70 hp 1975 Johnson - general problems

Warmmi

Recruit
Joined
Jul 9, 2007
Messages
4
Had a tune up done on a no problem motor - mechanic told us that the water pump was bad as it only blew water out of the bottom by the prop - got a new impeller put on by a different mechanic - and discovered that that is where the water is supposed to blow -

Engine seemed to run fine for a few hours - now it is lagging, vibrating and not getting up to speed - Noticed a really small oil drip under it in the driveway this morning - checked plugs and they are not fouled - not sure what to do next - don't want to take it back to the shop that had it - don't think they know what their doing and all the other places around here are either booked solid or don't want to work on my old motor.....
 

mikesea

Lieutenant Commander
Joined
Oct 1, 2006
Messages
1,830
Re: 70 hp 1975 Johnson - general problems

What did the guy do for a tune up,?if your impeller hasnt been changed in at least a yr.he likely thought it should have one since its a 1 a yr.maint.item.Many go longer,but not adviseable,as for the oil,what color,if its black and coming from center of prop OR behind the prop,its likely just carbon from running.Did the motor sit for a long period of time,your symptoms demonstrate dirty carbs.I will say that the mechanic should have given you a perfectly good running eng.after a tune up.Is it a reputable shop that does many outboards,your engine is very basic.The tune up should have consisted of a spark,and compression test to start.Does your boat have a water seperating filter?looks like an automobile oil filter,usually?you could have bad or watery gas
 

Warmmi

Recruit
Joined
Jul 9, 2007
Messages
4
Re: 70 hp 1975 Johnson - general problems

Oil was black and just a really small puddle - tune up was new plugs and not sure what else - the guy that did it is a real a** and had a serious problem trying to get him to tell me anything - he was more worried with the "bad water pump" then anything else -
Did replace the impeller though - and it ran like a top till yesterday - loggy and slow - vibrated more then usual also -
 

HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
Re: 70 hp 1975 Johnson - general problems

Your leak sounds like carbon build up, but find out where it is coming from for sure. Sounds like a carb issue, but start with a compression test and spark test to start, easy to do even for novice do it yourselfers. Compression testers and spark testers can be bought local hardware store for pretty cheap.

(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.
 
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