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Author: Subject: poor low speed running Weber DCOEs
Valtra

posted on 27/4/11 at 06:14 PM Reply With Quote
poor low speed running Weber DCOEs

My Fiat Twin cam is standard internally but running Sunbeam Ti 40 DCOEs and Lumenition Competition Sparks The car runs fine under acceleration and at high speed but has become very rough at idle and low speed poping and banging and (seemingly) running on 3 cylinders some of the time . the car was jetted and set up on a rolling road 2 years ago and has not been touched since . is it possible for the mixture screws to work loose ? will be heading to Stoneleigh this weekend ..Cheers
Ian





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britishtrident

posted on 27/4/11 at 06:52 PM Reply With Quote
Simplist things first check the jet holders haven't come unscrewed if not take the jets out and check they are clear by blowing through, if not check for water in the float chambers and check the accelerator pumps are working and pump jets are clear.
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Xtreme Kermit

posted on 27/4/11 at 07:25 PM Reply With Quote
I think there's been a couple of cases on LCB where the retaining screw holding the choke in place has come out/loose allowing it to rotate and screwing things up royally...

The screws are on the underside of the carb body.

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contaminated

posted on 27/4/11 at 07:29 PM Reply With Quote
Does one cylinder in particular seem to be worse? Are the plugs fouling and is one blacker than the others? Sounds like it might be over-fuelled a bit on one. Are you using the chokes? They can stick.





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contaminated

posted on 27/4/11 at 07:31 PM Reply With Quote
quote:
Originally posted by Xtreme Kermit
I think there's been a couple of cases on LCB where the retaining screw holding the choke in place has come out/loose allowing it to rotate and screwing things up royally...



That's the issue I was thinking of. Advice is generally don't use the chokes for that reason.





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Valtra

posted on 27/4/11 at 07:39 PM Reply With Quote
Thanks one and all for suggestions

I've had the jets out and all are clear . One HT lead (they were new 2 years ago ) had a rubber shroud on the dizzy end that had gone like chewing gum but I changed that and there are sparks aplenty when the plug is out , but forgot to mention that there was very little (but a bit) difference in the idle when that plug lead was pulled off . not checked the other plugs but the car accelerates like a rabid greyhound so there can't be anything too far wrong at the top end of the fuel and spark equation .

Cheers





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Valtra

posted on 27/4/11 at 07:40 PM Reply With Quote
quote:
Originally posted by contaminated
quote:
Originally posted by Xtreme Kermit
I think there's been a couple of cases on LCB where the retaining screw holding the choke in place has come out/loose allowing it to rotate and screwing things up royally...



That's the issue I was thinking of. Advice is generally don't use the chokes for that reason.


could you expand on that ?





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Xtreme Kermit

posted on 27/4/11 at 08:22 PM Reply With Quote
There's a bit of confusion here. I am not referring to the cold start device, I am talking about the chokes which fit in the body and set the amountorvair that can be drawn through. eg my 45s have 36mm chokes.
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perksy

posted on 27/4/11 at 08:34 PM Reply With Quote
Venturi retaining screws, Located underneath the carb

They can work loose/fall out allowing the venturi/s to turn in the carb body
You'll get all sorts of problems *If* this happens


Weber 40 DCOE Diagram http://www.webcon.co.uk/weber/40dcoe.htm

Item number 28

[Edited on 27/4/11 by perksy]

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Xtreme Kermit

posted on 27/4/11 at 08:38 PM Reply With Quote
Ahah! Venturi. That's what I was looking for. Cheers persky
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Valtra

posted on 27/4/11 at 09:16 PM Reply With Quote
Had a quick gander and the chokes AKA Venturis (32mm on my 40's) are lined up and tight and I'm Pretty certain the retaining screws are in , though it is like keyhole surgery to see under them .

Will check all the plugs to see if one is gunked up tomorrow then it may indicate at least which cylinder is creating all the fuss

but please keep the suggestions flowing it is much appreciated





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greggors84

posted on 27/4/11 at 09:38 PM Reply With Quote
My DCOEs lasted about 2/3 years before becoming rough on idle. A quick tweak with the idle mixture screws and they were fine. It seems the high speed mixture isnt so critical so it doesnt go out of tune as much.

Give all the idle mixture screws a 1/4 turn open until the revs start to rise, then back off a bit. The balance screw between the 2 carbs might need adjusting aswell.

If you dont know which screw is which search google for dcoe exploded diagram, there are a few out there.





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Valtra

posted on 2/5/11 at 09:53 PM Reply With Quote
Poped and banged my way to Stoneleigh even after adjusting the mixtures on my carbs to where they seemed to idle the best . once warm in the garage before I left it was idling smoothly . One person at the show said it may just be a leak in the exhaust and sure enough there is a bit of a blow before the silencer could it be sucking in a bit of air and igniting unburned fuel in the exhaust and causing a bit of back pressure (backfiring!) thus making it sound a bit lumpy (bit like a 3 cylinder Subaru) or am I adding 2and2 together and making 6 ?





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Valtra

posted on 3/5/11 at 10:09 PM Reply With Quote
just bumpin up
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Valtra

posted on 6/5/11 at 06:25 PM Reply With Quote
any further comments your honour ?

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Xtreme Kermit

posted on 6/5/11 at 07:29 PM Reply With Quote
So I guess the next thing on the list is the float valves and levels.

If a valve is leaking you will have too high a level in the float chambers which causes all sorts of issues!

If the float level is out, you will also get issues.

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Valtra

posted on 9/5/11 at 10:15 PM Reply With Quote
I think I may be getting somewhere . I worked out which cylinder was playing up by pulling the plug lead of , on no1 cylinder it didn't make and difference , checked the compression and it wasn't particularly low , changed plug and lead but no improvement . so removed the idle mixture screw there was a bit of grey deposit which I scraped of with my fingernail and squirted some carb cleaner down the hole . there is now a very significant improvement so maybe with a bit more fiddling it will be ok

cheers

Ian

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steve m

posted on 9/5/11 at 11:08 PM Reply With Quote
I gave up with the twin 40's as they seem to need balancing /tweeking on a daily basis

So back with a 28/36 dcd now, and all back to normality

Steve

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Valtra

posted on 10/5/11 at 12:14 AM Reply With Quote
Ah but the noise is addictive

Some smoke , some can't live without regular booze. some bet on the horses . I burn carboniferous deposits so making music in the most spectacular way I can .





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Oddified

posted on 10/5/11 at 07:57 AM Reply With Quote
I've run 40's/45's and the like on cars for 20 or more years now, and by far the single best mod/change i ever did was to fit a single large filter and one piece back plate across the pair. Prior to that i had the 'normal' seperate twin filters and the balance between the carbs was for ever needing a little tweek and once you've done that you normaly redo the idles. The only logical reason is that the single filter back plate links the two carbs together and reduces the chance of one or other carb dropping ever so slightly on the manifold because of the flex mountings which then puts the balance out.

Ian

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Valtra

posted on 11/5/11 at 12:27 AM Reply With Quote
Nipped across to Ashby Foleville tonight and car was still not right . Think i'll have to start from scratch with the idle mixture settings . then dig a little deeper . engine is thankfully still mighty under power





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