Time for some general and some DRLA 40 specific carburettor nerdishness. Progression ports There are different versions of progression/transfer ports. Bottom to top in mm 1.00 - 1.40 - 1.40 - 1.40 - 1.40 or 1.00 - 1.60 - 1.60 - 1.80 - 1.80 The smaller ones come from Alfa's DRLA 36 have the larger ports. Rumour has it that if I drill out my Alpha style small transfer ports to the larger size I'd be able/have to use larger 32-34 venturis. The ports also bleed air into the idle emulsion while they are above the throttle plate so it isn't simple. Changing one alters all. The lowest is the smallest, half covered at idle and always below the plate - a fuel port? There will be a lot of manifold vacuum on it as the throttle begins to open mixed with a lot of air from the three higher holes. The next one up is covered by the plate at idle, then exposed such that when the lowest hole is just uncovered, the next one up is half uncovered. A gradual exposure of the two lowest holes. Another fuel hole. The other three start as air bleeds and then turn into emulsion suppliers as the plate passes. By the time the throttle passes the top one the manifold vacuum is so reduced that the ports can't provide enough emulsion. This is the point. That's around 70mph in a sporty car but sadly only 30-35mph (constant speed, level road) in a bus. The port to the left of the progression holes is for vacuum advance - I suggest that stops working also at this point which ties in with lean cruising needing more spark advance. At this point the main fuel circuit should kick in...but...the larger the venturi, the later they start to work. This can leave a lean dip in the transition between the two circuits if you don't want to use smaller venturis...or bigger carbs (which cause different problems). Fitting a bigger idle jet can and will fill this gap, but at the expense of overly rich idle driving (poor mpg) up to the point of transition. I can see that making the top four holes bigger will generally provide more emulsion throughout progression but not how it extend this progression to meet the mains. Any questions (and answers)?
Not sure I understand enough to ask questions, but I will have a go... Are you saying that opening the holes out, will allow you enjoy the extra high rpm power gained from larger chokes, without loosing a bit of low / mid range power in transition?
It's a constant load thing. If the the idles dwindle out before the mains "kick in" enough you'd be bumbling along at 35-40mph with maybe a lean misfire or kind of fuel shortage juddering. Up to that point of lean(ness) as it's low load driving it doesn't hurt. Weeny idles for low load driving are good for mpg but combined with larger vents can be a problem. It's a problem peculiar to Dells because while webers can be tuned out with bigger main air jets, that tactic doesn't work with Dells, probably due to the different length and designs of each make's emulsion tubes. Alfa carbs just aren't quite right for VWs. They work great in some circumstances but resist fiddling for others. They want to be lean fast constant speed and rich at WOT which isn't quite good for a bus.
Wait until I try and figure emulsion tubes and fuel level into the equation. Here's something - the bigger the lower air holes in the emulsion tube are, the more fuel they they push up the well.
Here's another DRLA fact. The .2 emulsion is popular because the .1 is NOT. Both are almost identical but the .1 has an extra hole that will always pump out air. The hole is level with the passage to the aux vent. The gap around the tube at that point is 0.8mm, quite small. Most people fit it into the air jet randomly. Sometimes it could end up pointing straight at the passage, sometimes the opposite. A Dell design cock up. I suggest if you were to index them directly opposite the passage they'd behave and some say they bring the mains in earlier.
At this point, car manufacturers switched to fuel injection, as there is so much wildness going on in a carburettor, where you are using derived effects like vacuum from air flow sucks fuel up through a hole and an emulsion tube and an air jet, rather than just measuring air flow in a meter and calculating fuel injection off that. Most of the bus troubles come from low revving fairly low displacement engines with optimistically over sized carburettor venturis, dont develop proper vacuum from air flow so are always having to over-size the main jets and related hardware. Producing first a flat-spot as the progression hole bodge runs out and then overdoing it.
And it's also quite easy to have idle and main circuits overlapping too much creating a rich overlap with similar symptoms or at the least chucking petrol way for no purpose.
There is, particularly on Samba, a "law" that says if you suggest a smaller venturi to someoneone having jetting problems you will be completely ignored. If you tell 'em your 2l revved freely to 6500rpm with a 30 vent, again you'll be ignored, these answers are not acceptable in "mine's bigger than yours" land.
When we were apprentices, we built a simple steam piston engine without a throttle and showed that what a piston engine wants to do is rev to destruction. The very idea of controlling that by throttling it's air supply with a crude rotating flap that also had to control the fuel supply at all openings and revs could only end in complication. I had it going really smoothly the other day by setting the idle and running balances by ear...and jetting by driving...and indexing my emulsions. I'm back to using the AFR as a rolling road would, just to check it's not too lean under full throttle.
I have started some back of envelope calculations and they start from assuming a 34PICT-3 is about right for a 1568. Early shocking results seem to show that even a single progressive 32/36 a bit big for a 2332 at the same (about 4000) rpm... And your 4x 30mm vents need an 8500 cc engine under them to be working at the same rate as a 34PICT-3 !! Or you can rev a 2332 to 22K RPM. Simplistic calcs based on venturi cross section area.. Smaller vents, smaller main jets would be a lot smoother, and much less sensitive to fuel level because there would be a massive vacuum sucking air through the emulsion tubes. These guys went down an interesting path... http://www.willhoitautorestoration.com/documents/Engine Dev.pdf
I had twin DRLA 36's on my type 4 and they worked a treat but now days people poo poo the idea and say they must be 40's. My point is I never needed to change anything. But then, I'm not very nerdy
I liked it when after gradually going up the cams he put a stock one in, the graphs weren't much different but he raved about how much better the stock cam was to drive. Then I liked the FI to carb where FI really only won on cold starting. Finally when he found his twin spark wasn't doing anything I was chuckling as he claimed that was because of his brilliant head chamber design, conveniently not doing the same test with the original 2 spark head. Oh, and his custom exhaust that just made it noisy. I did like the honest way he'd backtrack with stuff like the cam and exhaust, continually surpised that his efforts other than increasing capacity, valve size and unshrouding the inlets hadn't done much if anything other than made it noisy and less pleasant to drive in exchange for some top end power at silly revs.
I hope 36's are ok ..trying to set mine up again shortly if I can stop that chirping... Would li,e to get some 28 venturies just to see what difference it makes too but a bit steep just for a play... Got another grotty seized up set sat in a bucket of parafin to see they loosen up to refresh too!
