Mixture adjustment - DCOEs + wasted spark

[millsn]

I'm trying to tune my zetec by looking at the spark plug colour.

I get a light rusty brown on the first two cylinders (one carb) and the other two plugs are actually as white as your teeth. What should the aim be? White or light brown? Please bear in mind that the wasted spark is likely to burn any excess mixture on subsequent strokes.

Should I be using a different procedure to setting up a xflow? I.e. running until engine spin best and then richening by 1/4 - 1/2 turn?

 

Nigel Mills - 2.0 Zetec carbs

[Robster]

The mixture adjustment on the Weber only does the idle mixture. The colour of the plugs indicates the mixture throughout the engines 'normal' speed and load, ie 'main' jets in use. You need to check that the jets, both fuel, air and emulsion tubes are the same, then check that the idle jets are the same in each cylinder. May sound odd, but my Xflow had a rolling road done a while ago, I was never happy with it afterwards, and when I checked the plugs, some were black, some where white. So, I looked at the jets and some moron had mixed them all up, three different size air jets, hence all cylinders running different mixtures!

 

The brass cover on top of the carb unscrews with a wingnut, and you will see two screw in jets, the larger being the mains. Get some advice as to what they should be set at (James Whiting or someone will have an idea) and it is very easy and obvious how you change them.

 

Good luck!

 

Robster.

[Asklepios]

My understanding was that the spark plug colour, as a tuning method ,is redundant with the demise of leaded fuel.

When practiced it was not a reflection on the mixture throughout the speed load range.Rather it reflected the status as of ignition off.Thus a plug check for mains would be done after tanking along on main jet,cutting the throttle and coating to the side for the plug inspection.Have I got this wrong?

[rjc]

Phone Weber ! They can advise you on what jets to fit. There is general guidance on their website.

 

You need to ensure that the two carbs are balanced with one another using a suction gauge so that all four cylinders suck the same. Use the air bypass screw under the white cap to equalise two chokes in the same carb if they are different.

 

Idle screw adjustment is important to normal driving. I use a 'colour tune' see thro' plug to check the tune on each cylinder in turn. There is a temptation today to fit too small an idle jet because it gives better economy. However the car is usually lumpy at idle and spits and coughs when accelerating from low revs or on part throttle. Make sure that when you screw open an idle jet from closed the engine speeds up after about two whole turns and then slows down again when turned further. If it doesn't do that you need to change the idle jet. Colour tune will tell you if its too rich or too weak.

 

'Main Jets' consist of a main jet at the bottom and emulsion tube and an air corrector jet at the top and the three work together under acceleration to provide the rich mixture. Again they need to be balanced with each other.

 

The four sets of jets need to be the same as each other.

 

A session with your local engine tuner will usually sort out any fundamental problems and Weber do a book on the DCOE with detailed explanations of how the carbs work. Demon Tweeks can sell you all the stuff you need to do the tuning.

 

 

[I.Mupferit]

Hi Nigel

I was told by Weber agents Southern Carburettors to ignore the plug colour. Not sure if its due to unleaded or what, but it is not right, however, that each pair of cylinders have a different colour. Assuming the jets are the same in each carb, I would suspect it is definitely a balance problem and needs to be sorted ASAP.

 

If you are interested I have a complete choke & jet listing for a standard 2L as recommended by James Whiting. This should only be regarded as a starting point however as due to manufacturing tolerances, no two apparently identical engines/carbs are alike in behaviour.

 

I played around with jetting (using Des Hammills excellent book as a guide) for ages before achieving a reasonable compromise between driveability/power, because that is all you can ever achieve with carbs I am afraid.

 

Hope this helps

 

Brent smile.gif

[stevefoster]

I have tuned the slow running jets by holding at a certain rpm for 30 secs

and then turning off. You must run long enough at a certain rpm for it to make its affect on the plug colour.

I have now got a similar colour across all the plugs.

 

I have done this for 1000 rpm, 1200 rpm and 1500 rpm.

 

The slow running mix jet affect the whole rpm range but less and less the

higher up you go.

 

A light grey'ish colour is the one to go for but expect a carbed car (esp Weber) to be a bit sootier at lower rpm.

 

I think engine tuners tell people that this method is defunct partly because it is

time consuming and IT IS defunct now with modern measurement equipment.

 

I can't see any reason why unleaded should affect the effetiveness of plug colour tuning. As I can vouch I have altered my plug colour from a range of soot colours on 3 cyl and one light grey to grey across the board and the engine runs better for it.

 

Nigel, I am not sure what the white means but try richening those that are white and see the affect.

 

/Steve Steve from sunny warm Sweden.

[millsn]

Thakns for the info all. I'm happy with the main etc. The progression has 4 holes only which I'll invstigate getting changed. I still don't know whether I can use plug colour to tune this engine though, or what the differnce from white to light brown is or what I should aim for. Any additional advice on this specifically?

 

wHATEVER ARE YOU DOING ON THE LIST OVER THERE Steve? PS, could od with using your lathe when you get back (ride height troubles continued - I aslo have a large lump of ally for the gearknob)

 

Edited by - millsn on 27 Aug 2001 00:07:40

[I.Mupferit]

Nigel,

I can't remember which size carbs you have, are they 40's or 45's?

 

Brent

[rjc]

You cannot use plug colour alone to tune the engine.

 

The plug colour just indicates how the fuel air mixture is burning in the cylinder. In your case there is an indication that one carb is significantly out of balance with the other.

 

Two light brown - OK

Two white like teeth - not OK, and potentially a serious problem the engine is too lean in these cylinders and you risk knocking and burnt out pistons.

 

White is too lean, brown is OK, Black is too rich.

 

Sorting the problem demands that you determine that the air flow, idle mixture and main jets are setup correctly. This cannot be done by the plug deposit colour alone.

 

You MUST get the carbs all sucking the same amount of air as the first step. You WILL NEED an appropriate suction gauge.

 

Then you can use colour tune see through plug to get the mixture right. The instructions with it will tell you how to test the idle and acceleration mixtures by the flame colour of the mixture burning in the cylinder.

 

You can mess about trying to get it right BUT I guarrantee you will not get it right by guesswork and in the worst case you could damage the engine. An idiot in the local garage tuned my old crossflow by listening to the carbs with piece of plastic pipe and twiddling the idle jets until he pronounced it OK.

 

It was awful. I bought a colour tune and a suction gauge from Demon Tweeks and set it up myself, it was miles out and needed bigger idle jets. Afterwards the transformation was simply staggering.

 

It's easy to do and the kit costs nothing compared to sorting an engine where two pistons have had a hole burnt through the top of them through running too lean over a long period.

 

 

[millsn]

ok. Perhaps I should clear some things up. The engine is on 45s. It has already been setup by Dunnell for power and ran fine when I first had it. Since then I've meddled with the idle jets and learnt about the potential 5th progression hole. A friend has set up his engine idle (which does affect all the rev range) using plug colour - which for idle seems just fine to me (assuming I can do this with my ignition) and he's had good results. Is the VX wasted spark?

As to carb balance, I've no problems there as I already have a sychronisateur as I recall it being named. I've no problem with anything I've been told so far. I just want to know about colour testing versus wasted spark really.

 

The colour tune idea is interesting. Is it designed for throttle response though or is it for economy? If you use one of these does it use the original spark plug or a surrogate one and do you need to richen the mixture after its use. It might be worth experimenting with it if you ever go the North Hants and Berks meet in Hartney Wintney?

Other than that I'll richen the two misfit white plugs for the moment. Thanks to all so far.

 

Nigel Mills - 2.0 Zetec carbs

 

Edited by - millsn on 27 Aug 2001 20:36:57

[rjc]

OK Sorry !

 

Wasted Spark means that two plugs are fired by one electronic coil fed from the ECU. That way you only need two electronic coils instead of four. As a result each plug fires twice every cycle, once for real and once (in synch with its pair firing for real) on the exhaust stroke - a wasted spark.

 

The colour tune plug is a pseudo spark plug that is inserted in place of the standard one and it has a toughened clear plastic annulus built into it which acts as a window into the cylinder. By looking down over it (or through the supplied periscope with a mirror) you actually see the mixture burning in the cylinder as the engine runs !

 

By noting the colour of the mixture as it burns you can make a precise adjustment of the idle strength and watch what happens when you rev the engine simulating acceleration.

 

You do each cylinder in turn and then the engine is set. The colourtune plug is not hardy enough for permanent running and is for tuning purposes only, but once set the engine is as perfect as you'll get it on carbs. Watch out for burnt fingers though - it gets hot !

 

 

[millsn]

Unfortunately the colour tune doesn't fit the zetec! Bugger

 

Nigel Mills - 2.0 Zetec carbs

[STEVE GILBERT]

I can give you my jetting programm if it helps. I have a @210 hp 2.0 litre Zetec on 45 dcoe webbers which runs very nicely with no flat spots, splutters, or backfires. Let me know and I will look tonight.

On the subject of the idle jets I was finding a flat spot at 2.8-3.2k revs untill i drilled them a bit to increase the size. They started at F8-55 and I drilled them to 0.65mm (f8-65)

Oh just worth saying i am on 3d ignition, dont know if it makes a diffrence

 

Edited by - steve gilbert on 30 Aug 2001 09:25:52

[russb]

colourtune can be made to fit a zetec, I have just modified mine to fit a VX after much frustration trying to get the mixtures right on dellorto 45's. I had to weld a steel tube onto the colourtune plug so that it acts as both the plastic extension tube and a box spanner to screw the thing into the plug hole. You will need a taper seat adapter for the zetec so will need to tack weld that to the colourtune body, failure to do this could lead to the embarrassment of removing the colurtune whilst leaving your adapter screwed into the head in a very inaccessible spot.

best of luck

[Fast Westie]

Millsn

The key is what you said about which two plugs where white. All plugs should be straw colour. In your case the carb balance is out so you will need to adjust the balance screw between the two carbs.

 

Put a length of tube in the trumpet of one carb with the other close to your ear and then put it in a trumpet of the other carb in the same position. You will hear a different sound. Adjust the balance screw until the two are as close to the same as possible. You may need to drop the idle speed once they get closer to balance.

 

Email me if you want more info.

 

The car in front is a Westfie1d wink.gif