Yet another Compression Testing topic!

[gltriker]

X

[desertrefugee]

I can tell you now (new knowledge) that the CBX compression is spec'ed by Honda at 170psi +/- 10. The thing is, all the CBX guys are saying that they've never seen one run that high. 160 seems to be tops and folks are happy at 130. Mine CBX runs 135 - 145. My GL1000 runs 140 to 150.

[gltriker]

X

[robin1731]

Featuring the fractured bump stop on the #1 carburetor's CV vacuum piston cover?

Now, I have to remove the carburetors to repair the damage.

No need to completely remove them. Just unbolt them and slide them over. All you need to remove is the air filter housing. Don't even need to take the cables off. Just a little over to get to the carb top screws.

.

[gltriker]

X

[chewy999]

I've never heard of doing a compression test without the carbs fitted and don't see why you need too, maybe I'm wrong (been wrong many times before)
I like to believe I think out issues in a logical way, which can be dangerous!
My understanding is that a compression test checks the sealing ability of the rings and valves, so with a gauge fitted, throttle fully open, kill switch on you press the starter, for a few seconds until the gauge reaches its max reading. That is because due to the engine cycle, the valves are opening and shutting in split seconds letting air in and out, but part of the four stroke principle is COMPRESSION, both valves shut and piston at the end of it's stroke. You need to operate the starter for a few cycles to allow the gauge to reach the max pressure, once you stop, the gauge should then maintain the pressure until it's reset.
It's similar to when you put your finger over the hole of an old bicycle pump, You only need the air in the cylinder to feel the compression!
Just my (logical) thoughts.

[gltriker]

X

[chewy999]

I've never heard of doing a compression test without the carbs fitted and don't see why you need too, maybe I'm wrong (been wrong many times before)
I like to believe I think out issues in a logical way, which can be dangerous!
My understanding is that a compression test checks the sealing ability of the rings and valves, so with a gauge fitted, throttle fully open, kill switch on you press the starter, for a few seconds until the gauge reaches its max reading. That is because due to the engine cycle, the valves are opening and shutting in split seconds letting air in and out, but part of the four stroke principle is COMPRESSION, both valves shut and piston at the end of it's stroke. You need to operate the starter for a few cycles to allow the gauge to reach the max pressure, once you stop, the gauge should then maintain the pressure until it's reset.
It's similar to when you put your finger over the hole of an old bicycle pump, You only need the air in the cylinder to feel the compression!
Just my (logical) thoughts.

okay, here we go......

As I'd offered in post #1, I had been wondering how much, if any, difference in compression reading numbers might possibly be observed when comparing an open intake port compression test, vs. carbs installed with wide open throttle compression test. That's all. Simple as that.
In my almost 69 years, I've professionally accomplished a large amount of gasoline and diesel engine systems troubleshooting, and repairs. anyway....

As you may have observed in posts 2 and 3, desertrefugee and I had both comparatively seen compression test results in the mid 140 psi range on our respective, very good condition, GL1000 engines. The factory GL1000 service manual standard is 171 psi.
(I'm not trying to rewrite the Honda service publication.)

As is commonly known, there's a significant difference in compression test numbers when comparing a fully open throttle , cylinder compression test, to a closed throttle, cylinder compression test. Agreed? Yes Sir, been there done that!

Certainly, even though the throttle plate might be fully opened, there's still a large restriction present in the carburetor throat with the lower end of the vacuum piston, sitting closed in its resting position. In my mind, the volume of air passing under the "slide", at engine cranking speed, most likely isn't sufficient to even slightly lift the vacuum piston.

Whether that resting "slide" 's prevailing air flow restriction is enough to significantly reduce air flow, during the compression testing procedure, is an unknown. For me, it is.

And, now that I have to unbolt and move the carburetors to remove the # 1 vacuum piston cap to repair the previously mentioned fractured bump stop button, anyway, what better opportunity to perform the open intake port compression testing could ever present itself, to do an open intake port compression test, too!

Respectfully speaking, Until I perform that open intake port, cylinder compression testing, today, and subsequenty report the results herein, let's let my topic rest 'til then. Pretty Please? Ok, be interesting what you discover.

Fyi, last time I did the check, 1980 1100 carbs on, all four readings came in at the low 160's, which pleased me a great deal.

[gltriker]

X

[robin1731]

Don't forget, all spark plugs removed.

.

[Oldewing]

I have a very rare 1100 engine. NOS it is, never fired (yet)

It will bring 170 on the second compression stroke, so 171 sounds very correct to me.

[gltriker]

X

[gltriker]

X

[Whiskerfish]

So no diff between carbs on with Wide Open versus carbs off correct??

[gltriker]

X