1978 Johnson 85hp wont run in water. Only on hose

[Clemak]

I have a 1978 Johnson 85 hp outboard. It will run perfect on hose/muffs. But as soon as I back the boat into water the motor will not run. It will barely start and then immediately stall. Go home and back on the hose.. no issues. I know exhaust back pressure can have alot to do with it. I have spark in all 4 cylinders. No fuel leaks or blockages. And compression is low 90s in all 4 cylinders. I'm at my wits end with this motor and even if I could afford a mechanic.. they are all booked with months waiting list. Can anybody please give me some ideas on how to get this motor running. PM's welcome or comment in the thread. I dont care. Please help.

 

[Joe Reeves]

Compression should be better than 90 psi on that engine BUT that reading may be due to a faulty gauge.

However, that 90 psi reading may also be due to a very slow cranking engine. The engine must crank over at least 300 rpm in order for the stator to energize the powerpack capacitor to produce the proper spark.

At cranking speed with "all" spark plugs removed, the spark must jump a 7/16" air gap with a strong blue lightning like flame... a real SNAP! Does it? (Nothing less will do!)

You obviously realize that back pressure has the effect whereas it slows down the cranking rpm... so... if too slow, that affects my above statements.

On a flushette, the idle rpm on that 1978 85hp model Johnson should be as follows, depending on its shaft length:

20" = 1000 rpm

25" = 1200 rpm

I assume your engine's shaft length is 20".

Spark plugs should be QL77JC44 Champion plugs gaped at either .030 (long life) or .040 (super strong spark).

Let us know what you find.

 

[racerone]

Sorry-----It is not " perfect "---It just sounds good on a hose to an untrained ear.-----It will roar / bark on 2 of the 4 cylinders.----You may have a blocked jet in a carburetor.--------Or plug wires on the wrong cylinder.----Test compression with another gauge.----Should be 120 + PSI.

 

[Clemak]

I will retest compression as last time I did it my battery was weak. I gapped my new plugs at 35. I dont know the shaft length of my engine. The model number is 85ml78c. Out of curiosity.. if the floats in the carbs are off adjustment would this problem occur or is my best bet still the back pressure or compression issue? I have alot of questions but this is my first outboard. I dont know how to diagnose outside of the basic spark fuel and air essentials. I will post my compression retest results on here later today

 

[Clemak]

Also my plugs do have a good blue spark. I didn't measure inch gap.. I just know I gapped at 35 and installed. But the spark when I checked because I thought i had a bad powerpack a couple weeks ago.. was a bright blue arch on all 4 plugs

 

[F_R]

Sorry if this is a dumb question, but you Are lifting the fast idle lever--aren't you?

 

[jimmbo]

Sorry if this is a dumb question, but you Are lifting the fast idle lever--aren't you?

It is not dumb. A lot of people today are used to Fuel Injected cars/trucks that require no actions other than turning the Key.

 

[oldboat1]

It's possible you are due for a starter rebuild, or maybe just a thorough cleaning. Work on the battery first -- good idea to take it to an auto parts store for free testing. Include cranking test/voltage drop test. Get a spark tester similar to this one (cheap) for measuring spark:


:

 

[Clemak]

Yes. I am lifting the fast idle lever and battery has already been tested. The starter is good. Motor will start in water but for how long is a gamble... 3 seconds? 20 seconds? Is completely up to the tempermental motor

 

[racerone]

Remove the bypass covers on the sides of the block.-----Held on with 6 screws on each one.----Lets you look at pistons / rings.-----There is something wrong with this motor.----You need to inspect find it.

 

[Clemak]

I'll try that. Thank you