Weberology 501: tuning cruise, transition and flat spots in the 44 DCNF Mondial

As an adjunct to the LM-2 thread, this is the Weber tuning of the new 44 DCNFs. Subjectively, they drive well with a small transition flat spot noticible only if you open the throttle slowly at higher cruise rpms. Large movements do not generate a flat spot. It is also subjectively a bit rich from starting behavior and smell. These are born out on the A/F meter which show a steady state cruise at 300 - 3500 rpm of 12.5 or so and an idle (once warmed up) of about 12. The transition flat spot also occurs in the mid 3000 rpms with slow opening of the throttle, which the AF shows a spike to an air/fuel of 16 -- and it has been my experience in the past that around an A/F of 16 is typically where burbling or lean miss occurs.
Weber 44 DCNFs with 38mm venturis jetted with 135/F24/190 and idles of 52 (or 55, I can't remember)

As most of the cruise fuel delivery is from the idle jet controlled transition circuit, we probably can bring down the idle jet to 50 or so, being VERY careful as we lean. This will widen any transition lean hole as well, so the choices are to richen the main jet slightly (or adjust the A/C) or provide a stonger velocity signal with a 36mm venturi. (Insert sound of Mark E. flinching).

But what about the accel pump, you say? Well, at slow throttle openings the pump does not squirt enough to make up for the increase in air flow, especially with our leaner accel pump cam optimized for power. the car runs great without flat spots if you use large throttle movements (as Hans and robert say we should do all the time anyway) - you only notice the flat spots in subtle movements at cruise. That said we can optimize the pump jet nozzel for slightly more volume by moving up from its current 135 without dumping the a/f down in the 10s.

We could also certainly speak of float height here, and raise it slightly to keep more fuel in the wells - wells which are already fairly full with the F-24 ETs.

So we will stepwise:
lean the cruise a/f by coming down to an idle jet of 50
moving up to a 140 main, then carefully tweak the A/C if neccesary //OR// try a 36 mm venturi to increase signal. The downside of a slightly smaller venturi is obviously while you will increase drivability with a stronger signal, you also can restrict the high end airflow and top end if the carb is the rate limiting component in the induction (and we know it is in my application from the flow bench). What we are trying also to avoid was a very rich initial main jet A/F which had a/fs in the 10 - 11 range and poked a big dip in the power curve on the original dyno jetted at 140/F24/170. We MUST keep the A/F on the main circuit between about 12.3 and 13.6 from 3500 rpm up on full throttle to produce max torque and power; this is where the A/C comes in. Even with that overly rich jetting the engine put out about 310 hp at the flywheel sae hp. (260 at the wheels) on a 100+ degree day, so if we can knock out the rich area, we should be at the target 320+ sae hp at the flywheel.

We'll see how we do
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Your flat spot sounds like the main jets are not kicking before the transition circuit falls off. The two methods you suggest will work. I have had very limited success with raising the float level, but yours is so close it may do the trick. I like to set it about 2mm below the spill port. 36mm venturis will also work, but at a price as you point out. A trick I have used many times is to change the ET's. Some ET's kick the main jets in earlier, this would allow you to keep the 38 venturis and get rid of the flat spot. Unfortunately the book I have does not list the characteristics of the DCNF ET so I do not have a recommendation to try. If it was a DCOE I would suggest a F7.

Cheers Jim

 

Yes - I agree. I rechecked the jetting on the carbs (as I did not jet them last) and the jetting it is carrying is 135/F24/170 with 52 idles. I am finding there may be as much as .5 AFR error in the LM-2 readings at lower flow rates, so will err a bit rich.
Looking at the first dyno with the 140 mains, you can see when the main circuit came on the A/F dipped to 11 and below, giving a dip in the curve that later recovered at the rpms increased and the AF climbed over 12.
With the current 135s, based on my experience, should raise the A/F about a point across the curve, which is what is suggested by the on board AFR showing in the 12s at WOT over 3500 rpm. I think with the new smaller pump cam and jetting the rich spot is probably about gone - it pulls like a mf above 4000 rpm like its never pulled before.

So while I will reduce the idle to .50 (and will get some 47s), I would go on up to 140 mains and use a 190 or 200 A/C (even 210). This is beginning to get closer how it was jetted before the porting, when it had 52 idles. With the porting, you'd expect more pull, and therefore impirically a smaller idle. As for the main jetting we would expect the same were it not for the fact we went to a larger 38mm venturi, which may allow us to use similar jetting to the non-ported engine jetting with a 36mm venturi. I really like to get on the main circuit as quickly as possible, so if I can reach down with the mains, I will.

I also really need back-to-back dynos with the 36 and 38mm venturis. I never heard from my flow bench guy if the larger venturis ever made a big difference in the flow; I can tell you from experience in jetting that there is a noticible difference in generated signal between the two.

I am also going to look hard at the float levels, but I like you, think it may not be as applicaple, but it's worth a look.

The jetting between DCOE and DCNF is amazingly similar, except for the crazy ET tube designation system.

Will order the new jetting Monday and try this week.
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amazing.

 

One other thing I have just picked up on looking at some other dynos. No matter which main jet is used, there is a definite rich dump around 4200 rpm and recovers above 5,500 rpm.The recovery can be controlled by the a/c jet but the what I am having to balance now is bringing the main in earlier, possibly by going up in size on the main, but keeping the that 4200 rpm dump in check.

On every dyno, this richness dump correlates with the dip in power.

OBTW - rechecked the notes and this dyno was made with 145 main jets - big difference.

I've been currently using F24 which keep a lot of gas in the well, and it might be worth trying to reduce the cas available in the well by going to F36ET. This way, hopefully, it will be easy for the gas to start flowing in the main cicuit sooner with the 140 main, but as the vacuum builds the gas delivery throttled by the ET. Then on top trim it out with the a/c, perhaps a 170 or 190.

Then I could always go from a 38 to a 37mm venturi...

I also really need to get that dang LM-2 rpm sensor working.


All ideas and advice appreciated!!!!
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These are the original carbs we're discussing?
Have heard the number of progression holes varies between the original issue and new carbs.

 

This is an conversion application - we are placing 44 DCNFs on a Ferrari 3.2 liter QV.

 

I have had some luck using an adjustable potentiometer to get the RPM to work on the LM-2 (see their web site for details). However lately I just watch the O2 sensor readings while I dive and don't bother recording the data. It takes to much time to plot it.

I would still try the ET for the lean transition issue. It may work or it may not, so much of tuning carbs is trial and error. As I said early I have made some major drivability changes by changing the ET. I used this trick to remove a huge flat spot in a DCO3 carbed car.

Looking at your dyno run I wonder if the main jet is rich (2500 to 3000 RPM and 3700 to 5000 RPM). The leaner spot inbetween could be a fuel stand off issue.The lean spot may be a condition where the vapor cloud of fuel standing over the ram pipe is not being reingested by the enigne. It may be worth trying a longer ram pipe. If you are running air cleaners you may also want to look at where the cover is relative to the inlet. It should be a minimum of 66 mm (for your carb, personally I like as much as I can get i.e. huge air filters) from the inlet. As long as it is over that you should be reasonalbly good shape, however the plate can reflect the pressure waves coming out of the carbs causing all kinds of weird dips in the power curve. So adjusting it position can sometimes be used as a tuning aid (it doesn't always work though).

I see at the very top end you are starting to lean off. It may be worth trying a 165 AC to richen it up a bit. At this point thou you are probalby wasting your time with the AC. Until you get the transition and the 2500 to 5000 RPM range sorted out the AC size means little.

Cheers Jim

 

Modeler has a good point. Some seeming indentical weber can have very different transition circuits. If you get a carb that was designed for a smaller engine the transition circuit can be undersized. I did open this circuit up once but I consider it a last resort trick. BTW the ET made a bigger change to the transition point.

Cheers Jim

 

Just out of curiousity here, what are you using for float height now (48mm or 50mm)?

 

Thanks, guys.
A little background. This car was converted to 40 DCNFs about 5 years ago and after many dynos and LM-1 time, was optimized with no transition flat spot with 145/F24/210 w/52 idles on 34 mm venturis with an excellent dyno of 240 rwhp up over the stock 214 rwhp (see attached dyno).
Recent new work included ported head with larger intake valves, port matched manifold and upsized 44 DCNF Webers (NOS originally intended for a Ducati), here with whopping 38 mm venturis which gave the dyno in the previous posts of 259 rwhp, well over 300 at the flywheel, on 145/F24/170 and 52 idles. Since then, we have been running on 135 mains which rally livened it up, and on the LM-2 suggest it holds A/F in the 12s where before it was in the 10-11 range. The car runs like a rocket and will throw you back in your seat (amazing for a Mondial), and the very slight burble is only felt at small throttle movements near around 3500 rpm cruise. It idles at about an A/F of 12 and trundles along at cruise, still in transition, at about the same.

The previous set up worked great, as I was able to lean out cruise to high 13s squeezing the idles and covering the transition with a sl larger main, then 'braking' it with a 210 air correction.

The quick and easy way to fix my current burble would be to go to a 36 or 37 mm venturi, but I am trying to avoid that to see if I can jet around it similarly. I expect with the stroget pull from the ported heads I can get away with slowly first backing off on the idles to 50s. I then want to reach down with a sl larger main, say 140, but progressively brake the enrichening effect with a larger a/c as it becomes more effective as flow increases. I've spoken before of also swapping to an F36, with a smaller well volume which might also limit the rich tendency.

I could also open up the accel pump a little and trade charge for duration, but I think the secret is in the transition jetting.

For the complete development, here are Dyno-jet dynos for an example stock 3.2 engine, the 3.2 with the 40 DCNFs, and the first dyno of the ported engine with 44 DCNFs. The last chart shows a projected cleaned up tracing, once the overly rich jetting is corrected - basically, a smoothing out of the dip.

Will speak with the LM-2 folks today on the rpm logging issue.
Thanks again, and all comments welcome.
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Good point -- 48mm

 

That's pretty high up, infact, if memory serves me correct (according to a few of the Weber books I have), that's the highest you can go with DCNF's? That'll be quite close to the lower edge of the auxiliary nozzles...

 

Spoke with Mike Pierce of Pierce Manifolds (very helpful guy) this am and reviewed current jetting and performance. He agreed that the ET tube may be the answer in combination with the larger jet. He also laughed and said it sounded great in theory, but we'd have to try it.
So, we are going from: 52 idles 135/F24/170
to the new: 50 idles 140/F36/190 or 200

He didn't think different accels would make a difference.

Ordered a set of 50 idles and 200 air correctors. In addition, they now have a nice set of air horns with screens for dcnfs I am going to try. They look like they definitely may add some velocity, but then again we're faced with the hot ambient under bonnet air with minimal filtration, even though the opened up starboard vent blows into the engine 'V'. It will be quicker to make tuning changes to, though, if I temporarily use individual filters - easier to get to idles and linkage.

Also attached below for comparison is a dyno of a Euro carb 308, showing how leaning out a rich mixture buys horsepower.
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Went for a drive with the rpm hooked up; have a lot of data, but here are two excerpts. Still thinking about it but good news seems to be nothing dangerous.
First thing is to work on idle/transition richness.
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Manly men did jetting, ET changes w/o steenkin' computers. 46IDAs on 2.8, 42DCOE's on a 3L 240Z, just me, my uni-syn, and the butt dyno. Bet we both can field strip a Weber blindfolded.
Leaning out rich doctor buys P8/4 new horsepower

 

you're killin' me over here...

 

Are these graphs with the new ET?

FYI, I find it easier to get the transition circuit sorted out by revving the car in the garage in neutral. The transition will take place at a higher rpm than driving, but the expanded operating range makes it easier to see what is going on (especially with an O2 sensor). If you look down the venturis you can see the main jets kick in. At least you should. I had one that would die like you cut the ignition off when it ran off the transition circuit and mains hadn't kicked in yet. Of course once it is working in the shop you need to verify it works on the road.

Cheers Jim

 

No, jets are in the mail. The one thing I need to do first is lwan out the idle/transition just a bit. In the past, I've had better results with idle and cruise a/f in the high 13s. Maybe could go to 14, but I've got a little pad in the 13s and it starts easier. I'll get a mirror on a stick and try to watch the transition as you say - that sounds very interesting.
Thanks!

 

Just for the hell of it...

I really do love the jetting I have in now, and have always felt that just a wee bit more velocity might do it to clean up transition. So, although everyone will scoff, I am going to try my longer velocity stacks. They do not fit in the stock airbox, so will try separate filters and see if the wide open ducted starboard vent directs enough cool air - I know the carbs will run cooler, anyway.

Hey, it's all in good fun - we'll see what happens.
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I always tried to run long stacks with the tall cleaners. Really does make a difference in midrange and seemed to smooth out invariable flat/transition spots. I threw out the CIS on my 3.0CS waaay back, and the car ran far better with Webers on it. Noises nicer too. Reminded me of a leopard fellating an elephant. Different kind of laptop tuning I guess.
Looks great Russ. When TF am I going to see P8/4 and the rajul?

 

I thought your Mondial was black, Russ? And now all of a sudden we see a yellow car???

 

If you will look closer,...
I painted my garage yellow with red tool pegboards like a Ferrari garage.
A bit over the top, I realize...

 

Needs some ideas here, please.

The current 135/F24/170s seem to work great once the main circuit comes in, with just a tiny transition stumble at 3500 rpm. The idles at 52 are too rich - you can really smell it. James had tried 45s but they simply did not respond. As I squeeze the idles, I want to reach down a bit to pick up the hole.

So, while I know the A/Cs are primarily working at the top of the range, they do have an effect throughout. Do you guys think squeezing the A/C down to say 160 will persuade the 135 main to come in a bit earlier with a leaner idle?

James Patterson at Norwoods also gave a historical note, saying they've noticed that cars need to be jetted richer now with modern gas than they used to years ago. This is a good reminder to keep the butt-o-meter as a validator on the A/F meter.

 

He's right, as gas formulations have changed radically over the years.....