engine and drivetrain Weber DCOE 45 Tuning on a lightly Modified Engine

[CaptJ]

Hello all!  About a two years ago I acquired a 1973 BMW 2002 that had been modded but then left to sit for a few years. As remembered by the previous owner, the specific engine mods were:

 

E12 head with:

  All casting removed from rockers

  Rocker lockers

  292 regrind

  Valve seats recut and hand lapped

  Port and polish

Engine:

  9.5:1 pistons

  Windage tray

  FLame Thrower coil

  Pertronix electronic ignition

Intake:

  Lynx manifold with (1) Weber 45 DCOE with a 36 choke

Exhaust:

  shorty headers

  straight pipe with resonator

  magnaflow/SuperTrapp interchangeable exhaust

 

The car does run and drive decently enough but is running rich at idle and possibly elsewhere in the rev range. Also if the throttle is opened suddenly it seems to momentarily stumble or hesitate, making me think the injector pump isn't working or the wrong size. Once I get that pulled out, I'll know the size. It may also be happening during the transition from the idle to main circuit as, if I recall correctly, higher rpm rapid adjustments do not show this issue. 

 

Doing some research I have replaced the 55F6 idle jet and 155 air corrector with 50F8 and 195 respectively as prescribed by this site: http://cnx.org/contents/oKbaEWQ3@2/Selection-and-Tuning-of-Weber-. I also have a 200 air corrector jet.

 

So currently it has:

Idle: 50F8

Air corrector: 195

Emulsion Tube: F16

Main: 145

Starter Jets: 150

Accelerator Pump inlet valve: 40

Accelerator Pump Jets: 45

 

Pump Bleed Backs (not sure what these are?): TBA

Pump Rods: TBA

 

 

 

 

 

I have found that adjusting the idle mixture screws makes no noticeable difference to the idle. If the car is left to idle for a few minutes it will start to stumble and eventually quit. This can be postponed by engine reving to, I am guessing, clean the spark plugs (haven't had a chance to pull one yet). It also the exhaust smells very rich. I am guessing I need to further decrease the size of my idle jet to correct this?

 

As for the main jet settings. Does anything stand out as being incorrect?

 

I have the timing set at about stock but I don't believe the distributor is a Tii which from research means I don't have as good of an advance curve as I'd like?  Any advice there?

 

I have currently pulled the carb off the car to give it a quick cleaning, look over, and basic rebuild. I plan on buying one of those choke delete kits and removing that assembly from the equation since I have yet to ever need it or find it helpful. Any advice for or against that would be welcome.

 

I am also planning on looking into an air/fuel mixture setup to assist in tuning and any recommendations or tips would be helpful.

 

Thanks in advance for the help!

 

Corey

Edited February 5, 2016 by Corey W.

[Simeon]

Have you checked the float level and whether it is leaking / sinking?

[Oldtimerfahrer]

Float level is a good bet, the compression and choke size on the venturis will also be determining factors on its behaviour. Low compression (9.5) and large chokes will make it run rich. Another factor are the number of bypass's the carb has. I had 45dcoe9 from an Alfa, switched to the 45dcoe152g with the right number of bypasses and 10.5 compression making a world of difference with the same jetting and emulsion tubes.

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[Hans]

Check timing for a start. I'm using about 33deg max advance, which come in around 3000rpm. Search on timing to find previous threads on this. Made a big difference to my engine. I'm not familiar with one DCOE settings, but generally if mixture screws have no effect, jets may be wrong size.

[MildSeven]

Hi,

 

I can't say that I know anything about tuning carbs but I disassembled the DCOE 45s on my 2002 and reassembled them not long ago (car isn't re-assembled yet).

 

Here's what the previous owner provided me in terms of engine "specs," mostly unverified...for now.

- 1969 M10 block w/ 121ti head

- Crane ignition

- Ireland 292 cam

- 10:1 compression ratio

- E12 valves w/ 1800TiSA valve springs

- mechanical timing chain tensioner

 

Here's how the carburetors are setup: one of the throttle plates has notches on it, but I can't remember which:

 

EDIT: ok pasting a table didn't work well so I attached an image.

 

Edited February 4, 2016 by MildSeven

[Oldtimerfahrer]

The first point is really to examine whether the carbs have right number of bypass holes, it's a pre-requisite for a proper setup. If you don't, you will be continuously trying to compensate various issues you encounter with different combinations of jets, emulsion tubes, etc. Getting this right will save aggravation. There is plenty of literature on it, the carbs can be modified by an expert.

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[TobyB]

 

 

I have found that adjusting the idle mixture screws makes no noticeable difference to the idle.

which means that the carb is running on the 'transition' (idle) jets, not the idle passageways.  Rendering the idle passageway drillings moot.

 

Getting the throttle plates to close farther will get it back onto the idle.

 

Float level and fuel pressure, for starters.  Jetting's great, but it's fine- tuning compared to those 2 variables.

 

t

[CaptJ]

I will definitely check the float level.

 

This right number of bypasses is new to me.  The top cover is pictured and is stamped "45DCOE 152" and below that "107 02". I have no letter after 152 like some of you seem to have.

 

The throttle plates seem unmodified. I seem to have three progression holes (these the bypass holes by chance?). I remember while on the car, the edge of the throttle plates could be seen covering the first progression hole so I don't think the progressive circuit being used at idle should be a problem but I could be wrong.

 

The car has an electric fuel pump with one of those adjustable dials ahead of it in the fuel line and I have it set to 2.5 psi, if it is accurate. I did find it to be too high at one point (oops) and turned it down but didn't seem to notice much if any difference.

 

Starter Jets are 150

 

Accelerator Pump inlet valve is 40

 

Accelerator Pump Jets are 45

 

 

For cleaning, how far should I disassemble it? There is an ultrasonic cleaning tank at my work. Are those at all useful for such things?

 

I've attached some pictures so I don't have to write thousands of words.

 

Thank you all for your suggestions! 

[TobyB]

Your last picture shows the progression drillings- the cap you have removed on the main body,

right next to the two flathead screws, is the progression cover.

From memory, that looks about right, as to hole size and position.

 

You should see the edge of the throttle plate through the first (most closed) hole when

the idle air, fuel, timing and celestial alignment is about right.

 

Also, that plastic cap outboard of the progression cap may be the idle air bypass screw- it's

a really easy way to get the throttle plate where it needs to be.  Add idle air, idle fuel, and 

work on getting the plate almost completely closed.

 

hth,

t

Edited February 5, 2016 by TobyB

[Simeon]

I have a general but related question regarding the use of a 45 choke carb on a crossover lynx manifold. The standard prescription seems to be to use a 45 but since each choke is only feeding a single cylinder PLUS you have the added plenum volume of the runner and port that aren't drawing on the carb, wouldn't it make more sense to use a 40 to try and keep the charge velocity higher?

It's obviously doing something right since everyone who has this setup (eventually) raves about it, especially on stock motors. It just seems a little bit counterintuitive to me.

[jmr_1602]

I have a general but related question regarding the use of a 45 choke carb on a crossover lynx manifold. The standard prescription seems to be to use a 45 but since each choke is only feeding a single cylinder PLUS you have the added plenum volume of the runner and port that aren't drawing on the carb, wouldn't it make more sense to use a 40 to try and keep the charge velocity higher?

It's obviously doing something right since everyone who has this setup (eventually) raves about it, especially on stock motors. It just seems a little bit counterintuitive to me.

I don't know the specifics, but I believe it is because of the range of venturis that can be used with a 45 (28-40mm) compared to the 40s (24-36mm.)

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[TobyB]

 

 

but since each choke is only feeding a single cylinder 

Each choke is feeding 2, on a Lynx, but they're alternating.  1&4 use one choke, 2&3 use the other.

 

But that shouldn't matter.  One half of a 40 on a well- designed common plenum (IN THEORY) SHOULD be able to run a 2002

engine just as well as 2 x40's on individual runners.  In fact, it would (again, theoretically) meter fuel better, as the intake

flow would be almost constant, as opposed to the pulsation that IR's give.  

Minus, of course, the losses in the manifold from de- atomization.

 

I think this is a case of "both work, and common wisdom is..." more than anything else.  The DCOE is so tunable and adaptable

that either would work just fine, with only a bit of finessing.

 

fwiw,

 

t

[Lee]

Considering the low build of your engine (9.5 pistons and "reground 292), I suggest you reduce your choke size to 32 for best drivability.

[CaptJ]

TobyB,

 

It seems that it was indeed then in the ballpark when on the car so that is reasonably comforting. Now to hope I didn't let all the black magic out when I opened it up...

 

 

Lee, thank you for your suggestion.

 

I don't have a huge understanding on all the aspects of engine and engine tuning just yet.  My basic understanding on choke size is that smaller is for better low end torque and higher is for better high end breathing. With that said, you seem to be suggesting that the combination of 9.5 pistons and a 292 regrind will not breath well enough at the top end to desire to rev past 5500 6000 rpm (based on charts of displacement vs choke size like the one in the link in my first post.)?

[Lee]

If you run four 32mm chokes, you already will have twice the breathing area as the original 32/32 carb.

292 is very mild while 9.5 was stock Euro pistons. Basically, that engine is lightly hotter than stock. But not built enough to require 36 chokes. By using 32mm chokes you will gain a lot of drivability. How often do you get to rev beyond 5k RPM? ;-)

 

If you feel you're missing from 1k rpm to 2,5k rpm, then increase idle jets by 5 points. A richer bottom gives you nice torque.