Compression testing a Yamaha OX66 ??

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DennisG01

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Hi all. I am very familiar with stern drives, but have never done a compression test on an outboard. I'm going to look at an older Grady next week that has a newer 2001 (might be 2000) Yamaha 250HP, OX66 engine. If anyone can give me some insight...

-- Do these engines come with a "kill switch" that would be mounted at the helm? If so, does that shut off fuel, spark or both?
-- Short of having that kill switch, is there an outboard-equivalent to a coil wire that I can remove and ground?
-- In doing some research on outboard compression, am I correct that "typical" numbers (#'s) are in the low-100's for larger HP OB's? The reason I ask is that the typical numbers for stern drives are in the mid-to-upper 100's. Obviously, consistency across the cylinders is very important, and sometimes more important than the actual numbers. But I was curious to hear what a good running engine is generally putting out.
-- Anything else that I should be aware of?
 
I know on my 1987 150 yamaha 2 strokes without the kill switch key i have no spark. The main thing with compression is they should be pretty much even between all the cylinders. I don't think yamaha actually has a spec. I know when i checked mine i had 120 to 125 in all cylinders. What type of grady did u find?
 
I like my 86 offshore esp with the twins she can move. When i had down to delaware a couple of weeks ago it handled the 3 foot waves of the atlantic pretty good.
 
It is best to use a remote start switch connected to the solenoid to crank the motor. The coils on those motors don't like to be fired without a load ( spark plug).
As mentioned, consistency across cylinders is important as are actual levels but note that the two opposing cylinders may have a bit lower compression than the other four. That is normal for SXs, at least for smaller hps but I assume that might also be the case for your motor. I can't remember (age again) if the two lower or two upper read lower. I am sure someone else remembers.
 
Thanks, Seasick. Yes, that was one of my concerns (spark load). Usually when I do a sterndrive (4-stroke) I will ground the coil plug and or flip the kill switch off. But with a 4-stroke, if the engine is turning, there's at least some oil supply to the cylinders. But if I just use a remote starter button, I won't be getting any oil supply, right? But that's where my dilemma (or inexperience) comes in with an outboard.
 
I know on my 87's there direct injection. A mechanical oil pump delivers oil directly into the cylnders. I would assume the OX66 is the same. Also i would think there would be no damage caused by cranking the engine over with all the plugs out and at such a low rpm.
 
I believe it is the two bottom cylinders have less compression than the other 4. Check with Yamaha. They sent me the specs when I had compression checked similar motors a few years back. They were designed that way.
 
Thank you guys, for the guidance. Much appreciated!

I called a local Yamaha dealer and he had not heard of any of the cylinders being specifically designed to have lower compression. I then called Yamaha and talked to, who sounded like, a very knowledgeable technician. He also verified that there was never any design call for any cylinders to be lower. He did mention that there never was any "spec" for compression, but that a good one (warm engine) would be in the low 100's (110-120, give or take) and 90psi would be a bit low, although it doesn't mean "game over".

He also verified that the low RPM's of a compression test would do no harm to even a cold engine.
 
DennisG01 said:
Thank you guys, for the guidance. Much appreciated!

I called a local Yamaha dealer and he had not heard of any of the cylinders being specifically designed to have lower compression. I then called Yamaha and talked to, who sounded like, a very knowledgeable technician. He also verified that there was never any design call for any cylinders to be lower. He did mention that there never was any "spec" for compression, but that a good one (warm engine) would be in the low 100's (110-120, give or take) and 90psi would be a bit low, although it doesn't mean "game over".

He also verified that the low RPM's of a compression test would do no harm to even a cold engine.
Click to expand...

"Sounded like" are the operative words. Was the representative of the male gender? Men (particularly the younger ones) are known to make up answers when they don't know them, rather than say "I don't know". Sounds like what this fellow did.

An SX250 does indeed have different compression ratios for the numbers 5 and 6 cylinders. See snippet from the OX66 service manual below.

He was also incorrect regarding a specification for compression values. For instance, included with the snippet is a section from a later model two stroke service manual that lists the minimum compression values. The motor that is covered by this service manual has a higher compression ratio than does the OX66.

 
Ono - thanks for posting those numbers and the video.

Boscoe - You're right that the person I talked to was male - I would guestimate in his 30's-40's. I will say that he didn't hesitate with his answers - he seemed to know right off the top of his head. I'm not saying he's right and anyone else is wrong - far from it. Just trying to learn more about this.

Out of curiosity, how significant is the difference between the #1-4 and #5,6 compression ratio that you showed in that graphic? In other words, taking out other variables, what could the actual difference be when doing a compression test? I do realize that it's a used engine and "all bets are off" in terms of actual numbers due to many factors over time. But, in "theory", what would the difference be?

By the way, you had also responded on that Yamaha forum - thank you for that, as well. Do you work for that business? The reason I ask is that if all goes well and I end up buying the boat, I would like to buy a service manual for the engine. I was curious if you guys sold them.
 
I called Yamaha again and asked some more. The gentleman I spoke to (different than yesterday) said "good question - let me find out for you, I'll talk to someone in engineering". When he came back on the line, he said there was no design spec for having different compression numbers between cylinders. I had also asked if there were any differences in cylinder or piston size - he said there was not and that the specs were for the ID of the cylinder wall to be between 90.0-90.2mm and the piston diameter to be between 89.65-89.8. I did forget to ask about stroke, though.

I've never read anything bad about these engines, just searching for some education! It appears there are two credible sources giving slightly conflicting information - certainly more credible than me - so I'm just a bit confused, is all! :)
 
DennisG01 said:
I called Yamaha again and asked some more. The gentleman I spoke to (different than yesterday) said "good question - let me find out for you, I'll talk to someone in engineering". When he came back on the line, he said there was no design spec for having different compression numbers between cylinders. I had also asked if there were any differences in cylinder or piston size - he said there was not and that the specs were for the ID of the cylinder wall to be between 90.0-90.2mm and the piston diameter to be between 89.65-89.8. I did forget to ask about stroke, though.

I've never read anything bad about these engines, just searching for some education! It appears there are two credible sources giving slightly conflicting information - certainly more credible than me - so I'm just a bit confused, is all! :)
Click to expand...
Remember that compression ratio is just that; a ratio of the maximum volume divided by the minimum. So if you have identical bores and strokes but the cylinder head chambers have different volumes, the compression ratio will be different. All that said, it won't make a lot of difference in your motor and as long as numbers are in line with specs and within 10% or so of each other, you should be fine.
I am a bit surpised that this post hasn't gotten a response about "forget compression numbers, you need to do a leak down test":)
 
I am 100% sure there is/was a design difference in compression for the lower two cylinders on the 225's that were on my Sailfish. I know this because I did a compression check when I bought it, the lower two were less than the other 4, but within what one would consider "within tolerance," 10%. When I sold the boat, the buyer was more concerned than I over the difference. Yamaha faxed me a copy of this design and the fact that the lower two cylinders were indeed meant to be lower. I had to provide that information to the purchaser and his mechanic (who didn't know either) to seal the deal. I just looked through my files to see if I still had the papers, but didn't find them. I don't know about your specific model, but if a Yamaha tech says that it was never a design feature, they are wrong. I read the papers from Yamaha myself.
 
seasick said:
All that said, it won't make a lot of difference in your motor and as long as numbers are in line with specs and within 10% or so of each other, you should be fine.
Click to expand...

Yes, you're right -- and I agree 100% with this. In the end, that's the main thing. I'm just after as much info (education) as I can on this engine.
 
georgemjr said:
I don't know about your specific model, but if a Yamaha tech says that it was never a design feature, they are wrong. I read the papers from Yamaha myself.
Click to expand...

And that's where/why I'm getting confused. A number of you say that you spoke to Yamaha directly (and I absolutely don't doubt you - that's not at all what I'm talking about). My first conversation was with a local Yamaha dealer's technician. OK, maybe he didn't know - I can understand that. But then I called Yamaha (the company) directly - I spoke directly with two different guys there who both said there was no difference. The second guy even went and talked to someone in engineering (his words, not mine) who also said there was no difference. Can you understand why this is me right now... :bang ...joking, of course... I'm not at that point, yet! :)
 
You are confused because the two gentleman that you spoke with Yamaha are confused. Or, maybe they don't know their own asses from a hot rock. Yamaha does not have real engineers in Georgia. The engineers are in Japan. I suspect that he talked to a misinformed technical type and then referred to him as an engineer to try and make you (or himself) feel better about the incorrect answer he was providing.

Call them back. Refer them to the Yamaha 2001 PIG (Product Information Guide). Page 83 specifically. It clearly shows that cylinders 1 ~ 4 have a compression ratio of 5.9/1 and cylinders 5/6 have a compression ratio of 5.7/1.

Here is a copy of the page

 
I will do that (call them again), armed with the extra info you gave. Maybe they were generalizing and saying that there is no appreciable difference in PSI? Or, maybe your answer sums it up better! :lol:

OK, follow up question... And please correct me because I believe I'm missing something here... If I multiply the ratio by the Stoichiometric of 14.7, I get 86.7psi and 83.8psi, respectively. Obviously this is not what we actually see on a compression gauge. I would "think" that even though different gauges will read differently, this still would not explain these lowers numbers, compared to what actual numbers are.

Again, I fully understand that at this point I'm dealing with theory as opposed to actual numbers - I'm just trying to learn something... ie: what would be the theory behind the reasoning for differing compression ratios and what would that theory equate to in terms of compression pressure?
 
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