Folks:
Got a mystery on my hands. I think I can figure it out, but I
would like some brainstorming help from the list to make sure I cover
all the bases.
As you know, I got my car put back together ( mostly. =:-o )
after a complete teardown last winter for paint and a suspension
rebuild.
It ran purrfectly before the teardown, but has had a low speed
miss since being reassembled. The engine, ignition, and carburetors were
not disassembled or serviced, they were merely removed and reinstalled.
I can recreate the misfire at will; Put it in 4th gear,
decelerate to around 1500 RPM, then use light to moderate throttle to
accelerate. It will misfire 2-3 times and then accelerate smoothly. It
misfires at other times while accelerating in any gear, but always below
2,000 RPM. Under these conditions it will not always misfire, rather it
is occasional. Cruising is smooth and powerful, but in 4th gear at
around 1,500 RPM it will do it every time. High RPM is fine - no breakup
or misfiring.
Items I have done to date: Regapped the champion plugs to 0.025"
(they are almost new). New points and condenser. Replaced a frayed
breaker point mounting plate ground wire, reset the timing. Verified
vacuum retard is working properly, replaced the spark plug wires with
new ones, topped off the dashpots, resynchronized the carbs, adjusted
the idle, and reset the bypass valves. The plugs are a nice even tan
color, and the "lift the slide test" works properly, indicating the
mixture is fine (?). The diaphragms were replaced when I rebuilt the
carbs last year, as were all the o-rings and gaskets in the carbs. The
float level was checked and reset at that time.
By the way, I am running B1AR needles, not the stock B1AF
needles, which are about 10% richer than stock. I had to change them
after I shaved the head, installed a 270 cam, and ported the head. The
stock needles were just too lean after all the modifications.
I am now suspicious of the carb dampers. The low RPM
acceleration misfire has me thinking that the air valves are rising too
quickly, and the engine goes lean briefly. I am running 10W-30 motor oil
in the dampers. I am thinking about experimenting with 80WT to test this
theory. If it is going lean, then 80wt should push it to the other
extreme and it should go rich, but not misfire. If that works, then I
can fiddle until I find the right viscosity for the dampers.
Am I on the right track here? I have wasted a lot of time
pooping around with the ignition, and now am thinking carbs. What else
should I be checking? The car looks so sweet with the new paint, and
it is frustrating that the motoring experience has a fly in the
ointment. Any comments or thoughts are welcome.
Thanks everyone!
Vance
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