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Long EZ Carb Aibox

Paul C

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I’m looking for a compact inline airbox w/carb heat. I suspect my current setup is restricting airflow to my #4 cylinder. I have the O235 and my #4 CHT is >450 at idle. I think the smaller inline setup with a full NACA air diffuser will help. Vans Aircraft has a filtered air box but they have a long lead time. I’m hoping to find/build something within the next couple weeks. Does anyone have one for sale or know of another source? Thanks.

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This doesn't sound like an airbox problem.  Restricted carburetor airflow would cause rich (cooler) running or less power output and thus lower cylinder temperature.  It sounds more like a gauge problem.  It could possibly be lean running from excess air like with an intake air leak but I would think that if your other cylinders are normal, it would be hard to get a huge difference in one cylinder, even from an intake leak.

Maybe you are breaking-in a cylinder which might run a little hotter but you shouldn't let it get more than about 300-350 for a ground break-in.

I would test the CHT indications dipping the sender into a tin can with an inch of oil heated with a propane torch and measuring the oil temp with a candy thermometer.  Compare a couple of cylinders.  Also consider that at idle, an engine is not making much power and not rejecting so much heat so it is harder to get temperatures that high. 

However, 450 is very high and parts of a cylinder at idle airflow can develop hot spots and be even hotter.  Lycoming says for best engine life, keep the temps below 400.  At 425 on the ground, I am shutting it down to cool off.

Edited by Kent Ashton

Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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Thanks Kent. What I would give to have one-tenth of your knowledge…

I’ve been chasing this rabbit ever since I bought the Long EZ in February. I thought I had it fixed by patching all the rips and holes in the silicone baffle seals and orienting the seals correctly. With that,  I’ve run the engine with a leaf blower aimed in the NACA and CHTs do not rise above 400. Without the leaf blower, CHTs rapidly rise to ~450.

My high CHT issue is isolated to #4 (although #3 runs in the upper 300s). #1 and #2 are in the lower 300s.

Take a look at the attached pics. I have up-draft cooling with no air diffuser from the NACA and the carb air inlet with no filter oriented below the #4 cylinder. I think this off-set orientation plus my carb heat SCAT, is blocking airflow from reaching the #4. I hope by moving the air inlet and SCAT, this will allow air to more freely flow to the #4 (similar to your set-up).

I spoke with the folks at Vans this afternoon and have purchased one of their filtered air boxes. It’s for a 320 so I’ll have to modify it to fit the 235. While I’m waiting for the air box, I’m going to test the CHT sender.

Thanks you your input.







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Some random thoughts:  I get the impression you are only ground-running this engine.  That will only tell you so much.  If the engine starts normally in 2-3 blades, has good oil pressure, idles smoothly, accelerates smoothly, you lean it as much as possible on the ground  and after it warms up a bit it appears to pass the "idle mixture check", then there can't be much wrong with it.  At Arkansas spring temperatures _in idle_, it will warm up slowly--I'm guessing maybe 10 minutes to reach 300-325 degrees.  It was an 1800 hour engine, right, so we know it probably ran OK and you're not seeing problems from a break-in.  An older engine might get a sticking exhaust valve and display "morning sickness" until it warms up, then appear to run normally.  Your compressions are good(?) but even a low compression engine should run smoothly.   There is not much point is ground-running beyond that unless you're making adjustments to idle mixture.

A miss-timed engine (too advanced) can run hotter than a properly timed engine but you are only complaining about one cylinder.

I don't think the airbox is your problem but you could run the engine without the air box--even fly without it for a while.  That mesh is only keeping squirrels out of the engine  🙂

A diffuser might help but the improvement can be marginal, so I wouldn't worry about one until you can fly it. 

An intake air leak might be noticed during a compression check.  I saw a video where I think they backed the prop up until the intake valve began to open and the pop as the compression was suddently released into the intake was seen as a sudden leak at the the intake piping.  The intake pipes are swaged into the sump.  I have also read about squirting propane around the intake swages.  If they are loose, the rpm will rise as the propane is sucked in.

It is often true that in flight, #3 and #4 run hotter by say, 25 degrees (or so)  because plenum air piles up against the aft bottom baffle and cools #1 and #2 well but until you fly, you can't tell.  The angle of your airbox and SCAT might be degrading the flow up to #4 but you wouldn't see that when ground-running.  You might take a look at what this guy did   https://alongwayroundtheworld.com/category/non-build-mods/ram-air-box-for-jze/

Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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