Compression test 90PSI???

[dem0nk1d]

Guys,

Just did a compression test on my motor since it doesn't seem to have the power it once had. I did a compression test and hit 90psi on the gauge. I'm thinking it maybe time for a rebuild and go big bore. Question is the starter doesn't continue to crank over and over. So i'm wondering if i should hit it with 12v's directly and get another reading. What do you guys think?

[dem0nk1d]

also note i tested on a cold engine.

[honkerman]

Guys,

Just did a compression test on my motor since it doesn't seem to have the power it once had. I did a compression test and hit 90psi on the gauge. I'm thinking it maybe time for a rebuild and go big bore. Question is the starter doesn't continue to crank over and over. So i'm wondering if i should hit it with 12v's directly and get another reading. What do you guys think?

I'll let you know the compression rating when I get home and can check my manual. Tbh, I think there is a compression release on the exhaust cam to aid in starting, in which case a compression of 90psi might be correct. I'll know better when I look at the manual

[homie]

YAY!!! dem0nk1d sighting
"PCX doesn't seem to have the power it used to"
could be donuts ya know, check the dunlap over the belt

[dem0nk1d]

YAY!!! dem0nk1d sighting
"PCX doesn't seem to have the power it used to"
could be donuts ya know, check the dunlap over the belt

Homie,
hahhaa yes im still around just lurking in the shadows. the manual states something about create a certain amount of compression at a certain RPM. But i couldn't find a static number in the manual for testing cold. I also figured out i could manually crank the engine over with the 22mm or is it 23mm socket directly on the variator side of the crank. Thus not using any juice from the battery. =P

honkerman,
let me know what numbers you get with the motor cold. I would like to know what my options are.

I also may have found an alternative option for a aftermarket 1.6 BAR Radiator Cap that was purchased locally at the yamaha dealership. I'll post an update when i get home.

[honkerman]

I'll let you know if i find anything in the service manual...of course, it may be a moot point. I just recalled seeing somewhere that you have a cam kit and bore kit on yours. The info in the service manual won't apply if that's the case.

[honkerman]

Ok, the service manual on page 10-3 should be 159.3 at 850 rpm.

As for the process, it's a normal compression test process. Warm the engine, unplug the spark plug, attach the tester and crank it 'til the guage stops. Make sure you've got the throttle wide open when you do the test.

[dem0nk1d]

Honker,

That is correct as i've read that page over and over. But what stumps me as you should be able to do a compression test while the motor is cold. But besides the fact, they state 159.3 at 850rpms. But i don't even think the starter can even crank the motor over that fast. I find the book to be kind of misleading on this subject. However i could be totally wrong. My bike is in pieces at the moment as I'm going to be doing some minor mods to it.

Also my motor is pretty much stock except for the variator, shorten stock exhaust, and intake. I do want to believe the motor has some kind of strange exhaust valve relief like you stated above a few post.

[honkerman]

Honker,

That is correct as i've read that page over and over. But what stumps me as you should be able to do a compression test while the motor is cold. But besides the fact, they state 159.3 at 850rpms. But i don't even think the starter can even crank the motor over that fast. I find the book to be kind of misleading on this subject. However i could be totally wrong. My bike is in pieces at the moment as I'm going to be doing some minor mods to it.

Also my motor is pretty much stock except for the variator, shorten stock exhaust, and intake. I do want to believe the motor has some kind of strange exhaust valve relief like you stated above a few post.

850 RPMs isn't that fast. My instructor here at school verified my suspicion that it's likely that they're expecting the starter to be providing that 850, so just ignore that bit. Basically, if you do the compression check per the manual, they're expecting you to get the 159 lb spec. Now, if your starter motor is going, that could certainly change your RPMs and give you a lower reading. Do a leak-down test and see if it holds pressure.

[dem0nk1d]

Honker,

That is correct as i've read that page over and over. But what stumps me as you should be able to do a compression test while the motor is cold. But besides the fact, they state 159.3 at 850rpms. But i don't even think the starter can even crank the motor over that fast. I find the book to be kind of misleading on this subject. However i could be totally wrong. My bike is in pieces at the moment as I'm going to be doing some minor mods to it.

Also my motor is pretty much stock except for the variator, shorten stock exhaust, and intake. I do want to believe the motor has some kind of strange exhaust valve relief like you stated above a few post.

850 RPMs isn't that fast. My instructor here at school verified my suspicion that it's likely that they're expecting the starter to be providing that 850, so just ignore that bit. Basically, if you do the compression check per the manual, they're expecting you to get the 159 lb spec. Now, if your starter motor is going, that could certainly change your RPMs and give you a lower reading. Do a leak-down test and see if it holds pressure.

Started the bike up for 6 mins at idle to warm the engine up. Tested the compression once more and came out with another reading of 90 PSI. I'll have to borrow a leakdown tester from my neighbor tomorrow. Sucks and I have a ride coming up soon which I cannot miss. Might have to take the Zuma 125.

[dem0nk1d]

So according to Page 10-9. It goes into very little detail about the "Decompressor system". So then how to i calculate the real PSI from 90psi if it's letting compression go??

[honkerman]

So according to Page 10-9. It goes into very little detail about the "Decompressor system". So then how to i calculate the real PSI from 90psi if it's letting compression go??

Since the manual indicates that the compression test should return a much higher value, it's possible the decompressor is staying engaged. A leak down test may tell you a bit more.

[sendler2112]

What makes you think the power is less?
.
It is not a normal starter. It is an AC generator which is run as a starter when it is not generating. There is no way to apply 12v to it.

[dem0nk1d]

What makes you think the power is less?
.
It is not a normal starter. It is an AC generator which is run as a starter when it is not generating. There is no way to apply 12v to it.

Anyhow what I ended up doing was use the 23/22mm nut on the variator and turned the motor over with a electric drill. =P I'm still seeing 90psi anyway i do it. Anyone do a baseline compression test for me on their pcx. I suspect the decompression lobe is giving me an off reading. I will borrow that leak down compression tester today...

[dem0nk1d]

Put a few cc's of oil in the cylinder, and no difference. Leak down in the morning. It would be great if someone could get me a baseline reading if possible. I suspect uncle Honda stated the compression test incorrectly in the service manual, and didn't account for the decompression lobe. More info tomorrow.

[sendler2112]

What is your current top speed? Is it lower than it used to be? By more than what normal belt wear would account for?

[Alibally]

I've a compression gauge but haven't got an adapter for the smaller thread plug as my one is used on cars that have a bigger plug. I'll have a rake about to see if I can get one.


Sent from my iPad

[dem0nk1d]

I've a compression gauge but haven't got an adapter for the smaller thread plug as my one is used on cars that have a bigger plug. I'll have a rake about to see if I can get one.


Sent from my iPad

Sendler,
i was hitting the back roads on my test. Best i could do was 60. I really need to put the panels back on the bike and hit the freeway. I would usually hit the limiter at 78mph. =P


Alibally,
I got the Harbor Freight version comes with three different spark plug thread sizes.

[sendler2112]

How did you ever hit 78 mph?, My 2013 PCX150 would do 66 when brand new. Started to dip to 64 mph by 4,000 miles with some belt wear.. And will sometimes hit the rev limiter 72mph down hill with new belt, variator , and sliders. but more usually run out of power before hitting the rev limit at 68 mph.

[dem0nk1d]

Im running the NCY variator, pod filter directly on the throttlebody, biggest tires you can go on stock rims, vented cvt case, gutted and shortened stock exhaust... and i think that's it for now. Also weights of course can't remember what weight.

Side note I'm drinking a beer by Ale Smith called My Blood Valentine Ale... OMG!!! It's really good!