VW engine - one sooty exhaust stack...

Gary T · Post by **Gary T** » Sat Feb 05, 2011 10:24 am

Hi all,
I have an interesting issue on my VW engined Nipper. One exhaust is sooty black, the other 3 are a nice light brown colour. The soot is not oily and the engine runs very well throughout the rev range - plugs are fine too. Both cylinder heads were refurbished an hour ago by a reputable engine shop and a leakdown test was carried out yesterday. The results were excellent; all four cyl's 80/80.
So, there is no fundamental issue with the engine and if the exhaust were 4-1 I possibly may not have been aware of the problem.
The consensus seems to be overcooling of the cylinder (it's the front left, viewed from the front - prop turning clockwise) is resulting in poor atomisation and a sooty burn - hence the black, sooty exhaust stack. It's been suggested that to find a cure will be hit and miss, simply adjusting the baffles to allow the head to run warmer (the engine is fully cowled by the way). To this end I've switched the CHT's to the front 2 cyl's to see what happens and will monitor them closely on the next flight.
Has anyone come across this before on a VW derived engine? Although not a major issue and perfectly flyable, I'd like the burn to be perfect in all 4 cylinders if possible.

Nick Allen · Post by **Nick Allen** » Sat Feb 05, 2011 11:45 am

One thought: you say the plugs are fine. Is this from a visual check or have you tested them? And the leads? Plugs and leads can do mysterious things but look OK from the outside. Might be worth replacing/swapping them around. If the sootiness wasn't there before you had the heads fettled, it might suggest that something has gone astray in the refitting process... have you re-re-checked the valve clearances (since the hour's run)? (We refitted heads last year, did a good long ground run...but soon thereafter one of the valve clearances went awry.) It would of course be interesting to monitor all the head temperatures -- but if you are measuring two simultaneously, then perhaps it should be the hottest and the coldest, so that you can see in normal operation if there is a big difference between them. Actually, that was several thoughts...apologies if any were of an egg-sucking nature!

Gary T · Post by **Gary T** » Sat Feb 05, 2011 1:05 pm

Hi Nick,
thought I'd managed to get everything down but realised I'd only given half the story!
The engine had the exact same snag before the heads were refurbished - in fact that's what prompted having the heads looked at. The plugs were replaced around forty hours ago and then new ones fitted after the head work, along with 8 new plug caps. There is no significant rpm drop when the mag's are checked, so I assume the leads are good - although I will be turning the engine over with the plugs out to check there's a nice fat spark.

I had the exact same thing with the valve clearances happen to me! I was meticulous in setting them up then checked and rechecked before putting the rocker covers on. After 1 hour of flying (3 flights) the leak down check was done and one of the valve clearances was almost double what it should be. I was mystified but I'm obviously in good company!

Nick Allen · Post by **Nick Allen** » Sat Feb 05, 2011 3:28 pm

The trouble with checking plug leads that way is that you might get a spark, but when the cowls are on, a faulty lead may then be routing differently and finding something to short to. (I had this on a friend's car, which was running roughly at higher revs, and she asked me to look at it. Open bonnet and check for the obvious: all looks fine. Rev engine: sounds OK. Shut bonnet and drive car: definite though faint sputter under acceleration. Stop, open bonnet and rev engine...hmm, seems OK... but then I got a little shock off one of plug leads as I reached over the engine. There was a flaw -- invisible to the naked eye -- in the lead, which was shorting to the underside of the bonnet, but only when the bonnet was closed!)

Gary T · Post by **Gary T** » Sat Feb 05, 2011 5:09 pm

I know what you're saying Nick and I'd love to replace the leads for the few pounds it costs. Problem is that the mag is totally inaccessible without removing the fuel tank, panels etc etc - which I'm loathe to do! I might have to in the end I guess...

G.Dawes · Post by **G.Dawes** » Sat Feb 05, 2011 8:13 pm

If you have an engine that runs well but gives a sooty exhaust then I would look at the mixture , which if it runs well must be about right . However there is a distinct difference in the manifold design and the amount of fuel going this way or that may be different. There should be an amount of turbulence in the system to enable mixing of the fuel. Get hold of a colourtune and stick it in the top plug holes and just look you will soon see what is going on.
Yellow or purple or red or worse still nothing.

Gary T · Post by **Gary T** » Sat Feb 05, 2011 8:30 pm

That's an interesting suggestion - will see if I can scrounge one up!
Fuel system is an Ellison throttle body (recently overhauled at quite some expense!) which feeds to each pair of cylinders via a common branch either side of the engine. i.e. the rear cylinder is fed from the same branch and is fine so you'd expect the front cylinder to be seeing the same mixture.
Much head scratching going on I can tell you!
Hmm, given that it's the front cylinder I have a problem with, Ican see me getting bashed on the head by the prop while looking down the Colortune's cardboard tube

Bill McCarthy · Post by **Bill McCarthy** » Sat Feb 05, 2011 9:04 pm

I would take along a couple of safety "lookouts" and do the Colourtune checks in the dark. Any HT shorts could also be observed - I found a faulty ceramic section on a spark plug that way - an nice intermittent blue spark to earth.

Ian Melville · Post by **Ian Melville** » Sun Feb 06, 2011 7:38 am

Gary T wrote:Hmm, given that it's the front cylinder I have a problem with, Ican see me getting bashed on the head by the prop while looking down the Colortune's cardboard tube

The've gone downhill then!! I brought mine in the early 80's and it has a plastic tube with a chrome plastic mirror that can be angled to see the window from a safe(er) position.
Still sounds high risk, esp at night

cardiffrob · Post by **cardiffrob** » Sun Feb 06, 2011 8:57 am

Have you tried the "red chalk" trick? The chalk turns white at a certain temp, so you can see in which order and at which rate the exhaust manifolds heat up by marking a point near the head on the bend from the port. I wondered if it might show up a different story to the EGT/CHTs.
There are also those £40 thermometers with a laser pointer on them. It would certainly thin down the number of possible causes and you can point it at the heads and barrels..

Again, apologies for potential egg-sucking lessons.

Have you got a photo of the cowlings on the Nipper? Breazy or cowled?

Gary T · Post by **Gary T** » Sun Feb 06, 2011 9:28 am

No apology required - I think I'm a fairly competent mechanic but am new to the VW engine - any advice gratefully received

It's a fully cowled installation - Not sure how to attach a pic on here yet...
Have borrowed some chalk from the kids too.

G.Dawes · Post by **G.Dawes** » Sun Feb 06, 2011 9:32 am

As it is the front one you might just be running it cool enough for soot to form. fan cooled after all.

jangiolini · Post by **jangiolini** » Sun Feb 06, 2011 9:46 am

I havent read the whole article so this may already been said! I did have similar on my motorcycle and it was down to oil leaking past the exhaust valve stem which then burnt in the hot gas causing soot. It doesnt taken much infact very little as any great amount would show up as white smoke! This was after rebuild and all tolerances were well in spec. It didnt have stem seals on exhaust!
John.

Gary T · Post by **Gary T** » Sun Feb 06, 2011 8:17 pm

John, valve stem oil seals good - both heads refurbished in reputable engine shop. Fault apparent both before and after refurbishment...

Am probably going to bite the bullet and remove the tank for access to the magneto then replace the spark plug cables.

Once plug leads are replaced, that'll be everything in the ignition circuit new, cyl heads refurbished and leak down test perfect. Will then start playing with baffles I guess.

That said, if there was a problem with the ignition, would it not manifest as when the mag check was done? It doesn't and the engine runs really well at all rpm's.

Gary T · Post by **Gary T** » Wed Feb 09, 2011 6:44 pm

I managed to get up for a couple of quick circuits tonight and the sooty cylinder's CHT was slightly colder than the non-sooty one. I think that rules out cool running and atomisation problems. Anyone disagree?

I guess the next simple step is to switch the ignition leads when I get the chance and see what happens with all new ones fitted. Fingers crossed!

Light Aircraft Association

VW engine - one sooty exhaust stack...

VW engine - one sooty exhaust stack...

Flown today - interesting indications