hello, I have a question for u all. I did a compression check on my 2000 V6 yamaha XO66. top and middle cylinders had 120 -125, but the 2 bottom cylinders had about 90. I thought that was a alittle low ,but my friend said that was normal for the 2 bottom cylinders to have lower compression than the middle and top ones. any truth to that thank you
low compression on 2 cylinders
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Although it is common for the two lower cylinders on SX motors to have lower compression, in your case the difference is more than normal. That said, if the motor idles and runs well, I wouldn't be all that concerned. You might want to check again with a different gauge. Always measure with all plugs removed. If the two lower cyclinders read low again, squirt in a tablespoon or so of oil and remeasure.. If the readings increase significantly, you probably have ring issues or worn pistons/cylindersagainst the wind said:
Doc Stressor
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As Doc stated, use 2 stroke oil. WD40 is not an oil nor is it a lubricant as many believe. It is basically a petroleum based solvent mostly benzene.
I spoke with Yamaha about this years ago when I ran into the same situation. They verified it is part of the design and sent me a document explaining this at that time. Hard for me to remember why, but I think it had to do with those cylinders not being used at idle or something. Either way, straight fromYamaha, it's the design of the motor.
WOW, unbelievable, I guess you guys are right !! I did shoot some 2 stroke oil in the cylinders today as seasick mentioned. compression was the same on the bottom 2 cylinders 90 to 95. I would have never guess that in a million years.I diff would love to know the reason !!
georgemjr said:
The combustion chambers are a little bigger on the bottom two cylinders making the compression ratio a bit lower.
I don't remember which two cylinders don't fire at idle but I am pretty sure it is not the bottom two. Regardless, that has noting to do with compression. The ECU cuts off the spark. I know lots of folks who didn't know that and bought new coils since the cylinders were not firing
thank u for that info.. like i said i was very surprise about that info, never would have known that.. I will call up yamaha to get the reason why
sooo.. i called yamaha about the lower compression on the bottom cylinders this is what the teck told me.. all 6 cylinders should have the same or close compression within 10 to 15%. He never neard of yamaha designing a motor with lower compression in the bottom cylinders. This is really a concern because what u said made sence to me.. :hmm
My bottom 2 just tested lower that the other 4. 120lbs on 1-4 and 115lbs on 5 & 6. I've also read that is normal but don't know why.
Yamaha not only told me about it, they sent me a document explaining it. I don't think I have it anymore, as I gave a copy to the new buyer of my boat who was also concerned about this situation. I will look to see if I have a copy. Either way, I am not imagining this, and just because 1 Yamaha rep isn't familiar with it doesn't make it untrue. I think it is because they do shut down those 2 cylinders at low rpms, and therefore the lower compression keeps engine smoother.
It's funny, if you call Yamaha on five different occasions you will often get 5 different answers.
Yes, the engine DOES have cylinder deactivation at low RPM's. And everything I have read about, and researched, also showed that the lower two cylinders (5 & 6) have slightly less compression. I noticed a small amount of difference when I compression tested mine. But, honestly, it was such a small amount, that if I didn't know about the lowered compression thing, I would just chalk it up to normal differences from one cylinder to another.
As mentioned above, though, the difference you're seeing is not because of the lowered compressing thing. I would run it real well (on the water) for a while and then check again. It could just be that the rings aren't sealing well from sitting.
Yes, the engine DOES have cylinder deactivation at low RPM's. And everything I have read about, and researched, also showed that the lower two cylinders (5 & 6) have slightly less compression. I noticed a small amount of difference when I compression tested mine. But, honestly, it was such a small amount, that if I didn't know about the lowered compression thing, I would just chalk it up to normal differences from one cylinder to another.
As mentioned above, though, the difference you're seeing is not because of the lowered compressing thing. I would run it real well (on the water) for a while and then check again. It could just be that the rings aren't sealing well from sitting.
Found this info in, of all places, my service manual (ignition section)...
Now, of course, this is for my model year, but I would be surprised if this changed. It also applicable to at least the 225HP and 250HP models.
From idle to about 850, spark is cut to #5. From about 850 to 1,950, spark is cut on 2 & 5.
Now, of course, this is for my model year, but I would be surprised if this changed. It also applicable to at least the 225HP and 250HP models.
From idle to about 850, spark is cut to #5. From about 850 to 1,950, spark is cut on 2 & 5.
thank u george and dennis.. As you mentioned at idle the lower cylinders will read lower, and that makes perfect sence.and i told that to the teck at yamaha.. He told me to hold on and he put their "quote" head teck on the phone. He said while the motor is idling the 2 bottom cylinders will have lower compression, but when the motor is put in gear ALL cylinders fire the same.