Compression Question

[RodneyCB750K1979]

I just installed new piston rings and lightly honed the cylinders. I spun the crank about 20 times using the electric starter with the engine on the workbench without the carbs.  Compression ratio ranged from 90 to 110 psi.  Valves have not been adjusted and the temperature was about 50F.   Is the compression expected before the rings break in?

[Lucky 1]

No. It will improve. Check at 500 miles.
Make sure the throttle is wide open, plugs out. No air-no compression.

[shinyribs]

Did you rub an oil on the cylinder walls during assembly? Dry rings wont seal well. Once everything gets lubricated and bedded in I  imagine the compression numbers will improve like Lucky said.

[seestheday]

Adjust those valves asap. Dohc valves can be prone to burning, and your numbers should improve dramatically once they're in spec.

[750K8]

I heard motorcycle compression gauges are more accurate. Is this true?

[TOOLS1]

750K8 wrote

I heard motorcycle compression gauges are more accurate. Is this true?

I did not know there was a difference. Unless it could be the price, because it is marketed, for a "motorcycle." A compression gauge is a compression gauge. They should all be checked for accuracy. The Harbor Freight ones are notorious for not being accurate. I use a Sunpro from Advance Auto myself.
TOOLS

[750K8]

There's a couple people on the SOHC forums that tend to think that motorcycle gauges produce more accurate numbers.

I'm not sure what I believe; I'm just raising the question if anyone has compared the two.

Rodney's same gauge was used on my CB750, and it also gave low compression numbers on my 12,000 mile CB750. Sometimes, I tend to second guess the tool I'm using.

Also just read that he hasn't adjusted his valves yet. Adjusting my valves on my CB450 (which were so tight that I couldn't fit any filler gauge in) raised my numbers by 15 PSI.

[Lucky 1]

A compression gauge does not care what it is on. It does not have a brain.

[Hoosier Daddy]

Lucky 1 wrote

A compression gauge does not care what it is on. It does not have a brain.

...and a bad gauge would be bad on anything. If you know it read lower than actual on another guys engine before, it probably will on others

[shinyribs]

750K8 wrote

 Sometimes, I tend to second guess the tool I'm using.

When it comes to measuring tools that can be a very good habit to have. I have two compression testers and I always use both before I consider something to be ''bad'' or not. Two sets of calipers,two levels....

[icerigger]

A guy with two compression gauges is never sure what the pressure is, but a guy with three . . . . . . . .

[seestheday]

You could make a manometer and calibrate your pressure gauges.....


Physics for the win

Shiney, how is that for dropping random weirdness into threads?

[Lucky 1]

Do not use a manometer.

Use a dial gauge and you will have a much easier life.

No bubbles.
No fluid getting sucked into the engine.
No expensive fluid to order and wait for.
No hoses and tubes to try to store in your tool box.
No calibration every time you want to use it. (Very time consuming)
No rubber caps and fittings to get lost or split.

[seestheday]

Dude, it was a [bad] joke.  

I also didn't say to use it to check the pressure of the engine, just to calibrate your pressure gauges.

[shinyribs]

seestheday wrote

Shiney, how is that for dropping random weirdness into threads?

Nailed it!  Two thumbs up!

BTW,my gf totally butchered one of your wife's recipes the other week. It was so bad...

[750K8]

Well, new compression numbers?

[Lucky 1]

I am with Hoosier Daddy.

The compression gauge does not know it is a motorcycle compression gauge. LOL..lol

It only knows how to do one thing. Show compression.
So all these years using auto compression gauges on motorcycles and now
they ask, "Doesn't the gauge have to be specially made just for my motorcycle?"

Now we will see ads for: "Suzuki compression gauge only $500."
Specifically made for your Suzuki for the best results." Hahahahhhhahhahha

[750K8]

Honda uses a special compression gauge as seen by many of the manual photos. It's quite a bit smaller in volume.

Hopefully Rodney posts his results soon. His new compression gauge which was built for motorcycles showed gains of 7-12 PSI over his conventional automotive one.