Mixture Screw

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pbar
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Mixture Screw

Post by pbar » Mon Feb 17, 2014 8:20 pm

I have the standard 32/36 Weber carb on my 2.0 Laser. As I understand it, the mixture screw should be something like two turns out (from bottoming). Mine won't run with anything like that, it has to be at least 8 turns out, preferably into double figures to run smoother. Is having to have it so far out masking an issue elsewhere? What should I be looking at first?

I do have some hesitiation, if the mixture screw isn't out a long way.

I have tried adjusting the screw to achieve the fastest idle, it has to be turned out around seven times to get this, and then continuing to turn it does nothing sound wise, but it does make the car run better and smoother.

I have set my timing with a strobe to 6 degrees. Points are new and set, new condenser, and plugs and leads are relatively new also. Same with the dizzy cap and rotor arm. I haven't done much yet to try to sort this, thought I'd ask first. Though I have cleaned the idle jets with a strand of nylon and the carbs fuel filter too. Neither had an issue. Accelerator pump works as it should, as does the auto choke.

Car runs great really, no real issues.

Or is this behaviour not that unusual on an old non rebuilt carb?

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stevemarl
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Re: Mixture Screw

Post by stevemarl » Mon Feb 17, 2014 10:13 pm

It sounds like the idle jet may be partially blocked or the wrong size (too small) A bigger one should give you a full range of adjustment so it will start to go too rich if screwed out beyond a couple of turns.
Good thing is that it only affects running at very low speeds/idle as above this main jet takes over.
(Also easy fix as it screws out from side of carb, I think should be size 50)
Have you checked the float level as if that`s low it`ll weaken mixture?

edit: just checked my records & mine is a 60, which gives correct range of adjustment and always passes CO test

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Re: Mixture Screw

Post by pbar » Tue Feb 18, 2014 7:22 am

Thanks Steve. I did consider the float bowl and level, is there a way to check this with the carb in place?

I did take the idle jet out to give it a clean with a nylon strand. I didn't notice it's size, I will have a look later now you have told me that. Though I assume mine is the original one, unless it has been changed at some point.

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Re: Mixture Screw

Post by stevemarl » Tue Feb 18, 2014 8:14 am

The float level is adjusted on the top cover, which can be removed in situ. Worth checking float has not got fuel in it as apparently brass ones can leak? It`s detaiied in Haynes. If you don`t have a spare top cover gasket, they`re symmetrical so if you turn it over it should seal (there`s a ridge on one surface only) There was a story back in the day (yes... I know......) that if you put it in boiling water for 1/4 hour it will `recover` to some extent. Best while wife`s out probably....

Whilst the jet may not have changed in 30 years, the fuel has, so it may need a slightly bigger one now?

The other thing is - what was the CO like at last MOT? Because if that was about right (1-2%)then the adjustment IS still correct (even if it IS at the limit of its adjustment) if you follow?

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Re: Mixture Screw

Post by pbar » Tue Feb 18, 2014 8:29 am

Thanks for the info Steve. The CO was too high initially at the last MOT, turning the mixture screw in sorted that and it passed. Although since then I have set the timing as it was very advanced when I checked it, so set it to 6 degrees, which is 2 degrees retarded for unleaded. Although the mixture screw issue that I have has been like this for some time, it's only now that I'm looking to sort it. Very good point about the possible need for a larger idle jet with modern fuel, I hadn't thought of that.

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Re: Mixture Screw

Post by DaveSimpy » Tue Feb 18, 2014 2:39 pm

I remember reading on a website, may redline PDF download maybe?

If you keep turning out and nothing changes then you need to go up in size on idle jet. There was some definite steps which were described.

Was this carb standard on the car or did it come off a cortina or something?

Ah, here we go:

Simple Rules for low speed calibration

If the mixture screw is more than 2 1/2 turns out turns then the Idle jet is too lean (too Small). When the mixture screw is less than 11/2 then the Idle jet is too rich (too large). These assumptions are based on the fact that the speed screw setting is not opened more than 11/2 turns. If the speed screw has to be opened 2 or more turns then this is also an indication of a lean condition usually requiring greater change. At times it may appear to be showing signs of richness or flooding it is really a lean condition. See pictures and notes in the tech 2 article supplied in the kit instructions, view and please understand the need to keep throttle plate as near to closed as possible so as not to prematurely expose the transition holes. This is what causes the visible rich condition, and confirms the need to increase the jet size. JET KITS are available if needed.

EXAMPLE With the speed screw set at no more than (1 1/2) turns in after contact with the stop lever; and the best idle occurring with the mixture screw set at 3 turns from bottom, indicates the need for a larger Idle jet. Achieving the best idle at under 2 turns indicates the need for a smaller idle jet.

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Re: Mixture Screw

Post by pbar » Tue Feb 18, 2014 3:22 pm

Thanks Dave, yes I did come across that whilst I was researching this. Really good site that. The carb, as far as I know, and I have no reason to think otherwise, is the original. What put me off just changing the idle jet is that it's all standard at the moment (or so I believe), which made me think that it's an issue elsewhere. But, as Steve said, modern fuel has changed so maybe changing the jet will simply compensate for that. I will take the jet out again later and see what size it is, and check the Haynes as I think the recommended jet sizes were stated in there.

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Re: Mixture Screw

Post by pbar » Tue Feb 18, 2014 7:55 pm

Well I had a look at the idle jet and it's a 60. Gave it another clean, but I'm happy for that to remain, don't think that's the issue. So, I guess the float is the next thing to look at then, unless there are any other suggestions as to what might cause this behaviour?

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Re: Mixture Screw

Post by pbar » Thu Feb 20, 2014 7:32 am

I will get a carb service kit I think, as it makes sense to give it a going over as I'm taking it apart anyway to check the floats as previously suggested, and don't think the carb has had much attention in the past ten thousand years. Definitely seems to be a blockage/running lean, needle valve maybe?

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Re: Mixture Screw

Post by stevemarl » Thu Feb 20, 2014 9:15 am

The only thing I would say is that it doesn`t sound THAT far out;
The CO was too high initially at the last MOT, turning the mixture screw in sorted that and it passe
So it does have a full range of adjustment, lean to overly rich, it`s just that the rich end requires several more turns out than it should be? At the end of the day, if the CO can be brought within limits, then it doesn`t really matter too much where the screw is. I`m not saying it won`t benefit from a service/needle valve etc, just that it`s not really very far out & you may npt get too much improvement. I would be more inclined to try a 65 idle jet qs that will affect the `range` of adjustment.
Just my thoughts....

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Re: Mixture Screw

Post by pbar » Thu Feb 20, 2014 9:36 am

Thanks Steve. Your thoughts are most welcome as always. It's not just the mixture screw position that bothers me, the car does seem to be hesitant and lean. And as the screw position (at the moment it's a whopping 18 turns out which is it's best position for minimal hesitation) is obviously not right, I am inclined to think that something is amiss carb-wise as I am happy that everything else is properly set. Carb service kit is about £20 from Fast Road Cars, it's likely it's never been done. I am hoping that I take the top off and instantly see the issue with the needle valve, one of your first thoughts was for me to consider the float!

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Re: Mixture Screw

Post by stevemarl » Thu Feb 20, 2014 9:43 am

Must be nearly off the threads??

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Re: Mixture Screw

Post by stevemarl » Thu Feb 20, 2014 9:53 am

There is a really good book about Webers, construction, function, tuning, overhaul which I bought when I got mine. Explains different circuits & how they overlap & affect running
This is the exact edition I have, covers to `79 - but although MADE later, our cars are sort of pre `79? I would strongly recommend.
http://www.amazon.co.uk/Weber-Carburett ... arburettor

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Re: Mixture Screw

Post by Ghia15 » Thu Feb 20, 2014 10:29 am

Hi,

Same sort of thing with the manual.

I have this one and it is cracking. Also as it is specific to ford only carbs it is fairly well focused. Covers from 1978-1991.

http://www.ebay.co.uk/itm/NEW-HAYNES-MA ... 0900110800

Either one will give you good info on the workings of the carbs and faults.

All the best.
Image

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Re: Mixture Screw

Post by pbar » Thu Feb 20, 2014 11:14 am

stevemarl wrote:Must be nearly off the threads??
Not far off. It can't actually be metering much at all.

Thanks for the book advice guys, it's right at the top of my shopping list.

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Re: Mixture Screw

Post by pbar » Sun Feb 23, 2014 1:01 am

Well I cleaned the carb up etc. and fitted a new needle valve, reset the float and so on. It was a little bit out and both measurements. Mixture screw is about 5-6 turns out now and gives a much more finite adjustment when finding the highest idle. Was still hesitant on opening the throttle so I looked to the points (should have checked them sooner I know, thought they were ok though), one contact had a 'pimple' on it so cleaned them up and reset them, cleaned the dizzy cap contacts too as, although replaced that not so long ago they were dirty. Car runs sweet now, no hesitation at all. Glad I did the carb as it did need a clean up and not looked at the float since I've had it, but most of the hesitation was down to the points and dizzy cap I feel. The combination of the carb and points has sorted it. Life's perfect once more.

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