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Discussion Starter · #1 ·
Hello, this is my first post on the forum. I did search and did not quite find an answer to my question but I did find on this forum that the oem compression ratio is 10.7:1 for this model and year.

I am using a relatively new and lightly used (I purchased new a few years ago) Crasfan compression testing kit. All the o rings are getting a good seal. When I roll the engine over, I am getting about 155psi top, however it drops quickly (over the course of maybe 15 seconds) and then stalls out at about 60psi before I have to release the pressure manually.
Is this drop indicating that there is an issue with the top end? The bike has just over 14000 miles on it, I’m not sure on the previous owners maintenance but I have been religious about it. It always seemed like the bike just is not as peppy as it should be, but I am not certain. I’ve owned the bike for 2 years now and decided to check into this and maybe rebuild it if needed.
Any information or direction would be greatly appreciated!
 

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Hello, this is my first post on the forum. I did search and did not quite find an answer to my question but I did find on this forum that the oem compression ratio is 10.7:1 for this model and year.

I am using a relatively new and lightly used (I purchased new a few years ago) Crasfan compression testing kit. All the o rings are getting a good seal. When I roll the engine over, I am getting about 155psi top, however it drops quickly (over the course of maybe 15 seconds) and then stalls out at about 60psi before I have to release the pressure manually.
Is this drop indicating that there is an issue with the top end? The bike has just over 14000 miles on it, I’m not sure on the previous owners maintenance but I have been religious about it. It always seemed like the bike just is not as peppy as it should be, but I am not certain. I’ve owned the bike for 2 years now and decided to check into this and maybe rebuild it if needed.
Any information or direction would be greatly appreciated!
"The bike has just over 14000 miles on it,"..."maybe rebuild it if needed".
In principle do not rebuild at such a low mileage.
Why did you decide that your engine was dragging weakly?

According to Honda Maintenance Book P. 8-6
  • You need to warm the engine to working temperature, and only then removed the plug.
  • The starter should be turned on for no more than 7 seconds, to not drain the battery
  • The built-up pressure should reach to 188psi at 490rpm (You measure the maximum, and do not measure the drop from the maximum).
  • The throttle should be full open.
 
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Discussion Starter · #3 ·
"The bike has just over 14000 miles on it,"..."maybe rebuild it if needed".
In principle do not rebuild at such a low mileage.
Why did you decide that your engine was dragging weakly?

According to Honda Maintenance Book P. 8-6
You need to warm the engine to working temperature, and only then removed the plug.
The starter should be turned on for no more than 7 seconds, when the built-up pressure should reach to 188psi at 490rpm (You measure the maximum, and do not measure the drop from the maximum).
thanks for the reply, I’m only suggesting if there is an issue that this test (or a future leak down test) suggests to rebuild it, not that it absolutely needs rebuilt.

I did do the compression test warmed up. Or warmed up and then tank removed to gain access, but still warm.

You are referencing that in the manual it is saying the compression PSI should be 188 warm? Certainly not going any higher than 155psi for me, that’s with a 7 second roll. I’m only mentioning the pressure release because I was confused by it, but I now realize that’s a tool issue, as the tool itself should hold pressure till the relief valve is hit. I’m going to replace the check valve and o rings on the tool, make sure it is as it should be, and then go from there.

To follow up, the compression should be 188psi when warm, approximately a 12:1 compression ratio?
 

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I’m only suggesting if there is an issue that this test (or a future leak down test)
NO issue
During the rotation of the motor pressure is built, the maximum pressure generated is measured, and there is no interest to measuring the speed of the pressure drop, it is not interesting, only the maximum at which the needle has reached is measured.
 

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Certainly not going any higher than 155psi for me, that’s with a 7 second roll
A limit of 7 seconds, according to the book, is for not drain the battery. Maybe your battery is weak so the pressure built up is also weak?
Therefore the book also mentions the RPM.
You can see if your battery is strong enough to reach 490 RPM.
 

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the tool itself should hold pressure till the relief valve is hit.
Not relevant at all, obviously the pressure goes down, all that matters is what is the maximum the needle has reached (what is the maximum pressure).
 

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the compression should be 188psi MAX when warm,
You measure only the maximum pressure
...approximately a 12:1 compression ratio?
Your device does not measure compression ratio. Compression ratio is not relevant at all for the compression test. The compression ratio is a theoretical figure calculated according to the dimensions of the piston, and it does not change due to leaks (This is a theoretical figure).
 

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post photo of your compression gauge.

The extra volume of compression-tester must be added to combustion-chamber volume when calculation CR. Depending upon design, this added volume varies based upon location of check-valve. If it's located close to spark-plug hole, only little extra volume is added and readings are high. If valve is located near gauge, volume of entire length of hose is added and readings are low.

The actual reading is measure of dynamic compression-ratio, not actual static calculated ratio. Dynamic compression is affected by additional variables such as cam-specifications, intake-tract design, exhaust, RPM, etc.

Also make sure you hold throttle WOT when cranking engine.
 

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the bike just is not as peppy as it should be,
This is common issue raised by many bike owners after having their bike for couple years.

Of course 25bhp feels slow once you get accustomed to riding and gain more skills and confidence. Then you get 50bhp bike and after couple years, it too feels slow. So you get 100bhp bike and after couple years, that too feels slow. Then you get 200bhp bike, and it too feels slow after couple years, etc, etc... I'm eyeballing H2R next...

Only true test to confirm power-output is dyno test. All others are irrelevant.

 

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Discussion Starter · #12 ·
You measure only the maximum pressure

Your device does not measure compression ratio. Compression ratio is not relevant at all for the compression test. The compression ratio is a theoretical figure calculated according to the dimensions of the piston, and it does not change due to leaks (This is a theoretical figure).
Ok so if 188psi is the MAX when warm, what is the expected?

A compression ratio tells us the PSI by multiplying it to pressure. The book is saying a 10.7:1 compression ratio, the atmospheric pressure in my area is 14.661 so 14.661 * 10.7 = 156.8727 and I’m getting a rough value of 155psi on my gauge. I am going to perform a leak down test to see if I have a different issue at hand, but I don’t think it’s rings, hoping not valves. Might be head gasket? Oil was fine on my change a few days ago, I’m going to check coolant shortly.
 

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Discussion Starter · #13 ·
post photo of your compression gauge.

The extra volume of compression-tester must be added to combustion-chamber volume when calculation CR. Depending upon design, this added volume varies based upon location of check-valve. If it's located close to spark-plug hole, only little extra volume is added and readings are high. If valve is located near gauge, volume of entire length of hose is added and readings are low.

The actual reading is measure of dynamic compression-ratio, not actual static calculated ratio. Dynamic compression is affected by additional variables such as cam-specifications, intake-tract design, exhaust, RPM, etc.

Also make sure you hold throttle WOT when cranking engine.
I don’t have a photo handy, but it’s a common craftsman kit. The valve is right by the spark plug, I am thinking the quick disconnect between the hose attached to the gauge and the hose adapter/extender (what has the valve in the spark plug thread end) is what is leaking. When I do the test again in a couple days (once I have the leak down test kit too) I will be sure it’s warm, WOT, battery is doing expected rpm, etc.

Right now I’m looking to get clarity on this 188 because if that number is fact for this engine then the rings must be gone.
 

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Discussion Starter · #14 ·
This is common issue raised by many bike owners after having their bike for couple years.

Of course 25bhp feels slow once you get accustomed to riding and gain more skills and confidence. Then you get 50bhp bike and after couple years, it too feels slow. So you get 100bhp bike and after couple years, that too feels slow. Then you get 200bhp bike, and it too feels slow after couple years, etc, etc... I'm eyeballing H2R next...

Only true test to confirm power-output is dyno test. All others are irrelevant.

It’s the lack of pep, and the fact that j cannot achieve the same top speed this summer as o did last summer. To maintain highway speed I have to have the RPMs screaming in 6th gear. I know the bike is not fast, est case top speed from the factory is 95mph, but I should be able to keep 70mph without pushing 7500rpms in 6th gear I would think
 

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A compression ratio tells us the PSI by multiplying it to pressure. The book is saying a 10.7:1 compression ratio, the atmospheric pressure in my area is 14.661 so 14.661 * 10.7 = 156.8727
I'm writing to you again. Your device does not measure compression ratio. Compression ratio is NOT measured in relation to atmospheric pressure. Compression ratio is the ratio between the suction volume (piston down) and the compression volume (piston up), and it is a theoretical ratio calculated according to:
  • The dimensions of the piston and how much it goes down + Combustion chamber volume (at the top of the engine).
  • and up + Combustion chamber volume (at the top of the engine)
Compression ratio is a ratio between two volumes,
(not between two pressures).
 

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Discussion Starter · #16 ·
I'm writing to you again. Your device does not measure compression ratio. Compression ratio is NOT measured in relation to atmospheric pressure. Compression ratio is the ratio between the suction volume (piston down) and the compression volume (piston up), and it is a theoretical ratio calculated according to:
  • The dimensions of the piston and how much it goes down + Combustion chamber volume (at the top of the engine).
  • and up + Combustion chamber volume (at the top of the engine)
Compression ratio is a ratio between two volumes,
(not between two pressures).
Alright, I just wanted to be clear on it thank you for explaining. I am well below the 188psi, I will check again when I have the leak down test setup as well. Have a feeling she will need at least rings tho I can’t imagine a 30psi difference just due to warm up on a single cylinder
 

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Ok so if 188psi is the MAX when warm, what is the expected?
If you measure 155 it is indeed a pressure that is considered low and means a sharp drop in engine performance. It does not take a dyno test to reach this conclusion.
As you have already written, this is due to one of the following 3 options:
  • Worn valves?
  • Worn rings? barrel? cylinder?
  • Engine head gasket?
And I have the impression that you are not doing the test correctly, maybe your battery is weak, maybe another problem? So I keep asking:
Why did you decide that your engine was dragging weakly?
 

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Discussion Starter · #18 ·
If you measure 155 it is indeed a pressure that is considered low and means a sharp drop in engine performance. It does not take a dyno test to reach this conclusion.
As you have already written, this is due to one of the following 3 options:
  • Worn valves?
  • Worn rings? barrel? cylinder?
  • Engine head gasket?
And I have the impression that you are not doing the test correctly, maybe your battery is weak, maybe another problem? So I keep asking:
Why did you decide that your engine was dragging weakly?
I mentioned in one of my posts that it does not seem to have the low end torque it did, but also I am unable to maintain a highway speed at what I think might would be a reasonable RPM
 

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I should be able to keep 70mph without pushing 7500rpms in 6th gear I would think
I can’t imagine a 30psi difference just due to warm up on a single cylinder
I think sixth gear does not reach at 70mph(112.65kph) to 7500RPM, and if it does then maybe the previous owners changed the gear ratio in the external transmission?
Maybe because he lacked off power?
Can anyone here please tell us what the speed is in the sixth gear when the engine is at 7500 rpm?
If the transmission ratio is different from the original in favor of added power, then even the speedometer does not say the correct speed (shows a slower speed, compared to what in reality).
For example:
  • When reducing teeth in the front sprocket, or...
  • When adding teeth in the rear sprocket.
 

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It’s the lack of pep, and the fact that j cannot achieve the same top speed this summer as o did last summer. To maintain highway speed I have to have the RPMs screaming in 6th gear. I know the bike is not fast, est case top speed from the factory is 95mph, but I should be able to keep 70mph without pushing 7500rpms in 6th gear I would think
This is correct. 70mph is approximately 7500RPM with factory gear ratios. This has nothing to do with compression or power. The gearing is static, unless your clutch is slipping, in which case you would turn higher RPM for a lower speed.
 
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