Tickover and fuel mixture screw/adjuster

[mister2mk1]

When I start my car it initially runs at around 2000rpm (seems a little high). I realise this is probably the auto choke, but after a few minutes it does drop but sits at around 1200rpm.... (Still seems to high - thought it should be around 800/900rpm)


Is there a simple way of adjusting this???

Also I have checked the ignition timing and all seems okay there at 10degrees.

In addition the exhaust fumes smell quite rich is this because the idle is too high??? If not is there a mixture screw??

Greatful of any help

Steve

[System-G]

There are two adjustments needed.

there is a screw in the throttle body (sometimes covered with a black lug). This adjusts idle speed.

There is another screw on/near the MAP sensor bracket on the boot bulkhead which adjusts fuel/O2 mixture (RHS of inlet manifold) - also often covered by a plack lug.

Cold Idle should be around 1800 - 2000rpm warm idle should be 800 - 1000rpm.

HTH

[PW@Woodsport]

Problem is you will never adjust it down if the waxstat is stuck preventing the air bypass from fully closing, which is extremely common on Mk1's.

Take your intake pipe off and you will see a 5p sized hole at 5 o'clock just in front of the throttle butterly.... stick your finger over this hole with the engine running and it should stall, if it does then your waxstat is stuck, invest in a new one or fit a manual choke kit.If the engine doesn't drop hardly any revs then you have an air leak elsewhere on the plenum.

[System-G]

Lets first try adjust idle and O2/fuel mixture. No need to jump at a problem that may not even be a problem in the first place.

4 MK1's I have owned or co-owned plus another 3 or so engines that have gone through them, plus a load of other MK1's I've worked on have not had any problems with waxstats - so not that common a fault in my experience

However, Paul does see a LOT more than I guess I ever will see, so...

[dopiaza]

Both of my Mk1's have had this problem, current one has been 'fixed' by plugging the hole Paul mentioned with an inverted dust cap with a 2mm hole drilled in it - this has allowed me to easily get the idle down within spec and also it only revs to around 1300rpm when starting from cold.

Not a true fix obviously but seems to have done the trick for me.

Incidentally, does anyone know what the CO level should be set to at idle?

Graham

[PW@Woodsport]

Lets first try adjust idle and O2/fuel mixture. No need to jump at a problem that may not even be a problem in the first place

G the reason i advise checking the waxstat to see if it is stuck first is that you may well be able to "artificially" lower the idle speed by adjusting the air bleed all the way in (it should not be all the way in) and adjust the idle mixture too to get the idle down.However this is not the correct way to do it, say for instance the waxstat becomes unstuck and shuts off bypass air altogether? with a fully wound in idle air bleed screw the engine is then going to stall every single time.This might not be annoying if it happens once but if the waxstat is being tempermental and sometimes sticking, sometimes not ,then it will frustrate the owner no end.

By all means give it a go, you might find turning the air bleed all the way in works for you, but it is not the way it is supposed to be and will give you further issues if the waxstat becomes free again.

Rule out the waxstat first and solve the main cause of the problem.

[System-G]

Yup - heard the dust cap solution before - have also heard them dissapearing too... into the throttle body/manifold/TVIS/Valves? Or do the get spat out into thin air through the induction system?

The way I see it is try adjust idle and O2/fuel mixture first and if you can't get them into spec, then look at other problems - perhaps starting with the waxstat. If the waxstat is goosed, then get it sorted but for Dog sake, don't put a dust cap in there. Sort it properly

[un1eash]

I have a dust cap in mine Along with a manual choke kit i made myself.

[PW@Woodsport]

The way I see it is try adjust idle and O2/fuel mixture first and if you can't get them into spec, then look at other problems

I've just explained why this would not be the best way to go about it, you will end up artificially masking the problem by just winding the idle screw in and letting the stuck open waxstat provide the idle air.... not good.That's like adjusting the ignition timing to mask an exhaust cam out by one tooth running problem, it will run ok but it's not the right way to do it.

Determine if the stat is stuck FIRST, cure that, and then when it is closed properly on a hot engine then and only then adjust the idle air tickover.Remember the waxstat is only supposed to be supplying auxilliary cold start air, not hot idle bypass air, that is the job of the bleed screw.

That is the only professional way to do it.

[dopiaza]

As Paul said earlier, first thing to do is whip off the inlet ducting and plug the hole with your finger. It really is a 5 minute job and gives a definitive result. Make sure the engine is fully warmed up for this. If your waxstat is working properly then plugging the hole should have no effect on engine revs - there should be no feeling/noise of suction at the hole either.

Yes, I have used the dust cap bodge myself, but mainly because my car is rarely driven. Theoretically the pressure differential across the dustcap should either hold it in place firmly or at worst case (WOT) there should be no pressure differential so bearing in mind it took quite a bit of force to fit it I can't see it coming out easily. Although the more I think about it the potential consequences the more I'm inclined to remove it now! If mine were a daily driver then a manual choke conversion would definitely be at the top of my list of things to buy.

[scomr2]

So far nobody has mentioned the idle up valve - this raises the rpm by ~200 whenever there is an electrical load. If you switch off everthing including the passenger camparment fan then the idle up valve should close and the revs drop. If you are going to fiddle with the idle speed then this should be done with the valve closed and a proper handheld tach.

[LimeyMk1]

Yup - heard the dust cap solution before - have also heard them dissapearing too... into the throttle body/manifold/TVIS/Valves? Or do the get spat out into thin air through the induction system?

Mine disappeared, I imagine it went into a combustion chamber and got thoroughly pounded, burnt and spat out of the exhaust. Altho I have pushed one too far into the hole and it popped through into the waxstat mechanism, so it may have gone that way.

Just don't use metal dust caps.

[phil mk1]

I have had a dust cap with a 5mm hole fitted for two years with no problems all it has done is lowered my cold start revs by 700 rpm it used to rev at 2000 from cold now revs at 1300 saving me quite a lot of fuel as it is my daily runner
cheers phil.

[mister2mk1]

Okay - latest update on this when I cover the 5p sized hole in the intake it doesnt stall but runs slowly and quite lumpy.... where next!

[PW@Woodsport]

It should definately stall, so it's getting auxilliary air from somewhere... time to get the wd40 out and spray all over the place to find the air leak.

[mister2mk1]

Okay tried this again and after running like a dog for a few seconds it then stalled....... I assume this then would suggest no (or little air leak) what does it mean for the WAX stat?

thanks

[PW@Woodsport]

If the idle was racing with a hot engine and it stalls when you cover the hole then the waxstat is sticking.

[mister2mk1]

Engine was relatively cold but idling at +2000rpm?

[PW@Woodsport]

The test won't work on a cold engine because the waxstat is supposed to be open during that period.... repeat the test on a HOT engine.

[mister2mk1]

Great thanks paul - will repeat test...

I assume that when hot it should still stall? And if it does then its just a tweak to the idle and mixture?

Sorry to be a pain....!!

cheers