Engine Wont Hit WOT RPM

[77Muskie550]

I have a 1977 Johnson 70HP loop charged outboard.....Recently While on the water I was at Full throttle while on my way back to the docks. I came down off of full throttle to go past a few fishermen and after 40 or 50 yards past them I again began to accelerate back to Full throttle.....I achieved WOT for about 15 seconds when I suddenly lost power and could barely even plain out....I've Checked the prop hub (tho this wouldnt be the issue because there was no slippage) and did a compression test and all the cylinders came back between 80 and 90 psi. What Could my issue be if its not a dead cylinder?

 

[racerone]

My compression tester would show about 130 / 140 on one of those engines.----Do you have good spark on all 3 leads ?

 

[77Muskie550]

Well The Tester I used came with those numbers 3 times and its one I rented from an auto parts store....Removed all my plugs and did each cylinder each time it came up with the same thing.....If its low on all cylinders What would be wrong? The motor starts and runs just fine but after it planes out its topped out at 20 or so and used to do 32-35.....how do I check for spark?

 

[77Muskie550]

I guess I could have not turned the motor over enough....Just read that Im supposed to let it climb till its done I only turned it over maybe 2 or 3 seconds

 

[Bosunsmate]

use a spark tester, or do a drop test on each cylinder, sounds like one cylinders down

 

[77Muskie550]

Ok so after doing a little research and calling all 3 marine service shops in the area they all told me that the year and motor that compression sounds right if I used just a cheap tester......so I dont know what to believe.....Im unfamiliar with a spark tester or how to run that test motor is a loop charged motor as well not sure if that makes a difference or even what that means

 

[77Muskie550]

Does it matter if I tested it in or out of the water

 

[Bosunsmate]

nope just make sure you keep the impeller wet.
spark testers are $5 from an auto shop.
I would do a drop test and not worry about the compression yet as the most important thing is they are even (ie no cylinders gone catastrophic)

 

[77Muskie550]

So if it was turned over with no water even near it would it give false reading