405 auto Tooo Rich ( unlike me! )

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mg46783
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Joined: 02 Sep 2002, 02:16

405 auto Tooo Rich ( unlike me! )

Post by mg46783 » 16 Nov 2003, 06:36

<font color="blue">Hi folks... LTNS.... I popped in to see if anyone may have a clue to fixing this problem Im struggling with... As the subject line says - 405 auto Tooo Rich ( unlike me! )
Had a new webber carb only 12 months back.. runs really rich,.cant get emmissions down below 7.5% - HCs ok ..
the mixture screw is set about a quarter of a turn from being screwed right in! In this position the emissions are the lowest I can get them (7.5) and the best "running" position for the engine. Obviously the plugs ressemble soot - fuel consumption way high. and blows black smoke like a diesel.
Ive run it with the air filter tubing off..makes no difference, Ive blown/cleaned the 4 jets in the top of carb and the hole where the mixture screw screws in... all with no joy... ANY suggestions would be greatfully received. Cant figure it.</font id="blue">
ps: Car is 91 with only 47000 on clock...gen. and other than the problem itself..the car/engine is very good nick.

alan s
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405 auto Tooo Rich ( unlike me! )

Post by alan s » 16 Nov 2003, 13:38

Presumably the Weber carb is just a standard downdraft twin throat carby? If so, turning the mixture screw will do absolutely nothing apart from alter the idle mixture; it has absolutely no effect whatsoever on the overall mixture settings.
When you cleaned the jets out, I presume you took them out of the carby blew them out, then blew out the holes that they screw into individually. The reason I ask is that I have seen these jets put in assabout; the two in the middle being the biggest culprits at his.
On the assumption that they are OK and your air cleaner is clean, the next step is to check your float level. I think it should be set at 7mm as the needle touches the seat when held on a vertical plane. The only thing that can really throw this out is wear so the next thing to check is the actual needle & seat to be sure it's not leaking. This can be due to wear, manufacturing fault or the usual thing which is a bit of dirt stuck in it & preventing it from fully closing off.
On the old type carbs, there was always a chance that the float may have leaked & petrol got in it causing it to overfill, however with the modern idea of using a solid buoyant material that is very rarely a problem these days.
Check also that there's been no malfunctions with the ignition system also as a centrifugal advance jammed solid can also give the symptoms of faulty fuelling.
Alan S

mg46783
Posts: 104
Joined: 02 Sep 2002, 02:16

405 auto Tooo Rich ( unlike me! )

Post by mg46783 » 17 Nov 2003, 06:47

Hi Alan, thanks for the reply... I have to think hard when it comes to carbs.. Im more into EFi and diesels these days.. Its the same as when you come across a car with points!!
Anyhow, the carb. Yes , its a twin barrel auto choke thingy.. Did I mention its only 12 months old.. not a second hand.. it was "new".. I dont understand the mixture screw thing you said though.. surely that does the mixture.. if not though, what does???
The jets at the top (4) are all ok.. Ive not opened up the carb and checked the innards yet.. I dont honestly know what else it can be..
I took the 'mixture' screw right out... it ran altogether better..if a little lumpy, but it was deff. leaner ( no black smoke etc )...but as soon as I popped it back in.. problems return..as I say, wherever its set at. Ive even thought about drilling a hole in the inlet manifold to intriduce more air!!!
Anyhow.. let me know if you will what you think..Im still no further forward with it...
regards
Miek

alan s
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405 auto Tooo Rich ( unlike me! )

Post by alan s » 18 Nov 2003, 07:19

Mike,
Sorry to be long winded getting back; been pretty busy.
The "Mixture screw" should be called by it's proper name "<b>Idle</b> mixture adjustment or screw." it works only on the idle jet and in all honesty I've never seen one so far out as to cause black smoke etc. When it reaches this stage it's way past that and the minute adjustment it allows won't ever stop the smoking. The reason it stopped smoking when you removed it wasn't due to it's altering the amount of fuel entering the carb but was due to it allowing excessive air to enter via the hole it fits into which means it didn't reduce the fuel flow but increased the air volume at idle. As soon as the car was beyond the range of the idle jet, the problem would be still there.
The compensator/intermediate jets and Main jet are a set size & cannot be altered nor will they go out of adjustment. The reason I asked about downdraft was that some side draft carbs do have a fully adjustable set of jets but these are only usually used in modded and racing cars and until you've had the thrill of setting a motor up with these on it for racing, you haven't lived. It's the stuff nightmares are made of which is why they don't use them on standard road cars.
You say it has an auto choke. This being the case, that's your first port of call. Be sure the auto choke is cutting out when the motor warms up. This can be visually checked to be sure the butterfly is open within a couple of minutes of starting & the engine warming. Usually, cars with auto chokes have a corrugated tube from a hot box on the exhaust manifold. If that's missing via a local greasemonkey (ie) "Yer don't need that mayte, dunno why they put 'em on in the first place; get better fuel consumption without it." type of mentality, you'll find replacing it will possibly rectify the problem due to the engine currently thinking it's in a permanent winter.
If that checks out to be there, then it could be the dash pot on the side of the carby attached to the choke flap that's sticking. Usually a broken spring or simply a clean & lube up will cure that. If all else fails, convert it to a manual choke.
Check the needle & seat to be sure it's seating and hence cutting off the supply of fuel at the correct height in the float bowl.
If the float level is too high either due to the needle & seat or float, then petrol will continue to dribble into the throat of the carby and hence cause this problem.
If ever you have a chance to read the principles that carby/coil ignition operates on, you'll wonder why or how we ever made it past the Model T Ford. It's very hit & miss and it only takes a simple thing like float level to throw the entire system out of whack. Although we all complain about the complexities of EFi, it's a million times more scientifically accurate than this type which was only reliable due to its vagueness. Could almost be compared to calling a guy a "crack shot" when he's shooting flocks of pidgeons with a shotgun and keeps consistently hitting only one pidgeon.[:D][:D]
Alan S [:)]

mg46783
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405 auto Tooo Rich ( unlike me! )

Post by mg46783 » 19 Nov 2003, 06:15

Thanks Alan.. I like the analogy of the pigeon shoot !!
( Ive thought about shooting the 405!!)
Right well.. update: Ive been looking at it today - and I noticed that right at the back, at the bottom of the carb, is a throughway for water - ie, an inlet and oulet water pipes.. which is obviously for water heating the base of the carb.. as there were no pipes going to these tubes Ive rerouted (and added ) athe pipes to include it in their path. That little trick seems to have an amazing improving influence on things! ( It seems this was a case of "dont need them anyway" mentality).. Ive managed now to get it down to about 3.5 on the co meter but it wont go lower than that. The auto choke is working fine.. There is actually an adjustment "inside" a spring attached to a plunger like setup going into a water valve type of thing that appears to operate the auto choke.. I messed with that a bit because I felt the auto choke wasnt coming off fully -(this was a few days ago ) but to be honest t didnt seem to make much difference to anything - that said..the choke flap is fully openeing now - so maybe it did!!
So, thats where Im up too at the moment - tomorrow morning should provide more insight when I try and start it and see how it runs from cold.
So, from what you say.. what is it that actually adjusts the mixture , co, on these things???
Thanks thus far..
Mike

alan s
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405 auto Tooo Rich ( unlike me! )

Post by alan s » 19 Nov 2003, 13:28

At idle, it's the idle mixture screw, above idle it can be other influences such as dirty air filters etc.
There is also what they refer to as a "throttle" setting which is usually set at a dimension which varies with ambient temperature (20 deg C from memory) but I wouldn't even try to explain this for several reasons including the fact that I've never had to do one & it takes about 3 sections of a service manual to describe.
i would suggest a close look at the timing as well as the vacuum & centrifugal advances, as these can have things going in all directions too.
The centrifugal advance mechanism has a nasty habit of jamming or seizing & hence the distributor is set at idle but doesn't advance & retard as the engine revs rise & fall & this too can have a devastating effect on fuel consumption & hence emission levels.
My CX dropped about half of it's fuel consumption when mine jammed & was freed up.
A vacuum type with a stuffed vacuum module will do exactly the same thing.
Alan S

mg46783
Posts: 104
Joined: 02 Sep 2002, 02:16

405 auto Tooo Rich ( unlike me! )

Post by mg46783 » 21 Nov 2003, 03:03

Hi again.. and thanks.. I'll have a look at the timing , advance weights etc..that could well be a good plan! I did what I said wiith the piping , not helped.. At the moment its ticking over ok on the emmisions ( 3 ) but when you take the revs up the emissions go to 11 - 13 !! So, thats where Im up too...tomorrow, as they say, is a new day ! We'll see what it brings .. needless to say - Im sick of it!
I'll let you know what happens... watch this space..

alan s
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405 auto Tooo Rich ( unlike me! )

Post by alan s » 21 Nov 2003, 10:11

Beginning to sound more like the auto advance system by the day.
It's a nice easy one to sort if that's all it is if that's any conso;ation.
Alan S