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Compression Issue Symptoms

R

renvette

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I have a 1991 60hp Johnson. What are they symptoms of low compression on all cylinders? My readings are cylinder 1 (89), cylinder 2 (90) and cylinder 3 (88). My motor begins to shake violently around 20 mph.
 
Those are very low readings UNLESS the engine is cranking over rather slowly.

Does your warning horn beep momentarily when you turn the key to the ON position? If not, test the horn as follows:

Engine NOT running. Key in the ON position. Ground the TAN wire that you see protruding from the cylinder head. The horn should sound steady. Does it?
 
joereeves said:
Those are very low readings UNLESS the engine is cranking over rather slowly.

Does your warning horn beep momentarily when you turn the key to the ON position? If not, test the horn as follows:

Engine NOT running. Key in the ON position. Ground the TAN wire that you see protruding from the cylinder head. The horn should sound steady. Does it?
Click to expand...

Engine is cranking over normally. The warning horn does beep. The engine cranks fine, idles fine, but begins to shake at 20mph or so and will not accelerate past that speed.
 
Sounds to me like it is out of sync. Is this a new problem what is the history. Check engine sync between ignition timing and throttle opening. If the pistons and rings are carbon fouled it could be the problem with compression.
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Make sure the carbs open exactly the same time and then put it into forward gear engine off. Watch the throttle cam as you advance the throttle slowly from slow to fast. When the pickup roller is center of the mark on the cam is when the carbs should just begin to open.
 
Does the tell tale hose come off the side of the block or off the top ?-------------My gauge would show 120 psi on one of those motors.-----------Has oil injection system been maintained / checked ?
 
The normal diameter of the black throttle roller is about 3/8" and it was a one piece item. A number of years ago, someone decides that it should be a two piece item... a smaller black roller (approximately 1/8" in diameter) , covered by a larger clear covering to bring the diameter up to the normal 3/8".

Problem is that eventually that clear roller breaks away, leaving a 1/8" diameter roller that throws everything out of synchronization..... Check the roller.
 
The Roller is good with no cracks or anything. Any other suggestions?

joereeves said:
The normal diameter of the black throttle roller is about 3/8" and it was a one piece item. A number of years ago, someone decides that it should be a two piece item... a smaller black roller (approximately 1/8" in diameter) , covered by a larger clear covering to bring the diameter up to the normal 3/8".

Problem is that eventually that clear roller breaks away, leaving a 1/8" diameter roller that throws everything out of synchronization..... Check the roller.
Click to expand...
 
Sounds like SLOW mode to me too.
Check for overheat. Disconnect tan wire from temp sensor on head.
Be sure it IS NOT overheating before running with the tan wire disconnected.

What are the RPMs when it shakes? 20 mph doesn't say much.....
 
daselbee said:
Sounds like SLOW mode to me too.
Check for overheat. Disconnect tan wire from temp sensor on head.
Be sure it IS NOT overheating before running with the tan wire disconnected.

What are the RPMs when it shakes? 20 mph doesn't say much.....
Click to expand...


RPM gauge has not worked in years, but it doesn't sound like the RPMs are increasing at all. Does disconnecting the tan wire increase compression? The motor appears to run fine with water muffs on and I put it in drive. However, it bogs down at 20mph when the boat is in the water under load.
 
MPH means absolutely nothing when troubleshooting. compression, spark gap, color of spark plugs/fuel, engine sync and link all need to be ideal then you can size the prop using WOT rpms. Once you get the motor close to 100% that is as fast as it goes. If you want faster get a bigger motor! disconnecting the tan wire disables the safety feature and done only for troubleshooting. With the engine running short the tan wire to ground the warning horn should sound and rpms should drop. Did you check engine sync? watch the throttle butterflys as you advance the throttle. The butterflys should just begin to open as the line on the cam passes center of the roller?
 
He says "shake violently" in his first post. He could be hitting 2500 rpm at the 20MPH speed.

He needs to make sure he is NOT overheating.
 
So what are my diagnostic steps? Once I disconnect the tan wire, what is the next step? I do not hear the overheat alarm whenever I'm running the motor in or out of the water. Also, we are taking about the tan wire coming from the power pack, correct? Thanks for the help. Normally, this engine runs about 37 -40mph with two adults.
 
The motor is not producing full power.-------I would be removing the cylinder head to inspect the pistons / cylinder walls.--------No amount of adjusting / fiddling is going to work to get full power if there is scoring in the cylinder.----------Take a positive step to determine condition of engine.---------So it will cost you a head gasket if all is well.---------Cheap lesson and you will learn a lot !
 
No removing the head until you determine if it is or is not overheating.
Use a digital laser thermometer, or hold your hand on the head for a count of 4-6 til you HAVE to pull it away.
It should run 140F or so.

If it is NOT overheating, pull the tan wire and see if the violent shaking goes away. If it does, use an ohmmeter on the temp sensor side of the tan wire, and see if you have continuity to ground. If the temp is 142 for example, and you have continuity to ground, then the temp switch in the head is bad.

Much easier than pulling the head at this point. Much easier and no parts costs unless you find a defective temp switch/sender.
 
It is not overheating (128F) and I pulled the tan wire, but the shaking is still present.



daselbee said:
No removing the head until you determine if it is or is not overheating.
Use a digital laser thermometer, or hold your hand on the head for a count of 4-6 til you HAVE to pull it away.
It should run 140F or so.

If it is NOT overheating, pull the tan wire and see if the violent shaking goes away. If it does, use an ohmmeter on the temp sensor side of the tan wire, and see if you have continuity to ground. If the temp is 142 for example, and you have continuity to ground, then the temp switch in the head is bad.

Much easier than pulling the head at this point. Much easier and no parts costs unless you find a defective temp switch/sender.
Click to expand...
 
Uh oh.
Now things are getting tough.
So, I guess the question is....is it really in SLOW mode? All I can go on is your description of shaking violently.
Can you add to this in any way to help me understand exactly what "shaking violently" means?
 
As I attempt to accelerate, all is well with the motor picking up speed, but once it hits about 20 mph (RPM hand does not work), the motor maintains the speed (20 mph), but does not accelerate. The motor does not rev up or anything either. The shaking is similar to an unbalanced washing machine on spin cycle I guess.

daselbee said:
Uh oh.
Now things are getting tough.
So, I guess the question is....is it really in SLOW mode? All I can go on is your description of shaking violently.
Can you add to this in any way to help me understand exactly what "shaking violently" means?
Click to expand...
 
Well, I will describe SLOW mode....see if this matches...

The engine hits 2500 rpm or thereabouts, and very rapidly begins fast up and down of the RPMs. Rev up, down, rev up, down, rev up, down....very fast succession.
This causes a shaking of the whole motor.

If you are hitting a 20 mph limit, could it possibly be a spun prop? Mark using a magic marker across the prop hub, thru the center of the prop nut, and on to the other side.
Run it, and see if the mark has moved after the shaking begins.

If this is not it, I am at a loss at this point. I really do not believe that low compression would cause violent shaking, just a loss of power...not shaking.
Even if you lose one cylinder, the engine will not shake violently.

If you can post a video, that would help.
 
Thank you for the help. I will check for spun prop and record a video when I take it out again.



daselbee said:
Well, I will describe SLOW mode....see if this matches...

The engine hits 2500 rpm or thereabouts, and very rapidly begins fast up and down of the RPMs. Rev up, down, rev up, down, rev up, down....very fast succession.
This causes a shaking of the whole motor.

If you are hitting a 20 mph limit, could it possibly be a spun prop? Mark using a magic marker across the prop hub, thru the center of the prop nut, and on to the other side.
Run it, and see if the mark has moved after the shaking begins.

If this is not it, I am at a loss at this point. I really do not believe that low compression would cause violent shaking, just a loss of power...not shaking.
Even if you lose one cylinder, the engine will not shake violently.

If you can post a video, that would help.
Click to expand...
 
You say it has a new pack on it. Is there any possibility that you have pinched the tan wire going into the pack between a mounting boss or any other metal part of the block?
That would make the pack think it was in overheat, and trigger SLOW mode.
 
I was thinking the same thing and I removed the pack to make sure and there is no issue. How do you check for spun prop?

daselbee said:
You say it has a new pack on it. Is there any possibility that you have pinched the tan wire going into the pack between a mounting boss or any other metal part of the block?
That would make the pack think it was in overheat, and trigger SLOW mode.
Click to expand...
 
You might want to figure out why your compression is in the sewer did you ever get a different gauge and retest. No amount of lines on the prop are going to fix your motor if the compression is that bad. You can play with the pack and temp guns all you want but if your compression is bad you are chasing your tail. You gave everybody the facts low compression and got steered off in every other direction and never bothered double checking what was wrong.
 
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