mid 80's 70hp running rough, help w/ diagnosis

[zio228]

This is my first outboard, just bought the boat a month ago. Took a little while to get it running, as it had been sitting for about 6 months. Installed new plugs, and after a squirt of starting fluid it fired... took a few tried but it finally started. Ran rough, but slowly "cleaned up" after warming up & revving it out for a while. If it sits for more than a week, it's hard to start & usually takes a squirt of starting fluid. But again, after it's warmed up it's fine.

Took it out for the first time yesterday on a very small little lake, 5mph speed limit. it ran fine for the first 20 minutes at the low-rpm speed. Shifted to neutral while we rigged up a pole to troll, and it started bogging. I revved it, it hesitated, but slowly picked up. Figured we'd better head back to the dock, & it took some feathering but we made it back fine. At the dock, I took the cover off & revved it... it was bogging & sputtering badly. It died a few times at about 1000 rpm above idle, and if I revved it high & chopped the throttle it died immediately. It would only go back to idle if I slowly brought the RPM's up & back down slowly.

I have a few things to diagnose, but I'm thinking it's a fuel delivery problem and should be fixed by a thorough cleaning of the carbs. I'll replace the fuel lines & filter as well as drain the fuel & put fresh in (last owner said the fuel was 6 months old, but he added stabilizer to it).

Also, when I swapped the plugs, the pistons had a lot of carbon build-up. Would the piston/rings have anything to do with my problem? I am very familiar w/ single cylinder 2-stroke engines... do all my own wrenching on my dirtbikes. All the same principles apply, right? Worn piston/rings would affect power & maybe make for hard starting, but once running it should run relatively smooth right? Or at least if it's running fine one day it should run the same the next?

 

[Joe Reeves]

Re: mid 80's 70hp running rough, help w/ diagnosis

(Compression & Spark Test)
(J. Reeves)

1st - Regardless of what problem one might encounter, always start with the basic troubleshooting procedure. Check the compression which should be in the range of 100+ psi (somewhat less on the smaller hp engines) and even on all cylinders.

2nd - With the spark plugs removed, rig up some type of spark tester whereas you can set a gap to be jumped. On magneto ignition systems.... and also the Battery Capacitance Ignition system OMC engines from 1968 to 1972, set the gap to 1/4". On the solid state OMC ignition systems from 1973 to the present, set the gap to 7/16". The spark should jump the gap with a strong blue lightning like flame..... a real strong blue snap!

If there was no spark, on engines from 1973 up, disconnect the RED main electrical plug at the engine. Remove the s/plugs. crank the engine via the starter solenoid (jumper bat term to small 3/8" term nut, not the ground nut) and observe spark. If spark is now okay as stated above, the usual cause is a shorted ignition switch.

If the above checks out as it should, and the engine dies out when throttle is applied or won't idle, it is usually due to one or both of two things. The carburetors are fouled, in which case the cure would be to remove, clean, and rebuild them..... or the timer base under the flywheel is sticking which would result in a retarded spark. If the engine will not fire/start at all even though you have the proper spark and compression, it's usually due to fouled carburetors.

To check the timer base for sticking (Engine Not Running), spin the prop to align the shifter dog with forward gear and while doing so, put the engine into forward gear. Now, while watching the timer base, slowly apply throttle clear up to full throttle. The timer base should move smoothly all the way up against the black rubber cap that is atached to the end of the full advance spark setting screw. If the timer base sticks, find out why and correct that problem.

Usually any sticking of the timer base is caused by one of the four retaining clamps being slightly too tight. Putting a very thin washer under the clamp cures that problem. Some boaters use a thin screwdriver to bend the clamp upwards slightly but I prefer the thin washer. The sticking can also be caused by having a faulty stator start to melt down, resulting in a sticky substance dripping down on the timer base nylon retaining ring.

 

[tashasdaddy]

Re: mid 80's 70hp running rough, help w/ diagnosis

i'm very surprized Joe didn't lambbaste you for using starting fluid, to 2 cycle engines worst enemy. use premixed gas in a spray of squeeze bottle. thats the best way to burn a hole in piston.

 

[zio228]

Re: mid 80's 70hp running rough, help w/ diagnosis

i'm very surprized Joe didn't lambbaste you for using starting fluid, to 2 cycle engines worst enemy. use premixed gas in a spray of squeeze bottle. thats the best way to burn a hole in piston.

Desparate times call for desparate measures. I had the wife, kids and brother-in-law all watching... anxious to call my beauty a pile of junk. It gave one good cough the first try, then hours of nothing. It's the first time I've ever used starting fluid for anything other than a friends Olds Delta 88 beater!!!

 

[zio228]

Re: mid 80's 70hp running rough, help w/ diagnosis

(Compression & Spark Test)
(J. Reeves)


To check the timer base for sticking (Engine Not Running), spin the prop to align the shifter dog with forward gear and while doing so, put the engine into forward gear. Now, while watching the timer base, slowly apply throttle clear up to full throttle. The timer base should move smoothly all the way up against the black rubber cap that is atached to the end of the full advance spark setting screw. If the timer base sticks, find out why and correct that problem.

Usually any sticking of the timer base is caused by one of the four retaining clamps being slightly too tight. Putting a very thin washer under the clamp cures that problem. Some boaters use a thin screwdriver to bend the clamp upwards slightly but I prefer the thin washer. The sticking can also be caused by having a faulty stator start to melt down, resulting in a sticky substance dripping down on the timer base nylon retaining ring.

Thanks, Joe. I'll check it out. BTW - Gapped the plugs to same as the ones I took out (can't remember), and shorted one to the case - nice strong spark. Don't have a compression tester... I'll have to go buy one.

 

[Joe Reeves]

Re: mid 80's 70hp running rough, help w/ diagnosis

Check the price on this one.

Spark Tester - Home Made
(J. Reeves)

A spark tester can be made with a piece of 1x4 or 1x6, drive a couple nails through it, then bend the pointed ends at a right angle. You can then adjust the gap by simply twisting the nail(s). Solder a spark plug wire to one which you can connect to the spark plug boots, and a ground wire of some kind to the other to connect to the powerhead somewhere.

Using the above, one could easily build a spark tester whereas they could connect 2, 4, 6, or 8 cylinders all at one time. The ground nail being straight up, the others being bent, aimed at the ground nail. A typical 4 cylinder tester follows:


..........X1..........X2

.................X..(grd)

..........X3..........X4

 

[zio228]

Re: mid 80's 70hp running rough, help w/ diagnosis

Well that sure sounds better than the heart-stopper method I use!

 

[peacekeeper6]

Re: mid 80's 70hp running rough, help w/ diagnosis

that is ****ing genious, i know this is an old thread BTW.