Re: I got the no/low compression blues

David Russel


I was just looking at getting a leak down tester from Harbor Ft, one on clearance for $25, Store purchase only.  OOPs, no local stores around here have any in stock.  Never thought to ask but will see if O'Reilly has them to lend.

Did a compression test yesterday on the single cylinder R27 and given the power needed to kick it over was surprised at 80 psi dry, and hardly up any with oil.  Was sad and was thinking about buying a more expensive leak-down than I need, just to track down the problem area.  Din't need a tester to ID the problem area as it was ME.  Neglected to open the throttle.  Tested today, throttle open and got 120 psi dry and didn't bother with oil as this reading is good enough

have a H FT borescope and yesterday discovered the camera/light unit is too fat to pass thru the spark plug hole.  Would not have bought the scope if I'd realized the no-fit size.  It was possible to use it to peer into the  cylinder but didn't get to see much and don't have the experience/knowledge to interpret what it showed other than the top of the piston being black and not having a hole, both of which I knew without looking.

Good luck on your cycle engine project.  Assume it is a 4 stroke so easy enough to verify that both pistons are sliding up and back, then with valve covers off the valve movement can be checked.  Leak down tester might do you a lot of good.


On Tue, Dec 1, 2020 at 8:44 PM Bruce Dewing via <> wrote:
Yes, I'm using a real compression tester (screw in). 

I've ordered a leak down tester.  A couple of days out tho.

The values given are a regular compression test dry (80 lbs)  2 minutes later wet (50 lbs) then again 20 lbs, then 25 lbs, then zero.  Used a 2nd (hand held, jamb it in the hole) compression tester showing 0 psi.  Two days later, I was cranking it over with my index finger sealing the spark plug hole, with little resistance.  It's a two cylinder upright, will try again with the carb off and both plugs out.

I don't have a smart phone but do have usb video camera/cable, will give it a go.

Thanks for your input,  Bruce

On Tuesday, December 1, 2020, 8:10:46 PM PST, Ken Nelson <citbuff@...> wrote:

Whatever David, but I sure wouldn't trust those nos.  Any parts store will loan a real compression tester free - 


On Tue, Dec 1, 2020 at 9:23 PM David Russel <djrussel@...> wrote:

Think he’s doing a leak down tester without a real leak down tester. First number is starting press and second is what it leaked down to. 

Did I get it right Bruce?


Sent From Mobile Phone

On Dec 1, 2020, at 6:09 PM, Ken Nelson <citbuff@...> wrote:

Hi Bruce, this is a 2 cyl right?  Did you do a spinning compression test with plugs out, carb throttle plate full open PER cylinder, if each cyl has its own carb - then spin with starter & compression test gauge which holds pressure?  That's the only way I know to get a true reading, and many guys forget to hold the throttle wide open to allow free movement of air to cyl.  Those pressures are just too wacky to interpret from my end.   How did you get the original 80, then 50, then 20 psi to zero??  
if you did nothing more than spin engine with both plugs out, and listened to the air pumping in/out of each cyl, and they sounded definitely different, then I'd suspect burnt valve, broken rings or something - 
I once had a Citroen DS19 4 banger basically stop wanting to run when No. 1 cyl dropped to 70 psi from 140 or so.  Turned out none of the 4 pistons had complete rings, just cracked pieces in each groove, except for no. 1 piston.  That one had NO upper ring at all - just carbon-filled groove.  My leadfooted son ran it so hard the piston crown melted thru at pt above the no - ring groove, and compression dropped as result.  But ALL the remaining rings were broken but in grooves except for that first one, and I'm surprised the engine ran for over a yr at all!  

Could a hole have melted thru the piston crown?  It's been known to happen - Got a borescope for your smartphone?  I've heard they're as cheap as 20 bucks on the web - 


On Tue, Dec 1, 2020 at 4:06 PM Bruce Dewing via <> wrote:
Some advice please.  I'm helping a friend with his 71 Honda 600 sedan (you know, those motorcycle engine cars).  Compression test right side, it went from 80 psi, to 50 psi (wet) to 20 psi to 25 psi to zero psi  in 10 minutes time.  With piston at TDC, I did a hillbilly leakdown test (no leakdown gauge, just connected a 3 gallon air tank w/gauge to the cylinder).  No change with the tank gauge, listened with a mechanics stethoscope & didn't hear anything.  Then adjusted loose intake valve and pressure is now 30 psi.
Am I missing something?

Grass Valley, CA

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