70 merc outboard(HELP)

[Barry&Lori]

I own a 1981 3 cylinder merc out board. we took the boat out and it took off like a rabbit throttled down for a bit and when I went to give full thottle it bogged down then stalled the fuel is fresh and the filter is clean and tank is vented. does anyone have any idea as to what the problem may be. it seems to fun good but under load it craps out when full thottle is aplied.. thanks Barry

 

[catfish]

Re: 70 merc outboard(HELP)

check the plugs first.if they are fowled clean them with some steelwool.if that does not fix it check the power pack for a loose wire i worked on my merc 70 for 2 weeks and thats all it was.

 

[halibut]

Re: 70 merc outboard(HELP)

I have a 1979 70 HP, 3 cyl. that is having the same trouble your engine is. It will run perfect for about 15-20 minutes at all speeds. After 15 minutes or so it will only make 1600-1800 RPM. I have taken it to a local Merc dealer 3 times so far. <br />1. First they thought it was the gas tank cap not venting properly, but problem remained with no cap on tank. (I also replaced the hose/primer bulb assy)<br />2. Next they checked the fuel pump and said it was OK and left original installed. Still problem remained. After this they decided it was either electrical or carbs needing rebuild. <br />3. Carb's were rebuilt with no change, so I guess I'm to the electrical problem? <br />My engine will run fine at idle up to about 1/4-1/3 engine speed after that it boggs down and if I try to go WOT it acts like it will stall until I pull back slightly to about 1600-1800 rpm where it will run fine. At slow speeds all plugs appear to be firing OK, but I haven't been able to check while U/W with problem occuring. I'm not sure if the problem is showing up after the engine is at operating temperature or if it is a "under load" problem. <br />I was advised on another bulletin board to check the stator resistance and also fuel pump diaphram for holes. Any other suggestions would be appreciated. Thanks in advance!!

 

[petryshyn]

Re: 70 merc outboard(HELP)

I know this post is kind of dated, but if you guys are still around.....<br /><br />- Ignition packs on these are notorious for intermittent misfires<br /><br />- Had nothing but fowling plug problems until i went back to champion OEM plugs<br /><br />Can one of you guys do me a favor? Have a look and see where the bleeder lines go on that engine. They are a small looking hose about 3/16" in diameter. I know there is one going from top of engine to bottom,(just behind the carbs) but where does the middle one go?<br />Thanx

 

[dick]

Re: 70 merc outboard(HELP)

If it runs good up to 1800-2000 while in the water and then craps out above that theres a good chance it could be the stator. Mercs use a two stage stator where after about 1800-2000 rpm the low speed windings provide the power for the ign. system , after that the high speed windings take over.You can do a ohms reading on the stator but the best way is to use a DVA(direct voltage reading meter)If you you don't have one , most people don't, take it to a dealer and get them to check it . DON"T throw parts at it ,Merc stators and switch boxes are not cheap.

 

[dick]

Re: 70 merc outboard(HELP)

Whops , I should of said under 1800-2000 rpm the low speed windings provide the power for the ign. system.<br /> Sorry.