Tuning 40 DCOE Weber 151 on an L24

[240260280z]

Well my experience with triple weber 40DCOE151's so far is that I can make them run rich or lean by playing the mixture screw against the throttle plate opening.

I read the blurb "DCOE INITIAL SETUP & TUNING" in the DCOE chapter in "WEBER CARBURETOR REPAIR AND TUNING BOOK" by Pat Braden. He suggests:

1. Once engine is warmed up and idling, set all mixture screws at same position out then synchro-balance airflow into all carbs (using throttle valve opening screws and air bypass screws). All 6 should flow the same.

2. Install air filters and connect tach

3. Reduce RPM to ~ 900 with the three throttle valve opening screws (for triples).

4. Turn out all mixture screws to find the point of fastest/smoothest idle. (If no peak RPM is found, repeat step 3 but use 850 or 800 rpm if possible).

5. Once the maximum RPM point is reached reduce RPM to ~ 900 (or whatever idle RPM you desire) with the three throttle valve opening screws.

I sort of did this when I first set up the carbs but I was pig rich. Now that I have read a few articles, I will try again... when I get the car back from the "panel tweaker".

I will also plot idle screw, throttle plate variations, A/F with 1100 rpm constant "for fun"... maybe a graph will talk to me. Especially if I also substitute throttle plate turns in with first pilot hole exposure.

Edited August 7, 2012 by Blue

[240260280z]

This excellent post notes the difference in DCOE mixture screws and provides good advice.

[Zedyone_kenobi]

Okay, I know this is a thread about the L24 that I started, but I thought it would be great to start with one weber setting for say a OEM L24, and trace the tuning required to get the same carbs to run well on a modified L28. Which is exactly what I am trying to do now.

My current settings are:

40 DCOE webers

30mm chokes

130 main jets

170 Air correctors

F11 Emulsion Tube

55F8

I forget the accelerator pump sizing, but for now lets not worry about that, as I think if you get the jetting right, the accelerator pumps will be fine over a very wide range.

My L28 specs are important to this thread as well. As you need to know what I am coming from and what I am going to:

2.8L N42 block

N42 head

.472 lift cam

10.5:1 compression

Ross forged pistons

OEM L28 crank

OEM L28 rods

6-2-1 ceramic coated header

Pertronix igniter and flamethrower coil

9 lb flywheel

Standard Cannon Intake manifold

When I first put the carbs on the new L28 from the L24, I did not so much as adjust anything. I wanted a straight out of the box run.

Well the L28 would not even start on the original L24 settings. I had to turn the mixture screws over 3.5 complete turns out from fully seated before I got it to idle. Obviously not the best setting. :stupid: My AFR at that point was about 13:1, and I could get it to 12.5 with some tweaking.

The engine idled fine and settled down to about 950 - 1000 rpm with ease. Driving at steady state speeds at low rpm in 4th gear you could feel the engine surge just a tad and the AFR's were about 15.9 - 16.5:1. This is way lean for steady state cruise. In 3rd gear the numbers would improve a tad to mid 15's, and in second it would be low 15's, high 14's. All of these values were at low rpm steady state cruise at around 2000 rpm. Giving it gas would bring the numbers back into the 12's but that is probably due to the accelerator pumps fuel input. When we got back to steady state cruise it would be VERY lean again.

So this afternoon, I started the rejetting process. I got the car started and back up to normal operating temp (which in my case is about 180 Deg). I was still at about 11.9:1 AFR at idle. If I tried to lean it up a bit the engine would slow down.

So the first thing I changed out was to change the 55F8 idle jet to a 55F9 idle jet. This basically means the fuel orifice was not changed, but the air orifice in the idle jet is smaller. Upon start up, the AFR was immediately 10.9:1. This makes perfect sense. Keep in mind the air corrector acts like a vacuum bleed. Going smaller means the vacuum signal will have more of an effect in pulling fuel up the idle jet. I was able to turn the mixture screws in to get the AFR back to 12.5:1. When I shut the car off, I counted the turns I was out from full lean. Keep in mind with the 55F8's I was 3.5+ turns out. Now with the 55F9's I am 1.75 turns out from full lean.

Next report will be to see how much effect the idle jets has on my lean steady state low rpm cruise.

When tuning webers, you must have the engine up to temp, and then only change ONE Thing at a time, and record the results. Write them down in a book or in a web forum (or in my case, both! )

I will go for a drive tomorrow, and let you guys know what the results are

Edited August 11, 2012 by Zedyone_kenobi

[240260280z]

Idle Mixture Screw Pitch on 40DCOE2 is 0.8mm

I'll measure it on the 40DCOE151's when I get close to them(100km away presently)

The other important part to measure is the taper of the screw. I'll measure that optically.

Edited August 14, 2012 by Blue

[Zedyone_kenobi]

I have been meaning to drive around and finally did right before a rain today. I was on vacation and the car had to sit waiting for tuning.

But the change from the 55F8 to the 55F9 produced a very rich idle and brought the low rpm mixture down from from a lean 15.9 to 16.4 to around mid 15's. I went driving around and noticed aside from having to change my mixture screws up at idle, not much else changed. So I went and installed the 50F9's I had. this was a smaller fuel orifice along with the same size air orifice. Well my 4th gear cruise at 55ish miles per hour is a golden standard I am using. Leaving everything else the same...

4th gear at 55 mph; which equates to 2500 rpm with very little pedal input.

55F8 AFR 16.5 - 16.9:1

50F9 AFR 15.5 - 15.9:1

55F9 AFR 14.8 - 15.1:1

The dramatic change in AFR tells me that my main circuit may not be coming in strong yet, as my idle jet seems to be still in the drivers seat.

The AFR at steady state cruise around 14.8 - 15.1:1 seem okay to me and mean I am around stoichiometric if not a tad on the lean side, but for steady state cruising, at little to no load, this could be a good economy for long trips.

Now lets us talk about what happens when I give her the beans! Well the AFR plummets. How low. I have seen as low as 10.7:1. This is obviously not what I am looking for, as that is far too rich, and the blurble out of the exhaust when shifting from 1st to 2nd, seems I have some extra fuel in there not burning off.

I have thought for a while that 130 mains were a bit too rich. I would like to see my AFR drop to 12:5 in WOT conditions, not in the 11's or 10's. I will keep going down jet sizes until I get the mixture where I would like it under WOT.

This is the only way to tune Webers as far as I can tell. You have to invest in jets and then set up some simple and controlled tests. Write down what you find.

I am happy with where I am right now on on the idle jet. I would like it be a bit richer though, as it is close to 100 deg F right now and when it gets cold, things will get a bit leaner. I like where I am with the 55F9 idle jet. However, I want to go richer. So would the right thing to do be to order the 60F9 jet, or the 55F13? I honestly do not know. You see the the 60 has a larger fuel orifice, while the F13 has a smaller air bleed orifice. Both will get more fuel into the system, but achieve it differently. Not sure which would be the right one. The easy answer is to buy both. But these get expensive, and I have not found a supplier of F13 idle jets.

So I used my favorite carb supplier (http://www.carburetion.com) to get some 120 DCOE mains and a 60F9 idle jet.

I am narrowing down on this tune. I will be darn close with this last order. The principle tuning method of 'suck it and see' is very very true, but I cannot even fathom how hard and how LONG this would take without a wideband O2 sensor.

Edited August 17, 2012 by Zedyone_kenobi

[d240zx2]

The seat of your pants would be worn slap-dad out!

[LeonV]

FWIW, Keith Franck's Hypojets have 4 selectable air-bleeds. This makes it into an adjustable idle jet, 4-in-1.

[Zedyone_kenobi]

OOOO, I am a Keith Frank believer...Are you using those Leon? If so, how are they?

Just checked them out..To purchase the hypojets and the hybojet insert it will run you 150 dollars plus shipping. The 28 Hypojet insert is about the same as the 60F9 idle jet I have ordered according to Mr. Franck. I am not sure if the 'improved tunability' of the hypojets would make me want to dump 150 bucks into this setup though. granted I have spent much more than that already , but I have options. If I buy the wrong idle jets, I am out 42 bucks. But if I buy the wrong hypojet inserts, I am out 150 bucks. Well, really just 90 bucks, as I can reuse the hypojet housings. I am tempted to try, as I am a carburetor fanatic just to see the difference.

Tell you what I am willing to do. I will get the 60F9 idle jet in, and I will see if it is as perfect as I think it is going to be. I will then order the KF setup and give it a try. I would be fascinated to know the difference.

Here is KF weber store. I see the Emulsion Tube 02 is for 30 mm choked carbs. I wonder how it compares, but at 40 bucks a copy, that is EXPENSIVE

http://www.webstore.com/82598,owner_id,other_items

Edited August 18, 2012 by Zedyone_kenobi

[LeonV]

OOOO, I am a Keith Frank believer...Are you using those Leon? If so, how are they?

Just checked them out..To purchase the hypojets and the hybojet insert it will run you 150 dollars plus shipping. The 28 Hypojet insert is about the same as the 60F9 idle jet I have ordered according to Mr. Franck. I am not sure if the 'improved tunability' of the hypojets would make me want to dump 150 bucks into this setup though. granted I have spent much more than that already , but I have options. If I buy the wrong idle jets, I am out 42 bucks. But if I buy the wrong hypojet inserts, I am out 150 bucks. Well, really just 90 bucks, as I can reuse the hypojet housings. I am tempted to try, as I am a carburetor fanatic just to see the difference.

Tell you what I am willing to do. I will get the 60F9 idle jet in, and I will see if it is as perfect as I think it is going to be. I will then order the KF setup and give it a try. I would be fascinated to know the difference.

Here is KF weber store. I see the Emulsion Tube 02 is for 30 mm choked carbs. I wonder how it compares, but at 40 bucks a copy, that is EXPENSIVE

http://www.webstore.com/82598,owner_id,other_items

I'll get back to you after I go to Keith's place to play with his new stuff, luckily he's local so we can try a few things without having to ship anything. I've been really busy with my EDIS install lately, but I plan to go to his place in the next few weeks hopefully.

[Zedyone_kenobi]

The weather is finally clearing up and I had time to get the old gal back on the road. I did install the 60F9 jets AND the 120 mains.

Low and behold the results were fabulous. Well they were good. My AFR at part throttle cruising in all gears is about 13.5:1, and when I give her the beans, there is no flat spot to be felt. EVER. Going WOT and up in the RPM I am getting about 12.5:1 and it gets more and more rich as RPM climbs, but slowly. I know the 30 mm chokes are going to make her run rich on the top end.

So for now I am going to order some 115 mains as spares, but drive it exactly like it is. When it gets colder in the fall and winter I should be just fine.

Right now this is the setup:

120 Main Jets

170 Air Corrector

F11 Emulsion Tube

60F9 Idle jets

Car feels like it found 30 more HP in the mid RPM band now that it is not so rich, and the pull up above 3000 feels just spectacular. Still I cannot really wring it out because the engine is so new, so I intend to put a hundred miles on her tomorrow weather permitting.

Edited August 24, 2012 by Zedyone_kenobi

[240260280z]

Excellent exploratory work! I am happy for you.

I get to see my 71 resto tomorrow and play.

Philip

[cygnusx1]

Going WOT and up in the RPM I am getting about 12.5:1 and it gets more and more rich as RPM climbs, but slowly. I know the 30 mm chokes are going to make her run rich on the top end.

So for now I am going to order some 115 mains as spares, but drive it exactly like it is. When it gets colder in the fall and winter I should be just fine.

Right now this is the setup:

120 Main Jets

170 Air Corrector

F11 Emulsion Tube

60F9 Idle jets

Car feels like it found 30 more HP in the mid RPM band now that it is not so rich, and the pull up above 3000 feels just spectacular. Still I cannot really wring it out because the engine is so new, so I intend to put a hundred miles on her tomorrow weather permitting.

You may want to try out a 190 air corrector to see if it counteracts the richening trend at the higher revs.