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Air Filter?

Posted: Wed Nov 25, 2020 7:25 am
by MGR
My VW engine has seized. The guy fixing it believes the problem was dirt and grit pulled into the engine due to the lack of an air filter. So, would like to know how many of you are running with or without air filter. Also, for people using an air filter, which one are you using? Clearly space is an issue, very little room under the cowl. Also, I have a rectangular air inlet attached to carb, which has a pipe from around the exhaust which provides a little carb heat. Not clear how I could fit air filter without major modification. Any thoughts most welcome. Thanks a lot.

Re: Air Filter?

Posted: Wed Nov 25, 2020 8:31 am
by Paul1954
Hi MRG, bummer eh?

I don’t know your VW engine type, so a few questions:

1/ How many hours since last overhaul?
2/ Did it stand idle and not run for long periods?
3/ What was the oil filter internals like when ‘your guy’ broke it open?
4/ Do you have a CHT gauge or oil cooler fitted?
5/ What was the oil like when drained?


Re: Air Filter?

Posted: Wed Nov 25, 2020 7:52 pm
by MGR
It's a VW 1835 Barry Smith ACRO with inverted fuel and oil.

1/ How many hours since last overhaul? 127
2/ Did it stand idle and not run for long periods? Absolutely not. 20 hrs in last 3 months
3/ What was the oil filter internals like when ‘your guy’ broke it open? Not broke it yet
4/ Do you have a CHT gauge or oil cooler fitted? Yes both
5/ What was the oil like when drained? Not drained yet, but oil in rockers black. 7 hrs since last change.

Analysis of pistons barrels valves & guides leads us to be extremely concerned about dirt/grit getting into the engine. This may not be the problem that directly caused the engine to seize. However I have been convinced that it has caused serious wear which I want to avoid once engine is fixed.

So perhaps you might like to answer the original question about whether you do or do not have an air filter, carb heat, and the design of the airflow into the carb.

Thanks a lot. Malcolm


Re: Air Filter?

Posted: Thu Nov 26, 2020 7:36 am
by Paul1954
Malcolm. You say this is an BS Acro 1835 with inverted fuel and oil, and you also that you have a carburettor and a carb heat take-off from the exhaust.

A true BS Acro 1835 built with fuel injection and inverted oil system has no need of a carburettor - I should know as I have flown behind them for hundreds of hours. What I suspect you have is a VW 1835 built by Barry that has had third-party work to install a carb for inverted flying and a system that hopes to return oil to the engine when inverted.

Would be interested in the ‘inverted oil’ system and where the inverted oil is picked up from and how it is returned to the internals. The most common cause for an engine to seize up is insufficient engine oil, which causes overheating and melting of the engine parts. That is quite likely to be a source of your engine seizing and I certainly wouldn’t be blaming a lack of air filter that this stage, that is unless you are operating in a particularly dusty or sandy environment.

RF3 and 4, the Motor Falkes' and all manner of VW engined aircraft didn't have air filters or carb heat when whey were built. Warm air was drawn from inside the well sealed cowlings. Slingsby Nippers fitted with the Ardem engine did have a carb heat system.

For clarity, a Barry Smith Acro engine built with inverted fuel and oil systems - ISTR this particular engine is the 2.1 litre engine that went onto be fitted to the Acro Advanced. The Nipper would loop from straight and level with it fitted, or do two vertical rolls from s/l if in need of a wider grin.

Malcolm. Unless the correct cause of the engine seizing is found, you risk wasting money on a repair, and the engine then needing repair again. The cause can only be found by examining the evidence. That is, inside the oil filter(s), condition of the oil (correct type?), sufficient oil, signs of debris on the mag plug, signs of moisture. Then open the top and bottom end and examine what is seen. Only then can the cause of the problem be though about, but may still require deeper examination.

Re: Air Filter?

Posted: Mon Jan 04, 2021 9:08 pm
by Neil Spooner
Hi Malcolm,

You say that the oil was black.....after 7 hrs that would suggest overheating of the oil. Does it smell burnt?

Do you have oil temp?

If so what was it before seizure?

Did the engine seize in flight or on the ground?

What was the oil temp' before seizure?

What was the oil pressure before seizure?

What part of the engine has seized?

Is it still seized after cooling?

I agree, unfiltered air should not be a short term issue unless the engine was operated in extremely dusty conditions. The BS injection system does not normally have a filter.

You are going to need to strip the entire engine and drill out the plugs in the galleries to ensure there is no debris left in the engine during its rebuild.

I have a built and tested 1834cc engine which is surplus to my requirements currently if you need it.