1978 Mercury 700 Weird Problem

[AJ in NH]

HI, I am having a problem with my mercury 700 1978. When I go full trottle it looks like the motor try to stauled, it shakes a lot, I put it in neutral and back to full trottle and I can go another 1-2 minutes and it does it again. First of all I tought it was a bad adjustment of the carburators so I played with the screws....
I don't know if it's normal If I turn completely clockwise the top carb screw the motor stop, If I do the same in the lower it doesn't, Also I bought some carb cleaner, when I spray the top carb for more than 1 sec at a time it staul. On the lower carb I can spray as much as I want and it almost doesn't do anything. Any Idea of what can cause my problem?
Also maybe someone can tell me a rought estimate of how to readjust the carb screws.
Thank you so much for helping.

 

[WillyBWright]

Re: 1978 Mercury 700 Weird Problem

Avoid spraying carb cleaner in a 2-stroke. It won't clean anything important in the carb, but it will wash the oil off the internals. Screws should be about a turn out. 2 carbs, 3 cylinders. Go figure. I'm betting you have no spark on a cylinder, probably the bottom. It could also be water in the gas, the water would settle to the bottom carb. Or there's a restriction (varnish) in the lower carb.

 

[AJ in NH]

Re: 1978 Mercury 700 Weird Problem

Hi, Here is an update:
I checked the compression cyl #1 140
cyl #2 140 cyl #3 145
Will check spark tomorow

 

[AJ in NH]

Re: 1978 Mercury 700 Weird Problem

Ok ,
I checked the spark ay 7/16" all three are good.
Also in the water when my problem occured I tried to spray a mix of gaz and oil in the carbs and it just tried to staul even more, so I guess this is not my problem.
Also I tried to lift my motor a little bit to change the angle When I did that It would run very rough even if I put it in neutral and back in throttle. Looks to me that when I run full throttle and when the outboard reaches a certain angle that's when my problem occurs. any Idea????

 

[tashasdaddy]

Re: 1978 Mercury 700 Weird Problem

ok time to check timing, linc n sinc. if that doesn't crue it rebuild the carbs.

 

[casi77]

Re: 1978 Mercury 700 Weird Problem

i had the same kind of problem.. carbs were re-done and new floats installed.. now runs like a charm.

ok time to check timing, linc n sinc. if that doesn't crue it rebuild the carbs.

 

[Kevin W]

Re: 1978 Mercury 700 Weird Problem

Ethanol based gas is cleaning out your fuel system and depositing junk in the float valves.
i bet your carbs are overflowing.
clean your carbs and install a good fuel filter just b4 the carbs.

 

[Texasmark]

Re: 1978 Mercury 700 Weird Problem

As far as a spray to clean up things, Sea Foam's in a spray can and it's called Deep Creep. Has oil in it and is recommended for what you are doing. Really does a number on dissolving gum, varnish, and carbon.

My '02 90 Merc is static set to 1 1/4 open. My service manual shows diffferent settings for different engines from '97 up and the range is like 1 to 1 1/2 turns. As it turned out, my engine runs best open 1 1/2 turns. Stopped hesitating on the hole shot.

The needles have to be perfectly conical. If you have been "on them" and screwed them in too tight they may be deformed (dimpled ring around tip) and will need to be replaced before you can get an accurate adjustment.

If you are getting different results performing the same function to your carbs, it is telling you that they are not all contributing to your engine's operation. Your job to figure which are behaving accurately.

HTH

Mark

 

[AJ in NH]

Re: 1978 Mercury 700 Weird Problem

Ok Now I have to remove my carbs and rebuild them check the floats, needles, Link and synk etc...

Where is the best place to buy the parts for this outboard? s/n 4877345 70hp 700 Mercury thunderbolt

Thanks for the help!

 

[Texasmark]

Re: 1978 Mercury 700 Weird Problem

Beins I'm lazy, I'd do a good job of decarbing first and it may just clean up a sticking float valve and you don't have to disassemble anything. Then if that doesn't fix it you can tear it apart.

Decarb instructions are on the site....."search" for it.

Mark

 

[tashasdaddy]

Re: 1978 Mercury 700 Weird Problem

decarb is now in the engine faq section.

 

[AJ in NH]

Re: 1978 Mercury 700 Weird Problem

Hi, Just got done decarbing as per your procedure, engine runs a little better but the problem is still
there!@#$ So I guess I'll need the floats parts.
I found out that if I sit on the pass seat it works fine.
As soon as I sit back in the driver seat Problem comes back. I tought about other solutions
1- never go alone (always sit someone in pass side)
2- always sit on the pass side
3- put 4-50lbs sand bags on pass side
But I guess the best solution is to fix the floats......
Any Idea of where I can get the parts?
Thanks Guys I followed your advise and now I am almost there, I guess within a week my outboard will be running as new...or almost new....

 

[tashasdaddy]

Re: 1978 Mercury 700 Weird Problem

old mercs.com. you may have to call them as they don't list all they carry on the site.

 

[ricksrster]

Re: 1978 Mercury 700 Weird Problem

iboats.com has tachs at a good price.

 

[tashasdaddy]

Re: 1978 Mercury 700 Weird Problem

depending on the boat 25 -35, if aluminum maybe more.

 

[Texasmark]

Re: 1978 Mercury 700 Weird Problem

Don't be in too big a hurry to tear things apart. Give the Sea Foam time to soak some more. It just keeps getting better. BTDT.

Mark

 

[emckelvy]

Re: 1978 Mercury 700 Weird Problem

Check the fuel pump and make sure the diaphragm is not torn and allowing excess fuel to be sucked into the bottom cylinders.

This would flood them out as well as a leaking carb. If your priming bulb doesn't pump up hard this would be a good clue. Also, if the bottom carb is blowing fuel past the float needle bad enough to make it not want to run, you'd see all kinds of fuel coming out the carb when you pumped the primer bulb.

I'd investigate the other issues before tearing into the carbs.

Another thing you might want to do is put a timing lite on the plug wires while it's running, to make sure there's good fire with the plugs in.

HTH & let us know what you find...........ed

 

[AJ in NH]

Re: 1978 Mercury 700 Weird Problem

I am still trying to get carbs kits

Some tell me I need MERC # 892593
and other MERC # 812953

Anyone knows which one is the right carb kit for
1978 Mercury 700 70hp Thunderbolt Ser# 4877123

 

[tashasdaddy]

Re: 1978 Mercury 700 Weird Problem

i don't have parts manual. did you try oldmercs.com, also BAMmarine.com. mercpartsexpress.com like i said you may have to do some calling, everything is not on line. also laingsoutboards.com
just checked laings site. looks like they have what you need. email Jeffl5798@wzrd.com give him serial # and what you need. bet he hooks you right up.

 

[mckeemm]

Re: 1978 Mercury 700 Weird Problem

1-Now , I have to do a link and sync but my tach is not working anyone knows what kind of aftermarket tach I should buy for the 1978 mercury 700 70hp.
2- After I am done with all the adjustment etc, what would be a normal speed for a runabout 15' with the 70hp , I know it is kind of difficult to estimate but let's say I am alonr in the boat ...are we talking about 15,20,25 mph.
Thanks for the precious help

Hi,

Had my 1986 Sunray 15.5 foot runabout out on Sunday for the first time. Power is a 1979 merc 700, a friend clocked me at 32 mph, with two gas cans two adults and fishing gear aboard. No hesitation out of the hole and on step at cruise speed in 5-10 seconds.

Mark