1981 70 hp evinrude piston wear and bogging down

[stelis]

Hi,

I am running through the drill to determine what might be the cause of my 1981 70 hp evinrdue bogging down when throttling up. It eventualy gets going but there's no punching it.

Thus far have:

changed and gapped plugs
rebuilt carbs and fuel pump
replaced ignition coils
checked compression (120 on each cylinder)

A local mechinc said compression could check out fine but what might be happening is that the internal end of the piston could be worn so that it does not produce enough vacumm to draw gas in. He also said there could be a vacuum leak somewhere.

Any of this sound plausible? I haven't really seen these reasons come up as a culprit for the bogging down problem in other threads.

Thanks.

Steve

 

[cbcmarine]

Re: 1981 70 hp evinrude piston wear and bogging down

Hi Steve vacuum maybe, but at 120 I doubt it, these motors did go through power packs you may have a weak one but usually when they go they go

 

[emdsapmgr]

Re: 1981 70 hp evinrude piston wear and bogging down

Any chance you are overpropped? What pitch prop are you running? Does the engine get up to 5500 rpm's at top end? If the engine runs fine at high rpm's the ignition is probably ok.

 

[stelis]

Re: 1981 70 hp evinrude piston wear and bogging down

Actually, I don't have a tach on the motor yet. Just purchased the boat this spring...But I can tell we ran it pretty close to WOT with no problem. I am not sure of the prop pitch...

I took the carbs off again the other night, opened them up to see if there were any deposits in the jets athat I might have missed during the rebuild. I plan to put them back on tonight and see how things go again.

 

[jtexas]

Re: 1981 70 hp evinrude piston wear and bogging down

"...It eventualy gets going but there's no punching it..."

in my experience, the fact that you can nurse it up to WOT implies possible link & sync might need tweaking a little. Make sure your throttles are parallel, and that the spark advance begins *before* the throttles start to open. The cam needs to hit the roller right on the mark. Also the timer base moves smoothly to its stop.

And, check your idle speed. Guess you'll need a tach for that.

Those plastic fittings that hold the linkages in place on the carbs and spark advance rod -- see if any of those are cracked. You can get some funny symptoms if one of 'em breaks.

I don't know if a vacuum leak can cause those symptoms, but to check the reeds, hold an index card up close to the carb throats with the motor idling -- you'll get the occasional droplet of gas, but if that card gets wet, the reeds are bad.