Just bought first boat...Having problem with 1986 Mercury 70HP

Oh Wow...What happened here...I just got first boat..seller put muffs on and ran water through the 86 Mercury 70hp motor..sounded great, water pump seemed to be working good as well, shifted fine and everything seemed to be working great....He (seller) swore by the motor, how he never had proplems with it and it runs great.

Put it in the lake, started right up, Idles nice, no smoke...

Bad thing...only goes about 5mph at full throttle...if I go WOT it will stall...also shifting to reverse it will stall.

Please help newbie....did I just buy a lemon?

Im also having trouble finding a manual for this motor..any suggestions?

Re: Just bought first boat...Having problem with 1986 Mercury 70HP

it would be best to light the sellers garage on fire youre in for costly repairs. the whole wiring is frayed on these things. some are worth fixing some not

Re: Just bought first boat...Having problem with 1986 Mercury 70HP

check to see if choke is on also have you tried to speak with the previous owner and see if he is willing to help you out on your issue? i would also try cleaning the carbs.

Re: Just bought first boat...Having problem with 1986 Mercury 70HP

You have learned a valuable lesson: how it runs in the driveway on muffs is not a good predictor of real-world performance. (In a barrel in the driveway is a slightly better predictor, but still not great).

Two questions: Did you put in fresh fuel? How high did your WOT rpm's go (even if you did not exceed 5mph)?

The seller did not necessarily cheat you, he just may not be too good at fixing stuff.
Eh, on second thought, he likely did know something was up and just didn't feel like dealing with it. :redface:

Either way, he's unlikely to be of much help, unfortunately. Assuming the seller has been truthful thus far, the only useful info I could see him providing is, "how long was it stored since the last time it never had a problem and ran great?" If that was more than a couple months ago, then hey, maybe it did run good then and now just needs a carb cleaning.

RaScLeS and I have similar motors and have gone through similar frustrations with them. Could be: dirty carbs, bad fuel pump diapraghm, bad fuel line, corroded wiring, bad ignition part, in need of a timing adjustment, etc., etc., etc. But the old saw goes: "if it has compression, everything else is just a matter of adjustment."

So, I say do a compression test and let us know what the results are. If compression is OK, we can probably get you motoring.

Re: Just bought first boat...Having problem with 1986 Mercury 70HP

its been a couple years these motors are 100% bricks of the century I shouldnt need to know my engine inside and out to be an owner LOL my johnson just did its job,

but anyways since this is the black motor forum id like to see it repaired my compression is only 120's and when the motor decides to work it really moves!!! but almost 1000$ later in parts it still wont go its junk and has had every issue in the book (im sure fuel delivery is still messed up AND maybe even a harness issue!) ugh

Re: Just bought first boat...Having problem with 1986 Mercury 70HP

im having the sam prob but i figer out that im pnly running one 2 cylinders and still havent figer it out

Re: Just bought first boat...Having problem with 1986 Mercury 70HP

I would start with a commpression test first. If all is well there then I would rebuild carbs. I had a problem with my 75hp mercury where it would idle great but had a lack of power above idle, rebuilt carbs and motor runs great now. turns out high speed jets were dirty. Hope this helps.