[Chrysler300] 300C suddenly idles VERY rich

["Ray Melton rfmelton@xxxxxxx [Chrysler300]" <Chrysler300-noreply@xxxxxxxxxxxxxxx>]

Hello All -

After a nearly five-year comprehensive restoration effort, my late 
father's 1957 Chrysler 300C convertible has been running and looking 
great for nearly a year - First place or "best of Show" at nearly a 
dozen local car shows, and lots of compliments, thumbs-ups and honks 
from other drivers!  But suddenly two weeks ago it began running VERY 
rich at idle, following an abrupt braking incident - now it almost 
wouldn't idle in gear, and I had to shift to neutral at every stop to 
keep from stalling.    I suspected the sudden deceleration had sloshed 
fuel from one of the carb bowls, and indeed I initially smelled raw gas 
under the hood.   But repeated attempts to "unstick" either of the carb 
floats by tapping  a wooden screwdriver on the tops were futile, and 
short bursts of full-throttle to 90 mph (shhh!) to perhaps flush any 
debris from a float needle jet did not clear the problem. However, once 
at full-throttle and high speed, the car runs GOOD and happy!  But 
nothing has stopped the return to severely rich idle.

It now starts sluggishly (whether hot or cold) and only if I hold the 
accelerator all the way down to give it maximum air, symptom of 
near-flooding.  Hard revving from idle gives black smoke from both 
pipes, but it seems to run OK once the revs get above 1500 - 2000 (no 
tach).  I don't suspect a fouled plug yet, since it runs good at top 
end, and BOTH sides of the dual exhaust show black smoke, instead of 
black smoke and sooty tailpipe from perhaps only a single fouled plug on 
one side.  Don't smell raw gas under hood any more, but too-rich idle 
persists.

Although I don't think it's relevant, it should be noted that I had just 
recently switched to 91-octane NO-Ethanol gas ($3.29/gallon) as soon as 
it became available in town barely a month ago (August).  I had 
previously been running 91-octane ethanol-adulterated pump gas with 25 - 
50% 100-octane low-lead aviation gas from my local airport 
($5.25/gallon)  But at this point, cost is not a consideration, and I 
may go back to the avgas mix as soon as I run out the new ethanol-free 
stuff, just to see what happens.

I can't yet find the info in the repair manual, but I need to clarify 
whether the engine runs on the FRONT or the REAR carburetor at idle and 
low speed, because that's where the too-rich running occurs; when the 
second carb kicks in, it runs fine!  Can anyone tell me which carb to 
start trouble-shooting?

NOTE:  Jim McGowan restored both carbs and performed the modifications 
outlined in Service Bulletin 1002-CH and subsequent correction in 
Technical Bulletin #364, which involved drilling new holes, lowering 
float levels and installing different jets and metering rods, which I 
had custom-made by the fellow who actually bought the original machinery 
that Carter used to fabricate the jets and metering rods back in the 
1940's and 1950's - no modern carb specialist could hold the necessary 
tolerances, whereas this fellow says he can hold 0.0005", just like when 
Carter was making them over 60 years ago!

I appreciate any help!

Ray Melton   Las Cruces, New Mexico  white/Gauguin convertible S/N 3N572517





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Posted by: Ray Melton <rfmelton@xxxxxxx>
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