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Air filter

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Air filter

Postby hobson » Tue Jul 24, 2018 5:05 pm

Hi all, had a problem with acceleration recently, basically the engine was struggling when I went full throttle on take off so I aborted.

I suspected fuel or air and started with the filter, no real issues found, old style oval filter...

The scat hose wasn't the best and no doubt contributed.

I removed the cowling and fitted a performance K&N style motorcyle filter directly to the carb to confirm my suspicions, wow - what a difference... the plane now had a real kick up the backside on full throttle rather than pondering its way to take of speed.

Sadly I can't fly it in this configuration but was wondering if anyone else had improved the breathing and how, aside from usual cobra etc, has anyone fitted a performance filter in the airbox? Or bypassed the airbox with a filter located near the air intake?
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Re: Air filter

Postby diablo » Wed Jul 25, 2018 11:17 am

What airframe is the jab engine on ? Do you have an air scoop on the airframe/ cowling ?

strange, never had a problem with my SK or SPL ...
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Re: Air filter

Postby Steevo » Wed Jul 25, 2018 7:07 pm

You have to be a little bit careful when changing bits like this. If it is stuttering at full power, that could be for a number of reasons. Too rich, too lean mixture etc. I suspect using a filter in the way you have has increased or decreased the airflow which could have a big leaning/richening effect on the engine. That will give extra power but at the expense of heat in the case of a lean mixture. Too lean and the EGT temps will rise considerably so you will need to check these if you want to run that configuration.

I have not tried different filters myself but have heard of a number of people who have and have had mixed results. Some of these filters are required to be sprinkled in oil which has caused problems.

I think I would start by trying to find the initial problem which could be airflow some how. It could also be things like float height in the carb and at full power it is far too rich and by increasing the airflow you have leaned out the mixture a bit. It could even be a jet that is too big or too small. It could even be the new filter is not providing as much airflow and the mixture was too lean and restricting the airflow has made the mixture more rich.

Lean of peak will provide better performance but at the expense of raising EGT temperatures which may cause longer term damage.

Also, in the UK it wouldn’t be an easy task getting a change like this through the LAA etc.

But it is always a great learning experience trying these things and there have been great results in the past with small changes.
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Re: Air filter

Postby hobson » Thu Jul 26, 2018 1:09 pm

Airframe is SK so bog standard install. I believe the scat hoses were contributing to airflow restrictions couple that with a dry and dusty summer and no doubt more clogged than normal air filter.

As soon as I fitted the performance filter the change (albeit ground runs only) were significant. Plan is to simply replace the filter with new and improve airflow through the hoses, was just wondering if anyone had modded the filter though. Stock filter is I believe off a Briggs and Stratton lawnmower engine (correct me if wrong), if correct - when you think about what its designed to do it would have a restrictive flow due to the dusty work load. Can anyone recommend an alternative oval filter?
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Re: Air filter

Postby Pops » Fri Jul 27, 2018 9:01 pm

They are very sensitive to dirty air filters - if I let mine get dirty it very quickly affects the running of the engine. But with normal maintenance it's never been a problem on my SK.
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Re: Air filter

Postby pilottrev » Sat Jul 28, 2018 4:06 pm

After reading your post I decided to change my air filter, it too is the oval type. Oh my God what a tight squeeze, I don't have very large hands but struggled to get a spanner on the retaining nuts inside the air box, I would think captive nuts on the back plate would make for an easier solution but hey I didn't design it, the forecast is for a few days of bad weather so a little more tlc passes the time.

safe landings

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Re: Air filter

Postby hobson » Sat Jul 28, 2018 9:12 pm

Yes its a ridiculous fit, but that's what you get when you fit a Briggs and Stratton lawnmower filter into a small airbox... Anyway, I bought a new filter, replaced all the hoses, filed out the air intake a bit as it's restrictive, ran it until warm, problem returned... sudden full throttle = engine dies/choked. No issues at all with the performance filter connected directly to the carb.
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Re: Air filter

Postby diablo » Sun Jul 29, 2018 8:54 am

It sounds like your engine is flooding. This could well be down to airflow.

Are you using a Briggs and Stratton air filter or an original jabiru obtained filter ? They are different and not a straight swap. I have personal experience and reports of engine runningvery rich and not reaching max RPM with the B&S filter.

https://www.ebay.co.uk/p/Air-Filter-Fit ... 1680825107

Is only rated for a 125 engine.

The jabiru filter is bigger and is rated for a much larger engine.

https://skycraft.ltd/acatalog/Air-Filte ... ml#SID=349
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Re: Air filter

Postby hobson » Mon Jul 30, 2018 9:24 am

Good intel, the one I took off is identical in fit to the new one. Agree with your assessment though, when I fitted a K&N style performance filter I had full RPM and no restrictions.
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Re: Air filter

Postby hobson » Mon Jul 30, 2018 9:34 am

Incidentally its the B&S filter that's quoted as a direct fit on the parts4aircraft site -

"The Oval filter used in earlier Jabiru air boxes is the Briggs & Stratton BP399806S. "
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