JRV,
Perhaps you've come across this driveablity issue...
Light throttle-Runs well.
Full throttle-Runs well.
Partial throttle tip-in (4000rpm, 2nd gear)...stumbles, light bucking, hesitation....then off she goes.
Seems like the "typical Weber transition" phase, but worse than I rememeber...
Even going up a moderate hill at 70kph, when you give her just a bit of throttle to maintain speed...you'll experience the symptoms described.
My gut says she'll feels lean...
Carbs are fresh...with new accel pump diaphrams.
Pump jets are spraying...
Timing set..and curve within specs.
New engine has 200km on rebuild.
You know those dang progression ports gave nothing but trouble under the conditions you mention, because the motor is not quite running on either circuit......my instincts say to change the Emulsion Tubes to get a little richer, better atomization in this mid range...the other route would be to step down slightly with the AC Jet, but I like that as a second step because it fattens it up across such a wide range, when you may not need that broad of a fix.
JRV,
I was afraid that your answer was going to be my suspicions...though I'm surprised that Ferrari never got this worked out. The car has always had this stumble...though now it is worse than before the engine overhaul. Unfortunately, I'm very familiar with the Weber transition phase, as I cut my teeth building hypo Afa engines...and rememember, all to well, the PIA involved in tuning this bug out!
The idle jets are .50mm, as specified in the WSM and OM. Oddly...and I think I posed this scenario before...the WSM states that the idle mixtures screws are to be out 4 turns (min)....if I'm translating the Italian correctly "Apertura vite registro minimo: 4 giri circa"
Currently, my idle mixtures screws, on average, are out 3 turns. I set the idle mixture using the lean enrichment method...a shot of carb spray, or propane...adjust idle mixture to the point where there's no/little raise in idle speed (coming from the lean side to rich). My analyzer went kaputo...
In the past, as the engine breaks in further...the problem will get somewhat better..
This is from a novice so it may not even be worth 2 cents.
First, my carbs were set up by Terri Gerome in Atlanta who is pretty good at this stuff about 2 1/2 years ago. I have always had a small stumble when getting started, say from 1000RPM to maybe 2000RPM. Seems to bog down a little then really gets going. Everything else is like a jet plane.
I have always believed that this is just a function of the imperfection in the carburetor design. There is just not a perfect transition between the fuel being added by the idle jet transitioning through to the progression jet.
The only thing that people never seem to discuss, even though it is discussed in detail in the Weber Carburetor Manuals by Haynes and Braden, is the initial idle position of the throttle plate relative to the idle and progression holes. As I understand it the plate must be just above the idle hole so a vacuum is created but not that far open that it uncovers the progression hole. The only way to do this that I know of would be to remove the carb and check/set it on each carburetor. Then, when you adjust the idle screw you have an idea of starting position of the plate.
I guess in reality this is such a pain that unless you've just done a rebuild you assume the position is approximately correct, and do the carb tune up from there.
This is what I plan to do next week thanks to all your help using the JRV method of adjusting the primary carbs first, then the secondary, adjusting air bleed to balance individual throats (though from a quick synchrometer check they seem to be real close)and finally mixture via colortune.
Hey, let me know if I'm really off base on some of this stuff. Reading and understanding, though a good start, is one thing. Actually doing it, especially the number of times JRV has done it (tuning carbs, not sex), is another.
you brought up an interesting point and my next Q to David...are those 3 or 4 progression port carbs on your engine?
second item of mention...one of the things to try has always been playing with idle jets...like stepping up to a .55 to try and mask the lean off idle condition.
Of course for the very brave (or foolhardy {}) drilling the progression ports out to a slightly larger size was also another attempt to alleviate this stumbling condition. Can't say if it is the fix because I never tried it. But based on the fact that PMO went to all the trouble to redesign & manufacture Weber knockoffs that specifically addressed this transition problem I doubt there is one answer to this age old problem.
I appreciate your thoughts and input. Your comment "There is just not a perfect transition between the fuel being added by the idle jet transitioning through to the progression jet.." is true, though with careful jetting and carb sizing, this "transisition" can sometimes be tuned out completely. I owned a repair/mecanical restoration shop for many years...and we did alot of Weber carb work. Often times, more effort went into dialing out this transition issue, than jetting the carbs for the rest of the powerband...
Likely, you've gathered that I'm very serious about having my BB run at her best...and value the input of other owners. Novice or not, either reading the words of others...or putting my thoughts to words, often helps me to see the problem from another angle that I may have overlooked, or didn't appreaciate its signnificance.
One of the reasons that Weber puts those brass plugs over the progessions hole is so one can observe the throttle plate position, in reference to the progression hole. In theory, if many factors are correct (timing, carb size, jetting, etc. are correct, the throttle plate will be in the correct position.
Going back to the issue at hand, I checked my carbs....and there are three drilled progession holes, with the 4th one having a factory plug in it. As I put more miles on the motor, she does appear to run a bit better...but not quite right yet. A good excuse to drive this "bad girl.."
Other than the "normal" Weber WSMs, the book "Weber Carbs" by John Passini has been m reference reading for a long time.
Update...
I took here on a 60 mile highway trip yesterday...Although the stumble is still there, it is noticeably better. More driving today...just hit the 300km point on the new engine.
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JRV,
Perhaps you've come
}) drilling the progression ports out to a slightly larger size was also another attempt to alleviate this stumbling condition. Can't say if it is the fix because I never tried it. But based on the fact that PMO went to all the trouble to redesign & manufacture Weber knockoffs that specifically addressed this transition problem I doubt there is one answer to this age old problem.
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