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DCOE rich idle - help please!

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I have read and searched and tried to self help but I am stuck and need another set of eyes to zero in on what I am missing.


Here is my set up:


- 1600 @ 9.5 with 292 built by Korman many years ago

- was running pretty well on a weber 32/36 and Bavaria fuel pump when I swapped in a ignition 123 and then it ran better

- decided to take this extra time at home and dive into a DCOE conversion

- Sent italian dcoe to Carbs Only for rebuild and they came back looking great

- I got everything mounted up with halboyles linkage kit and a korman manifold - fired right up and I was super happy until...AFR gauge is at 10:1 on idle!


I then proceed to play with timing & idle mixture and finally started to give into changing idle jets and float level check etc - long story as short as possible - here is the current configuration:


- Still at 10 / 10.3 -1 AFR with the following

- Timing 12 degrees at 1000rpm

- Venturi 30

- idle jet F2 45 ( started with F9 then F8)

- F11 e tubes

- 200 airs

- 115 main

- plastic float set at 12/25 on the bench and measured fuel from from the etube with a zip tie at 23-24mm / 200 needle


The idle mixture screws seem to have little effect so I am missing something here - other than the black smoke and fouled plugs I can get it to run reasonably smoothly


Other clues that may help those more experience than I - there seems to be some minor fuel leakage around the small fuel filter inline with the banjo fittings after running awhile - I tightened them  bit and it got better but they are a bit wet / weepy. Also after the final try today I noticed once I shut the car off there was some vapor  coming out of the hole just above the forward venturi - it was cold? It also was present when I took the jet inspection cover off - this was only on the rear carb...


Thanks in advance for any thoughts about what I am missing / things to verify and check.


Stay safe all!


Edited by ibewebbman
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DCOE whut?  They were made from 38 through 55 mm.....


Which venturi?  depending on size, again, you can be anywhere between 28 and 50...


You'll need to get them to idle lower, from the sound of it.

And you don't mention balance- getting the airflow equal between all 4 barrels is critical.

The Synchrometer is the only thing I've found that really works well, so far.  


Then pull the progression plugs- you want to get all 2 or 3 holes covered or on the intake

side of the throttle plate.  THEN your idle mix screws will kick in, and off you go.


How are your mixtures at load?  It's kind of important to get the mains set,

then you can tune the transitions, then finally, the idle mixtures.





"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Thanks guys,


@Son of MartyI’ll check fuel pressure tomorrow, I did not think the Bavaria pump put out more than 3psi? But I should definitely check that.


@TobyBthese are 40’s with 30mm Venturi, I have them close to sync with the tool you showed. They are not perfect but pulling close to the “10” on the gauge for both carbs. I have been focused on getting the idle set because it’s so rich but it seems the mixture will lean out a little for a short time when I rev it up but goes rich again quickly. I have not road tested it yet it’s so damn rich I can’t imagine driving it yet. I educate myself on progression plugs but are you thinking the throttle plates are out of position?



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@StevenolaI’d like to do that but shouldn’t I be able to get it to at least 12:1 at idle in the garage before I subject others to this ? I mean it’s freakin disgusting. 

the mains should not even be in play at all at idle from what i understand...and my air is big for the main already so I’m kind of expecting once i get the idle circuit sorted I’ll be lean under load

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How many turns on the idle screws? Too many turns can activate the mains. Also tune in the idle circuit with your linkage disconnected from each carb. My air meter shows 6-7 during idle. 

1976 BMW 2002 Chamonix. My first love.

1972 BMW 2002tii Polaris. My new side piece.

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@Stevenc22I can actually close the mixture screw and there is no effect! I’ll try disconnecting the linkage. Once warm it will idle at 1000 rpm or so, it’s got to be pulling fuel off another circuit somehow.

how big are your chokes? Could mine just be too small? I have never seen below 10 on mine? I am assuming a larger choke would slow the air down to a lower number on the meter but increase volume, is that correct?

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Thanks for the numbers- they are good, as in,

those chokes in those carbs should give you decades of easy tuning.


They have almost no effect on the idle, however.

And your jetting seems like a reasonable starting place.




Go drive it.


10:1 is in the lawnmower category, I agree, but you're idling, not driving.


You're synched on the transition jets, so you should be close enough for running around.


On a 1600, your jetting is a decent start-


if you can find a long, straight hill, get it into 3rd at 2500 or so, and get the throttle full open.

Then watch the AFR- 12:1 is good.  You may find it heads lean as revs rise (the 200 air corrector) and it's

ok to about 13.5.  If it goes leaner, lift off, and see what it does.  If it's above 4k when it's leaner,

try a couple steps smaller air.  If it's leaner down lower, try the next larger main.  If you can go up by 2.5 steps on the main, do that.


Once it stays in the 12- 12.8 range from 2500 to above 5k, throttle open, THEN we'll fuss with your transition (idle) jets,

and THEN, I bet, you find that the carbs aren't closing all the way.


But you may need different transition jets, and that'll affect idle mix, too.  

Since your transition jets are what you're idling off of right now, anyways.


here's some 'light' reading, btw:




Go for a drive.  Don't wear a mask in the car...

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"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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5 hours ago, ibewebbman said:

@Stevenc22I can actually close the mixture screw and there is no effect! I’ll try disconnecting the linkage. Once warm it will idle at 1000 rpm or so, it’s got to be pulling fuel off another circuit somehow.

how big are your chokes? Could mine just be too small? I have never seen below 10 on mine? I am assuming a larger choke would slow the air down to a lower number on the meter but increase volume, is that correct?


Sounds to me like what Toby says, you're idling off your progressions. If your idle is 1000 or above you need to back out your idle speed screw to close your throttle plates more, until your mixture screws are making an impact on your AFR. 


He's also right in that getting your Mains/air correctors correct first is the most important, but sometimes hard to do if you're still struggling with just getting outa' the driveway.  I've been there. 


Lotta' what everyone has said applies, in my experience. And listen to Toby, he tends to be right and allot of his advice helped me to get to the level of 'novice' with DCOE's. 


2 years on and I'm still learning/tinkering. DEFINITELY no expert, but I've learned a few strategies that work for me.


 And yep, gunna need a bit more info to help out, mainly AFR's in different driving situations. Idle AFR & RPM, and Initial timing are key


I've got no experience on how it translates to a 1600, but I've got a 292 on a 2.0 with 9.5:1-ish compression, and I've got 21 degrees advance at 900 RPM with 36 degrees all-in, tapering up to 37 at a 6500RPM redline). I'm usually shooting for a 12.5-13 AFR at idle. 


Carb sync is very important, using the squirrel cage Toby referenced. 


And yah, you should be able to get it to idle at whatever AFR you want. 10, 11, 12.5, 13.5, 15.... Whatevs, all in the screws.


I'm gunna assume you've got an idle jet prescription that's in the ballpark? I've had no issues getting an idle jet that's too rich or too lean to idle wherever I want as long as the throttle position is correct, but you'll know if you have the wrong idle jet as soon as you try to drive around the block at light throttle. If it's in the 10's you're too rich, if it's above 13.5 you're edging on too lean. Note- your mixture screw setting will affect your progressions to a degree as well. There is no 'start-stop' to each element, they all effect each-other to a degree, which makes it all the more fun to figure out exactly what's needed for your motor. 


I've been trying to aim for a 13-13.5 under light acceleration/cruise, 13 at idle, 11.5-12.5 when punching it, 13-13.5 cruising, and 12.5 at WOT... But again, that's me. I'm very much subject to correction, (so if this thinking is off, those who know better, please correct because I'm here to learn too! :)


When I've lost track of where things are, or are starting from scratch because for some reason I pulled the carbs or adjusted things into oblivion...


1. I start by visually checking the position of the throttle plates on the progression holes. I always started with the first hole being appx 50% covered by the leading edge of the throttle plate. This can be done by shining a flashlight down the throat of the carb and looking at the little circles of light exposed.  If they're off carb-to-carb you need to adjust your balance. If you have different amounts of light on two different throats on the same carb... well... things are about to get allot more fun and sketchy. 


2. If the car had been running ok-ish with a known AFR, slowly and LIGHTLY thread-in the mixture screws until you just feel a little bit of resistance that lets you know they're bottomed out, and note the number of turns it takes you to do so for each cylinder. Depending on your carb bodies, and if you have long or short taper mixture screws, that number can vary. 


3. Figure out a feel-good median average of turns across the cylinders as a starting point. If they're all around 2.5, start there. If ya notice your front carb is 2.5 but your rear is more 3-ish... Your carbs might be slightly off from each-other in their MFG tolerances, unless they're sequential serial numbers.  


4. Re-set your mixture screws to your chosen starting-point, If you were 3 turns out but were idling at 10.5, maybe start at 2 turns. With the help of your right foot or a friend on your linkage to keep it running around 900rpm, do so and note where your AFR is at as ya do. If it's popping and spitting and lean, richen it up evenly across the board in 1/4 turn increments. If it's ungodly rich, turn it in a little bit. If it's idling super high, bring it down with your idle adjustment screw.


Don't go crazy here, just do so in even, 1/4 turns and adjust each barrel the same and keep note of where you are. You just want to get close because as you get some heat into the motor your idle speed will go up, and your mixtures will lean out. My car starts and lumps around at 11AFR at maybe 5-600rpm on cold start and barely runs on it's own without a foot on the throttle, and levels to 900rpm at 13ish when hot. 


At this point, ya just want it running well enough to get some heat in it.  If the car is drive-able I'll go for a spin around the block to help warm things up-but ya might not be there. just keep it steady till things do. 


5. Now that you're up to temp, I'll assume you're relying allot on your idle speed adjustment screw to stay running more than anything. back your speed adjustment out till the motor is just barely lugging, and then bring it back up till it smooths and see where you're at. If you're rich, and above 1000RPM, slowly drop your RPM little by little, let it settle, and give your mixture screws a slight tweak in even richer or leaner amounts edging closer to 12.5-13.5 goal.


If you're rich, you'll find that as you lean things out, the RPM's will rise. So say-

Your AFR is 10.5 but your idle is 1100rpm- drop it back to 900 mechanically, then turn your mixture screws IN till your AFR gets into the climbs back up into the 12's, but doing so means your idle SPEED will likely go up, and should. Do the same thing, mechanically drop your speed back down closing your throttle plates to your target idle, and adjust your AFR with your mixture screws until your throttle plates are as close to closed as you can while hitting your target RPM and AFR. 


If you here, your throttles are as close to close as possible, and you notice on your AFR meter that your AFR is jumping between say, a 12.5 and 15.5 in semi-even pulses, you likely have one carb out of sync, or richer than the other. (again, assuming your carbs are in-sync). You can confirm this thru the squirrel cage and see what carb is pulling more or less air (leaner will pull more to a point, richer less) and adjust.


If you notice that your AFR is a relatively steady number, with an intermitted high or low pulse, it's likely one cylinder that needs tweaking on it's mixture screw. Do the same thing and look for the higher reading cylinder-or- you can simply go cylinder-to-cylinder and turn your mixture screws a half-turn in or out, and see if you can get that high to become low, or the low to become high and you have your culprit. Just be sure to wait 5-10 seconds, or give the throttle a light blip between tweaks and let it settle down to get an accurate reading. once you're close, an 1/8th of a turn or less can be the difference between .2-.5 AFR I've noticed. 


AAAAND, that should do it. If you're settled into 900, with a good 12.5-13.5 AFR, and you slowly pull away and shit goes way lean, or way rich.... Then you need to adjust your idle jet to be where you want it while driving on the progressions. 


And again, as Toby says, and I'm recently VERY aware of.. If something else in the recipe is off, it'll tip your whole jetting prescription in the wrong direction and you gotta figure out the hows and whys. I'm currently figure out how to get my Pump-jet/Spill valve combo dialed in to allow for an ideal leaner progression circuit and not stumble when mashing the throttle, and make for a smooth transition into the Mains. 


Fun stuff these carbs! Not for those who don't like fiddling :) Hope the above made sense. Might re-read and adjust later, but I wanna make some pizza.

Edited by 2002Scoob
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Thanks Scoob for the Weber manual. Just add covers. A few things: sort out fuel pressure. You may need a regulator to get 2.5 to 3psi.. And the squirrel cage. I thought Italian Webers idle screws were 1.5 turns out to start.  I use 50F8 with your formula.  I can't remember float settings for brass vs plastic, but those are correct for one of them.

I get about 10 - 12 deg at idle with about 33 deg full advance . Scoob, You run 21 deg at 900rpm? Yikes.

Edited by Hans
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Weird, my post wiped out my text when I dropped it in... re-writing. 

1 hour ago, Hans said:

Scoob, You run 21 deg at 900rpm? Yikes.

So i miss-spoke, I'm currently at 16@700, and 22@1000, so somewhere in there?

IMG_8041 2.PNG

I've shifted the curve up and down and that's where it seems to idle best. I run high-test and is currently a little rich on the idle jet to compensate for a leaner accel. pump circuit. But it pulls hard, and doesn't over heat or ping. Drove thru to Stuttgart last weekend climbing grades at 140 kph and sustained stretches at 160kph with no issues. 


I'll likely look pull a bit of timing as I look to either up the pump jets, or step down the spill valve while leaning out the idle jets a little more. 


Today's fiddling was to put 8 degrees of vacuum advance back into the equation above 1500 rpm. Seems to liven up the light-throttle inputs around town, and overall make the engine's demeanour feel tighter and more composed. Gotta better study out what's affecting what and how, but could definitely notice a difference. 



Edited by 2002Scoob
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