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elmo
25-04-2003, 12:49 PM
Some advice needed.

I have recently purchased a second hand 3HP Johnson 2 stroke. The motor would not turn over at first so I disassembled it.

After removing the head I gave the cylinder a tap with a nylon hammer and you could turn the engine by hand.

I replaced the head gasket and proceeded to reassemble and test run the motor. All went well, the motor started within the first few pulls and water was spitting at the back of the skeg indication that the pump was working.

I then decided to test run the motor on the boat and after approx. 2 hours of sporadic trolling (freshwater), I couldn’t have been happier.

The problem,

We beached the boat for some lunch and after about 30 mins we decided to get back to some trolling.

The motor started first pull (as per every other start that day) and we motored for about 5 mins. The motor lost revs and we decided to beach it again. There was no sign of rope etc on the propeller, so we decided to run it again and after a dozen or so attempts the motor started and all was well again for approximately ten minutes then the motor stopped dead.

The motor would not turn over and we used the electric to get back to shore.

We proceeded to pack the boat on the trailer and upon removal of the motor from the boat (probably 5 minutes after stopping), attempted to pull start and the motor turned over.

It would appear that the problem is heat related.

Can someone shed some light as to what is happening and what will prevent it from re occurring.

Some of my thoughts are

- The petrol is unleaded with a mixture of 50:1 oil. Maybe the higher octane unleaded and the lack of lead (motor was built before unleaded was even thought of) requires and additive or a higher concentration of oil.
- The water pump has stopped working, will check to confirm.
- There is a blockage in the water jacket around the casing. The small amount of mineral deposit was cleaned scrapped during the first dismantling.

If anyone can advise of similar encounters or a fix, it would be greatly appreciated.

Heath
25-04-2003, 04:26 PM
Not trying to be a Mechanic, but I have seen the internals of a boat engine :'( :'( :'(

When you say it won't turn over, i assume you mean it is stuck. Somthing is stopping it. Something in the piston chamber? Failing or seized bearing? I would be pulling it apart & taking the crank out & checking the block. If you find nothing then your only up for a gasket kit, or you might find the problem. ;)

elmo
25-04-2003, 05:09 PM
I think that it is all related to heat and expansion. So, yes, a bearing warped casing, piston, lack of cooling etc. is the cause.

What I'm trying to ascertain is which piece is causing the problem and why it ran okay for a couple of hours than gave up the ghost and when it cooled down appeared to be fine. (yet to be verified)

clutter
25-04-2003, 07:07 PM
I'd be pulling the piston out and having an inspection of it and the bore, particularly since the motor would not turn over when you first bought it. Not knowing the history of it, it could have been run lean previously and picked up in the bore and now doing the same when it gets hot. I'd be inspecting the water pump as well and at least replacing the impeller.

Cheers, Clutter.

elmo
26-04-2003, 02:03 AM
Thanks Clutter, that makes sense.

I'll let you know what happens.

blaze
26-04-2003, 02:22 AM
Hi elmo
Nearly sure to be the piston, running hot can be caused by running lean and also the water pump. the orignal siezure may have been caused by just left sitting to long and a piston ring may have rusted to the bore (also probable not free to move in the ring groove on the piston). remove cycinder, check piston, replace rings, clean piston with a 600 grade wet and dry, check ring grove and clean (pay attention to the locating pin for the ring when reassembling), ring grove in piston may be cleaned of carbon build up by snapping an old ring and running around the grove), hone the bore with a cycinder hone taking care not to damage the hone on the ports in the cycinder. Take perticular note of the condition of the cycinder bore for deep grooves from damaged rings (the old motor you have will tolerate a bit of damage)
Last of all reassemble and check compression ( should be somewhere around 120psi) Now I gota go and get ready for a weekend fishing charter
If I can be of any further assisance pm me
cheers
blaze

jaybee
26-04-2003, 08:42 AM
my bro had an old 33 johno many years ago, we would get half way across the bay it would stop, once cooled would go again, off to tempest we went, every time it go hot it stopped. After a couple of months of this he took it to an out board mechanic, turned out the head had the slightest crack in it, let water in which stopped the motor, while it was cooling the water evaporated.

Spaniard_King
27-04-2003, 08:39 AM
elmo,

one major thing people over look when looking for trouble when heat related is cylinder head warpage.

If the head is warped more likely than not exhaust gas will pass into the cooling cavities and gradually overheat the engine causing piston siezures. After a while/time to cool they will free up and seem to be ok. Once you seize a piston there is a good chance you will need to replace the rings and possibly the piston. On a bad seizure the bore will be scored and may need boring out to oversize and then new oversize pistons and rings need to be fitted.

To be realistic if you have had a siezure you need someone to determine what needs to be done, do you know anyone who has rebuilt a small engine before. As this problem could be expensive, check out the wreckers for parts, pistons/ rings and even gasket sets.

elmo
29-04-2003, 08:45 AM
Thanks for the replies.

For everyone's information, I ran the motor on Friday and Saturday without incident. Though I am still skeptical.

Parts are still available and the head gasket was reasonably priced so, I will follow the advice of the forum and have the motor checked, new rings fitted etc.

Thanks.

Elmo.