Hi,
I'm working with a friend on his 1931 Marmon 88 straight-8 engine. He
pulled the fully functional engine about a year ago, had the whole
thing miked out, replaced all the parts that needed it etc. in prep
for a rebuild to fix leaks and generally restore the engine. The
engine is now all back together, and we can get it to turn over and
run under its own power, but are having some serious complications
with timing it properly.
The problem we're running into is somewhat complicated and very
confusing, so I will explain it in detail in the hopes that we get
some useful advice.
1. The distributor worked perfectly in the car before the engine tear-
down. It's a dual-point distributor, and he's getting his
synchronization equipment ready to double-check, but it was
synchronized when the thing was taken apart and hasn't been touched
since.
2. We have verified that the cam and crank are timed correctly. They
were installed to spec using the embossed teeth on the timing gears to
align them for the installation of the timing chain, and we have
verified that at Cyl. No. 1 TDC (hereafter referred to simply as TDC)
the intake valve begins to open at exactly TDC on the intake stroke,
which is exactly as the Marmon manual (which he has, correct to his
year and model) specifies.
3. We have verified the firing order against the Marmon manual. I
believe, off the top of my head, that it is 1-6-2-5-8-3-7-4, but can't
confirm this- he has the book since it took him about 20 years to find
one in good condition. In any case, we have verified that the
distributor cap is wired correctly in that order.
4. We have verified that the distributor sparks Cylinder No. 1 at just
before TDC on the power stroke- in other words, we have statically
timed the engine to two flywheel teeth BTDC for the power stroke
according to the spec in the manual. We've verified that we're not 180
degrees out of time (of course, the engine wouldn't run at all if we
were, but we checked anyway).
So, the problem:
We can get the engine to start and run on its own. The starter motor
labours intensely to turn the thing over at correct static time, and
it barely starts and runs very ragged with *lots* of partially-
combusted gas in the form of black smoke in the exhaust. This is
frustrating but could be attributed to any number of things, but the
odd thing is: if we loosen the distributor bracket and *RETARD* the
timing by approximately 30 degrees of distributor rotation, which
corresponds (unless I'm mistaken) to 60 degrees ATDC, the engine runs
much smoother and cleaner, and the starter turns it over easily
instead of labouring. That's as far as we can retard the timing before
the condenser hits the retard arm spring screw and we can't turn it
any more. I would normally think that we should get better performance
out of the engine by advancing the timing, albeit at the cost of
hotter and less reliable valves.
Additionally, the more we choke the carb, the easier the thing is to
start and the better it runs, even with the throttle wide open it runs
better if we partially block the air intake.
The two "shots in the dark" we're going to take next time we drive the
50 miles to where the engine currently lives are a) checking the
timing with a dynamic timing light to see if it's flashing TDC when we
retard the distributor way past where we think TDC should be, and b)
checking the synchronization on the points.
Any ideas on what could be causing this? The distance between us and
the engine makes it inconvenient to go back and forth a lot, so we'd
appreciate any tips on things to look for when we try again after the
weekend.
Jon Harvey
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