Hey Guys ....I'm looking for some suggestions from those of you who are experienced with the tuning woes of a running single IDF on a AC VW.
I just built a mild 1915 for a customers street Bug...stock German crank, Engle 100, AA 94's, 12lb flywheel, new 040 heads, solid shafts, 009, Compufire w/ Pertronix coil, etc. Carb is a Weber 40 IDF, Bugpack aluminum 1-1/2" ( p/n 1018-11) intake (no pre-heat), and port matched dual port end castings. I'm running paper gaskets and new intake boots and I've checked for leaks. Timing is currently set at 32deg @ 3000rpm.
Engine runs great at idle and top end and it will lug and pull no problem, but when the throttle is cracked it completely falls on its face. If rev'd slowly with a controlled throttle it takes fuel fine.
I've done single Weber setups in the past and they have all took some time to dial in but this one is getting the best of me.
Carb has 5/64 (2mm) holes in the throttle plates as I've had good luck with that in the past. Currently running 28mm venturis, 160 airs, 140 mains, F11's, 65 idles, and 80 pump shooters. I also have a O pump bypass valve in it. Best starting and cleanest idle is with 55 idles. I've tried everything from 60 all the way to 100 pump jets with no discernable difference. Fattening up the idles to 65 seemed to help the problem but it will still literally bog to the point it dies if the throttle is cracked open. It runs just like a carb with a bad accelerator pump, but I know it's shooting fuel.
When moving the car around such as backing out of the shop or moving around in the parking lot, it will load up and puff black smoke out the tailpipe...but when you crack the throttle it's like an instant lean out.
I'm starting to wonder if I have some kind of air speed problem with the stock size valves and big intake...but then on the other hand, I've seen people running 40IDF's on stock 1600's....

Anyone else experienced this and overcome it? Suggestions?

What intake?
Aluminum?
Does it have a balance port between the runners under the carb?

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Intake is a Bugpack 1018-11...aluminum center, 1-1/2" tubes. Same intake that comes in most single IDF carb kits and the same one that EMPI copied. No balance tube under the runners. runners do come together in the center but come to think of it I have no idea whether they are siamesed / crossed over or not.

Also, this same intake and carb came off of a good running 2276 that I put together last year. Owner stepped up to dual 44's so I figured I'd use the single setup on the 1915

Needs a bigger shot of fuel. ..turn the pump nut in to give it more fuel ..then check the float .but i'm no pro.

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OR. timing not advancing down in the distributor. Those centrifical weights could be rusty crusty. So try to slide that plate back and forth that your module sits on. Then spray some break free oil down inside and see if it freed it up any and made a difference. Put a timing lite on it and see if the degrees are moving like they should. I'm guessing the reason it smokes is because the fuel is lighting off real late and does not get to burn good because the compression has done lowered some.

Is it popping back through the Carb? If not I'm gonna agree with passat you have a timing issue or an ignition issue period. Every time I've seen a lean mix issue it always backfires through the carbs.

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If it ain't broke ill fix that

This is a new engine with new parts...no dirty crusted up stuff. Distributor advance is working properly, timing is set to 32deg @3000rpm. Yeah Freak, it's leaning out / backfiring through the carb when you crack the throttle, yet it runs way too fat at idle. 55 idles is where it needs to be to run clean but it makes the flat spot even worse. Darrell, I put a O pump bypass in it, and I have tightened up the accelerator pump rod nut.
I've had as big as a 100 pump nozzle in it and still can't cover the flat spot. One of the bad things about the pump jets is the bigger you go (more volume) the less the pressure you get behind it, so instead of shooting a hard stream it just pisses it out.

I'm making some headway on this... I had a 48IDF here that was set up for alcohol. I noticed that it had a different type accelerator pump rod. Instead of the typical Weber long or double dampening spring setup, this rod was a solid .180 dia rod and it had a shoulder with a very short dampening spring, maybe 5/16" long.
I swapped the rod and Shazam! .....it'll wind up when you snap the throttle now.
Now to clean up the idle circuit and tweak this thing in.

Nice find ...i thought you would find it in the pump

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Yeah, I knew I was having a issue with the accelerator pump circuit but I couldn't understand why. I guess the original springs were soaking up too much of the stroke and I wasn't getting a long enough duration on the pump shot.
What really baffles me is that this same carb ran good as a single setup on a 2276 with an FK7 and 044 magnums with 42X37.5 valves.....why the hell would a very mild 1915 want more of a pump shot?
I've setup a lot of carbs and I've never beat my head against the wall as much as I have on this one.....I'm just glad I had that 48 alcohol carb laying around to look at,

Posted using the WoodsBuggy Mobile App I have had the same problem on a set of 40's I had to shorten the spring grind on the rod but washers in front of the pump spring and finally fixed the problem . And same deal here carburetors ran fine on a bigger motor

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