Float height for Webers - Part II

I love these discussions, God help me I love them so

would be interested in the numbers on your 'lean' high end of 11.2:1 - A 205 a/c with a 140/F24 combo is not usually that lean. The range you should shoot for is in the 13's from 4000 all the way up with perhaps a slight richening to the high 12s above 7000 (but it will do fine in the 13s). Your low end richness and poor cruise mpg may also partially be those monster 57 idles reaching up into the high transition. Your 140s already reach pretty far down for main circuit entry so there is every reaon to expect your numbers and performance.

Although I by no means guarantee the numbers, my crystal ball would say bring the idles down to 50 - 52, keep mains at 40 ( to keep earlier main circuit transition - a good thing) and lean up the upper end slowly and progressively first with 210 a/c.

 

The balance we, all true ferraristi w/carbs, seek is the idle size and main size and transition overlap. I once had a hole right at 3500 rpm at cruise and lemme tell ya you can feel it and it is annoying. My bet is at a 70 mph cruise you are burnin' gas away at AFR 10 - 11 (I've been there, done that on my portable LM-1 AFR) with those hose pipe idles (**those sizes are more typically used with larger venturis and more aggressive cams that generate a lower idle vacuum signal**). With 140 mains and F24s (my favorite ET, and I'm assuming 32mm venturis) your main circuit should reach down far enough to allow a bit smaller idle.
You can go to a smaller main, but your cruise mpg are still going to be poop as you will still primarily be on the idle/transition circuit at cruise.
I hope you have quite the hot plug at these AF's as well.

It was my experience that with 34mm venturis, a change of 5 in the idle jet size translated into a full point or better at cruise - and Lord knows you have some room as you should target AFR 13 - 14 at cruise. And your numbers would be possibly a bit more exaggerated with the higher velocity 32mm venturis.

Don't worry about being too lean on dumping the throttle - the accell jets will hold it down near 10 - 11 for several seconds until you are well into the main circuit to protect from an overly lean situation.

As you are also finding as I did, an overly rich mixture is tolerated by the Ferrari, at least for a while, and covers a lot of other ills. It is still not too difficult to get it a bit tighter, and you will feel the difference as you will pick up some response and umph once you get closer to the 13's.

best of luck - and have fun!

 

Some thoughts on the effects of lowering the float level on DCNF's.

All the owners manuals I've seen for 308's specify a float level of 48 mm, with the exception of the 40 DCNF /45-/48 series (early US spec GT-4's), which specify a float level of 50 mm, or 2 mm lower than other versions of this carb.

So how does the difference in float level affect the carb and it's settings?

With fuel at a lower level, it will have to be lifted higher for a given vacuum signal. This means all circuits should be leaner, but there will also be other effects.

1. idle jets and idle circuit - the idle circuit will be a little leaner. Work around: richen idle mixture by backing out on the idle mixture screw.

2. progression circuit - the over all effect will be leaner progression. Enrichment by the idle screw only affects what is in reality the first of several progression holes. Also, the progression circuit range will be limited as it approaches where the mains come in. In effect, it will cut out earlier. Work around: Fit a richer idle jet which affects all progression holes. This will not likely have much affect on the cut out point, only the volume up to the point of cut out.

3. main circuit - The mains will be leaner throughtout their rpm range, as if fitted with a smaller jet size and the main circuit will come in later. Work around: Fit a larger main jet. The larger main jet will not likely have much affect on the point at which the mains come in, only the fuel volume.

4. Accelerator pump - refilling of the pump chamber will be slowed slightly. Probably not an issue.

5. Cold starting circuit - A leaner mixture will be delivered if the choke is used, making the choke circuit less sensitive and less prone to flooding if you use the choke.

6. If the engine starves for fuel in tight right hand turns, it will now be worse.

7. If the engine floods on hills, the situation will improve.

Effects 2 and 3 suggest a method for dealing with the meshing of the transition and main overlap. If it is determined that this transition area is running richer than desired, because both the progression circuit and main circuit are feeding more fuel than desired, lowering of the float/fuel level could be an effective tool in fine tuning this portion of the operating range, both by decreasing the fuel volume and the points at which the circuits are disabled and enabled. Other affected areas could be compensated if necessary by increasing jet sizes or enrichening idle mixture adjustment.

Higher fuel levels would have opposite effects.

Bill