intermittent strange problem

[jeffntanya]

ok 1981 Johnson 70

when we went to buy the boat 4 weeks ago and test drove it there was a problem. you would get on the throttle wide open and it wouldn't go over 3000 rpms and there seemed to be fuel coming out from the intake area. the guy that owned the boat at the time said it had happened a few years ago and it was just dirt in the carburators. so he took it to the marina and had them clean the carbs and we went back later that day and it ran great. apparnetly they checked compression as well and it was good.


now about 6 or 7 times out of the 20 times we've had it out it's done this where it won't rev over 3500 rpm. there's no trace of fuel leaking this time. everytime it's done it is when we pull out of the marina and put it wide open. then we'd remove the cover to check things and all looks good. put the cover back on and start it up again it would be ok for the rest of the day. we could burn 3 tanks of fuel and never have the problem all day.

today it happened again and we stopped and took the cover off, checked the fuel filter, also remove the plugs (they looked black - we replaced them 2 weeks ago) put them back in and tried it and it's still only pulling up to about 3500rpm max. in neutral when you put the throttle up it shakes a lot and won't rev up very fast.

I'm thinking it's an intermittent ignition coil? would that make sense?

can we unplug one plug at a time when it's doing it to see if one of them doesn't change the performance?

is there an air filter on this engine? I've never taken the intake cover off.

anyone have any thing or ideas for us?

thanks

Jeff

 

[HighTrim]

Re: intermittent strange problem

maybe start with a spark and compression test, post ur numbers and we can go from there.

(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: intermittent strange problem

don't rev over 1500 in neutral, can cause a runaway, that can't be shut down till it runs out of gas, good way to throw a rod. do the things above, and post results.

 

[jtexas]

Re: intermittent strange problem

To answer your specific questions:
Can we unplug one plug at a time when it's doing it to see if one of them doesn't change the performance? yes.

Is there an air filter on this engine? no.

All three plugs look the same?
Black as in carbon deposits, or black as in a light coating of oil?

 

[zzzzz]

Re: intermittent strange problem

start diagnosis by isolating the motor from the boat...rig a 6 gallon portable fuel tank and run out the boat using it...if the problem happens then you know that the problem is on the motor and not a fuel system issue on the boat