Weird Erratic Idle - Dual DCOE Webers

I'm stumped. Before tearing into them, thought maybe some ApexSpeed Weber guru can help.

Dual DCOE 40 Weber set (on a 1600 for pushrod). When cold or even warming up in the pits before going out, idle is "normal, i.e. can be adjusted fine for 600-800 or so rpm. Even as it gets up to temp (before going out) idle is fine.

But after coming in after a race session idle way too high, maybe 2000+. We just shut of off rather than readjusting. Then after it sits waiting for next session (probably 90 minutes or so), idle will return to "normal" even if pre-warming it up before going out.

Quick inspection doesn't seem to be a return spring(s) problem That is, the linkage and springs return solidly to the idle screw.

I'd sure think it was linkage related but at this point it doesn't seem to be the case and wondering if anyone has seen something similar caused by something other than linkage.

Check for an air leak when hot.

I've had that same symptom before. You can try leaning or richening the idle jet a step or two. You could try adjusting the slow running air bleed screw leaner or richer when it's doing it's trick. You can try adding or removing idle ignition timing if you have a mechanical advance distributor. I have fixed it by adding idle ignition timing (by restricting the total mechanical advance inside the distributor). The added timing makes it want to idle higher but you can then adjust the carb idle speed back and that seems to help. I had one engine that I ran with a locked distributor and 42 degrees of advance, it idled and started great after I locked the distributor.

I guess the point would be that with dual Webers on a highly modified engine the actual linkage is only the starting point of what can get screwed up...

Brian

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Originally Posted by Pop Chevy

Check for an air leak when hot.

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That's what I'm going with.

Loosen up the throttle shaft nuts one flat and bend the lock tab back down. See if it gets better. What can happen is that one retaining nut could have been overtightened and "pulled" the throttle shaft to one side a little bit causing the throttle plates to come off center in the bore. When the body of the carb heats up, it can cause the plate to hang up in the bore just enough to raise the idle.

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Originally Posted by stonebridge20

What can happen is that one retaining nut could have been overtightened and "pulled" the throttle shaft to one side a little bit causing the throttle plates to come off center in the bore. When the body of the carb heats up, it can cause the plate to hang up in the bore just enough to raise the idle.

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Interesting and good info, but wouldn't that prevent the linkage from falling all the way back to the idle set screw? Or is there enough flex along the linkage system to allow one carb linkage to fall back to the idle set screw while the other carb is cracked open ever so slightly?

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Originally Posted by Daryl DeArman

Or is there enough flex along the linkage system to allow one carb linkage to fall back to the idle set screw while the other carb is cracked open ever so slightly?

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BINGO !

God bless ApexSpeed. Good suggestions so far. Keep 'em coming. I'll research each and let everyone know the outcome.

Found Solution:

Thinking that a progressive air leak (cold vs. hot) was the best probable explanation, I removed, inspected and made sure all the gaskets carb-to-manifold and manifold to block were OK. They were. While at it, completely disassembled carbs as if doing a rebuild, thoroughly cleaning passages, looking for any bad little o-rings, etc. Nothing found. Did find a rotor that seemed a little sloppy on the shaft so replaced it.

Alas, didn't fix the problem.

However, for some reason I decided to read the manual again regarding adjusting four idle mixture screws. Before, at idle, I'd turned each down gradually until idle was compromised, then backed of 1/2 turn. Turns out the manual says to back off more like a full turn from that point. Did that with all four idle mixture screws, then readjusted the butterfly (shaft) idle and re-checked the balance of air flow at idle at both carbs.

Problem seems to have gone away. Why those four idle mixture screws being a bit too tight (Like 1/2 a bloody turn!) would result in dramatic idle change, cold vs. hot, I have no idea. But...... life is good again. Lesson learned -- RTFM and don't rely on memory.

Never, adjust a Weber carb cold. If you do, they do all kinds of weird things once they heat up.

No doubt, good advice.

I have just fitted DCOE 45's/36mm to my Ford 2.0L S2 engine and I can't seem to get it to idle correctly... Here is a list of some of the things it is doing: (btw, engine was running well with the 2 barrel weber prior to assembling the DCOE's)

Doesn't idle well (won't idle down)
randomly spitting/popping back thru the carbs
Popping and flames from header on accel and de-cel
random popping in exhaust at idle

Here is a list of some of the basics:
According to documents I have read all the jettings in these carbs should be a good staring point. (possible leaner idle jet?)
Air Correction Jet: 155
Emulsion Tube: F16
Main Jet: 145
Idle Jet: 55F8
Starter Air Jet: 150
Ignition timing 36 degrees at 4000 rpm
Standard S2 cam
Stock mechanical pump (checked/regulated from 2-5psi)

I have tried a number of different things, checking, replacing, cleaning etc but with no success...

I'm starting to wonder about the plastic floats? I seem to remember in other carbs I have run that brass are preferred; less susceptible to vibration? Following one of the other threads on float adjustment I can't seem to get the proper reading at the specified float heights as well as trying different heights... I'm thinking that the level may be to high and it is bleeding over thru the air passage?

I know there is lot more that I could add here, but hoping this is a good starting point. Any help or suggestions would be appreciated.

Regards,
Vince

Check that all throats are synchronized with a UNI-SYN. Do this at about 1500 or so RPM. If the linkage is off a tiny bit multiple carburetors will act like crap.

Used to have one in my toolbox all the time.

This applies to multiple throats that are supposed to open simultaneously (not primary and secondary type set ups).

No one has mentioned the little bungie washer units that are used to isolate the carbs from the manifold. Tendency is to make them too tight. I believe the measurement is .040 gap between the springs (from memory) and most of us feel that is way too loose so we crank on another turn of the nut for good measure.

Bad idea. As someone said above - RTFM.

Pleas see below, answers in blue

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Originally Posted by vlamanna

I have just fitted DCOE 45's/36mm to my Ford 2.0L S2 engine and I can't seem to get it to idle correctly... Here is a list of some of the things it is doing: (btw, engine was running well with the 2 barrel weber prior to assembling the DCOE's)

Check your float level first on each carb. It should be 25mm from the top of the machined surface of the jet well to the fuel level in the main jet well. Use a thin piece of paper, marked at 25mm. Remove one main jet per carb. Put the strip of paper in the jet well while the engine is running and the fuel should just wet the paper at 25mm. If not, raise the float level, if the paper is wet before 25mm, lower the fuel level.

Doesn't idle well (won't idle down) - may be synchronization, may be a vacuum leak. It could also be the butterflies are not opening evenly together. Airflow must be even across all 4 barrels at idle through 1500-2000RPM. Remove the brass screws over the idle progression holes. Shine a bright light down inside. You should be able to see the edge of the butterfly at the first progression hole closest to the air horn. You can check both carbs and get the positions of both butterflies close to get you in the ballpark.

To find vacuum leaks, spray carb cleaner around the throttle shafts at either end and at the flex mounts at each carb barrel. There should be leather seals at each end of the throttle shafts. They are notorious for turning hard and leaking through.

randomly spitting/popping back thru the carbs - Too lean - try a larger idle jet.

Popping and flames from header on accel and de-cel - Likely lean and the unburned fuel finally lights off in the pipe.

random popping in exhaust at idle - Too lean idle mixture. Idle mixture is easily set with a "colortune" a spark plug with a window that lets you set each barrel by the color of the flame. You want the flame where it just turns yellow, not like the instructions that say "bunsen blue".



DCOEs are known for "off idle stumble" and low RPM lean running unless the right idle jets are used. The idle jets are in play from just off idle to 3500 RPM or so. I must admit, I have never used F16 emulsion tubes though.

Most of what you are dealing with sounds like synchronization, idle mixture and idle jets. A small error in float level really messes with tuning.

Your main jet sounds close. I can email you a pdf of the procedure.

Log into "Sidedraft Central" on Yahoo Groups. It is a moderated, private group. Keith Franck runs it and has helped many to get their DCOE set just right.

Here is a list of some of the basics:
According to documents I have read all the jettings in these carbs should be a good staring point. (possible leaner idle jet?)
Air Correction Jet: 155
Emulsion Tube: F16
Main Jet: 145
Idle Jet: 55F8
Starter Air Jet: 150
Ignition timing 36 degrees at 4000 rpm
Standard S2 cam
Stock mechanical pump (checked/regulated from 2-5psi)

I have tried a number of different things, checking, replacing, cleaning etc but with no success...

I'm starting to wonder about the plastic floats? I seem to remember in other carbs I have run that brass are preferred; less susceptible to vibration? Following one of the other threads on float adjustment I can't seem to get the proper reading at the specified float heights as well as trying different heights... I'm thinking that the level may be to high and it is bleeding over thru the air passage?

I know there is lot more that I could add here, but hoping this is a good starting point. Any help or suggestions would be appreciated.

Regards,
Vince

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I often found that poor idle characteristics were due to ignition timing being wrong for the combination of carb and cam at idle, so I would recurve the distributor. You could try swinging the timing at idle to see if it calms down, measure the best timing and re-jig the distributor to have that timing at idle and still get the full timing you want at full advance.

Thanks for the responses; I think I have found what may have been a major contributor, the throttle cable/butteryfly shaft was not allowing the them to close all the way, thus it wasn't really running on the idle circuit...

I still have a couple of questions:
Above someone commented that the poppin/spitting back thru the carbs and the flames out the exhaust is a "lean" condition? That is exactly opposite of what I thought... Aren't the flames due to extra unburnt fuel?

Also, my carbs have the air bleed adjustments, are those suppose to be adjusted as well? If so, how?

What about the stock 2.0L mech pump, is it sufficient? has anybody run them? Volume seems decent and I've place an inline regulator...

Thanks,
Vince