Hello, Listers...
Recently, someone enquired why their engine ran best with the ignition
timing set at some huge advance figure, around 60 degrees if I remember
correctly. In reply, I theorised that maybe the crank pulley was
beginning to separate and suggested a way of checking whether the timing
mark was still in the right place. (No. 1 plug out, screwdriver through
plug hole to see if piston is at TDC when TDC mark is by the pointer.)
Another possibility occurred to me later as to why the man's engine
needed so much advance. If the camshaft timing had been set one tooth off
when the engine was put together, the engine would run poorly but this
could be partially compensated for by setting the ignition timing a long
way from where it should be.
So, I surmised, what is needed is a method to quickly check whether an
engine's cam is
timed properly. So, boys & girls get out your notepads - here's the
procedure....
1. Remove spark plugs
2. Remove valve cover
3. Measure diameter of crank pulley ('63 to '74 MGB should be just over
5", '75 to '80 should be just over 6")
4. If pulley is 5", cut a piece of 1/2" masking tape 4-7/8" long
5. If pulley is 6", cut a piece of tape 5-5/8" long
6. Stick tape on outer edge of pulley, starting at the TDC mark, going
counter-clockwise
7. Rotate the engine until No.1 cylinder's intake valve is fully open
(that is, until the second valve from the front is pressed down as far as
it will go).
8. Look at the tape on the pulley. The end of the tape should be right by
the TDC pointer on the timing cover. Within 1/16" is OK; if it's 1/2" or
so away, then it's a good guess the camshaft is timed one tooth off.
9. Adjust cam timing if necessary (or talk to your engine builder!)
10. Replace spark plugs and valve cover.
This procedure works on the assumption that most stock cams - and even the
ones with mild competition grinds - all require the intake valve to be
fully open around 108 degrees after TDC. Since a B cam sprocket has 40
teeth, being off by one tooth affects the cam timing by 9 degrees.
Lawrie
British Sportscar Center
PS. If you have a Midget, you can follow the same procedure but I don't
have a Midget crank pulley to measure. So, take the diameter of your
pulley, multiply by 3.142, multiply the result by 108 then divide by 360.
That'll give you the inches of masking tape you'll need.
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