Weber flat spot diagnosis amp fix with LM1

Of course I could be all wrong, but for you weberphiles out there: Picked up the car with new cams from Norwoods. Drives great, but there is a new slight short miss in the 3500 to 4000 range during cruise with very small throttle movements. Was always for just a moment, and would not occur with larger throttle movements. It was mostly annoying as I was cruising around 70 + mph and small throttle application would cause a little quick miss the the mixture would catch up. Initially I thought that perhaps the new cams needed a richer idle jet to richen up transition. To check where we were, put on the LM-1 in car A/F meter.
Turns out that the mixture on the circuit is fine, even a bit fat. What looks to be happening is that at smaller throttle movements, the small accel pump jets I have don't respond well enough initially unless the pressure of a large throttle deflection loads the accel pump circuit. To review how this works in a DCNF, the accel pump shoots in to cover any transient leanness until the main circuit catches up to the new vacuum situation; when the pump diaphragm is deflected from throttle movement, that volume is the distributed through the metered accel pump nozzel and a fixed bypass opening in the carb. It is the combination of these two 'resistances' that determines how much fuel goes where. If you look at the graph below, made while highway driving around 3500+ rpm, you see the give and take firing of the accell pump. When the throttle is opened slowly, you see the lean miss up to 17 (my 3.2 begins to audibly miss above A/F of 16) then the mixture recovers as the normal circuit catches back up to the new vacuum level. Sooo, what I need to do is at low throttle deflections, I need to get more fuel to the accell pump jet. So, I reduce the resistance by using a larger accell jet taking some of the flow away from the bypass. The idle/transition/main circuit seem to be delivering enough fuel at steady state, so the accel pump is the object of my attention. Check out the LM-1 plot from today, with time on the x avis and A/F on the y axis.
Will put in a larger accel pump and see what happens. I makes sense as similarly jetted DCNFs (e.g. Ghibli) have much larger accel pump jets than I am using.
best
rt

If anything else is fine, looking at the log you should try a smaller pump bypass valve. But as far as I know these are not changeable at the DCNF (sorry, I’m more a DCOE guy).
So you have to go for a larger pump jet. Maybe this will cure it.
If this is the only problem you have, IMO you should first make changes to the air filter if you still plan to improve its flow. If a setting is at the edge sometimes small changes make big difference. So the solution could as well be found in the idle jet…

Chris
Thanks
You are right that DCNFs have fixed accell bypasses - my best option is to go to a richer jet
As the idle and cruise mixtures ( a function of the idle jet at low throttle openings) are pretty good, I think I will hold with the current 53s. From the other dyno thread, it does look like the car is getting flow restriction around 6500 rpm or so. Since I was planning on going to a slightly larger (34 to 36mm) venturi, will start with a richer pump jet(60)and back down until the problem re-appears. I am definitely thinking about individual carb filters as the flow improvement would more than compensate for the increased temp; another guy told me also that the individual filters apparantly help the carb quartet stay cooler helping against vapor lock. So, even if I held the same hp, an increase in reliability (and peace of mind) is worth it.
Many thanks
rt

I have fitted the "pan cakes" K&N to some Alfa DCOE´s and removed even more of them. In the type 105 Alfas the stock filter element with the resonator tube removed and a direct hose to the front side of the radiator shows huge performance gains over the incredible loud K&N filter.
This is not a crowded, tight engine bay and it is in the front of the car, so the air is not even heated up that much.

I removed these mushroom type air filters mounted directly on the MAF meter on turbo cars and replaced them with filters placed in the fender and connected via silicone hose. The difference was like a whole tuning stage.

It should be even more noticeable on a Ferrari if the intake air is taken from above the engine. You will not only loose power but may run into pre ignition problems. A drive with a remote thermometer can easily show you which temperatures are to be expected under the hood. Might give you a big surprise.
Expect up to 200° F on full load, which is the point where it really counts.
I don’t know the space over your air filter box , but I think it is limited. Would it be possible to cut out the lower part of the box, weld in two rings of thin sheet metal and use two filter elements stacked over each other? You could keep the look, use a stock element and have doubled the filter area.

A good heat shield can prevent the gasoline from boiling and great care of this point is taken on production cars. Usually a insulating bakelite shim is under the carb and the shield sandwiched in. But may be a large thermostatic controlled electric fan could be even more effective in your application.

I really love your project cause I have seen 4 cylinder engines that made double the fuel injected power when converted to large DCOE´s. I always thought about doing it to my “T”. If you find your final settings the response of your engine should be terrific! If you use the LM-1 and a EGT for tuning the economy at part throttle should be even better than the injected car, as you don’t have to look at emissions.
If you have sorted all carb issues, there is usually some tweaking of the ignition advance possible where some more power can be found. As electronic ignitions are constant in there settings, you don’t have to build in that much tolerance.

Test run with 36mm venturis, 60 accel pump and upped to 140 main.
Accel pump seemed to better cover the lean part, but even with the 36mm venturi cruised richer almost a full a/f point.
Will back down on mainjet returning to 135 and keep the 36mm venturi and 60 accell jet.
More graphs soon. Closing in on the solution - this is fun.
best
rt

Mitch and Chris - thanks for the resonses. I spoke with Gron today at Pierce; He agreed mostly with theory 'B'. When told what I was running, he stated that my main circuit was not coming in soon enough leaving a hole. When told of my venturi and main jet size, he said that could be part of the problem, as a 36 mm venturi needed about a 145 main jet to start flowing soon enough. What he suggested was to go to a 145 main to see if that brought the main circuit in sooner, and consider going back down to a 34mm venturi to increase the air velocity if the hole was still there. He relayed an almost exactly similar experience on a hot cammed 356, where you went up on the main jet, then backed off on the idle/transition.
Son of a gun - it worked!!!!!
Gron was right.
Went up to a 145 main (F24;210;53) and you could feel the main circuit come in earlier!! Clinically, the flat spot is about 80% gone - if you try hard you can induce it sometimes a little, if you are looking for it, but the anoying recurrent flat spot at 3500 - 3800 rpm 5th gear cruise is GONE. I almost don't believe it. I am now backed off on the idle/transition from 53 to 50 to lean out the low end cruise richness of the mid/low 12s - Sure enough, the cruise A/F, while still variable came up generally across the board about a point. The car now seems to like about 75 mph in 5th where it settles out around 13.7 to 14.2 depending on load. Another temptation is to go back to a 34mm venturi which would lower the transition to the main circuit a couple of hundred rpm, but the current improvement is amazing. While slow throttle opening before poked a hole to 17, now it just hits 15 and the main kicks in, you really do not feel it unless specifically looking for it, and even then just under very occaisional circumstances.
The last thing I may do is back down on the accell pump jet from the current 60. When I floor it, the A/F drops to 10 or so and lasts for a while. When I flash the throttle sitting still, I do get the blue smoke out the back suggesting very rich mixture. I have a 35 and 40, and my records show even a 308 LeMans with 36mm venturis used a 40 accell pump jet. May do that tomorrow, and if all goes well will disconnect the LM-1 and hold what I have for a while.I would like to get to a dyno for a quick check of the high end full load mixture to look at the air corrector jet, now at 210, as it is not safe to do that on a public road.
best to all
Russ

Quick follow up - just tried the 40 accel jets - seemed to work well with less 'dip' and smooth wide open progression.

Just a summary: Had a flat spot in the transition to main circuit after installing new cams with 36mm venturis, 135 main jets, F24 ET, 190 A/C, 53 idle and 35 accel pump.

Turns out that can lower the transition to main circuit by increasing main jet size as well as increasing velocity with a smaller venturi. Also allowed use of a smaller idle/transition jet for better cruise.

After some variations, going to a larger main jet of 145 eliminated most of the lean hole without downsizing the venturi:
Final: 36mm venturi; 145 main, F24, 210 A/C, 50 idle, 40 accel jet.

I am getting very efficient at rejetting DCNFs, lemme tell you . And what a sound!
best
Russ

Below is a LM-1 A/F graph primarily showing cruise with the new 50 idle/transition jets. The difference in 53 and 50 is actually impressive - certainly more than I would have predicted. This Ferrari does not have any lean missing until above A/F of 16 under load.
.
Hope this is entertaining for some
best
rt

Russ,

This is great information!
Once I get settled (OK, move into) my new house, I've got to get an LM-1 installed on my BB.

I, too, found that a fatter main jet greatly reduced the transition stumble...

Great post!

Regards,
David

I dont know that "entertaining" is the right word! Russ we all are learning from your efforts and enjoying it as well.
I've been thinking on the A/F meter idea. There are some "lesser quality" units around. and I have beenthinking about trying to convert one into a temporary type. That is, have the sensor clip deep into the tailpipe for good readings, and I would be able to get closer in adjustments before the dyno (road is free,LOL). Your thoughts?
Thanks for sharing!

Kermit