Mike
I got into this thread late, and don't know what cam you have chosen.
Mine is kind of a long story, and is prompted by my refusal to proceed to
the next step of my engine rebuild until I fully understand the issues of
the previous step.
I purchased a Piper BP270 cam from BP Northwest. Unfortunately, the spec
sheet that came with the camshaft is a bit terse. This is the result of the
Brit propensity for treating their cam grind specs as "trade secret", so
they refuse to disclose details of their cam grind specs. The clue was the
spec for "LTDC" - lift at TDC, which was .039 I-valve, and .045 E-valve,
which I've assumed is valve lift at the end of the exhaust-stroke.
To make a long story short, Piper's cams are asymmetric, meaning the center
of the exhaust and intake lobes are not equidistant before and after TDC,
respectively: thus the Bentley instructions on valves 11 and 12 "on the
rocks" are null and void.
This may not apply to your cam. Most cams are symmetric with 110 degrees for
both E and I valves, and for that case the Bentley 11 and 12 valve procedure
applies.
If there is any question, buy a crank degree wheel and follow instructions
for verification of cam-to-crank timing.
Jerry Shaw
Date: Thu, 26 May 2005 16:14:05 EDT
From: DaCRANEz@aol.com
Subject: Cam fixed?
Dick
Since rebuilding the engine about this time last year I have had issues with
the idle. Recently Vance had a post about how to verify that the cam timing
was correct. I did the check and found that 11 and 12 rockers were not even.
By
rocking the crank 16 degrees clockwise I could get them even with 1 and 2
closed. Called Vance, he agreed that I was probably off one tooth on the cam
gear.
Corrected that. Reset cam drive gear to Bentley specs. I am running 9.5to 1
CR. Premium gas . 400 ft above sealevel, no anti-run valve, 150 PI cam,
Good's
1.65to 1 rockers. As to the spark exchange I don't hear arcing but the
leading
tip of the rotor is a bit discolored and the posts in the cap are a bit
burnt. I am going to try your suggestion on shortening the spring on the
retard
capsule. I need a small tap as I am going to install a set screw to make it
adjustable. Sorry about the long tirade last night. I was really frustrated
at not
making any headway. GeeZ-I hope this just isn't due to some bad petrol.
Thanks All
Mike C
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