All,
A while ago, I posted a plea for help in solving the
following "post-smog-check" running problems on my 1974 TR6:
1) very rough idle when cold with choke
2) missing/popping when cold with choke
3) "hunting" when warm on constant throttle
4) lots of popping on overrun
followed a couple a weeks later by:
5) misfire at 2000 rpm
There were many responses from those on the list. Thanks
to all. Being a scientist, I had to solve this using the "scientific
method". This involved making one change, testing the car from cold,
making the second change, testing the car from cold, etc. So here goes
with my Presidents Day weekend saga.
Hypothesis 1: New spark plugs (NGK N9Y's). The misfire at 2000 rpm
(item 5 above) completely dissapeared, but symptoms 1-4
remained.
Hypothesis 2: I had also ordered a new dizzy cap and rotor. Put them on
but symptoms 1-4 remained.
Hypothesis 3: Changed the carb damper oil from Marvel Mystery oil
to engine oil. Item 1) above became more pronounced. The
engine seemed to idle on 6 cylinders for a few seconds, then
run on 3 cylinders for a few seconds, then run on 6 for a
fews seconds, then run on 3....... Symptoms 1-4 remained.
Mmmm I thought, must be an air leak somewhere.
Hypothesis 4: Disassembled both carbs, cleaned, and reassembled.
Balanced carbs, set mixture and idle speed. Symptoms
1-3 solved! Now the car starts and idles smoothly when
hot or cold, pulls well, no flat spots. A new car! However,
item 4) (popping on overrun) remains.
Hypothesis 5): The popping on overrun to me means that unburnt fuel
is leaving the cylinder and igniting on the hot exhaust
manifold and/or pipe. Does this sound reasonable? I tried
setting the mixture on the carbs leaner and richer trying to
get rid of the popping, but no such luck. I guess I still
have a leak sonewhere, perhaps in the idle circuit. 1974
and later cars have a mechanism to shut down fuel flow when
on overrun which is clearly not working in mine. Can anyone
tell me what part of the carb I should look at next?
When I had the main needles out, I checked them against the older
needles and they were 1/4" (yes! 0.25") longer then the original needles!
The original needles are marked B1AF. The top of the code on the
new needles are obscured, but seem to be marked B1AF. After research,
there are a set of needles out there that are marked B1AP (used in Volvos
I think). Has anyone out there ever seen their replacement needles
being a different length?
As an aside, the after-market needles appear to be of inferior quality
to the originals.
Shane Ingate, hummin' but poppin' in San Diego.
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