Eureka!!
Thanks Bud. I think you may have solved a long standing problem I have had with
my 912S. After a prolonged climb at full power I often notice some engine
roughness
and the odd stutter as I level out and pull back the power. It soon smooths
out and begins to run nicely again. I have tried a number of things to identify
this but I have noticed some sootiness on one of my carb bowls so this
may well be the answer. I will try sealing the joint and making a shield.
Richard Iddon
G-RIXS
> On 7 Feb 2017, at 04:15, Bud Yerly <budyerly@msn.com> wrote:
>
> I was reading the Europa Flyer on Ruedi Vogal's engine failure in HB-YJF.
> Excellent
airmanship making a safe forced landing.
>
> By the sounds of the failure, I must bring up a recurring problem with the
> 912S
with XS firewall forward and its exhaust problem.
>
> I highly suspect an exhaust leak may be another culprit to consider, as with
red Aeroquip or Parker fire sleeve, fuel vaporization is rare in the XS cowl
with
the boost pump on for takeoff. I have had two recent experiences with engine
roughness due to exhaust leaks and left to continue the engine would have
failed at full power. Exhaust leaks don't show up during initial ground runs.
When the engine is cowled up, shortly after takeoff, if the exhaust leak is
on either aft cylinder or its downpipe, the exhaust will jet out of the leak
onto
the float bowl and boil the fuel out of the carb in a very short time. This
causes roughness then failure normally not a pure just dead silent engine.
>
> The post flight analysis usually is seen as small amounts of soot on the carb
body or bowl, upon disassembly, the carb may have a sticky lacquer type feel
due to the baked fuel residue. It is really frustrating as during any engine
roughness emergency we normally do the following:
> Aux Boost Pump - On
> Fuel selector - switch to unused full tank.
> Reduce power-As required to obtain smooth operations.
> Land as soon as possible.
> This throttle reduction action refills the float bowl at reduced power (as the
jet of exhaust is now diminished) and there is no evidence of a boiling
incident.
Drat.
>
>
> I did a poor job of explaining in my Troubleshooting guide on how to look for
and solve an exhaust leak. Look for the tan soot trail out of the junction of
the pipe to cylinder head joint or the ball socket at the muffler for sure,
but one should also make a heat shield to protect the carb from the possible
future exhaust leaks. I am not a fan of the 914 drip tray on the older models
we all suffered with, but take the idea from the new tray design for the 914.
A simple bent piece of aluminum attached to the foot well will work also. I
personally have my clients fab a tray, bolted to an Adel clamp on the engine
frame and to the 5mm threaded hole in the cylinder head and shaped to deflect
any exhaust leak from the carb and wiring. Crude but effective.
>
> I apologize for not including this in my miscellaneous ramblings from one
> night.
>
> Best Regards,
> Bud Yerly
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