Planning a Motorcycle Engine Leak Down Test

[Braden, L]

Hey, Moses, I've been watching your xr650r videos. I was wondering how much psi you input into your snap-on device to perform your leak down test on your motorcycle engine. Would too much air force the piston down in the cylinder, open the valves, etc? 

I'm looking at doing a leak-down test on my bike soon and thinking the whole process through. Thanks!

[Moses Ludel]

Braden, L...Glad you're into the videos, is your bike an XR650R?  If you have an auto-decompressor on the starting system, it's difficult to perform a leakdown test without first loosening the valve adjusters enough to keep the decompressor from interfering with the valves and releasing compression.  Note: I got around this with the use of the HotCams Stage 1 camshaft that eliminates the auto-decompressor mechanism from the valvetrain.

You're right that the valves need to be completely seated.  If the valves are seated, you could actually test the cylinder with the piston at any position.  However, you do want the piston at TDC (top dead center) for this test, and here is the reason:  The maximum wear on a cylinder is at the top of the piston ring travel, where compression is the highest.  By design, the piston rings expand with increasingly more tension as cylinder pressure increases.  Combustion plays a role here, too, adding to this ring pressure against the upper cylinder wall.

Compression rings are designed with an inside ledge that provides a space for cylinder gases to apply outward pressure to the compression rings.  This provides a cylinder seal as the piston rises on the compression stroke, and the cylinder pressure builds.  This design feature contributes to "taper" wear;  envision the pressure increasing as the piston compresses gases or when combustion takes place on the power cycle.  The maximum taper, as you would imagine, is at the top of the piston ring travel.  

This is where a ledge or "ridge" builds on iron or aluminum block engines.  On your motorcycle cylinder, there may be a hard Nikasil coating, which still would be prone to taper, though very minutely.  What you see with Nikasil wear is reduced crosshatch near the top of the cylinder.  L.A. Sleeve Company's Moly 2000 chrome-moly iron alloy liner on my XR650R engine build will show that kind of honing wear after long use.  I will likely be able to re-hone the cylinder and re-ring the engine (new piston,too) without re-boring, although I'm at standard bore diameter now and could re-bore and hone the cylinder years from now.

So, you want the cylinder reading to reflect seated valves and a piston at its TDC.  If at actual TDC, the leakdown compressed air should not force the piston downward.  (As the piston passes over the TDC point, the crankshaft pin is straight upward and not "compressible".)  If not at precise TDC, the piston will want to go downward from the leakdown tester air.  

As for pressure, the Snap-On tester I use calls for a line setting of 60 psi.  This is supposedly enough to get an accurate seal, but I find this figure unrealistic.  Some engines will not get an accurate ring seal reading at low input pressures.  True cylinder pressures while cranking a modern engine can reach 160-plus psi, which helps expand compression rings and seal the cylinder.  Compressed air tests at reasonable pressures will not create an issue.

Caution:  It does not hurt to use 100 psi unless you suspect a leaking head gasket or cracked casting at a liquid cooling jacket.  You do not want to introduce high pressure into the cooling system of an engine.  Cooling systems are designed for 20 psi or so.  Also, we're not talking here about "zero-gap" piston rings used in all-out racing engines, which call for specific leak down testing procedures.  

Think of a compression test.  Normal cylinder pressure might be as high as 160-170 psi.  It certainly will not hurt to apply 100 psi—with one caveat:  As you introduce leak down test air, you will be blowing the oil film off the cylinder wall and the rings if you apply pressure for any length of time.  If you suspect that oil film/ring seal is declining, try cranking the engine over several times then return the piston to TDC on the compression stroke.  Reapply your leak down air pressure.  You'll quickly discover how oil provides a sealing film.

My rule-of-thumb is 8%-12% leak for a fresh or good sealing engine.  (8%-10% leak with production type, quality piston rings is a new engine standard.)  Four-stroke street/trail engines will typically deliver decent performance with as much as 18%-20% leak.  Here, however, you know the parts have wear and need to plan for a rebuild at some point soon.  Beyond this degree of leakage, a performance decline will be noticeable.  

What you will discover from leak down testing is how much better you can pinpoint trouble and wear when compared to a cranking compression test.  I've had engines produce somewhat normal cranking compression with a compression gauge and test 40% or more cylinder leak down.  With a leak down test, you'll know when the engine is on "borrowed time" and have a good idea what to expect at tear down.

Moses

[Braden, L]

Moses, as always your thoroughness is must appreciated. I have a 2001 xr650r with "low miles". I'm performing a leak down test with the same snap-on machine you used in your video, however the manual wasn't included in the used purchase. I'm doing this so I can eliminate an engine rebuild on my list of must do's to the bike right now. In the future I plan on street legalizing it, with baja headlight and tusk products. Another quick question, I saw someone on YouTube doing a leak down test on a motorcycle engine during the process he hit the top side of the valves with a piece of wood and a mallet to break up potential carbon build up so there wasn't as much of a leak. Is this something you suggest if the leak is greater than 20%? Doesn't sound right to me. Thanks!

[Moses Ludel]

Braden, L....It's not far-fetched for carbon to prevent full valve seating.  With the engine stationary, the risk applies.  The block of wood protects parts, a light rap on the wood with a hammer would unseat the valve with spring force rebound to crush loose carbon.  

I prefer rotating the engine with the leak down tester's air applied to blow away any carbon flakes.  If much carbon buildup is present, there's already a hint of rich fuel mixtures or low compression, items you would want to consider.  Again, make sure the valves are seated when running the actual leak down test.  

I'm very pleased that you have a Honda XR650R, you will value its versatility and willingness to perform all of your chores and expectations.  The flexibility, to me, is quite remarkable.  Adjustment to the weight and mass, especially with a 6.3 gallon Acerbis fuel tank, has not been that difficult.

I've been riding Honda XRs for two decades and can readily change between the LRP (Little Red Pig 1984 Honda XR350R) and the BRP (2000 Honda XR650R) without a lot of consideration—other than 55 versus 23 horsepower of these tuned and prepped bikes!  The XR650R's dual-sport weight gain has not been an issue in any riding situation yet.

Moses

[Braden, L]

You said in your leak down test video, that some of the damage was caused by the filter not seating properly. Would a filter that sits directly over the carb be a better option? Thanks-Logan B.

[Moses Ludel]

Good question, Braden, L...I do not like an exposed air cleaner element at the carburetor.  

First, it will collect more dirt when drawing air there.  If you do ford water, even moderate streams, the engine can suck water through the filter at the carburetor and "hydraulic" (actually, hydro-lock) the engine, which can result in severe damage, including a broken piston and/or connecting rod.  You might also fly over the handlebars as the engine locks up.

I prefer the factory remote air box as a buffer against extreme dust exposure and water intrusion.  Some do run an air cleaner at the carburetor—I don't.  On my XR650R, I even opted for the complete factory air cleaner and fire trap assembly from Honda.  All of the original pieces are new and in place.  The beast hasn't starved for air since the rebuild, and I'm sure there's plenty of CFM flow...even with the Stage 1 HotCams camshaft.

Moses