Air Leak Down test reveals porous inlet manifold

[Dale]

I had heard of this but had not experienced it myself. I performed an air leak down test on a fresh Combat Wombat build. The motor would pass the "no more than 1 psi lost in a minute" but was slowly leaking pressure. Typically, fresh builds with all new seals do not leak any perceptible amount. So, I was determined to find the source of the leak.

It turned out that the body of the carburetor inlet manifold was leaking right through the surface. A very small leak, but a leak none the less. A good cleaning and a skim of JB Weld has cured this. It seemed like a really good use for JB Weld to me.

Air leak through porous carb inlet manifold

[matt glascock]

Interesting Dale. Did the manifold look grossly normal or was there a degree of corrosion?

[Dale]

Interesting Dale. Did the manifold look grossly normal or was there a degree of corrosion?

Totally normal. No sign of corrosion. The flaw was literally invisible before and after its discovery. Only air pressure and windex revealed it.

[matt glascock]

Well well. I've heard casting flaws are always a possibility - in theory - but I've never see it in the real world. Very interesting. I've always thought it was more of a problem in prototype motors and parts.

[Darrell]

Porous castings happen. I've had factory GM alloy wheels that leaked air through the metal; as well as an auto transmission case that sweated fluid also due to the porosity of the casting.

[matt glascock]

Hi Darrell. Is this a metallurgical or a technical problem?

[taber hodaka]

Machined aluminum would not leak, poured cast aluminum could. Look at the rough surface, like concrete not vibrated ??. ---- Clarence

[Darrell]

Poured concrete is the best analogy for cast aluminum porosity. When concrete isn't completely consolidated/vibrated inside the formwork we call it "honeycombing" (at least where I come from). If the concrete isn't vibrated well enough then it doesn't flow completely around restrictive areas like rebar cages and formwork corners.

An overly dry, or stiff, concrete mix can exacerbate honeycombing; so in light of this I suspect that porous aluminum castings might have been poured when the molten aluminum was too close to cooling off. I'm a carpenter rather than a mechanic, but I think the crossover comparison is apt.

[Dozerash]

What are you using for a leak down tester? I’m in the process of building one and using my small engine carburetor leak tester to pressurize it.

[Dale]

What are you using for a leak down tester? I’m in the process of building one and using my small engine carburetor leak tester to pressurize it.

My leak down tester is a crude combination of plumbing pieces and parts that I have adapted to fit to every model of Hodaka.
Various sizes of freeze plugs to block the exhaust port.
Various types and sizes of connectors for the carburetor inlet depending on flange or spigot mount.
My air input valve is on the body of the unit that also has the low pressure gauge.

All crude, but works nicely.
I keep thinking that someday I will build a nicer looking kit that would also test vacuum. Someday...

A tip for sealing connections: A thin coat of grease on any rubber or PVC connector will make them air tight.

[Dozerash]

I have enough stuff to almost build two leak down testers. The aluminum piece I’m planning on turning down on my lathe to fit in the intake manifold.

[ossa95d]

I like the idea of turning an aluminum piece to fit the intake manifold. It allows you to include the rubber manifold and all the connections between it and the engine in the test. The Motion Pro leakdown tester has several aluminum connectors to fit every size spigot carburetor mount.