1985 70 johnson runs great WOT only the first minute of the day

[lazeronu]

Hey all,

Boat was running fine for 2 years now. Ran into this issue on the lake today.

Went out, fired up no problem, went full throttle and was cruising. After 1 minute the motor sounded like it was dropping a cylinder. I throttled down and was able to maintain about half speed just fine. I nursed it back in to safety.

On the way back in, I noticed middle cylinder did not have spark. (pulled wire quickly to check to see if any rpm dropped on all cylinders). The cylinder would get spark after I let the motor sit for a little bit. I had new coils and spark plugs sitting around so changed those.

I launched the next morning and ZOOM full speed, thought i fixed the problem. 3 minutes later, same thing. Felt like it dropped a cylinder, it slowly idled itself down and would not be able to have power past a slow troll. I checked spark again and spark was there on all 3 cylinders. Possible intermittent spark problem?

It seems like when the boat gets warm, something happens. It will sputter a little and then just not go.

New impeller last year, tell tale was strong and not hot. I could touch motor without it burning me.

I ran the boat and squeezed the ball throughout, did not fix the problem. Changed the fuel pump cause i had a new one laying around.

Any and all help is greatly appreciated.

 

[sutor623]

Sounds like intermittent power pack failure to me. When faulty electrical components get hot, they fail. Eventually it will lose spark entirely on that cylinder.

id slap a pack on there if you don’t have a dva meter to check timer base. Only three things that could cause one cylinder to lose spark on that motor. Timer base, power pack, coil/wire.

 

[racerone]

Perhaps gaskets on the exhaust manifold are leaking and spraying water into one cylinder.-----Those exhaust plates warp with time.

 

[Bad Al]

After 1 minute the motor sounded like it was dropping a cylinder.

On the way back in, I noticed middle cylinder did not have spark. (pulled wire quickly to check to see if any rpm dropped on all cylinders). The cylinder would get spark after I let the motor sit for a little bit. I had new coils and spark plugs sitting around so changed those.

I launched the next morning and ZOOM full speed, thought i fixed the problem. 3 minutes later, same thing. Felt like it dropped a cylinder, it slowly idled itself down and would not be able to have power past a slow troll. I checked spark again and spark was there on all 3 cylinders. Possible intermittent spark problem?

You were not actually checking the spark... you were performing a drop test of the individual cylinders to see which cylinder wasn't functioning.

Check the compression! although you may have water entering via the exhaust manifold... If the #2 cylinder compression varies from #1 & #3, that may indicate a slight head gasket failure which would also allow water to enter the cylinder.

 

[lazeronu]

Ran compression -
120/120/115

the wire harness looked pretty shot. I took it apart and whoever had the boat before definitely jerry rigged it together. Wires were showing signs of wear and barely connected with wire connectors. I have a 1982 75 evinrude for parts so swapped out the Harness.

i was tempted to swap out the power pack as well but it looks like the 1985 power pack has an extra wire from the stator (3wire plug) compared to the 2 wire plug on the 1982. Is that 3rd wire necessary? It looks like a kill switch wire.

I checked the stator and timer base with a meter. If anyone has some better instructions I’ll do it again and post readings.

going to take the carbs apart again and do another thorough cleaning. They looked great the first time when I cleaned and rebuilt but want to make sure.

im going to run it on the muffs later today to really observe the spark and make sure it’s strong and firing.

is there anyway to check the power pack while running on muffs?

 

[sutor623]

You can check the power pack on muffs. 1.) Check for good blue spark at 7/16". 2.) Index the flywheel and check for crossfiring or intermittent miss.

Hardest part for me as a mechanic is not having a test tank and all of the test wheels to diag the older motors for WOT and issues that only show under load.

 

[lazeronu]

Ok so update - solved then new problem arose

replaced the power pack and problem was solved! Ran for about 2 hours no problem. Started great, WOT great, cruised great.

Pulled in midday and tried to go back out and ran into problems again. When going WOT motor would bog out and die unless I hit the choke. I could nurse it to WOT but it was running 5mph less at top end. I suspect this is a carb issue? I just rebuilt the carbs so maybe I missed something or maybe bad gas? Any suggestions are greatly appreciated.

 

[oldboat1]

Check the fuel for water (let it set in a glass jar overnight, and see what settles out (water will be on the bottom).

If no tank contamination, replace the head gasket after resurfacing. This may be a motor that required moving the tell-tale elbow (air lock at the top of the head) -- don't have the info. in front of me at the moment.

 

[racerone]

This motor is not on the list for moving the telltale !!

 

[oldboat1]

Thanks. Scratch that possible fix.

 

[oldboat1]

Might replace the plug wires if you haven't done that (wires, boots and connectors). Need 7mm solid core wire (solid or stranded -- not automotive type.)

 

[lazeronu]

Ok pulled off spark plugs. Top spark plug is milky. As mentioned above likely the head gasket?

Will replace the head gasket and hopefully get out on monday.

Any advice on resurfacing?

Anything else to check before getting out especially knowing that the top cylinder spark plug is milky?

Thanks!

 

[racerone]

If there is WATER inside the engine then corrosion takes hold immediately.-------Get the problem resolved / engine running ASAP.

 

[lazeronu]

pulled head and gasket is blown. noticed scoring on the inside of the top cylinder. very slight rust as well.

obvious this is not good. what are my choices? will this motor still run with scoring on the top cylinder or is it just a ticking time bomb?

 

[racerone]

It will run but is not going to heal itself.----There was a time that I would rebuild those motors ----But today owners of old motors want it done yesterday ,for ess that what they paid for the motor.----Factories put out NEW motor for almost less that a proper rebuild.----But these electronic / plastic motors will not last like a 1985 vintage Big Johnson.

 

[lazeronu]

well im going to replace head gasket and do compression test and see what its like from there. If things looks ok and within spec of each other, I will take it out monday and see how it runs. Obviously a rebuild would be best, just not sure if it is worth it right now. If I can get another couple seasons out of it that would be a huge bonus. Just not sure how long this will last with the top cylinder being scored.

 

[oldboat1]

Ok pulled off spark plugs. Top spark plug is milky. As mentioned above likely the head gasket?

Any advice on resurfacing?

Thanks!

Use a flat surface (piece of glass or similar), and a full piece of 200-400 grit wet/dry sandpaper. Wet the bottom of the paper so it stays in place. Rotate the mating side of the head in a figure eight pattern until all sides have an equal shine.

 

[lazeronu]

It looks like the head gasket is caked on the head pretty good. What is the best way to take this "blue" left over residue off?

 

[racerone]

Razor blade / carbide scraper / or any good heavy duty scraper.-----With the proper feel you will not do any damage to the block or the head.

 

[oldboat1]

^^ Agree. Might try some lacquer thinner on a shop rag as well.