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1990 Johnson 48SPL Running On One Cylinder

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Practical Outdoorist

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I have a 1990 Johnson 48 SPL. 90PSI on each cylinder. Spark through both plugs. Yet only top cylinder is running. If I pull bottom plug wire... NO difference. Put it back and pull the top wire.... and it shuts off. Unspent fuel in bottom cylinder. I swapped the carbs to see if the problem would follow, it did not. Getting milky oil from exhaust. BTW I have a Jet Pump on this motor. Could this problem be Exhaust related ??? I appreciate any help on this . Thanks.
 
Possible reed valve problem.------Or bottom seal problem.-----Or sleeve position issue.-----Or spark not strong enough.-----At 90 psi you should get another gauge.-----Something wrong with that value.
 
Did you check the compression with motor warm and throttle wide open, both plugs out and cranking fast? Spark should not only occur as shown on the plugs, but MUST be checked for minimum potential arc. This should be 7/16 inch on these modern outboards. "Milky oil" in exhaust is likely your unburned oil/fuel mix blended with the discharge water and exhaust from "good" cylinder. You have to focus on the cylinder with weak spark......I do believe.
 
That's really true for most 2 strokes, but I certainly notice a difference on some motors, especially 4 strokes. So I make it a general rule. It won't hurt, or give a false reading by opening the throttle to allow the air to get in with only the reed restricting flow. The goal is to get these folks to supply us with the most accurate compression number possible.
 
On 4 strokes yes.-----On a 2 stroke the exhaust ports are WIDE open at the bottom of the stroke.------When piston goes up into the cylinder the compression of air that is trapped starts when ports are closed off.
 
In theory you are 100% correct. What happens if the motor is in the water, Racer? Well it depends on how much exhaust is directed above water level, but for the most part, a motor in the water will not be able to suck much air into the combustion chamber via the exhaust route as a motor on dry land. It will be fighting atmospheric pressure. I just like to clear any variables for those doing compression tests, that's all.
 
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Air is sucked into the crankcase and compressed.----Then transferred under pressure as it scavenges air out of the cylinder.----The crankcase does NOT behave as a supercharger !-----Air is at atmospheric pressure when compression starts in the cylinder..
 
Ok I remember now. When it fires, the exhaust ports are higher in the cylinder so spent gas is discharged, there first, then as the crankcase has been loaded on the firing stroke by vacuum, the downstroke forces that charge which is now trapped by the reeds, or in some cases a rotary valve, that creates pressure in the crankcase. Then as the top of the piston clears the intake ports, that pressurized fuel mixture is "forced" through the intake ports and then the piston returns on the compression stroke for a fire. Lets go back......then while the piston is heading back on the compression stroke, it creates a vacuum or suction into the crankcase. So yes, the suction part of the operation is what gets into the crankcase, the pressure side of the operation is what gets into the cylinder. Its a 2 step process. Withou one, we don't have the other..
So, if air can't get "sucked" into the crankcase as the piston is rising, then we have little hope of a good or accurate compression test. Right? The ports are not configured to get an air charge by way of the exhaust side, brother.
My old science teacher would be proud. I can make em run, but how the he## does it work?
 
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Bingo Tim, that's the deal on a 2-stroke. It's kinda like the Suck-Cut from the Wayne's World movie.....It Sucks, As it Cuts!!

The webbing on the crankshaft does all the work pulling air in as it sweeps past the reed cage, the reeds trap the air in the crankcase, and on the the down stroke the piston sucks it into the cylinder.

This is also where exhaust tuning comes into play. When you have individually 'tuned' exhaust ports they have expansion chambers that create sound waves which generate mild back pressure in the exhaust pipe to prevent air escaping the cylinder during the intake stroke. This minimizes unburnt fuel/air from leaving the cylinder into the waste exhaust.

Here's a case in point. In the 90's Polaris made a 3-cylinder, 750cc jet ski that made 80HP using a single exhaust pipe. In 1997 they came out with the Pro785. It was the exact same crank case and cylinder bore with a slightly longer stroke. The key was bigger carbs (more air & fuel in) and mechanically actuated exhaust port valves to change port size. The most important feature was it had 3 individual exhaust pipes that were shaped (i.e. TUNED) to each cylinder so the acoustic waves maximized air/fuel charge. The result was a 140HP fire breathing monster that had ridiculously instant throttle response. The increased stroke gained 35cc over the original design, but the tuned pipes were the real magic. I know in the outboard world 140HP doesn't sound like much, but you put that HP in a boat that weighs 450 lbs and revs to 7,000 RPM and you got a whole bunch of 'hang on to your butt'. The low end torque from that design would just about rip your arms out.

Okay, I know we're in a Johnson/Evinrude forum but just for grins, here's a video of a Pro785 running. If you watch close you'll see the ski literally jump out of the water when he grabs WOT. That is some serious torque you'll never see coming out of a single pipe 2-stroke engine.

https://youtu.be/eIlTB7O_C1w
 
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Throttle plates have calibrated openings.----Throttle plates are not completely " closed " lots of room for air to get in at compression test RPM.------Throttle plates do not have to be wide open for a compression test.
 
Practical Outdoorist said:
1990 Johnson 48 SPL. 90PSI on each cylinder. Spark through both plugs. Yet only top cylinder is running. If I pull bottom plug wire... NO difference.....I swapped the carbs to see if the problem would follow, it did not. Getting milky oil from exhaust. BTW I have a Jet Pump on this motor. Could this problem be Exhaust related ???
Click to expand...
Okay so back to the OP's real question.

Did you test spark with an air gap tester? This is very important. Just holding the plug near the cylinder and looking for an arc doesn't cut it. The sparks needs to jump a 7/16" gap. If it does not you have an ignition related issue.

Have you replaced the plugs? They do go bad and have internal resistance that will reduce spark under load.

Compression is a little low, but even across both cylinders is good.
 
You guys are awesome. Keep up the good deeds, we got the best boat forum out there. What makes it good is experience like Racer, Mr. Reeves, that other guy in Kentucky, the professor in UK, Gator, Bobby, Kevin, not to mention all the Merc gurus.....Bullet, Scott, Pappy. Hundreds of years experience, maybe thousands. I'm pushin' 50 years myself.....still learnin'.
 
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I will pick up a spark checker and a different compression tester and report back .Probably mid this week. I did OHM test on the coil.... Each primary tests out to 000. Bottom secondary (which is the dead cylinder) tests out at 266 OHMS... Top secondary tests out at 344 OHMS.
 
If your compression tests good. I would look at your cdi module as i believe yours is under the flywheel. Those are problematic.
 
Kevin...when you increase stroke you increase displacement, that why motor was labeled 785 and not 750 and the extras helped even more. Back to subject......its either a reed or seal problem with a possible bad head gasket. I would pull head just for giggles as these motors are bad about scuffing exhaust side of piston.
 
Right, Pappy. Good advice. Kevin, saw that video, awesome. I had an old "King Cat" 760 Rockwell Arctic cat. It was a rocketship. Sold it to a retired pilot. He totally restored it. Its the Panther with twin chrome dual exhaust, coming out the front and down each side. There was a three cylinder version of it too with a 793. Think it was a Hirth. I shiver just to think of another ride on one someday. They race modern snowmobiles on water (oval course) up here in the summer. An old lady asked me "how they can do that". I said "put a 200 horsepower motor in a 400 lb. machine....and you can do ANYTHING". Used to make hydroplanes out of a sheet of plywood in shop class. Hang a super 10 Merc on 'em. WOW! TALK ABOUT A RIDE! Sure loved those 👍 ole days. Should be a corner in this forum for nuts like me.
 
Ok Guys, I got another Compression tester, Both cylinders the same at 125PSI. I also got the air gap spark tester, Top spark tested good. Bottom did NOT meet the required 7/16 test. I just want to restate that bottom coil test 266 OHMS, Top coil test 344 OHMS . So is it the coil?? or could I also need the power pack??
 
Did you switch coils to see if weak spark changes to the other cylinder.....just swap the imput wire.
 
Follow RacerOne's advice. Also, it looks like the plug wires are separate from the coil. You could swap the plug wires and test again with the open air spark tester. If the problem follows the plug wire then it might just be the wire itself. If the problem stays on the bottom cylinder then also try swapping the orange lead.
 
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