Today's Message Index:
----------------------
1. 05:07 AM - Re: Chugging on start-up (Owen Baker)
Message 1
| INDEX | Back to Main INDEX |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Chugging on start-up |
12/26/2013
Hello To All of You Lycoming Engine Operators, There were two very meaty
postings on the yahoo lycoming digest today. They are copied below for your
information.
OC
==================================================================
1a Re: Chugging on start-up Wed Dec 25, 2013 8:42 am (PST) . Posted by:
bobatjicdotcom >Has anyone experience or knowledge of a Lyc chugging for a
few seconds when starting? Was not particularly cold at >50+, smoothed right
out with a little throttle, run ups were fine. Did it two starts in a row.
Pipes are clean and >light tan. Am concerned about it being a valve since a
fouled lower plug wouldn't completely stop a cylinder's >firing. Thanks for
any advice and MERRY CHRISTMAS!
I'd concur with Mahlon. Classic "Morning Sickness" symptoms.
Time for SB 388C Valve Wobble check.
http://www.lycoming.com/Portals/0/techpublications/servicebulletins/SB%20388C%20(11-22-2004)/Procedure%20to%20Determine%20Exhaust%20Value%20and%20Guide%20Condition.pdf
Sorry for Lycoming's horrible wrapping URL.
And here is a link to a much less expensive (and more likely to be in stock)
tool to check the proper clearance on the exhaust valve(s), if you can't
find the Genuine Lycoming tool through a local mechanic.
http://www.aircraftspruce.com/catalog/topages/valvewobble.php?clickkey=315805
Assuming that all the cylinders are the same age, you'll want to check the
rest of them, since they are likely to be very close in wear to the #3 you
have identified as "cold" in your testing.
It is possible to ream the excess carbon from the guide, per SB 388C
instructions, to prolong the life of the cylinder, but ultimately the answer
is pull the cylinder and replace the guide (and probably the valve), because
the root cause of the carbon build up is wear in the guide, allowing exhaust
gases to shoot up the guide and coke oil that lubes the guide, and as it
builds up, the valve then sticks open until heating of the cylinder allows
the clearances to expand and the valve is temporarily freed.
An early warning sign that takes only minutes, and costs nothing to do, is
to pull the valve cover(s) and look at the color of the spring and upper
retainer. Those with exhaust leakage up the guide will have a different
color than the intake valve right next to it. Usually the "oily varnish" one
sees on mid-time engines will have a charred or sooty gray color from the
super-heated exhaust gases escaping the guide and impinging on the spring
and retainer. When you see this, you know the guide has exceeded Lycoming's
allowable clearance (and may have begun to build up burnt oil in the guide
to cause it to stick.
This is also the warning sign and precursor to broken exhaust valves, which
occur after the valve looses its centering and slams shut offset on the
tapered seat innumerable times. The head bends slightly on each contact, and
eventually micro fractures occur in the metal at the head-stem joint. As the
micro fractures grow and intersect, eventually the head breaks off. Hence
the mention earlier of needing to replace the valve when evidence of valve
sticking or excess guide wear occurs.
Also look closely at the hydraulic tappet and pushrod. When the valve sticks
OPEN, the pushrod can come out of the socket and do damage to the cap on the
tappet, etc. If it were to stick CLOSED, the cam will attempt to open it,
and bent pushrods and other damage can occur. Roll the pushrod on a piece of
glass to be sure it has no warp in it.
Best Wishes and Merry Christmas to all the Lycoming list.
--Bob Steward A&P IA
Birmingham, AL
==============================================================================
1d
Re: Chugging on start-up
Wed Dec 25, 2013 10:43 am (PST) . Posted by:
kwe5252
Another possibility to this "Chugging"; problem, especially if it does not
clear up as the engine warms up...... is that there is an intake leak at or
near the "cold" cylinder. On the Lycomings it is usually the paper intake
gasket that has failed. Most common on overhauled (vs new) engines because
of the threads in the head being clogged. When the bolts that hold the
collar that holds the intake tube to the cylinder that smashes the paper
gasket are tightened they torque out on the threads, not from the clamping
effect on the gasket. After a few hundred hours the gasket fails, making a
small gap in the gasket.
This intake leak first shows it's self as a dead cylinder at closed throttle
(low manifold pressure that is really high pressure differential inside to
outside of the manifold) because of a lean, lean mixture. With the throttle
plate opened the pressure diff is less, so the leak is less and the mixture
is closer to correct (enough to fire) and the engine smooths out. At wide
open throttle there is very very little pressure differential between intake
pressure/vacuum and atmosphere pressure.
Before I spent much energy taking apart my valve train, I would check
closely for an intake leak. A broken primer line or missing primer/fuel
injector plug at the intake port in the cylinder is also a intake leak.
If you have an all cylinder egt system you can "see" the cylinder come
"alive" as the mixture en-richens when the throttle opens due to the
differential reducing and the fuel/air ratio comes into "burning range".
Bob's info is excellent if the problem is a sticking valve.
Good luck on your search for a cure.
Ken (A&P IA)
Other Matronics Email List Services
These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.
-- Please support this service by making your Contribution today! --
|