No idle without choke

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

Reply to
Tony Jones
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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, 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. 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 haven't heard any. It's possible there's clag in the fuel tank, but it happens even with a new fuel filter.

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

Reply to
Tony Jones

clean the idle jet and check that the idle jet solenoid is actually working. there is no provision to alter the mixture at above idle speed. also the dwell and timing will need to be reset after the points have been replaced.

Reply to
Mrcheerful

Thanks. According to Mr Haynes:

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I have two solenoids: 10. the idle cut solenoid valve and 11. the coasting leaner solenoid valve and idle switch. Am I right in assuming it's number

10? The idle cut solenoid has a spring and plunger, but which one is the idle jet?

Cheers, Tony

Reply to
Tony Jones

Does the solenoid click nicely when the ignition goes on? No.14 is the idle jet.

Reply to
Mrcheerful

Vacuum advance diaphragm on the dizzy split? Remove the vacuum pipe going to it and block with a bolt and see if that cures it.

Reply to
Conor

Sounds like it doesn't have an air filter in it, or the air inlet is leaking badly making a lean mixture .... I think I'd give it a major service ...

Reply to
Paul - xxx

It does have an air filter - I've taken it off enough times to get at the carb :) The air filter is one thing I haven't replaced (it has '2005' scrawled on the top, but the car has only done about 5K since then) and could do. I also cleaned the remains of the carb gasket off with solvent as best I could before replacing it so there was a flat surface and I hope there are no vacuum leaks.

When you say the air inlet is leaking, what do you mean? The top of the carb is exposed to almost free air (there's only the filter in the way between it and free air, though the free air side of the filter is also supplied with exhaust from the EGR).

Well, it's kinda getting one through all the bits being replaced... anything else I should do that might help?

Tony

Reply to
Tony Jones

he means is there an air leak on the engine side of the carb. below the carb at idle there should be a considerable level of vacuum, if there is a leak (crack, gasket, hose fallen off etc.) it causes a very weak condition which you may not be able to correct by idle mix adjustment. The way to check for an 'air leak' in this situation is to have the engine running at idle (or as near as you can get) then spray carb cleaner along joint faces, over the manifold and breather pipes etc. if the idle speed changes (usually increases) then you have got an air leak and a bit more careful spraying can pinpoint it. I have also seen cracked pipes to the brake servo and faulty servos cause air leaks, so spray around those too. My bet is still on a blocked idle jet.

Reply to
Mrcheerful

I don't know the car specifics at all ... but it sounds like it's running weak and one thing that makes a car run weak is too much air in the mixture ... (doh!!) so there might be a possibility that air is getting in between the carb and engine/plenum/however it's fitted, before it gets to the combustion chamber. Thus making whatever you do to the carb almost immaterial, other than you'd need to set the carb richer (choke on) to try to compensate.

As I say I dunno the car .. but I'd take the carb off again, then take off any pipes or manifolds or plenum chamber that connects between the carb and engine-block and check them for air-tightness .. cracks/holes etc etc.

Then I'd re-assemble them all using smears of gasket goo or gaskets (depends on what's correct) and service the carb .. set float height, clean jets, set throttle cable lengths/linkages properly etc. and work from there towards the fuel lines.

Air could be getting in the fuel lines too, which means it's already a weak mixture before it even gets to the carb ... hence the carb needs (again) setting richer (choke on) to compensate for the weak fuel mix.

In essence, check for 'incoming' air leaks after the carb and incoming air leaks in the fuel system before the carb.

Reply to
Paul - xxx

Ta, that makes sense. The carb cleaner trick sounds like it's the place to start and then work from there.

Tony

Reply to
Tony Jones

Tony Jones wrote

Reply to
Roger Hunt

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