Help please any carb gurus .

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fozworth

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Fozzy
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Twin DRLA 36 carb problem ,guru help please
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Post by Fozzy » Thu Jan 11, 2018 9:09 pm

Hi all ,so I've just fitted a pair of newly rebuilt / as new condition DRLA 36's to my 1641 twin port which is stock apart from a vintage speed exhaust.I have all new manifolds and decent thick gaskets and have faced the manifolds before hand
The carbs according to what Euro carb recommended being,
115 mains
180 air
55 idle
33 pump
150 needle
9164.2 emulsion
28mm chokes.
I'm running a Bosch svda dizzy and have a anti pulse valve fitted inline between left carb and dizzy.
Before I fitted these I had new Webber ict's which I covered 500 miles and had no running issues and the timing and valves etc have not been touched since removal.
I have a facet fuel pump and filter king regulator still set as per Webber ict recommended .
I have set the idle on the Drla's and balanced both carbs .
The main jet circuit is fine and it revs nicely within that circuit.
Both pump jet levers are opening at the same time and same amount of travel ,about 5mm of movement .action finishes about 1/4 throttle position.


Now the problem .While it sits at idle pretty well at around 850-900 rpm As soon as you press the gas pedal down gently it nearly stalls .It you imidiatly come off then press the gas it picks up and then you can Rev it fine but if you let off the gas and allow it to idle again for a couple seconds and then press on the gas it has that boggy flat spot. If you are in the flat spot and gently pump a couple time it puffs and you get a flash out of the venture ,mainly the right hand carb. I
I've tried re setting the mixture and tweaking the timing very slightly with no joy on both.
I'm really stuck as to what it might be .
Before I take the carbs off and double check that all pump jets are working as soon as the throttle spindles are turned and also check the pump jet non return valves are not letting fuel back into the bowels ,is there anything else I could try to save taking the carbs off ?
I will also get some wd40 to spray around the manifolds just incase they are leaking but they really shouldn't be.
Cheers
Ja
 

fozworth

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It's fine on main jet circuit ,it's an issue with a dead/ flat spot when yo roll off idle.
I'm booked in at a rolling road on the 27th but would like to see if I can get rid of this flat spot just off idle beforehand.
I had another play tonight and this is what I found
I double checked for leaks around the manifolds and carbs tonight and had zero issues .It ticks over nice and evenly and one thing I noticed tonight was that if I slowly increase the throttle by turning the bellcrank linkage by hand on the engine it revs up nicely but if I do the same but press on the accelerator pedal it nearly stalls coming off idle .Cant understand it ?
 

fozworth

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You can hear it sound ok when you Rev it .Im thinking take the carbs off and check the no return valves and pump jet squirt action
 

SiDev

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First check is to make sure you haven't got a blocked / partially blocked idle jet. Take it out, blow it through with an air line or some carb cleaner and put it back in, see if it's better.
 

Dave.P

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Get some aerosol compressed air cans (electricians use them, can get on eBay or electrical supplies place) and blow out all jets, I always carry cans of air , they’re a must with twins.. my idles block every now and then due to dirty fuel tank, I can whip them out roadside with angled needle nose pliers and a mirror and all blown out in 10 minutes.first thing I do if running lumpy.
 

Dave.P

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Reading your post again, it does sound like not enough squirt from pump jets. I’ve had this before ,years ago but forgotten what it was 🙄. Do you have the dell or to book by Tomlinson ? Great fault finding pages in there. If I can find mine over weekend I’ll look and let you know.
 

fozworth

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Hi Dave ,I've checked my valves,static timing,point gap and inspected my pumpjet action and all four squirt the same and ok as far as I can see ,I've also adjusted the fuel pressure to 2.5psi .its ticking over fine and revs fine but if you flaw the throttle it dies or pops/flashes out of both carbs .it holds steady at high revs but you can hear it hunting a little .
My mates del book leans towards either upping the main jet size or airs if you get these symptoms on acceleration?
So I think I need to wait till I get it to a rolling road as I font have any larger mains .
 

fozworth

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https://1drv.ms/v/s!AlYeK-ATh-yogntbnePrCuaWLxFM
Hopefully this will give you an idea although I'm sure you do now
 

fozworth

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Hi Dave ,
This morning I did as someone else advised who suspected I had to small idles which was to wind my mixture screws out 3 1/2 turns from seated to see if it ran better .It did ,in fact I turned them out nearly four turns and the idle was smoother and it did seem to loose most of its flat spot although it still did it a bit but then it was probably due to the mixtures being all that way out .So I think 60 idles are a defo .
I would like to try possible 118,122 mains just to see as well
 

Dave.P

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FYI if it’s any help, mine are wound out 3&3/4 turns and I’m running 180 airs and 60 idles. If it pops it’s too lean so I would maybe up idles and mains a bit. I’ve always found vent size x 4.1 -4.3 is good for main jet size. Hope that helps
 

Zcat7

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Idles operate up around 3000 revs before the mains kick in so it's surprising how much you rely on them.

Another idea - I don't know much about Dellorto carbs but are you sure that the vacuum takeoff you are accessing is above the throttle plate? If your timing is set to expect advance on opening the throttle rather than retard then unintentionally connecting to the wrong side of the throttle plate will give you a load of retard when you opened the throttle. If you connect to the manifold/lower side of the throttle plate you get max vacuum/advance at idle/off throttle. So imagine you then tweak fuel and tickover etc to get a nice idle, when you open the throttle quick it will cause you the issues you've been seeing as it immediately "releases" the advance, which = retard. Not what you want!
 

fozworth

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The vac pipe can only go to one port on the left hand carb on the dels as seen in my pic I've posted .its just above the mixture screw near the linkage
 

fozworth

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Ok quick update guys.I ordered 58 and 60 size idle jets and tried the 58's this evening . I preset the mixture screws to 3 turns out .Once warm the engine ticked over nicely and so then I tried to dial in the mixture screws .I slowly turned each one a quarter of a turn in but on each one the engine started to drop off so backed out to 3 turns again .at this setting the engine no longer pops or flashes but it does have a flat spot still if you floor it of idle ,but if you back off and floor it again it pick tight up ,no pops. So I wonder if I should try the 60 jets to see if it looses the flat spot .? It does smell quite fuely/rich but maybe that cus I'm in a garage with only the back door open .I think with the 60's I might have to wind the mixture screws in a bit .
I'm really hoping it will solve the flat spot..
 

Dave.P

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Yeah I would bung in the 60’s and see what happens. I used to get my jets from fast roadcar on eBay and I think years ago he told me you can make up thedifference of 2 (58-60) with mixture screw adjustment? Dunno if that’s true though! but I found 60 were good for me. I’ve played about for hours in the past doing this, enjoyable if you have the time for it 😆 if you can get it running ok a rolling road will sort you out I’m sure which would be wise anyway to get the right fuel air ratio etc, have you balanced them with a snail gauge by the way?
 

fozworth

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Hi all ,well after trying some different combos I've ended up with the 118 mains and 58 idles in place .Its not much different to the 115 mains and 58 idles and both combos still give me a small flat spot off idle if you floor the pedal .it smell quite rich to .I suspect there is another issue that I'm not finding and will now leave it till I take it to the rolling road next sat to get it properly hooked up .
Will keep you posted .
 

Bluesnailman

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Dave.P said:
Reading your post again, it does sound like not enough squirt from pump jets. I’ve had this before ,years ago but forgotten what it was 🙄. Do you have the dell or to book by Tomlinson ? Great fault finding pages in there. If I can find mine over weekend I’ll look and let you know.

Different carbs I know..... I had the same problem as you with a 34 pict3 and it was the pump squirt on rapid throttle opening that was the problem. Mine was blocked, yours might be undersized.

I think your idle jets might now be oversized and you're now masking the problem by running rich (not that I'm an expert, but just attempting to give another perspective) - hence the smell
 

fozworth

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Ok so I took my dizzy up to a friends vw garage today and he stuck it in a customers bay with a 1600 stock motor and single carb. The dizzy showed to be running just as it should .we hooked up the strobe and The advance responded fine ,and wouldn't go over 32 degrees.Im wondering if the refurb company free'd it up when bench testing it ?
I know a single carb will create mor vacuum as it's feeding from four pots rather than the ne one the twin carb setup but even my Drla's should respond well enough based on this mornings test.
I'm going to take my bus up to him next week and he's going to check to see if my setup responds the same and most importantly doesn't do what it did at the rolling road ,which if you remember resulted with the dizzy not advancing off idle and then spiking hard at high rpm to 44 degrees with vac off and 53 connected.
I checked the vacuum port on my Drla to see that it was defo no blocked and it's fine .
The only other thing Ive noticed is that when you Rev my engine and let off when stationary the revs don't always drop completely back to idle imidiatly ,sometimes they hover just above 1000rpm before settling at 850 and then sometimes the just drop straight back down smoothly .maybe it's because I've got over sized jets at the moment ??
 

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