Compression Check - Review??

77SceptreOB · #1 10-21-2009, 03:40 PM

What do yall think of these compression #'s????

These PSI #'s are shown as you look at the motor from in back of it. (orientation)

130 130
127 126
120 121

Seems like the cyl. in the top of each bank are higher and the lowest cyl. in each bank is the lowest. Is this normal for the lower cyl to wear more / faster. Could it be due to higher heat down low??
The total spread is 10 psi, so I thought that was pretty good for a 20 year old Yamaha V-6 (225hp)

bridgeman · #2 10-21-2009, 07:51 PM

a little de-carb will probably tighten the numbers up but they don't look bad at all

stumpie2 · #3 10-21-2009, 08:34 PM

normally the top will have more heat but away from that... the decarb idea might bring the numbers in a tighter group. if it doesnt i would start thinkin about repower or rebuild because manufacture spec says no more than 10 lbs. or 10% and you are at 10 lbs ( your max difference)... you get the point... just my .02

gofastsandman · #4 10-22-2009, 09:27 AM

1 2
3 4
5 6
I`m guessing you did the test hot. I`ve been told to always do this cold. I was told that by the time you get to 5 and 6 the air pump {motor} can cool off the cylinders and give a low reading. Would you mind trying again with the motor hot on 5 and 6. I`ve also never done a hot vs cold comparo.
My Rude is 87-92 cold. Your numbers seem fine to me.
GFS

77SceptreOB · #5 10-22-2009, 11:52 AM

Quote:

I`m guessing you did the test hot. I`ve been told to always do this cold.

I did it cold. But now from what I've read, you SHOULD do it hot/warmed up. Oh well. Also, I did it dry (no gas), another mis-step on my part. They say, from what I read you should NEVER crank a 2 stroke engine over w/o fuel. Because you apparently then don't have any oil either. Not sure about a direct oil injected engine. Well the test is done now anyway. I should have read up a little more BEFORE I comenced to doing the test. A "rookie" mistake on my part and I don't like to read! I better learn before I really tear something up:-(. Also they say you should ground the spark wire, it protects the ignition system. Live and learn.

77SceptreOB · #6 10-22-2009, 04:25 PM

FWIW, the local Yamaha dealer mechanic sayes the critical spread numbers for compression are from bank to bank at the same cyl. elevation. If so, I'm golden!! only 1 psi difference from cyl. to matching cyl.

Okay "motor heads" on the forum, does that sound right?

I wonder what the firing order is??

1 2
3 4
5 6

??

stumpie2 · #7 10-22-2009, 07:51 PM

anytime you do any test on an engine you try to simulate a running engine. that being said, a properly running engine not run cold under normal operation, thermal expansion will make the pistons to "swell" due to heat and this is how the engine will run under normal conditions. ALL cylinders should have no more than 10 lbs. or 10% difference from highest reading to lowest reading. if you look at the cyl. heads on the engine one side is higher (normally port side), the highest cyl. is #1 and is also the odd side. as for grounding the spark plugs, yes, not doing so is an easy way to fry an ecu, switch box, icu, whatever you want to call it... this info is and industry standard across the board. not exact specs. but general rule of thumb.

77SceptreOB · #8 11-23-2009, 10:30 PM

Well I went ahead and had it De-carb and it seem to run better. I also had the 3 carbs rebuilt. The better performance I'm sure is due to the carb rebuild not the ring cleaning, but together the motor runs very well for a 20 year old V-6. Soon I will check the compression again. I'll post the results then.