Rich
Have you checked the manifold gasket? A leak here could give the
symptoms you describe. It's an easy job to replace it.
Nick (73 B Roadster)
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From: Rich Mason
To: mgs
Subject: backfiring / HIF4's
Date: 14 January 1997 17:15
Help!
In November, I purchased a '73B that was running a bit rough. It was
drivable only if you maintained about 1/4 choke (rpm boost). The PO felt it
was the temp compensator in the rear carb. I decided to replace all the
soft goods as I suspected leakage of the enrichment valve.
-------------------------------
What I've already done:
Replaced all soft goods in both carbs, replaced float needles and seats,
both carbs, and adjusted float height (which was off quite a bit). Cleaned
and installed existing points, gapped to .015", new condenser (old one was
allowing points to arc during static timing check), new plugs, new wires,
timing was set statically to 10 degrees BTDC, dwell is 61-62 degrees.
-------------------------------
Symptoms:
Backfire when the throttle opens - mostly through exhaust, but sometimes
through carb. Problem appears to be the rear carb only as I can set mixture
running on front carb, but not on rear carb.
-------------------------------
Vehicle:
73 MGB - SU HIF4's, no smog pump, injection ports crimped off, gulp valve
outlet capped.
-------------------------------
Details:
In diagnosing the two carbs, I tried running the engine on one carb to see
if I could find the problem (slackened linkage bolts and backed out on idle
adjust screw for carb I was not running). With the front carb only, I was
easily able to set the mixture using the "lift the piston" technique. With
the mixture properly set, the engine ran well at all speeds. The rear carb
was a different story. The "lift the piston technique" indicated the rear
carb was running rich - the same problem that originally prompted the carb
rebuilds. Backing out the adjustment screw 1/2 turn each time, I continue
to get the indication that the mixture is too rich. Eventually, the engine
begins to backfire in the exhaust when I lift the piston slightly. Leaning
it out further gets it to backfiring at idle.
The only internals that were not replaced were the needle and jet, neither
of which I thought of as "wear items" when I ordered the original parts for
the rebuild. I have a needle and jet on order now, but thought I'd see if
there are other thoughts from the list members. I also have a valve cover
gasket on order - as I have not yet done anything to the valve lash
adjustment (although I doubt an out-of-spec valve clearance would cause
backfire with one carb and not the other). Any suggestions would be greatly
appreciated
Thanks,
Rich Mason
'73B (The machine gun)
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e-mail-> rmason@ro.com
www----> http://ro.com/~rmason
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