Colin:
10 BTDC with the vacuum retard disconnected.
Is it really a backfire or is it a miss? If it is spitting back
through the carbs then I guess it must be a backfire, yes? A miss would
not spit back through the carbs... Hmmm.
One possibility is that you have an intake valve that is
misadjusted, independent of the fact that your timing is slightly
advanced. The advanced timing would cause knocking (or "pinking", as the
Queen's subjects would say). Backfiring occurs when the intake valve
doesn't close, allowing the combustion flame front to propagate back
into the intake manifold.
You didn't say when the cam went into the motor. Was it a recent
addition? If so, it is possible that the camshaft is misphased. It would
need to be waaaay off however, so unless it idles like home made dog
food, this seems unlikely.
If your distributor rotor or cap is bad, the spark could
occasionally be jumping to the wrong plug tower I suppose. As the engine
revs the spark timing changes because of mechanical advance, making it
more difficult for the spark to jump to the wrong tower and the backfire
goes away.
Dunno, just a couple of ideas to check out.
BTW, my 6 runs very happily on 86 octane with 9.6:1 compression,
270 degree cam, Zenith carbs/B1AR needles and stock exhaust.
Vance
-----Original Message-----
From: 6pack-bounces@autox.team.net [mailto:6pack-bounces@autox.team.net]
On Behalf Of COLIN THOM
Sent: Wednesday, September 22, 2010 12:48 PM
To: 6pack@autox.team.net
Subject: [6pack] Ignition Timing
Hi Gang..what's the verdict on ignition timing? Here are the details:
270 deg. duration cam with 108 deg lobe separation, stock rocker arm
ratio, compression upped to 10 :1...maybe a bit higher, vacuum system
disabled, running 86 octane smoothly enough, Pertronics...no points,
Lucas high-output coil. I'm getting some occasional back firing through
the carbs when pulling away from a dead stop and am trying to figure out
what's causing it. Currently the timing's set at about 14 deg BTDC at
825-850 rpm then idle is upped to 900-ish, which seems to have helped a
bit. Thanks as always, Colin '75
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