I have run into an engine problem that I can't diagnose, and am
wondering if any out there have had a similar experience.
Situation....freshly restored car with essentially a brand new engine.
New 1725 factory short block, which I have been carrying around with me
for the last 25 years, fitted with all overhauled/reconditioned
accessories. Head and cam overhauled by a reputable machine shop, engine
built up by me (I've done several before), distributor and Stromberg
carbs overhauled by me, etc, etc. New spark plugs, clean fuel lines with
an in line filter. Various vacuum hoses checked and found leak free.
Points and spark plugs correctly set, as were the tappets.
Engine initially started, and distributor set to approx 70 BTDC. Carb
mixtures adjusted to give reasonable running. Car idled at about 1200
rpm. Drove car for about 2 miles at modest rpms, then shut down.
Next day, started car OK, went for a drive, and after about 2 miles,
engine started to miss, and get a little rough. Got worse as I returned
home. Found that I could remove number 1 and number 2 spark plug wires
with no change to engine running, though still with a big spark on the
leads. Fiddling with throttle and choke had no positive effect. Both
carbs seemed to be working, i.e. as I blipped the throttle, both pistons
raised a little.
Let engine cool down, then attempted to restart. Started fine, ran
smoothly for about two minutes, or until things started to get warm.
Then the misfiring returned, same symptoms...nothing from #1 and #2 cylinders.
Let the engine cool down, and repeated. Same results as before...ran
fine for a couple of minutes or so, then went off.
I assumed that the front carb was acting up, so removed the carbs, and
replaced the front one with another which had not been overhauled, but
which had worked successfully on a 1725 a couple of years before.
Replaced the carbs, and started the engine....exactly the same symptoms.
While fiddling around in this condition, I blocked off the air intake to
the front carb with a leather work glove....the engine smoothed out
noticeably (perhaps not completely), but at a lower rpm than when the
engine was started.
At this point I am a bit stumped. Perhaps I have some sort of vacuum/air
leak which only occurs as the engine warms up, perhaps there is
something I haven't thought about. If it is a leak, how come it only
affects the front two cylinders? The problem does not appear to be
electrical, since there continues to be good sparking at the front two
cylinders. Since I replaced the front carb with another, it seems not to
be directly front carb related.
Any comments and/or advice will be appreciated. I'd like to get this
sorted so that I can go driving this weekend!
John Slade
Manotick, ON
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