Worldly Scions,
I turn to you for the collective wisdom -
I have been experiencing a perplexing problem with a carb for
some time now. I know the carb is at fault because I've tried it
on multiple cars.
Problem: Once the car warms up, it wants to idle at 2500-3000 rpm.
This carb has always wanted to idle a little high. When the carb was
new, the idle speed would creep up to 1200-1500 rpm I could only bring
the idle speed down by "dragging" the motor down with the clutch and
car in 4th gear. Lately the idle speed has been inching higher & higher.
This is getting kind of hard on my clutch.
The Carb: I bought the carb from TRF a couple of years ago. It was
new old stock in the original hermetically sealed Leyland bag. It's a
stock ZS carb for a MkIV Spit. I'm running it on a '78 Spit 1500.
I suppose it might be a little "small" for the motor. Could this be the
problem? It has a manual choke (set properly & unused this time of
the year). The idle speed screw it set as low as possible without the
set-screw falling out. It is *not* sitting on the choke fast idle cam.
I can find no vacuum leaks. I've tried plugging both vacuum ports (no
change).
It's got oil in the dashpot. The piston moves freely. I've tried adjusting
the mixture but get little change in CO according to my (semi-cheesy)
heath-kit exhaust gas analyzer (works OK otherwise). These "emission"
carbs typically have little range of mixture adjustment. I have not taken
the carb apart in any way. It has less than 4K miles total on it. I've been
trying to not introduce any more variables by disassembling it. It looks
like new both inside and out. Spark plugs look good. No smoke from the
tail pipe. I'm stumped.......
The car(s) start and run just fine in all other respects. Engine braking
is not too good of course. I'm certain the carb is at fault. I do not have
this problem with any other carbs. There are relatively few (3) possible
adjustments on the carb. Am I overlooking something?
Thanks in advance for any Worldly Wisdom you might provide.
Bob Sykes (s1500@aol.com) - - - __,@_\____
'78 Spitfires - -- }-0-----0->
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