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tamburello
04-21-2008, 10:10 PM
I had my local shop run a compression test while they were tuning up my boat this Spring. I was wondering what numbers are good and what numbers indicate the need for a rebuild?

Here are the results for my stock 1994 350 Mag Carb 250 PSHP:

Cyl#1 - 175
#2 - 180
#3 - 165
#4 - 185
#5 - 175
#6 - 180
#7 - 180
#8 - 175

Looks like cyl. 3 is the lazy azz of the bunch. :smash::smash:

- Jeff

BUIZILLA
04-22-2008, 06:56 AM
those numbers seem high across the board for an 8.5-1 motor....

Cuda
04-22-2008, 07:03 AM
Let her eat!

9 times out of 10, low compression indicates valve problems, and even then, a total rebuild is not required.

My first car burned so much oil, you'd think it didn't even have rings, but it still ran good compression. Unless half the piston is laying in the oil pan, the compression should be adequate, which your's is.

BlownCrewCab
04-22-2008, 07:39 AM
those numbers seem high across the board for an 8.5-1 motor....



I Agree, I had the same range on a 10.1 motor with gapless rings.

Cuda
04-22-2008, 07:43 AM
I Agree, I had the same range on a 10.1 motor with gapless rings.
It's my experience, rings have little, or nothing to do with compression. I do think the readings are a bit high.

Carl C
04-22-2008, 07:47 AM
I agree that the #s are high. Rule of thumb is above 100 is good and, like MP said, within about 10%. I think you are good to go. Was test done with engine warm and carb open?

ky-donzi
04-22-2008, 08:30 AM
I did a test the other day on my engine and the numbers were lower. I've got a mag engine which I would assume had a higher C.R. my numbers ranged from 135 to 145.

Jamesbon
04-22-2008, 08:35 AM
Tumburello,
Those numbers are FINE!

Go have fun with your boat!

MOP
04-22-2008, 09:58 AM
Compression tests should be done after the engine has been run for at least 10 minutes and before it fully cools down, the carb should be at full throttle, all plugs removed. The coil must be disconnected or grounded, many of our boats have a hot start circuit that will fire the engine even with the key off when the engine is cranked, so that step is very important. True the plugs are out but it can and will shoot a wonderful flame from a cylinder at times that will stain your shorts or worse. Uneven readings are very common and like mentioned above 10% variation is fine, a shot of oil in weak cylinders and a re-test will show a valve problem if it exists.

Phil

Carl C
04-22-2008, 11:50 AM
Run It!!!.......like you stole it!:smash:

tamburello
04-22-2008, 12:09 PM
Thanks for the feedback, I'll be runnin' it like I stole it to keep up at AOTH! :smash::hyper: