Michael:
Well, as Ricky used to tell Lucy, "You got some 'splaining to
do".
Your engine is obviously not stock, as your compression is much
higher than a stock '76 ever dreamed of. I am assuming that the camshaft
is also non stock, so it could be exactly as your mechanic stated, that
the valves need adjusting. The higher performance the camshaft, the
fussier it becomes. If that doesn't do it, you got a couple of
possibilities:
1. Bad rings. Use the "squirt oil in the cylinder" test to see
if this is true.
2. My favorite: head gasket blown between cylinders 4 and 5
3. Bad valves or valve seats.
Let's hope that if there is something wrong, it is #2 as it is
*relatively* easy to fix. You need not pull the motor, and the fix can
be done in a weekend.
It is not unheard of for the wrong head gasket to be installed,
or the correct head gasket to be installed upside down. Either of which
will ultimately result in head gasket failure. If your DPO did the
engine modifications, then it is possible that he/she boffed the head
gasket install. It is very easy to do.
Later engines need the head gasket with steel reinforced fire
rings, and the gasket needs to be installed with the fire rings mating
with the grooves cut into the block. If the gasket is installed upside
down, it will not seal and sooner or later it will blow. Likewise, if
the DPO mistakenly installed the earlier gasket without the fire rings,
it will eventually fail as well. The higher than stock compression would
also stress the gasket more, so it is much more likely to fail when
improperly installed.
My DPO installed the correct gasket upside down, and the
compression in #6 was about 60 PSI. Tsk, tsk. Not good. To cut some
slack to my DPO, the gasket is NOT marked "This side up", and mates with
all the holes in the block just fine when installed upside down.
BTW, the reason that I favor the gasket theory is that it
requires only one failure and explains both low cylinders. Anything else
requires TWO failures (two misadjusted valves, two sets of bad rings,
two burned valves, etc) and those two failures must give identical
results - which is unlikely. A blown gasket on the other hand, only
needs a single failure to explain both cylinders AND it explains why the
compression test gives the same results on both of those cylinders.
Let's hope I am wrong.
Vance
-----Original Message-----
From: owner-6pack@Autox.Team.Net [mailto:owner-6pack@Autox.Team.Net] On
Behalf Of samuelsma@aol.com
Sent: September 10, 2007 7:39 PM
To: 6pack@autox.team.net
Subject: [6pack] compression
Listers:
While I have my '76 in for carb tuning, the mechanic did a compression
test and got the following readings:
Cyl 1: 197
Cyl 2: 182
Cyl 3: 196
Cyl 4: 161
Cyl 5: 165
Cyl 6: 190
Does this discrepancy between cylinders 4 and 5 compared to the rest
indicate a problem?? The mechanic plans to adjust the valves first to
see if that is the cause.
Ideas?
Thanks.
Michael
'76 Tahiti blue
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