Jerry:
If your 75 has all the emissions stuff installed, then you most
likely have a bad diverter valve. This valve shuts down the air
injection system when decelerating and shifting. If the air injection
operates during these times, you get a backfire or a pop - hence the
need for the diverter valve. Disconnect the belt to the air pump and
drive the car - does it still backfire? If not, it is the diverter valve
and/or its associated plumbing. Do NOT leave the belt disconnected as
exhaust gas will flow back into the pump, and eventually it will seize.
If you don't have any emissions controls, then you have two
possible causes, timing and valves.
For the valves, set the lash following the procedure described
in Bentley. Do this first, because you can set it and forget it in terms
of trouble shooting. Check the lash BEFORE and AFTER you set it. You are
looking for any valves that have really sloppy lash, which might
indicate a flat cam lobe or a dead lifter. You can't rely on a
compression test to catch this sort of thing...
Assuming you check and set the lash and find nothing, move on to
the ignition. First set your base timing, 10 BTDC with the vacuum retard
disconnected. Reconnect the retard. The timing should fall back to 4
ATDC and the idle speed should drop noticeably. If the timing does not
fall back your vacuum retard is bad. While observing the timing, rev the
engine. The timing should advance smoothly as the engine speed
increases, and should likewise fall back smoothly as the engine slows.
If the timing does not fall back smoothly, or remains advanced then your
dizzy bob weights are sticking.
Use a dwell meter to check your dwell angle. A less reliable
method is to check the point gap with the engine not running. Also,
check to make sure you have not swapped a couple of plug wires.
Cheers,
Vance
-----Original Message-----
From: owner-6pack@Autox.Team.Net [mailto:owner-6pack@Autox.Team.Net] On
Behalf Of Jerry Greenberg
Sent: October 13, 2007 10:46 PM
To: 6pack@autox.team.net
Subject: [6pack] TR6 backfires
My 75 TR6 with 69 engine has recently (the past 2-3 months) developed a
tinny sounding backfire upon deceleration, either when downshifting or
when dropping back down from a stopped fast idle. Can't seem to find the
problem. The car runs at very low, very rough idle and gets lousy gas
mileage, although this has occurred only since the tinny backfire began.
It has a Crane XR 700 electronic ignition and high output sports coil
installed about 10 years ago. I always keep plugs and wires maintained
and gapped.
Does anyone have any suggestions about the cause and the solution?
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