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From: Tony Jones on 1 Jul 2010 17:27
I have a 1987 Mazda 626 with 1.6 carburettor engine and a manual choke.
I've been having some trouble with it idling - it'll only idle when the
choke is pulled out all the way. Otherwise it runs OK, though fuel
consumption is a bit poor (25-30mpg, probably due to the choke). As soon as
the choke is pushed back in and there's no pressure on the accelerator, the
revs fall too fast and it stalls. The trouble is, when it's warm with the
choke out it idles at about 3000rpm which is a bit annoying (and eats lots
of fuel). It'll quite happily drive without the choke, but as soon as it
comes back to idle it stalls.

I was also having some problems where it would run fine, but that all of a
sudden the revs would fall. A push on the accelerator sometimes kept it
going for a bit, but it would usually stall - even at 70mph. I've taken it
for a few 20/30-mile runs on the motorway without much change to the idle
behaviour (and it's been driven on some hundred-mile runs since the problem
first started to arise, so I don't think it's cobwebs in the engine).

I first tried replacing the fuel filter and cleaning the points and
rotor arm, with no luck. Then I removed and cleaned the carb (with spray
carb cleaner) and replaced it with new gasket. That didn't seem to help
much. I've also replaced the fuel pump and the spark leads. That may have
helped the random stall problem, though as it's random I can't be sure.

I did manage to keep it usable for a while by tweaking up the idle screw to
2000rpm, which it will idle at OK when it's hot. Below that it starts
misfiring. I've played around with the mixture screw and a dicky Gunson CO
meter (reckons a range of about 1.3-1.5% CO but I'm not sure I trust it) but
I can't seem to prevent it misfiring below 2000rpm. The mixture setting
doesn't seem to make a lot of difference to the revs or the CO output.

After putting the carb back together it's behaving slightly differently...
now it will only start from cold (and we're talking summer cold here -
15-20degC) when the choke is held out (that's about 2.5 stops), while before
it would start OK in the winter (-10degC) on just 2 stops. There's not much
I can adjust on the choke cable other than a millimetre or so, which I can't
see making a difference. I've listened for vacuum leaks around the carb and
head gaskets, but can't hear any. It's possible there's some clag in the
fuel tank, but it happens even with a brand new fuel filter and drives quite
happily apart from at idle.

Sadly I can't seem to source a replacement carb or one to compare mine
against, which might be the simplest option.

It probably needs a decent tune up, but I need to fix the idle problem
first. Any ideas?

Cheers,
Tony
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