe measure. Gunk in a high speed jet could roast one of your cylinders. The fact that you have to keep the revs up or it will stall, tells me that there is a fuel issue. I'd check/rebuild the fuel pump diaphrams and carbs as well as change the fuel lines, primer bulb, fuel filter and fuel/water separator.

These motors are great runners, and if mechanically sound, are well worth the time/effort to keep on the water. I had a 1967 650 and a 1986 1150, and these engines were real screamers. Keep the fuel system clean and they will really flyyyyyyyyyyyyyyyy.

 

[bashunov456]

Thanks for the reply. Took a second look at the boat this weekend and noticed the problem with reverse was what you said- the lock wasn't engaged (had to put it in forward to get it to lock). I also changed the spark plugs and saw spark on every cylinder (did not use a spark tester, just the old fashioned way). I am going rebuild the whole fuel system like you said over the off season since I can only take her out a couple more times this year anyways (I am from Connecticut). I already saw the carbs need rebuilding when I bought it.

Another issue I saw is there is literately water coming from the top spark plug so I will also have to replace the gasket in there also. Other than that she starts right up every time and shifts good.

Thanks for the info, I will keep you undated on how rebuilding the fuel system goes within the next month or two. Question-should I bother fogging the system if I am going to rebuild the carbs and fuel lines or should I get them rebuilt than run it with muffs to test than fog for the winter? I am planning to have it all rebuild by end of November or so. Or is this playing with fire?

 

[bashunov456]

also wanted to give an update. took her out this afternoon and she runs flawlessly! put in 2/3 of a can of seafoam into 4 gallons and it took it out on the lake, engine was real quiet and was back to normal in terms of the power, even idled really good! The auto choke doesnt seem to work so I think I will prewarm the motor before heading out. Kind of annoying having no power tilt at the launch, but hey thats just one more thing to break, right? lol.

 

[sutor623]

Hey bud. I am glad that the seafoam cleaned ya up a bit!! It is known to do that, at times. Sometimes it just doesn't seem to cut it.

Personally, I wouldn't bother fogging the engine until you get the carbs rebuilt. That way when you get em back on and fire her up she will burn clean. After that, go ahead and fog the motor.

Those reverse locks can wear out and be a pain. If you don't feel like fooling with it, just make sure the motor is locked (by yanking on her) just before launching. Like you have discovered, sometimes moving the gear shift lever into forward and back into reverse can lock her in.

Can't remember which choke system is on this motor. I think there is a magnetic choke solenoid that pulls a cable and when tight, it will close the chokes on all three cylinders. What you can do to check it is take a small battery and run 12v directly to the choke solenoid (+ and -) and see if it engages. If it does (and I imagine it will) you will need to replace the choke switch. The switch is either a button on the control box or push in with the key. (I think yours is the latter.)

BEWARE!!!! Those older control boxes can be finnicky, and hard to put back together. The electrical connections on the switch and very close together and can be hard to solder up without shorting. Still, worth the effort to fix the choke. If you dont feel like doing that, you run a new push button switch to the solenoid. Last option would be to get a lawn mower style choke pull and install it on the lower motor housing. Then you could just pull the choke, walk to console to start her up, and go back to the motor to open the choke. Having an operating choke on these motors is almost a necessity

The only thing that scares me here is the fact that you are getting water from your spark plug hole. If you pull the plug do you see water in the cylinder? This needs to be figured out NOW before you put any time/money into this engine.

 

[johanw]

Hi bashunov456, sorry to hijack your thread, but where did you order the manufacturers manual? I have only seen the third party manuals thats all over the web... Got a 1979 70 hp Mariner that needs fixing...

 

[bashunov456]

Hi bashunov456, sorry to hijack your thread, but where did you order the manufacturers manual? I have only seen the third party manuals thats all over the web... Got a 1979 70 hp Mariner that needs fixing...

I bought one on Amazon but sutor623 actually has a link to it for free to read online:
Free Online Outboard Service Manual
http://www.boatinfo.no/lib/library.html

Seems to have just about every manual on there