Too rich on my DCOEs

Too rich on my DCOEs

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tomtrout

Original Poster:

595 posts

162 months

Tuesday 19th August 2014
quotequote all
I'm running a pair of sidedraft webers with my 1700 x-flow and I'm strugging to lean off my idle circuit. Before everybody tells me to whizz off and find a rolling road specialist, I have already invested in a session and although some progress was made I'm still running too rich. The car is brilliant if I start of with fresh plugs, fire her up and steam off over the horizon. A plug check suggests the car is running rich - not unusual i guess when running these carbs but the trouble starts if I let the car idle for any length of time because the plugs will foul up and the car misfires when I pull away.

At the moment the carbs are fitted with 30mm chokes which from what I can gather are great for increasing air flow through the carbs at low revs but might be one reason why I'm having trouble leaning off the mixture. I don't really want everything to come in over 4000rpm so I don't necessarily want massive air flow from big chokes but I wonder if I went up a size if would help me rain in the fuel ratio mix at low and medium revs. Also any thoughts on emulsion tubes. F16 is what I have but there seems to be a school of thought that F11 tubes perform well when mated to a cooking crossflow?

Just for info I'm currently running 30 chokes, 120 mains, 195 air correctors, 45F8 idles jets. I know this is a massive can of worms but it's all part of the fun! Any thoughts/advice from any weber gurus out there?

bluezeeland

1,965 posts

158 months

Tuesday 19th August 2014
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Haven't got a Vixen, side-drafts, can't answer on definite sizes of jets, I'm far from a guru and worms are for angling, but....

My 3000s had extensive work done on the engine, had a rolling road session maxing out BHP and torque.

After the session however I still changed the main jets, as it felt running better on somewhat bigger.

What i want to say is;
- no engine is the same (and so on all, jet-sizes are different...)
- you spend good money and time on a rebuild, why not spend time on a tune
- you need to be sure what the character of the cam and heads are
- cam degreeing and pre-timing need to be factored in
- your drive style/prefs need to be acknowledged

In other words if all criteria are met, buy some sets of jets/emulsion tubes and try out what you prefer

Frank (family name sake...)


Cerberus90

1,553 posts

212 months

Tuesday 19th August 2014
quotequote all
We're running 30mm chockes on the dellortos too, and get bogging down at idle and slow speed pulling away.


As an aside, we've just fitted a new set of HT leads, as the old ones were a good 5 years old and of unknown quality. Whereas before the garage would stink of unburnt fuel when starting and driving out and then backing it back in, it now smells much better, more like just exhaust fumes, rather than exhaust fumes and lots of petrol.



Also, Ian Daniels reccomended us Bosch Super 4 plugs when we asked about the Taimar, he said on his race engines, everything else just fowls up, but the Super 4s are fine. Something else to possibly try, although they seem to be getting harder to find, and working out which type is required in the first place is hard enough, especially if your running NGK 7s instead of 6s.


Have you spoke to eurocarb? They were really good in helping us get the starting point for the 1600M once we'd had the engine rebuilt.

sjwb

550 posts

207 months

Tuesday 19th August 2014
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Sounds the same as my experince with a pair of side drafts; could not get it to idle consistently. I found that the fuel level was not stable and rising (on idle) due to leaking float valves. New float valve assemblies, problem cured. Simples!

heightswitch

6,316 posts

249 months

Tuesday 19th August 2014
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Tuning webers never used to be a black art. Finding an old boy who remembers how these days is often the issue since all people now seem to use dongles, lap tops and maps on omex fuel injection. I would have thought a 30mm choke slightly to big for a low tune road car. I am sure in the distant past I was running 28 and even 26's but I have not had side drafts on a car for a very long time so may be having brain fade From memory I slightly undercooked the car because you rarely run at peak power on a road car.

Have a look here which may be helpful

http://cnx.org/contents/a0a6da11-6437-404f-8cd3-10...

http://www.dvandrews.co.uk

N.



Edited by heightswitch on Tuesday 19th August 20:09


Edited by heightswitch on Tuesday 19th August 20:13

tomtrout

Original Poster:

595 posts

162 months

Tuesday 19th August 2014
quotequote all
Thanks guys, lots to think about. The Aux chokes are 4.5s I think.

I realise that every engine has different characteristics and I think I will do some "suck it and see" trials as Frank has suggested. I would be surprised Neil if you were running 26mm or 28mm chokes on a crossflow unless you were on a standard head with little valves, which would be unlike you! The standard set up for the Caterham super sprint had 32mm chokes. Quite a few fast road xflow owners seem to get good results from twin 45s and these must be running at least 34mm chokes and probably bigger otherwise there is no advantage in the bigger barreled carb. The 30mm chokes certainly give the car plenty of low down grunt and the tick over and pick up is smooth following some recent fettling but I need to lean off the mixture a tad through the idle and progression phase and perhaps a set of bigger chokes will be a starting point, possibly along with some bigger air holes in my idle jets.

I know the original downdraft twin choke carb was always going to be less of a headache but my car came with the twin 40s and I rather like the sound and the look as well as that little extra punch. Not quite so keen on the mpg!


Fiscracer

585 posts

209 months

Wednesday 20th August 2014
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The idle circuit has a massive impact on running especially at roadcar speeds/revs. Usually the problem is that it is leaned off (so the bugger coughs and pops and bangs on cold start up) in order to get the midrange mixture weaker because to maximise power at high revs you have big mains.

It is not a black art at all, although you obviously didn't go to the right rolling road tuner. Have a word with Joe A-B at Throbnozzle Racing as he is the best chap I know on 4 pot Fords in our general area. IIRC he uses a man at Gatwick but he may be able to point you in the right direction. BTW if your plugs are fouling have you thought about changing the heat rating?

plasticpig72

1,647 posts

148 months

Wednesday 20th August 2014
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I have the following in my 1600M which is now 1700cc and a piper 285 cam.

Twin 40 dcoe
CHOKES 31mm dia
MAINS 73401 120
AIR CORRECTOR 77401 190
IDLE JET 50F11
EMULSION TUBE F15
SLOW RUNNING JET
ACC PUMP JET 35
I must admit it is flat if i floor the throttle but once above 2000 rpm it takes off
Alan

Monkeythree

511 posts

228 months

Wednesday 20th August 2014
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Camshaft, cylinder head and advance curve are all going to influence the setting up of the carbs on your car so it's impossible (or at least highly unlikely) to be able to determine the optimum settings over the internet. Having said that, the settings you are using sound about right for 30mm chokes so you are probably not far away from what your engine wants. I suggest changing one thing at a time and assessing effect with each change.

Given that you seem to be running rich on idle, leaning off the idle jets and the idle air mixture adjustment screws is a good place to start.

Edited by Monkeythree on Wednesday 20th August 11:08

tomtrout

Original Poster:

595 posts

162 months

Wednesday 20th August 2014
quotequote all
Yes lots of variables, even on engines with the same basic build formula.I guess that's why the consensus is always a session with an expert on a rolling road with emissions kit.

I have already gone for leaner idle jets but it didn't seem to make much difference. I will try one stage at a time and then go back and try a few different combinations and if still no progress will stump up for a second opinion - possibly "throbnozzle". Thanks for the tip off Richard. Don't think I will google them on my wife's computer just in case I get more than I bargained for!

Monkeythree

511 posts

228 months

Wednesday 20th August 2014
quotequote all
Have you stripped them down and given everything a surgical clean? It only takes a small bit of grot to foul up one of the circuits or block one of the smaller drillings. The pump jet circuit is susceptible to dribbling fuel into the airflow if it's not perfectly clean and the jets properly seated (alloy washers in place).

tomtrout

Original Poster:

595 posts

162 months

Wednesday 20th August 2014
quotequote all
Think so but I will check. Thanks.

bluezeeland

1,965 posts

158 months

Wednesday 20th August 2014
quotequote all
Think the suggestion re heat rating of the plugs is valid, also the pre-ignition should be observed and changed if so required.

Comparing with Alan stats, his idles are an increment bigger, his aircorrectors one smaller and there is other emulsion tubes.
That is sort of opposite what you should think....

Think changing aircorrecter sizes changes not only idle but also main circuit (?)

Setting of the idle screws should be done in all of the test cases in order to give a proper assessment of the change made

Kindly report findings, highly interesting !

tomtrout

Original Poster:

595 posts

162 months

Wednesday 20th August 2014
quotequote all
I think the air correctors have very little impact on the primary circuit and regulate the fuel ratio as the revs climb above the mid range. I'm told the big danger is that if you select bigger (numbers) air correctors you run the risk of leaning off the mix just when you really don't want to to put additional stresses on the engine. I'm quite happy if my car runs a tad rich over 4000rpm because it will only be for short duration during acceleration. Probably better to be too rich than too lean although I guess the engine response tells you long before any damage is inflicted.

Will update once I've had a play.


bluezeeland

1,965 posts

158 months

Wednesday 20th August 2014
quotequote all
tomtrout said:
I think the air correctors have very little impact on the primary circuit and regulate the fuel ratio as the revs climb above the mid range. I'm told the big danger is that if you select bigger (numbers) air correctors you run the risk of leaning off the mix just when you really don't want to to put additional stresses on the engine. I'm quite happy if my car runs a tad rich over 4000rpm because it will only be for short duration during acceleration. Probably better to be too rich than too lean although I guess the engine response tells you long before any damage is inflicted.

Will update once I've had a play.

the same is true for bigger 'chokes', that is, venturi's actually....

Which sort of narrows it down to emulsion tubes (providing all the rest is checked and found OK(ish))

btw; in comparison; on the Griff forum is a thread re the 14CUX ECU, or the 'modern' variant of similar issues, it has a zillion posts, none of which I understand....... (have a read, its hilarious !)

tomtrout

Original Poster:

595 posts

162 months

Wednesday 20th August 2014
quotequote all
David Vizard, who knows a little bit about carbs, suggests that emulsion tubes can play a very significant role in regulating the air fuel mix depending on the number of holes and their distribution. Emulsion tubes are not cheap but I think I will give a set of F11s a turn around the block. F11s have more holes and should lean out the mix when combined with the increased flow that will come with bigger chokes.

My strategy will be to make some single changes and run checks on the plugs over low, medium and high revs - I might even give my old colour tune plug an outing. I know this isn't a substitute for a rolling road diagnosis but it should be fun and will be a real buzz if I make some progress. I can always go back to the current settings if my fettling makes things worse.

Fiscracer

585 posts

209 months

Wednesday 20th August 2014
quotequote all
tomtrout said:
David Vizard, who knows a little bit about carbs, suggests that emulsion tubes can play a very significant role in regulating the air fuel mix depending on the number of holes and their distribution. Emulsion tubes are not cheap but I think I will give a set of F11s a turn around the block. F11s have more holes and should lean out the mix when combined with the increased flow that will come with bigger chokes.

My strategy will be to make some single changes and run checks on the plugs over low, medium and high revs - I might even give my old colour tune plug an outing. I know this isn't a substitute for a rolling road diagnosis but it should be fun and will be a real buzz if I make some progress. I can always go back to the current settings if my fettling makes things worse.
Yes DV has forgotten more than most of us will ever know and he would never dream of testing different approaches in such a haphazard way. You will be going around the houses until you are retired unless you can assess the fuel air mixture under acceleration and load. How often do you drive at constant revs, because I never do?

tomtrout

Original Poster:

595 posts

162 months

Thursday 21st August 2014
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Very true Richard and I know the rolling road is the only way to pick any spikes in mixture. It's just a little painful that I've already been down that route and haven't made much progress.

Grantura SWE

64 posts

205 months

Thursday 21st August 2014
quotequote all
The E-tubes has everything to do with how the air/fuel mix is regulated, however not so much in the idle circuit. On idle the idle jet itself works as an E-tube. There are different idle jets with different sized holes and different nubers of holes on the side (air mixture holes). Dellorto carbs are different with two E-tubes, one for low speed and one for high. In my opinion a better solution but more expensive.

If your car is rich on idle but "ok" on the rest you should start with smaller idle jets and working on adjusting the idle screws first. Changing chokes and/or E-tubes should come later.
If possible try to get hold of a wide band lambda meter. It makes the process so much easier! You can see exactly how the car switches from lean to rich depending on load, throttle and revs. Otherwise it is more of a guessing game….

Apologies for possible language errors. It's not my native language.

LLantrisant

995 posts

158 months

Thursday 21st August 2014
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i would first start with the cam timing.

is the cam timed correctly?
who has timed your cam?
who has built the engine?
do you have an adjustable chain-sprocket fitted?

valve clearance: setup for the cam or used the ford´s standard values?

next:

give us the exact type of carbs e.g. weber 40dcoe32 or weber 40dcoe72 & 73
incl. pic of the carbs.


30 chokes...i have driven lots of x/flow powered kitcars..99% came with 32chokes..and the engines have been tuned similar to yours.

have you ever tried 32 chokes?

the actual fitted ones are original 30ies or "somebody" has widened e.g. 28´s on a lathe?

original chokes are conic, this is often not respected when people widen them to safe some money.