Tuning 40 DCOE Weber 151 on an L24

[240260280z]

Stephen, I think you are going in the right direction. New chokes will change things in the upper end for sure... not sure about idle as the air flow is too low for the choke to affect anything...probably during progression that you will see a change. I read a huge post on HybridZ on DCOE configurations but most of it is useless as there is very little useful reference data coupled with a possible "hillbilly" element.I figured the "Use a configuration that someone else took a lot of time and energy and money to achieve" would be the way to go but when the fuel level or initial timing or idle throttle plate opening, or # of turns out on the enrichment screw for someone's "perfect setup for XXX engine" is not disclosed,who knows if they set it right or their set-up is fixing a problem they caused. For me, the only way forward is to do it scientifically and try to control/understand the variables.Here is a start for the idle circuit and the inter-relationships of the parts.Here is a crude drawing I made to represent the variables:

I did notice that I could play the Idle Enrichment against the Throttle Valve stop screw to get a variety of A/F ratios so here is what I think is going on along with unknowns:Idle Circuit Interworking Analysis "Talk-Through":

1. Fuel level 25mm below shoulder of main jet.

2. Timing >=15 degrees at 900 RPM

3. Throttle Valve stop screw 1/4 turn in from the plate resting in the throat (this is to prevent wear on the plate and throat and only allows a wee bit of air into the motor). Assume all linkage and throttle plates are set correctly and all 6 work the same. In theory this should be completely closed so that all air is from the idle needle port however it can not be completely stopped due to throttle-plate-to-throat wear.

4. Idle enrichment screw. This allows air and fuel mix into the motor. High manifold vacuum "sucks" both air and fuel out of the small hole below the screw. The amount drawn is dependent on manifold vacuum and the number of turns on screw and the taper of the screw's needle. (pitch of the screw thread is the same for most if not all DCOE carbs).

Gotta go to lunch. More later.

Edited July 29, 2014 by Blue

[cygnusx1]

Zed, in short, you discovered, like I did, that the idle jet size IS your progression mixture, not really your idle mixture. Those Italians, I'll tell you!#! :stupid: 32mm chokes will have nil effect on your idle, and nil+1 effect on progression. I like the 55F9 that I just put in. I think that's a good idle jet for the 151's on my L28.

Expect your mains to go up to 135 or so, and maybe up the air corrector to match. Got a micro drill set and a soldering torch yet? I make my own jets.

Oh BTW, my Webers talk to me too. You are not alone.

Edited September 12, 2012 by cygnusx1

[Zedyone_kenobi]

I fully expect my mains and airs to go up with the 32's. I think I will end up with the 55's.

I am glad to hear the 32mm will have zero effect on my idle and progression. As they are very nice right now!

I do not have a micro drill, but I do have a nice soldering torch... man you are an enabler!

[240260280z]

4 (continued). The progression holes upstream from the idle enrichment needle valve also allow air to mix with the enriched fuel from the idle jet.

5. Depending on the location of the top of the throttle plate relative to the first progression hole:

i. If the plate is far from the first progression hole and inboard towards the block,the progression holes will only add air to the idle mix (assumption)

ii. If the plate is very near the first progression hole, the first progression hole may draw fuel and air from the idle jet due to the venturi effect at the throttle plate. This may decrease the flow out of the idle port or it may have minimal effect on the idle port's flow (unknown). Of course there will be more air past the throttle plate that dilutes the fuel mix.

iii. If the plate is under the first progression hole, the first progression hole may draw additional fuel and air from the idle jet due to the venturi effect at the throttle plate and from more exposure to manifold vacuum.(assumption) This may decrease the flow out of the idle port or it may have minimal effect on the idle port's flow (unknown). Of course there will be more air past the throttle plate that dilutes the fuel mix.

iv. If the plate is past the first progression hole, the first progression hole may draw fuel and air from the idle jet due to the venturi effect at the throttle plate and from more exposure to manifold vacuum.(assumption) This should decrease the flow out of the idle port however the combined flow out of the idle port and the well exposed first progression hole may flow more fuel or less fuel. (unknown) Of course there will be more air past the throttle plate that dilutes the fuel mix.

From the above, as the the throttle valve is opened more and the idle enrichment screw is correspondingly closed, the idle mix will become weaker AND the idle enrichment screw somewhat balances how much fuel is pulled from the idle port vs the progression holes (i.e. taken to the extreme, a huge 2mm idle jet fuel hole will cause fuel to pour into the progression holes but the idle enrichment screw can completely block any fuel from reaching the idle port....thus the balance effect)

It is clear that as the throttle valve is opened, the idle port's flow will decrease thus the idle jet will certainly affect the progression enrichment thus there is an important transitional relationship between idle and progression that must be addressed by idle jet selection.

As the throttle valve opens further, the main circuit will come in. From experiments, the fuel level in the bowl will affect when the main circuit comes in. When the fuel level is high, the main will come in earlier and reduce the amount of time during progression.

My intuitive recommendation for tuning the idle and progression is:

1. Keep the throttle plate as closed as much as possible.

2. Assuming timing, linkage, fuel level, mains, air flow, valves, etc are all correct, turn out the idle enrichment screws until smooth 900 rpm idle is reached. Adjust throttle plates minimally to achieve 900 rpm.

3. Measure idle A/F

4. Drive and measure A/F at low rpms and plot the transition to mains.

5. If too rich or too lean at idle and progression the switch idle jets and repeat.

6. If rich at idle and lean at progression then open throttle plate more and turn idle enrichment screw in to have more fuel pulled from the progression holes. (The acceleration pump bleed back and related piston/throw can also be given attention)

7. If lean at idle and rich at progression then close throttle plate more and turn idle enrichment screw out to have more fuel pulled from the idle hole. (The acceleration pump bleed back and related piston/throw can also be given attention)

Edited September 12, 2012 by Blue

[240260280z]

Correction:

In theory this should be completely closed so that all air is from the idle needle port however it can not be completely stopped due to wear AND if the choke circuit upstream is to be functional! Wrong as the choke is down stream

[240260280z]

Something interesting from this site: http://www.webercarburatori.com/?p=handbook&s=2

The main features of accelerating pump operation are the amount of fuel injected at each stroke and the promptness and duration of each injection. When tuning up for proper adjustment settings, the pump jet and drain diameters are determined by trying to minimise, as far as practicable, the amount of fuel injected. Often, also the direction of the fuel spray proves to be a significant factor.

Generally, when engine operates at high rpm rates the pump jet (diameter between 0.35 and 1 mm) is subject to a vacuum sufficient to produce an uninterrupted flow of fuel, that is, it performs as a high speed jet and its role falls under the adjustment setting data.

I had brought this up to Keith Frank on his forum when I saw it in data and another forum member did a few additional runs to verify what I saw (a "leaky accelerator circuit"). I even suggested turning the jet nozzle around. Now it is documented in a Weber manual so I am happy:)

My solution for a Weber mod was to put small springs rather than weights over the ball valves as vibration and mass cause the balls and weights to jiggle and bounce and not seal effectively. A spring would not do this and keep the ball down until fluid pressure was applied by the plunger.

Eureka! A video to prove it! Not a DCOE but the same principal:

Look at the leaky accel jet spraying at the Philips screw head. I saw this on a graph nearly a year ago... now I see the theoretical! That's a Higgs Boson to me.

Edited September 12, 2012 by Blue

[LeonV]

That's a great video!

[Zedyone_kenobi]

That is a holley carb if I am not mistaken.

[Lazeum]

I read your posts and I don't understand why you rely on AFR at idle. I always setup idle at best lean condition then play with idle screw to set the idle speed correctly.

I also was having issue with unproper idle jets but change in size fixed everything on my case (I'm having DCOE 18, maybe that's why)

AFR at 2500rpm cruising has always been around 14:1 / 14.5:1 this way.

I'm measuring AFR with a clamp at exhaust tip so it makes even less sense to bother but would it be crazy to believe you might have reversal flow even close to the engine because of engine low operation? That could lead to false reading.

So to come back to idle adjustment, idle is always ok once set - whatever the idle mixture screw settings are - but good progression needs to have proper jets. It appears progression is almost always perfect when I'm near 1.5 turn from fully closed with the proper idle jets (50f9 with previous skock N42 head on L28 & now 45f9 on modified P79 head). So in my particular case, I'm just experiencing what the weber books are stating.

I also check plugs after some gentle sessions, Comparing plugs allows me to fine tune AFR balance also.

Regarding sealing of the throttle plates at idle, I observed they are not really sealing fully the bore.

I've adjusted & sync'ed my carbs while they were all on a bench; I was looking thru them while closed with a light behind. You can then clearly see the light spreading around the throttle plate allowing me to get similar pattern accross carbs.

BTW, this method did not bring any good result to me (so don't waste your time )

Regarding accel jet leakage at WOT, should we be worried? if it is repeatable, we should just take them into consideration while tuning, shouldn't we?

Last comment, I've also checked the bleed jet at the bottom of my bowls, they are sealed to avoid any bleeding operation with solder & I've got great response from my engine at tip in (with some limits though such as super quick tip in at 1500rpm in 2nd gear). Maybe that's something to investigate as well on your side if you have issue while accelerating.

I love webers for the same reasons as yours but ultimately, the most enjoyable moment is the reward to have a car running great or making progress

They never let me down so it helps also since I've never been frustrated with them.

Edited September 12, 2012 by Lazeum

[conedodger]

So Blue, that would mean you are Blue280ZBlue on the Franck forum... I have followed your travails.

[240260280z]

Yeah I got off it a while ago because interesting exploration was mostly by one or two chaps and often poo-pooed by another if their findings were not within his personal limits. Some great data in the archives there.

I just jumped in today for a much delighted "told you so...and so did the data from last year". That forum has so much potential.

[240260280z]

I read your posts and I don't understand why you rely on AFR at idle. I always setup idle at best lean condition then play with idle screw to set the idle speed correctly.

Snip

So to come back to idle adjustment, idle is always ok once set - whatever the idle mixture screw settings are - but good progression needs to have proper jets. It appears progression is almost always perfect when I'm near 1.5 turn from fully closed with the proper idle jets (50f9 with previous skock N42 head on L28 & now 45f9 on modified P79 head). So in my particular case, I'm just experiencing what the weber books are stating.

Snip

Hi Lazeum,

The point I was making is that a/f at idle and progression depend on where you initially set the throttle plate AND covering /revealing the first progression hole. I am not sure what is ideal. In my case, I can set 900rpm idle with little idle enrichment and run lean at cruise or set 900 rpm with lots of idle enrichment and run rich a cruise, all with the same 55F11 idle jet. There is important interplay between the two screws. As well, my transition to mains is not very different in both cases as the fuel level is high at 25mm down and the mains comes in quickly.

I have more experiments to do but plotting a/f against initial throttle plate position is one that I look forward to measuring.

Yes you are correct, the acceleration nozzle can be part of WOT tuning. Here is a plot of its effect. The second plot has the accel/ circuit completely blocked and out of service.

Edited July 29, 2014 by Blue