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timing posted 07-12-2005 02:50 PM ET (US)
Would greatly appreciate [advice]:
[What is] the [cylinder compression in] PSI specified for a 1999 [Yamaha Saltwater Series] 200-HP? Other [than] 10% or less tolerance accross all cylinders, what PSI reading should I expect if in good condition? What PSI reading would indicate a near term rebuild? And lastly, what PSI when brand new?
Thanks
Timing
jimh posted 07-12-2005 05:31 PM ET (US)
It is greatly appreciated if you use the actual name for products, like YAMAHA instead of YAM or other variants. This helps people locate information. People seldom search on terms like "YAM" or "Zuki" etc.
Also, abbreviations like SWS are not exactly on everyone's short list.
I do not recall ever seeing a published specification for the compression readings in an engine cylinder. Almost universally the specifications just call for a close similarity of readings between cylinders.
Getting a reading of compression depends on many variables, so it is hard to predict how a particular engine will measure.
Some engines turn up numbers in the 90-PSI region; other engines have numbers in the 120-PSI region. It is probably better to compare readings among cylinders and between two engines of similar age and design using the same pressure gauge and technique to measure them. Depending on the technique used and the actual pressure gauge, it seems reasonable to believe there could be quite a variation seen in measurements of cylinder PSI.
where2 posted 07-12-2005 08:05 PM ET (US)
Jimh speaks well. As an example, the combination that I bought last fall which was in need of an engine rebuild (seller and buyer were both aware of the engine condition). The readings on a conventional compression gauge indicated that 3 of the 4 cylinders were in the 110 psi range, while the 4th was in the 85psi range. As you will note, the low reading was more than 10% off of the other 3 readings. This is the typical failure mode of a 2-stroke engine. They rarely fail from wholesale compression loss, but rather fail in general on one piston from improper fuel supply (fuel brings the lubrication oil).
All the service manuals I have consulted over the years indicate a % differential as a threshold, and not a particular low pressure reading for assessing engine health.
jimh posted 07-12-2005 10:06 PM ET (US)
For more reading on this topic, see:
Compression Measurements Technique
http://continuouswave.com/ubb/Forum3/HTML/007199.html

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Author	Topic: Compression Measurements Yamaha V6
timing	posted 07-12-2005 02:50 PM ET (US) Would greatly appreciate [advice]: [What is] the [cylinder compression in] PSI specified for a 1999 [Yamaha Saltwater Series] 200-HP? Other [than] 10% or less tolerance accross all cylinders, what PSI reading should I expect if in good condition? What PSI reading would indicate a near term rebuild? And lastly, what PSI when brand new? Thanks Timing
jimh	posted 07-12-2005 05:31 PM ET (US) It is greatly appreciated if you use the actual name for products, like YAMAHA instead of YAM or other variants. This helps people locate information. People seldom search on terms like "YAM" or "Zuki" etc. Also, abbreviations like SWS are not exactly on everyone's short list. I do not recall ever seeing a published specification for the compression readings in an engine cylinder. Almost universally the specifications just call for a close similarity of readings between cylinders. Getting a reading of compression depends on many variables, so it is hard to predict how a particular engine will measure. Some engines turn up numbers in the 90-PSI region; other engines have numbers in the 120-PSI region. It is probably better to compare readings among cylinders and between two engines of similar age and design using the same pressure gauge and technique to measure them. Depending on the technique used and the actual pressure gauge, it seems reasonable to believe there could be quite a variation seen in measurements of cylinder PSI.
where2	posted 07-12-2005 08:05 PM ET (US) Jimh speaks well. As an example, the combination that I bought last fall which was in need of an engine rebuild (seller and buyer were both aware of the engine condition). The readings on a conventional compression gauge indicated that 3 of the 4 cylinders were in the 110 psi range, while the 4th was in the 85psi range. As you will note, the low reading was more than 10% off of the other 3 readings. This is the typical failure mode of a 2-stroke engine. They rarely fail from wholesale compression loss, but rather fail in general on one piston from improper fuel supply (fuel brings the lubrication oil). All the service manuals I have consulted over the years indicate a % differential as a threshold, and not a particular low pressure reading for assessing engine health.
jimh	posted 07-12-2005 10:06 PM ET (US) For more reading on this topic, see: Compression Measurements Technique http://continuouswave.com/ubb/Forum3/HTML/007199.html