1977 70HP evinrude running on 2 cyl

[Jwhit612]

I have a '77 Evinrude 70HP. It started showing symptoms last summer where it would on occasion run very sluggish. And by "sluggish" I mean when it was full throttle it felt like it was only half throttle. However, if I left it full throttle for a couple of minutes, it would often, very suddently, jump up and start running perfectly. the last run of last summer it would never come out of the sluggishness and I put it away for the winter. When I pulled it out this summer I changed the spark plugs and took it out but it did the same exact thing. It runs smoothly, it's just like its only running at half throttle. I'm thinking it feels like I'm only running on 2 cyl.
I'm thinking that it could be the coils or the powerpack maybe? I don't want to just start replacing parts without knowing for sure what the problem is.
any ideas?
Thanks!

 

[ctgmopar]

Re: 1977 70HP evinrude running on 2 cyl

Hi, I have the same engine & had the same issue. There are a few things to check. Do you have good compression on all cylinders, good spark on all, & fuel to all...in other words you don't have 1 carb that is sticking. In my case the coil was on the way out & would intermittantly spark & then all of a sudden boat would get up & go. Try the above....good luck. Chris

 

[jtexas]

Re: 1977 70HP evinrude running on 2 cyl

yup, sounds like a cylinder dropping out at high speed. Actually, it could be idling on two and, unless you're really tuned in to it you might not even notice.

FYI, for future reference, you don't want to keep running a motor like this--if one or more cylinders is starved for fuel, they are also not getting any lubrication.

Compression test first. PSI should be about the same on all 3, within 5 or 7%. I'd expect around 110 to 120 or so, but anything upwards of 100 would be good.

Spark test: inline spark tester (about $5 at autozone), set the gap at 7/16", clip it to the block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it won't jump a half-inch gap in the open air without a spark plug, it won't fire in the hole under heat & pressure. That will rule out all the ignition components.

Report your results.

Check the fuel tank for water & debris, make sure the fuel is fresh, the fuel line holds pressure and the tank vent is open & clear.

If all of the above checks out, rebuild the carbs. There's a good how-to thread over in the Engine FAQ section of iboats.

I highly recommend the factory shop manual; it has paid for itself many times over.

 

[Challenger84]

Re: 1977 70HP evinrude running on 2 cyl

clean, clan, clean out the carbs!!!