No idle without choke
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From: Roger Hunt on 4 Jul 2010 06:08 Tony Jones <tonyjones(a)example.invalid> wrote >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 -- Roger Hunt |