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HELP please! DCOE’s stumbling (leaning out) at ‘tip in’...


sam1904

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1 hour ago, Stevenc22 said:

 

Actually it is relevant. You want to hit the right AFR with the right number of turns as that then sets the rest of low speed driving. If you have too small an idle jet but set the right idle AFR with 4 turns, when you start driving you are going to go lean because you are already using some of the progression holes just to make IDLE and there wont be enough progression holes left to cover the rest of the low speed driving.

Now I want to go look at my carbs... I have a very light throttle lean spot. Might just be the jet. size *thinks*

-Nathan
'76 2002 in Malaga (110k Original, 2nd Owner, sat for 20 years and now a toy)
'86 Chevy K20 (6.2 Turbo Diesel build) & '46 Chevy 2 Ton Dump Truck
'74 Suzuki TS185, '68 BSA A65 Lightning (garage find), '74 BMW R90S US Spec #2

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Unless you are lucky enough to have a set of 15/16 carbs your transition holes are not going to be right, and the 151 type could be closer to ideal  than they are, leaving your transition less than optimal and using a richer idle jet is your only answer other than redrilling the carb.

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If everybody in the room is thinking the same thing, then someone is not thinking.

 

George S Patton 

Planning the Normandy Break out 1944

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1 hour ago, Stevenc22 said:

 

Actually it is relevant. You want to hit the right AFR with the right number of turns as that then sets the rest of low speed driving. If you have too small an idle jet but set the right idle AFR with 4 turns, when you start driving you are going to go lean because you are already using some of the progression holes just to make IDLE and there wont be enough progression holes left to cover the rest of the low speed driving.

I think this is a somewhat bizarre theory. Ideally, the throttle plates should be directly beneath the first transition hole at idle. It is primarily having the throttle plates past the first transition hole at idle that causes the lean hole just off of idle.  If you have that situation, fattening up the idle mixture may allow you to adjust idle speed to position the plates under (not past) the first hole. In any event, this is just an experiment, right? The longer term solution is likely to be installing a fatter idle jet.

Chris B.

'73 ex-Malaga

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2 hours ago, Chris_B said:

I think this is a somewhat bizarre theory. Ideally, the throttle plates should be directly beneath the first transition hole at idle. It is primarily having the throttle plates past the first transition hole at idle that causes the lean hole just off of idle.  If you have that situation, fattening up the idle mixture may allow you to adjust idle speed to position the plates under (not past) the first hole. In any event, this is just an experiment, right? The longer term solution is likely to be installing a fatter idle jet.

If you could just tune the AFR using the turns, there would be no need for different size jets. I explained it badly. If you have too lean a jet, yes you could correctly tune the idle AFR but you will run out of fuel as the RPMs increase since a jet has a finite amount of fuel it can deliver.

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1976 BMW 2002 Chamonix. My first love.

1972 BMW 2002tii Polaris. My new side piece.

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The idle circuit has very little effect at tip-in. What we are discussing here is some minor tweaking just to see if it affects the dreaded lean hole. He said he is running his idle at 13:1. That is pretty lean. With a little tweaking, he can likely get it up (by up, I mean fatter) to 12:1. That will affect not just the idle mixture, but will fatten the mixture farther up in the transition. That may well be enough to noticeably fatten up (to correct) the lean hole. This is not a fix, but may indicate whether a bigger idle jet is in order, instead of mucking around with E-tubes. If you are correct, then the idle circuit may indeed "run out of fuel" at higher RPMs, but these lean holes typically occur just off of idle, between 1500-2500 RPMs.

Chris B.

'73 ex-Malaga

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Hi all, great info here. I’m running dcoe40-151s, 292cam, headers, 123 distributor on the ‘ti’ curve, red Bosch coil.  I have an AFR gauge. I have the synchrometer. Fuel pressure is 2psi. I’ve found that my smoothest running jet combo was 55f11- 130/f9/195 -50p50b. Great drivability, but it’s rich (and stinky).  A trusted local 02 guy says not to worry about what the afr says, that how it runs is the thing.  He showed me on his famous 2002 race car, that 11-13 is ‘normal’. I can rationalize rich idle by the cam and I’ve tried 50f8 and 50f11 idles, which were worse running. I have read the Weber book. And the Des Hammill book. And the older Weber book. And the Haynes book (Humble brag). Here’s the question.  Why not closer to 14.7? The leaner jets don’t get anywhere close before the engine stumbles.  Even through the rev range it’s rich. My concern is that I’m leaving air (power) on the table, and maybe washing the cylinder walls.  11-13 to 1 on the gauge is something I can ignore (maybe) but the DGAS38 I ran before the upgrades lived happily between 13-15 all day. I see sensible folks on here saying the same thing that 11-13 is fine, normal, right.  I’m not disagreeing, I just want to know why side-drafts ‘should run richer’?

91367E44-774F-4EFD-86BF-1F6CAE15A64B.jpeg

Edited by HeyBikey
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So, you are kind of hijacking the thread here. The short answer is that carbs ere not precision instruments to the extent that FI systems are, which can meter fuel flow far more precisely using adjustment mechanisms like 02 and other sensors and computer control. 
 

You might be able to tune your carb(s) to flow at 14.7 AFR at some specific RPM, temp and load condition, but it would be almost impossible to achieve that under different conditions. Tuning your carbs for optimal fuel mileage will undoubtedly cause you to sacrifice something else, like optimal power. 

Chris B.

'73 ex-Malaga

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2 hours ago, HeyBikey said:

I just want to know why side-drafts ‘should run richer’?

 

I believe it's down to the size and the even-ness of the aerosolised droplets created by DCOE's.

 

Since they primarily rely on vacuum to pull fuel from a larger orifice, The carbs don't do a great job of creating a fine and even enough 'mist' to burn at a high enough efficiency to run well at higher AFR's, so it takes more fuel to create power, and what's lost to efficiency leaves your tail pipe as unburned fuel.

 

That's part of why EFI is so much more efficient, you're delivering fuel at a higher pressure, with smaller opening, creating a finer, more evenly distributed mist and a more efficient burn. 

 

Just think of one of those adjustable spray bottles filled with alcohol... if you tighten the nozzle down tight so it makes a fine mist and spray it above a lighter, most of that fuel is going to burn right as it leaves the nozzle and won't even reach the ground.

 

Loosen up that nozzle so it makes a heavier mist or even a stream (basically a DCOE), the same amount of fuel is being released, but less is able to burn within a given time due to less surface area, and you end up with a burning puddle on your floor.

 

Or as emissions 'richness' out your exhaust pipe.

 

The finer, higher pressure mist is just more efficient. 

 

Not an expert. Fire is fun. Don't light your house on fire trying this. 

-J

Edited by 2002Scoob
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Other factors affecting this situation is that a side draft Weber is a universal carburetor and it doesn’t have a “progression” circuit. 
 

I remember dyno tuning a bone stock Honda CBR600 (carbureted) to get a baseline for a before and after on a full exhaust system and jetting install. It was truly amazing how tight the AFRs stayed throughout all conditions, I can’t imagine how many hours Honda must have been spent making it That good. 

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12 hours ago, 2002Scoob said:

since they primarily rely on vacuum to pull fuel from a larger orifice, The carbs don't do a great job of creating a fine and even enough 'mist' to burn at a high enough efficiency to run well at higher AFR's, so it takes more fuel to create power,

Scoob, this entirely ignores the emulsion tube function, which entrains atmospheric pressure bubbles into the fuel flow and when this emulsion is drawn into the low pressure venturi the air bubbles being at higher pressure explode turning the entrained fuel into a very fine mist (although not as fine as high pressure injuction). In the main circuit this is limited at low throttle opening but in that case the fuel hits the partially close throttle plate and is evaporated or fine droplets are pulled over the edge breaking it up.

 

5 hours ago, Lorin said:

Other factors affecting this situation is that a side draft Weber is a universal carburetor and it doesn’t have a “progression” circuit. 

The dcoe 15/16 subtype are set up for the m10 engine and work well, the Spanish carbs are type 151 or 152 and yes these are universal carbs.      

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If everybody in the room is thinking the same thing, then someone is not thinking.

 

George S Patton 

Planning the Normandy Break out 1944

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5 hours ago, Lorin said:

Other factors affecting this situation is that a side draft Weber is a universal carburetor and it doesn’t have a “progression” circuit.

It is a bit misleading to say that the Weber DCOE doesn't have a "progression" circuit. The so-called "idle" circuit includes a transition function that provides provides fuel well into the mid-RPM ranges until the main jet circuit "tips in"- with overlap between idle and main circuits.

Chris B.

'73 ex-Malaga

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1 hour ago, Son of Marty said:
14 hours ago, 2002Scoob said:

 

Scoob, this entirely ignores the emulsion tube function, which entrains atmospheric pressure bubbles into the fuel flow and when this emulsion is drawn into the low pressure venturi the air bubbles being at higher pressure explode turning the entrained fuel into a very fine mist (although not as fine as high pressure injuction).

True yes. Perhaps I was over-generalizing :) and might very well be wrong, but that was always my assumption. 

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Thank you for all the comments which I need to read and digest. Between waiting on some different tubes to arrive and then spending three days searching for gas so I could drive the damn thing it's been a while since I gave an update so while I digest the content here is a quick update on the last few days.

 

I changed to F16 tubes as suggested by a couple and worked on fuel level. I am still struggling (or over thinking it) as measured with the tip of my calipers I am now sitting at around 28mm fuel level but this required going to 10mm (not 12mm) float clearance at center. To get in the 27 range will move me to 8-9mm which I will try later.

 

With the new F16 tubes, jetting as shown at start of thread and idle screws out 2 turns my A/F at idle is close to / slightly above 11:1. I still have the stumble but it is slightly better and can now be (mostly) driven around. I also have Chris Joyner here in town and as I have a couple of other things I have worked on I would like him to cast an experienced eye over I will see what his highly calibrated ear makes of this next Thursday!

 

Will review the thread above and get to 27mm and update further.

 

Thanks

Sam

2022 Defender 110 30th Edition

1976 BMW 2002 - Evolving...

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