Johnson 25 HP 1990 Can't get it to run properly

[jhudnall]

I bought a used Jon boat last summer with a 1990 Johnson 25 HP short shaft key/battery start and I haven't been able to get it running properly and have tried everything I can think of and also some things I read in other posts here. I can't afford to throw a bunch of money at this motor if it's junk, would rather put it towards another one. Here is what it does:

-Starts right up and idles fine
-When the rpm's are turned up it 'backfires' with a small puff out the exhaust and continues doing this randomly every 5 to 10 seconds but then generally goes away when at idle
- Compression is around 70 on both cylinders
- Upper plug is wet after running
- Lower plug is black after running
- Engine will continue running on just the bottom cylinder but not the top
- Put a spark tester on both cylinders and visually it looks fine on both but this is the first time I've used a spark tester
- Purchased 1 new ignition coil and tried it and the old ones in every possible combination on both cylinders with no change

- Carb was rebuilt
- Fuel pump was rebuilt
- Fuel tank was emptied and cleaned out and fresh gas added
- New spark plugs

Seems like upper cylinder isn't running but looks like it's getting spark.

Would the low compression cause this?

Any help would be appreciated, thanks, John

 

[racerone]

Compression is way too low.---Test with another gauge first.-----Could be a blown head gasket.---And you did install a new water pump impeller as that is the first thing to do with a " new to you " motor.----The " back fire " means that low speed fuel mixture is too lean .----Or throttle plate is opening too early.

 

[jimmbo]

70 is low. Did you keep cranking the engine until the gauge went no higher? If so, the gauge could be inaccurate, try a different gauge.
Since the upper cylinder appears not to be doing its share... Can the spark jump a 3/8" gap?
Fuel appears to be getting there as witnessed by the wet plug

Edit: I see Racer beat me to it

 

[lmuss53]

Swap the plugs, does the problem move with them? Could be a bad plug.

Good spark is a clean, blue, snappy 1/4 minimum spark with an open gap tester.

 

[jhudnall]

I used one of those spark testers that you attach to the spark plug and the other end to the spark plug wire and it shows you the spark in the clear plastic part in between. Appeared the same with both cylinders and didn't appear to change when it 'backfired'. Yes I swapped the plugs and the symptoms were the same, top wet, bottom black. I also played with the adjustment screw on the carb and moved it in small increments all the way in and all the way out and it had no effect on the 'backfire'. It is the only adjustment screw on the carb, is that what changes the 'low speed fuel mixture'? The compression tester I have is good as far as I know. I've used it on some cars and it read accurately. 70 is the max is goes to on both cylinders. I haven't tried to see what kind of gap the spark will jump yet. I can try that tomorrow. I haven't done anything with the water pump as I haven't had it in the water yet due to this problem. It does pump water properly with the hose and water ear muff attached to the motor. Are there other symptoms that would show if it was the head gasket? How hard are they to replace on these engines? I've done it plenty of times on cars but not on an outboard.

 

[jimmbo]

The Spark needs to be able to jump a 3/8" gap at Atmospheric Pressure to ensure it will be able to jump the Plug Gap when the pressure in the cylinder is above Atmospheric. A board, a couple of Nails, a bit of wire and a clamp can construct a fine Spark Tester
When using a Hose and Muffs, the water pump is having a Nap, it isn't doing anything.
If your compression Numbers are accurate, some detective work is in order. Pull the Head, check gasket. Also remove the Transfer Port Covers to allow inspection of the Piston skirt, and check for tension on the piston rings

 

[racerone]

Remove the cylinder head.------It is either a blown head gasket or an expensive overhaul is needed !!