78 Johnson 115 low compression 1 cylinder

[Deuce24]

I recently purchased a 78 Johnson 115 v4 model 115ETL78C. Boat ran great for approximately 2 months. Idleing back to the dock and it started missing. I have good spark, clean fuel, but low compression on one cylinder and that cylinder is not firing. Good spark from all coils, will all jump a 7/16 gap. Compression numbers are as follows: 122, 120, 117, and 103. Cylinder at 103 will not fire. This boat has never been in saltwater previous owner states motor always ran strong. First question is..I know that’s around 20% lower than the other 3 cylinders, but should it be firing? Question 2..with it being a 78 is it worth the money to have new sleeve, rings and piston on that lower compression cylinder replaced? Thanks in advance for any help/info!!

 

[harringtondav]

Howdy. You've hooked up with a great resource here. First thing that comes to mind is if the reed valve for the low compression cylinder is faulty. Blowback on the downstroke will limit the transfer charge and may cause low compression. Also it will basically blow the fuel charge back out. leaving nothing to ignite.

More advice to come, but do more trouble shooting before you tear it down.

Edit: I think I found your motor. Item #11 is the leaf plate assy. I was referring to. p/n 0389823.

 

[racerone]

Take bypass cover or cylinder head off for inspection of the low compression cylinder.----Post pictures off what you see.----Broken rings is what will happen to all of these crossflow motors.----Has it been rebuilt before ??

 

[racerone]

Reed valves have nothing to do with compression in the cylinder !!

 

[harringtondav]

Reed valves have nothing to do with compression in the cylinder !!

???? If the crankcase primary compression is low - due to a leaky leaf plate in this case, the transfer charge will be low and lean. 103 psi of a good fuel/air mix should ignite.

 

[racerone]

A bad reed valve does not cause low compression in a cylinder.

 

[Deuce24]

Guys thanks for all the reply’s. I had the cylinder head off yesterday and did find some scoring that ran with the travel of the piston. I can post some pix this weekend but won’t have time the rest of the week to get much done on it. There was slight scoring around 7:00 and more at 12:00. I could feel it with my finger no need for the fingernail test. This is the lower cylinder on the starboard side. Previous owner said to his knowledge it was never rebuilt. He owned the motor for 7 years. I did however find newer electronics, both power packs had a date of 2014.

 

[harringtondav]

Check out all of the easy stuff before you get into open heart surgery.

 

[Deuce24]

Will do. Thanks again for the info fellas. Is there a way to notice if the rings are broken? Would there be significant scoring?

 

[racerone]

Remove exhaust cover and the 4 bypass cover to look at the rings.

 

[Joe Reeves]

Harrington.... Racer is correct. The entire intake manifold could be removed... it would not affect the compression.

 

[Faztbullet]

Compression would be the same even if you took of intake manifold.....1978 was last year of high ring pistons and if original pistons all 4 will need replaced.

 

[Deuce24]

Another question. I just got home and noticed Unburnt 2 cycle oil running down the midsection of the motor. I recently rebuilt the carbs. I squeezed the primer bulb to see if I had a leak somewhere and the bulb was hard and no fuel anywhere to speed of. The motor ran for about 15 minutes yesterday when I was trying to see why I had no fire on the low compression cylinder. Could this be unbutnt fuel from the cylinder that wasn’t firing? If so, how is it on the outside of motor, bad seal somewhere? That seems like a lot of oil, and same on the other side. Trying to upload pic but says it’s too large. How does one upload a photo to a reply?

 

[Deuce24]

[No message]

 

[jimmbo]

Broken rings will show drop in compression. You said there was scoring on the cylinder walls? That pretty much hammers the nail in the coffin. Do what Racer suggested, I bet you will find scored pistons and rings with no springiness

 

[Bosunsmate]

Remove your HT lead for that cylinder and then run the motor for a few seconds. Check that sparkplug tip is wet with fuel. Ignite it to check it goes poof ie check it isnt water contaminated.
Your cylinders are sounding pretty bad but it should still fire with that compression. How are you checking whether its firing or not?
That fuel sprawl looks similar to mine but yours is a multitude of magnitude more. Ive put mine down to it being a bit of leaking from the carbs when i tilt her up and it takes a while to seep down there

 

[Bosunsmate]

Ps Ground that spark plug lead while the motors running for that test, otherwise you can harm the electrics

 

[Deuce24]

Remove your HT lead for that cylinder and then run the motor for a few seconds. Check that sparkplug tip is wet with fuel. Ignite it to check it goes poof ie check it isnt water contaminated.
Your cylinders are sounding pretty bad but it should still fire with that compression. How are you checking whether its firing or not?
That fuel sprawl looks similar to mine but yours is a multitude of magnitude more. Ive put mine down to it being a bit of leaking from the carbs when i tilt her up and it takes a while to seep down there

I was testing whether it was firing or not by pulling off the plug wires while running. Also, when you say, “Your cylinders are sounding pretty bad”, are you implying they should have much more compression, I’m assuming? Thanks!

 

[harringtondav]

racerone . Joe Reeves . Faztbullet You are all right, me wrong. I went to sleep thinking about this and realized the piston will get plenty of air for a compression test through the exhaust port. I participate here to both learn and contribute. I learned.

But a bad intake check valve will result in a weak air/fuel charge. 103 psi should fire a good mixture. I still urge Deuce24 to check that leaf valve during his tear down. Also to keep digging for the cause of the lower compression. A good rebuild will last a long time, and cost much less than an equivalent condition new motor.

 

[racerone]

harringtondav-----I have rebuilt enough of these crossflows and own a bunch more for parts too.----The owner of this motor should now know that a teardown is in order.----Then an evaluation of the motor and pricing of parts ( perhaps labour too )-----Then a decision can be made.