Kevin,
just a few thoughts (cold, rainy day in Northern
California...stuck inside reading email and giving
long-winded replies):
Do you have a repair manual for the engine (or the type
& year of car it came from)? If not, this is a must
have item.
On the head gasket, this is easy to check, if you have
(or can borrow) a compression gauge.
1) If a head gasket has gone you would see white steam
in the exhaust and water dripping or running from the
exhaust pipe (you did not mention either of these).
Also, you should not get coolant in the carb since the
engine produces vacuum during operation. The coolant
could only get to the carb in this situation if the
fuel system is inverted (under the engine - I don't
think this is the usual Subaru set-up) since it would
trickle down by gravity after the engine is shut down.
2) Get the specs for the engine, it should tell you how
high the compression per cylinder should be and what an
acceptable difference between cylinders should be (for
example - it could state nominal compression is 160 psi
+/- 10 psi with a difference of no more than 15 psi
---From the strongest to weakest reading....this is just
an example - the book will give proper values). If one
cylinder (or more than one) has an unusually low
reading the gasket MAY be the problem...or it could be
that a valve is not seating properly from severely worn
seats or burned valve edges (from running too lean, too
long) or bent valve stems (usually from constant engine
over rev) causing sticking. I think that neither is
the problem because you state that it runs rough only
intermittently at low RPM settings...but these are good
tests to show engine wear, in addition to a leak-down
test that checks for minor valve seat and piston ring
wear. A decent compression tester (looks like a large
dial type tire (tyre) gauge with a tapered rubber plug
over the Schrader valve to stick in the spark plug
hole) should be about GBP 35. You may also want to get
(or borrow) a Vacuum / pressure gauge, that way you can
see what kind of vacuum your intake produces and what
pressure your fuel pump creates.
It does sound (to me) more like a fuel or ignition
hesitation problem (because of the intermittent
nature). Glad to hear you cleaned the grit and goo out
of the carb but don't like to hear that it was there in
the first place. It could be that some of this goo or
grit has gotten to the metering system of the carb
which "meters" the air-fuel ratio (17 to 22 pounds of
air for each pound of fuel) which could give unsteady
mixture. Obviously why we need fuel filters, sumps,
drains and gascolators. If the engine sat for more
than 6 to 12 months with fuel in the carb, the fuel can
separate and the solids can gum up (or varnish) the
metering needles or jets, which translates to poor idle
and poor low speed running (if it will idle at all),
but this is from fuel starvation - yours sounds like
too much fuel (flooding). If some grit did find its
way to the low speed jet (or needle) it could have
enlarged the jet or needle seat which would admit too
much fuel.
It could more likely be a combination of fuel and
ignition. Sort of like a race car engine running at low
RPM - it sounds like it is running very rough due to
the timing trying to digest the larger than normal
(passenger car engine being normal) air-fuel mix that
the large carb and camshaft are introducing. As the
engine speeds up with increasing RPM, the distributor
advances the timing to trigger the spark at the most
efficient time and the mixture can burn more thoroughly
due to the residual heat of the previous ignition
cycle.
You could also check the ignition system as the engine
runs, pull off a plug wire and listen for a difference
in how the engine runs. Do the plug wires one at a
time so you only have one plug wire loose at a time.
You may want to loosen the wire with the ignition OFF
and GENTLY pull the wire off the plug with a clean
rubber-handle pliers while the engine is running...some
poorly shielded (or even greasy / dirty) plug wires can
give you a good shock (more startling than dangerous
unless you have a pacemaker) and a sore elbow. You can
also check the impedance (resistance value per length)
of the plug wires to ensure spec and also make sure the
end connectors (the little metal bits inside the rubber
end boots) are securely attached and have contact with
the center conductor wire. There is a 74 MG Midget
1275 in the garage that once a year in the Spring I
take the distributor cap with all of the plug wires
attached and run it through the automatic dishwasher to
get all of the previous years grime off (after cleaning
with lacquer thinner first....don't let your better
half catch you doing this...big, big trouble). You
could also try changing the coil (this assumes you are
using a coil-distributor ignition system, NOT a magneto
system) with one of the high voltage racing coils (for
on-the-ground testing only) to see if there is a
change.
On the voltage: 12Vdc systems are actually about 13.8
Volts dc and operate OK from about 10.5 volts to 16
volts. BUT, due to the critical nature of what we are
doing (no one wants to fall from the sky or smoke their
expensive avionics) these tolerances should be
tightened up and monitored with a current regulation
and an voltage over/under limiting system. So it does
NOT sound like this is a contributing factor since you
get 12.5 volts from batt only and about 14 (pretty
close to 13.8) from the alternator. If it were under
11Vdc I would correct this...I had a Thunderbird Turbo
Coupe with electronic ignition and electronic fuel
injection that would run funny when the battery got
weak. This is also where I ran across the plug wires
that were cooked inside (from the heat off the
turbocharger turbine side) but looked fine on the
outside.
Testing with meters and gauges is the only way.
Unfortunately you have your work cut out for you since
the problem is intermittent, but I would concentrate
first on the fuel system since you say leaning-out and
carb heat helps...sounds like it is running too rich in
the lower RPM band (could be that the idle jet or
needle is letting in way too much fuel) since leaning
reduces the fuel load and carb heat increases air flow
(since it is not going through a restrictor like an air
filter).
What do the plugs look like? After extended running
they should have a small amount of TAN scale on the
electrode. If it is black, the engine is running too
rich, if it is white too lean, if they are oily it
could be worn or stuck rings or worn valve guides, if
they are wet but not oily could be head gasket or
cracked cyl head or cracked block (cracked block only
if the cylinders are in the block like an I-4, V6, V8
or V12 car engine but generally not a boxer engine like
Rotax 912/914, BMW R-engine Motorcycles, air-cooled VW,
Porsche 911/912 type, Ferrari BB512 and of course
Subaru).
Check the no-load (idle) and load (usually around 2200
to 2300 RPM, but the book should say) ignition timing,
as stated in the instructions. The book may tell you
to disconnect the vacuum hose if it is a vacuum advance
system or to disconnect a certain wire - usually for
electronic ignition. Also check that it is advancing
properly (and smoothly as you increase RPM from idle to
the load value) as seen from the difference of no-load
and load timing
Is it a single carb or dual carb set-up? If it is
dual, the carbs may need to be synchronized.
Do you have single or dual ignition? If dual, is there
a difference in how the engine runs with one system
shut down?
Is it coil-distributor or coil-distrib. with CDI
(capacitive discharge ignition) or magnetic impulse
(magneto) ignition? If it is CDI, I had an Audi 100LS
with CDI that when the box heated up it would not
function properly - you may need to change the CDI box,
if there is one, and re-test.
In the repair manual, there should be a section on the
carb system. There may be specified adjustments for
the idle (low RPM) screw or jet or metering system that
you can work with. It may look like a set screw with a
spring between the screw head and the carb body at the
base of the carb (nearest the manifold, not the air
cleaner). The book should tell you the initial setting
for it (might be something like 1.25 or 1.5 turns out
---From seat) so you could try turning it in
(clockwise-CW) SLOWLY & GENTLY until it hits bottom and
back it out (CCW or anti-CW) 1 turn and test from
there. Check the book!
If you have someone close with the same engine set-up
(either in a plane or a car) ..trade carbs and test,
trade coils and test, trade plug wires & distrib cap
and test, etc. Just remember to trade back (sometimes
it is nice to have friends)...mark the ownership of
original parts for easy ID.
It could even be excessive back pressure in the exhaust
system, but I doubt it since this usually lets an
engine idle but not rev high. While the engine is
running, hold your hand (open palm) near the exhaust
pipe end...is there slight to good force pumping your
hand away? Or does it suck your palm in toward the
pipe? Should be a slight pressure holding your palm
away in pulses as the cylinders fire. What does your
palm smell like after this test? Smells heavily of
petrol...too rich, palm gets really hot...too lean.
Best test gear: sight, sound, smell and touch....leave
tasting out of these tests.
Has the engine been rebuilt? If so, was the original
(or original dimension) camshaft used? Or was it
changed to create a different power band? Passenger
cars have cams that produce good low-RPM Torque and
good mid-RPM horsepower....racing or special use cams
are altered (more valve lift for longer duration) to
create good low- to mid-RPM torque and high-RPM max
horsepower. An engine's maximum efficiency is near red
line, just prolonged operation there leads to short
engine life.
For safety, you can run the engine without the prop
(just like running your car with the trans in neutral),
but NEVER without the alternator and coolant
pump...there must be some kind of load (even a modest
one) on the engine (Alt & coolant pump take 1 to 5 hp
to operate).
You should do all of the tests and keep a log for
future reference to monitor the health and life of the
systems (for you) and for your periodic maintenance
check-ups (for insurance) and annual inspections (for
CAA). But whatever you do, write things down.
You can email me off-forum if you rather.
I am self taught...I am too cheap to pay someone for
shoddy work...that, I can do myself!
just my two (1.3 after tax) cents worth,
Dean Wiegand
Sacramento CA USA
dwiegand@surewest.net
kit A259
-----Original Message-----
From: forum-owner@europaclub.org.uk
Kevin Taylor
Subject: That rough running Subaru
Folks,
Just an update on my rough running Subaru, please feel
free to throw ideas
at me if you have any.
I spent the day messing around and didn't really get
too far, let me tell
you the symptoms and feel free to throw in any ideas,
although I realise
most of you have Rotax.
Its intermittent rough running at low rpm tick
over-3000 rpm, resulting in
bad running?/vibration?/misfiring? (Engine cuts if I
pull throttle back to
idle)
I drained and cleaned out the tank as the carb had what
appears to be water
and crap in!
The voltmeter is odd and is reading 14-15 volts!? (If I
take out the fuse
for the alt charge it drops down to 12.5 ish where it
should be)
Leaning and carb heat appears to assist in easing the
problem but doesn't
cure it.
Have checked the Diaphragm in the carb and it appears
to be operating OK.
Not sure where to start really.
1. Could junk in the carb but I have flushed it through
now a good few
times?
2. Could be ignition problem?
3. Could be a mixture problem
4. On the way home had an awful thought that the head
gasket might have
gone, water blowing back in to carb? I need to check
the water level and oil
I guess to confirm this is not the case.
For info last set of plugs were blacker than normal,
Oil change appeared to
be OK, fitted new plugs and just had a though tonight I
should check the
gaps to make sure they were set right, although this
problem was apparent on
the last set of plugs.
Maybe I have 2 problems? I've just had the panel out
and something could be
wrong there? giving the discrepancy in voltage.
As I'm sure you have gathered from my description I'm
not an engine man and
am trying to ascertain the potential problem before
deciding on my next
steps to call in some reinforcements.
Regards
Kev T
(http://www.grisoft.com).
09/10/02
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