1976 85HP won't start when hot

[petertay]

The 85HP Johnson outboard starts when cold. Runs good at 3000 to 6000 rpms pulling a skier. But when the skier goes down, either it won't start again, or if it does, it will only run if the 'Lift Up to Start' lever is up, and then it barely puts. Giving it any gas will kill it. After putting for several minutes, it might smooth out and pick up rpms again. It seems as if it is running on one or two cylinders.

The motor was in this condition when we quit. 24 hours later inspected the new spark plugs. #2 was dry, the other 3 were wet. The plugs are Champion L77JC4; 10% ethanol gas is new, mixed 50:1 with Penzoil Marine XLF synthetic blend.

Thanks for your thoughts. I'm wondering if a hotter plug might work. --Petertay

 

[oldboat1]

compression check first. (all plugs removed, screw-in style tester, crank until the needle stops and record result).

 

[Tassie 1]

Also when was the carbs cleaned last?
float height checked,
fuel pump diaphragm checked

 

[boobie]

Check your spark with an open air gap spark tester set at 7/16:"

 

[F_R]

Back to the 3 basics. Compression, fuel & air in right proportions, and spark. The problem can be any of the 3.

Actually, on a two-stroke engine there is a 4th basic often overlooked. Crankcase compression.

You mentioned spark plugs. You could have a defective one. But don't be putting hotter ones in it.

 

[petertay]

Thank you for the diagnostic help. Compression on all four is 110 or above.
With glass tube spark tester, all show strong 1/2 inch spark. All four plugs are working.
Carbs were rebuilt last year. We assume fuel and air are in correct proportions, because it is a fixed needle.
--Peter

 

[Bosunsmate]

when it doesnt run check spark, i suspect you have a failing electronic component, these happen to fail intermittently mostly.
Try ohmns checking them with a cheap multimeter. Resistance values are at cdielectronics.com

 

[petertay]

Of course when it fails, we are out on the lake. Hmm. Check the resistance of what?

Anyway, here is the recent history of repairs:

Power Pac was replaced by me in 2014 due to no spark on one side.
New spark plugs in 2014
Fuel line from bulb to engine replaced 2014. Old line was separating from its internal liner.
Carb rebuilt in 2015 by marine shop
Spark activator (name?) under flywheel was replaced by marina in 2016 due to no spark.

 

[Bosunsmate]

Lucky its a lake and not sea with off shore wind.
Resistance of timerbase, stator and also check for chaffed wiring

 

[petertay]

Thanks again for the help. I have the manual (for all Johnson motors) and am going through the ignition tests for Type 1, Capacitor Discharge. Are you familiar with this?

Sensor Coil Low Ohm Check.
Removed the leads from the PowerPac. Ohmmeter set at 200k. Resistance across 1 and 3 meter stays at 1. Resistance across 2 and 4 is 12.7. Book says meter should indicate in the range of 8.5

Sensor Coil High Ohm Check. Ohmmeter set at 2000k. Resistance across 1 and 3 is 0. Resistance across 2 and 4 meter stays at 1. Book says a zero reading indicates short to ground.

So it appears that these tests show failure. Before I buy a new timerbase with sensor coil, any advice from you?

 

[Bosunsmate]

My only advice is that that book knows more than me.
Check the units external feed wires for any chaffing. As if they chaff they can short to ground. Also in that age motor the wires can break internally.
Try cutting the wires near the unit and retesting. If they work inside acceptable parameters then, then it was the wires and you can just run some replacement wires, otherwise it seems it is a faulty unit.

 

[petertay]

I appreciate you. Thanks.