I'm sure you have already had lots of advice so far. Here is my $1 worth
(inflation you know)
A cylinder pressure test is a good indication of how "balanced" the motor
is. In other words, how well the motor has worn over time. The best test
is to use a leakdown tester (about $75) that will actually test how much
leakage is in the cylinder. With a good rebuild you should see something
less than 10%. My new rebuilt racing engine for a Sprite has 2-4% leakage.
Anything below 10% and I won't touch it. Anything above 20% and...
If you want a cheapy way of testing for leakage, read on...
Leaking Valves
1) If you don't have a leak down tester, the next best thing is to take an
old spark plug and rip out its guts and epoxy in a fitting to attach to an
air compressor. Remove the intake and exhaust manifold. Set the pressure at
something around 40-60psi. Run a cylinder up to TDC so both valves are
closed and connect the air line. Use a small piece of vacuum line and shove
it into either the exhaust or intake port and the other end into your ear
and listen for any leakage (valve seating problem). Check each cylinder.
Problems
a)valves not seated because of worn guides or the seats were lowered to
the point where the seat pressure is not good enough.
Rings
2) 110psi seems to be low, but I don't know what the pressures should be.
You could have installed compression rings wrong. STUCK RINGS/NOT SEATED:
These are harder to test. More than likely you wouldn't have a stuck ring
unless the pressures were 90-115-115-115. The pressures you gave are well
within the 10% range for good cylinder pressure. I don't know what the
cylinder pressure should be for your motor so they could all be weak as far
as I know. 170psi seems to be closer to what they should be.
Hints
3) Even with 40-60psi, the air will tend to spin the motor around for you.
Make sure you set it at TDC (using the timing mark or 180 deg). Use a
socket and wrench to hold the front pulley from spinning. Don't let the
motor move more than a few degrees. Test each of the cylinders and then
retest to make sure the timing didn't slip giving you false readings (good
or bad).
This should be a start...enjoy all the other responses you get.
Brian Carter
brianc@elonex.com
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From: healeys-owner
To: all-cars
Subject: spitfire engine rebuild
Date: Wednesday, July 19, 1995 10:18AM
After only 6000 miles, my rebuild of the 1500 spitfire (1980 model) engine
is all shot to hell. The compression across the board
is 110-115 psi dry and 155-170 wet (oil squirted in the
cylinders). I don't know what's going on, I suddenly started
getting blowby and decided to re-check the compression. The following
is the essentials of the rebuild carried out a year ago:
1. rebuild done at 60K miles using haynes manual
2. no new pistons since they were in spec.
3. rings changed
4. crank cut to .020
5. new cam, stock
6. oil pump not changed
7. block not boiled out since no overheating was seen.
8. cylinders honed as there was no ridge at the top
9. connecting rods not machined
10. no engine bolts changed
11. head rebuilt
12. freeze plugs not changed
13. all gaskets changed
14. new cam followers
15. webers instead of cd4t carb.
I before starting the engine I squirted a lot of oil in the cylinder
which I suspected was a mistake but I hand cranked the engine several times
before starting and also it did'nt start for a long time. I figured all
the oil had drained out by the time it started.
Before I tear the dadgum thing apart again, could any of you tell
me what I did wrong? I am thinking about going .020 oversize on the
pistons this time, will I need to run on octane if I do this?
best wishes
Zahid
ps: you can mail to the list or to zahid@mozart.sps.mot.com
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