Group, Hilborn sells their own version of a Leakdown Tester, one gauge
only, which reads directly in Percentage Leak. Dead simple to use.
Used for:
Preliminary setting of Metering Valve (also known as barrel valve).
With mechanical constant flow fuel injection (Hilborn, etc.) the pump
flow has to be cut way down for starting and idling, or it will be
vastly too rich to run. Hence the metering valve, which has to be set at
6-8% of full flow for gas on my engine as a preliminary figure. After
starting and warmup the metering valve is adjusted with its turnbuckle
to the best idling setting according to the particular engine. As the
throttle is opened the metering valve quickly goes to wide open and the
engine then runs on the pump and the bypass jet size.
With fuels other than gas a different percentage of "leak" figure would
be used (given in manual).
The leakdown tester is used, of course, with line pressure from your air
compressor, commonly at 80 psi on your compressor regulator.
Next thing your leakdown tester can be used for is "flowing" your
injection nozzles, with an adaptor in the Hilborn kit. Using this, you
can make a quick check that all your nozzles are flowing about the same,
proving none are clogged. In my case, they all show around 9% leak on
the gauge, the exact figure is unimportant as long as they are close to
equal.
Last thing you do with your leakdown tester is leak your cylinders to
verify sealing of rings and valves. This requires an adaptor (provided)
to your spark plug hole. Had to make my own adaptor for the (hemi)
Ardun with the plugs buried down in tubes. Once again, dead simple with
the one-gauge system. Plug your air in, the dial reads in percent leak
(piston at top dead center, valves closed, of course).
People report with ideal sealing they see 2-4% leak. "gapless" or
"total seal" rings supposedly, and super-good valve jobs. Traditional
rings and valve jobs, probably not this good. A pilot friend of mine
recently told me FAA standards for piston engine leakdown inspection
call for a minimum of 60 psi on the "cylinder" gauge with 80 psi on the
"air line" gauge, using the two-gauge leakdown system. I take this to
mean 25% leakdown on the one-gauge (Hilborn) system described above.
With a used engine, your valves and rings are going to leak some.
Might mention the time element too, you could get a certain percentage
of leakdown on a test, which keeps steady pressure on the valves and
rings, and still get a high figure on a compression test, which shows
the pressure the piston can create on a quick squeeze basis, which is
like actual running.
Using the two forms of testing together will tell you everything you
need to know about the mechanical condition of your cylinders.
Quality traditional compression testers with various adaptors are
available from NAPA stores.
Cheers from ArdunBill in the Great Dismal Swamp, Chesapeake, Va
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