1984 70 HP Evinrude - Top End Power

[Tiger2255]

I finally had the opportunity to put my 1984 70 HP Evinrude in the water this weekend after my rebuild and water pump change. It fired right up and I let it idle for 1 hour with no problems on the "double-oil" recommendation for break-in.

During the test drive, the motor had just a small problem with low idle but I attribute that to the "double-oil" break-in mix. When I throttled it up, it seemed to lack top end power with no RPM or power difference between 1/2 and full throttle. I pulled the cover off to ensure the linkage was engaging a full range and it was; however, poor power and the boat would not plane out.

I did not experience any sputtering or bogging down of the engine. The glass fuel filter did appear to have minimal gas in it, so I tried to pump the bulb which did not produce any change in rpm's or performance. I did notice a consistency in bubbles in the filter though.

Before I jump into fuel pump rebuilding kits or carburator rebuilding kits, I would really appreciate some expert advice on which way to go on this father-son project.

Even though we couldn't plane out, the kids still had fun plowing through the water at 1/2 rpm.

 

[wlbolerdo]

Re: 1984 70 HP Evinrude - Top End Power

I recently rebuilt my powerhead on my '92 70 horse. I was told to break it in like it was new, running 50:1 mix and never running at the same RPM for long periods of time. I adjusted my Rpm's constantly through two tanks of 50:1 fuel. A mechanic said 10 hours, I may have done 20. Sitting at idle for an hour is not good and certainly did not break that motor in properly.
Carburators, all three, were soaked and rebuilt with new kits including floats.
I also replaced the reed valves though that may not need to happen in your case.
A properly done sync & link and having a manuel.
But most of all, a wealth of knowledge from this forum, alot of individuals who know and the FAQ section that explains how to do the above.
If I'm wrong I'm sure that I be corrected but it's all part of learning.
Wayne

 

[Tiger2255]

Re: 1984 70 HP Evinrude - Top End Power

Well I decided to move forward with the carburetor rebuild kits and the fuel pump rebuild kit on this project. Hopefully, this will assist in increasing the performance at top end. Any additional comments/recommendations would be helpful.

I forgot to mention the rebuild consisted of boring the block .030 and installing new Sierra .030 pistons/rings and bearings. Do I have to adjust the carburetor's since I now have move volume with slightly larger pistons?

Again, any assistance would be helpful.

Thanks...

 

[Boatdood]

Re: 1984 70 HP Evinrude - Top End Power

Tiger I sure do like father/son projects and wish you the best on this. Just curious, but how old is the son? It reminds me of the time when my dad and I built an 8' plywood boat. We had a blast with that little boat. Back to your problem though.

It sure does sound like your motor is running on two cylinders to me. Of course it could be either fire or fuel at this point assuming your compression is good with the recent rebuild. I see you're going to do the carbs anyway and that's a good idea. Make sure the high speed orifices are open before you put it back together. With the bowl off the carb body it's easy to see through the orifice by looking into the drain hole at the bottom of the bowl. You'll see the brass fitting sitting an inch or so inside that threaded hole the drain plug screws into. It takes a special tool to remove the orifice so be careful you don't bugger it up trying to take it out. If it is clean and open there's no real need to remove it though. Hopefully the problem is in the carbs and a good clean and rebuild will get things moving for you.

Based on the fact that your engine seemed to idle OK before you throttled up I'm betting your fire is OK. A clogged high speed orifice in one of the carbs will still usually allow the engine to run on the idle circuit at low speed.

Good luck with it. That's a sweet engine that is usually easy enough to trouble shoot when there is a problem.