First a little history and then please help if you can.
I'm rebuilding a TD engine that was previously rebuilt some 25 years ago
by a truck repair shop in Arkansas(!). Already I have had to reverse a
couple of things that were obviously wrong, like all the con rods being
put in backwards. According to the receipts I got with the car, the
crankshaft was reground to standard and the camshaft was also reground.
I've disassembled the engine, had it hot tanked, and then reassembled,
checking all the clearances along the way.
Now I've got the new cam and crankshaft sprockets in place, along with
the timing chain. I believe I've got them on correctly, with the proper
number of links on either side of the 'T' marks. I've checked this
numerous times to make sure, matching my setup to the picture in the
manual.
I'm now trying to make sure that the cam timing is correct. I'm relying
heavily on Horst Schach's instructions in his TD Restoration Manual. I'm
using a homemade degree wheel made out of two protractors (actually
pretty accurate, I think), a dial indicator on a valve lifter, and
turning the engine clockwise as I face it. I'm putting my lifter in the
first tappet hole from the front of the engine for the exhaust valve
readings and using a checkpoint of 0.020" of lift to determine valve
opening and closing. Of course, I'm using the second tappet hole from
the front of the engine for the intake valve readings.
I've gone through the process Horst describes numerous times, checking my
procedure again and again, and the results I'm getting just don't make
sense.
Here's what I've got:
1) Intake valve opens 13 degrees _After_ TDC
2) Intake valve closes 21 degrees _After_ BDC
3) Exhaust valve opens 39 degrees _Before_ BDC
4) Exhaust valve closes 6 degrees _Before_ TDC
Now 2) and 3) don't particularly worry me, they are a bit off the specs,
but at least they are largely doing what they are supposed to.
1) and 4) are the problem. According to specs, the Intake valve should
be opening either 5 degrees _BEFORE_ TDC or 11 degrees BTDC, depending
upon whether the cam was ground to late TD or early TD specs (and I
don't know which).
Even more concerning is that the Exhaust valve is closing 6 degrees BTDC,
when it should be closing 5 degrees ATDC. According to Horst Schach's
book, the Intake and the Exhaust valves should both be open for about 10
degrees, the middle of which should be TDC. The Intake and Exhaust
valves in my case have _no_ overlap and the Exhaust valve is closed for a
total of 19 degrees before the Intake valve opens.
Any suggestions? I hate to have to buy a new cam, particularly since I
finally got the other one installed with all the pistons, con rods, and
crankshaft. I hate to tear all that out again, much less pay for another
cam. However, it appears to me that the truck repair place in Arkansas
may have done a pretty lousy job of regrinding this one, yes?
Thanks in advance for any help you can provide.
Yours in exasperation,
David Littlefield
Houston, TX
'62 MGA MkII
'51 MGTD
'88 Jaguar XJ-S
|