Topic: cold compression test

[KRONUS0100]

hi guys looking for thoughts, not to start an argument.  got my 78 750F3 motor somewhat apart in barn for valve adjustment, cam chain adjust, etc, basically tinkering for spring riding season.  thought to myself,  I have the pugs out and carbs off....lets do a comp test........on a cold motor  with last seasons oil still in it on a 35 degree day.  got 60 across the board..highest was 63.  book says to do a comp test on an engine at operating temps.  anyone got a guess as to what the percentage difference might be cold vs hot?  last time i did a comp test was back when i assembled this engine 3 years ago, and i have misplaced that info, but it seems to rattle my memory banks it was a bit higher than that, but it was late spring when i did it then.  keep in mind im using a cheap ass harbor freight  comp tester with a roughly foot long 1/4"  rubber hose as compared to HONDA's prefer metal tube tester.  appreciate any input from ya"ll.

[joeyvans]

hi guys looking for thoughts, not to start an argument.  got my 78 750F3 motor somewhat apart in barn for valve adjustment, cam chain adjust, etc, basically tinkering for spring riding season.  thought to myself,  I have the pugs out and carbs off....lets do a comp test........on a cold motor  with last seasons oil still in it on a 35 degree day.  got 60 across the board..highest was 63.  book says to do a comp test on an engine at operating temps.  anyone got a guess as to what the percentage difference might be cold vs hot?  last time i did a comp test was back when i assembled this engine 3 years ago, and i have misplaced that info, but it seems to rattle my memory banks it was a bit higher than that, but it was late spring when i did it then.  keep in mind im using a cheap ass harbor freight  comp tester with a roughly foot long 1/4"  rubber hose as compared to HONDA's prefer metal tube tester.  appreciate any input from ya"ll.

When I did my test last spring, I was using a cheap Acutron tester and was getting numbers in the 120's for my 550.  For me, the most important thing was that all readings were within 10% of each other for both the dry and wet test. 

I'd be curious to know what kind of testers most guys recommend for this.

Kyle

[DohcBikes]

The HF testers are notorious for reading low. If you have the right fittings and an air compressor that you trust the gauge on,  you can check the gauge against the indicated psi on the compressor.

Also remember to use the pop off button on the tester before you unhook it. Taking it apart at the coupling with pressure in it is not good for the calibration.

[70CB750]

I remember reading that Honda achieved the manual numbers by using tester with metal tubing.

60psi cold is low, the motor would not run with that compression.  Do you have another engine/vehicle to try the tester on?

[flybox1]

When its hot, you'll get some expansion of the cylinders, rings will seal better.
As joeyvans stated, you are looking for no more than 10% variance across cylinders, no matter what tester is used. 

[KRONUS0100]

did not know that about the HF testers, good to know, i will check it against my truck which i know has really good compression.  Maybe i can borrow the brother in laws snap on tester this weekend.  Bike runs great, very comfy at 70 ish.    but yeah, got 60ish across the board, well within the 10% range.  when i get done cleaning the carbs and get them back on, and an oil change, i will recheck on a hot engine and see.  Might be able to check my comp guage against my father in laws air compressor tonite or tomorrow.  thanks guys.

[bochnak]

Here is the tester I have been using for years and comes with 14,12, & 10mm adapters:

https://www.howtomotorcyclerepair.com/compression-tester/

Don't buy the HF unit! It does not have the schrader valve on hose end and results are lower. Ask me how I know.

[KRONUS0100]

well tested the guage with a known good air compressor..........HF guage is pretty darn accurate up to about 90psi, then it actually reads about 4 psi higher after that.  tested it up to 125 psi...the max of the compressor.  so, now i reckoni will finish assembling the carbs and bolt the valve cover back on, and heat up the motor and take new readings off a hot motor and see where i stand.  really hate to think i need new rings and or pistons on this motor.

[DohcBikes]

well tested the guage with a known good air compressor..........HF guage is pretty darn accurate up to about 90psi, then it actually reads about 4 psi higher after that.  tested it up to 125 psi...the max of the compressor.  so, now i reckoni will finish assembling the carbs and bolt the valve cover back on, and heat up the motor and take new readings off a hot motor and see where i stand.  really hate to think i need new rings and or pistons on this motor.

Bad rings are not the only thing that causes low compression.

You said you were out to do a valve adjustment,etc. Did you do it? Valves that are out of adjustment can cause low static compression.

[bochnak]

well tested the guage with a known good air compressor..........HF guage is pretty darn accurate up to about 90psi, then it actually reads about 4 psi higher after that.  tested it up to 125 psi...the max of the compressor.  so, now i reckoni will finish assembling the carbs and bolt the valve cover back on, and heat up the motor and take new readings off a hot motor and see where i stand.  really hate to think i need new rings and or pistons on this motor.

The HF tester has the schrader valve up near the gauge. So the LONG hose adds to the cylinder head volume, therefore reducing the reading.

The actron gauge has the schrader valve right where the spark plug resides, so readings are correct.

I too tested the HF gauge with my air supply and found it to be accurate. It's where the schrader valve lives that determines how well the gauge works.

Watch this video and note where the schrader valve is:

[bochnak]

Also, if you want to take things a bit further, you can do a leak down test. I made my own and this is how to perform one:

[scottly]

The HF tester has the schrader valve up near the gauge. So the LONG hose adds to the cylinder head volume, therefore reducing the reading.

I too tested the HF gauge with my air supply and found it to be accurate. It's where the schrader valve lives that determines how well the gauge works.

Correct, a 1/4" ID hose 12" long adds over 9 cc to the chamber volume.

[seanbarney41]

pull 1 or 4 plug, put your thumb over the spark plug hole, crank engine, if you don't hear a loud pfffft, you got problems...other than that, unless it runs bad or is smoking, who cares about a damn compression tester