Tuning dual Weber 40 IDFs... and a noise

[boondocksdrew]

I'm trying to tune the 40 IDFs on this new engine and having some difficulty. It doesn't really want to idle at the default settings (idle mixture screws 1 turn out, idle speed 1.5 turns in from point of contact with throttle lever). I have to turn the idle speed screws in a decent bit or it kind of putters slow for maybe 30 seconds and then dies before it can really be tuned any further. My understanding is that indicates the idle jets are too lean so I have some 55 idle jets ordered to try swapping in place of the 50s that are currently installed. I've tried some other starting points but nothing has really seemed to put it in a state where adjusting the mixture screws yields a predictable or consistent result.

If just leave the mixture screws at around 1 turn out and try to drive the car the throttle response is really terrible in the lower rpm but it feels more like bogging then leaning out. There's no popping or other weird sound it just kind of falls on its face until the rpms pick up and then it accelerates a bit more reasonably. I also noticed a kind of burbling sound with the engine off coming from (I think) one of the carbs which you can hear in the video below. The intake and carb weren't too hot to rest a hand on and none of my gauges or warning lights suggested it was running hot.

https://youtube.com/shorts/GuwZhrMNg8A?feature=share

I'm also wondering now if this is borderline too much engine for this exhaust but wouldn't that be more of a high rpm than low rpm issue?

1600cc
Compression ratio ~8.6
Dual Weber 40 IDF carbs
Pertronix SVDA distributor
ACN L3 heads with 40x35 valves
Webcam 218
Tri-Mil Euro 2 tip exhaust (1 1/2") w/ j tubes

[APPLEGREENVW]

What is your fuel pressure? A lot of info here to read.

https://www.thesamba.com/vw/forum/search.php?searc..._chars=200
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[boondocksdrew]

I'll try to get a measure on the fuel pressure when I get a chance.

After some more thread digging I'm coming up with this:

My current 28mm venturis are almost certainly too small for this build particularly in relation to the 40mm intake valves?

And the 50 idle jets are probably not my problem? I have the 55s to go up and some 47s to go down if it seems otherwise.

[madmike]

If it was to lean on the idles it will 'Pop'
I'd try another/different Distributor
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[oprn]

The idea that the idle screws should be X.X number of turns out baffles me. Every engine is going to be different, that is why they are screws and not a fixed drilling! It matters not if when you are done tuning and the engine is running well if the idle screws are 1/2 turn out or 5 turns out!

My engine ended up with the idle stops at about 1 turn and idle mixtures at about 3.5 turns. Runs and drives sweet!

If you exhaust is too big then yes it could effect the low RPMs.
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[Alstrup]

[oprn]

My brand new Pertronics III distributor stuck at 16* BTDC and would not advance or retard after running for a while.
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[fclmscott]

Look up the tuning and sync instructions on Redline Webers web site. You stated that you had the idle speed 1.5 turns in from point of contact with throttle lever). That's mistake number 1, at this point your way into the progression ports and your idle mixture screws are becoming ineffective. Follow what redline weber says and you'll be much closer. You shouls also pull the carbs apart and check the float levels. Way too many things to go into here.

Frank
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[boondocksdrew]

[boondocksdrew]

So this thread has a setup that worked on a stockish 1600:

https://www.thesamba.com/vw/forum/viewtopic.php?t=...last+night

stan_tichomirov ended up with this:

[modok]

Not a jetting problem so far. MANY MANY sets of weber 40IDF-70 have been sold with 50 idle jets, and on average that ends up a half a step rich.

Should be able to tune-in the idle just fine with idle jets that are one size too big or one size too small.

If NOT then you have to TRACE the idle system and find out what is wrong with it.

Now some of the weber CLONES, or different IDF weber models may require a larger idle jet size sometimes, but even so I think they ALL would idle fine with 50 idle jets.

Now all this "one turn past contact" I never have thought that was a good setting. What if the arm is loose? What if the throttle does not close fully?
Better to set the throttles so they are open even amounts OFF the car, then keep them that way. Or use a snail, or tune with VAC gauge, or check power balance, But just counting turns alone, not really mean anything IMO.

[boondocksdrew]

[modok]

Yeah that's good thinking, sure, but you will probably have them off again anyway so lets just take them off and look without needing a camera.

If you get the throttles set so they are at the same position PHYSICALLY, maybe the first transfer port half open, then put them on and use the snail you should see all four at least similar flow, and idle even on all four.
If it's WAY off then you have some OTHER problem.
The more you know the more you know.
Your thinking they are set the same from counting the turns.....probably, but better to be certain.

I bet you overlooked something basic with all the excitement....like...
Make sure the manifolds will go on flat without the gasket (not hanging up on some corner), make sure the surfaces are flat.


Like.... why does the left ball joint looks to be not fitting INTO the hexbar?
and... why is the carburetor hitting the side of the shroud?

Who put larger valves in the ACN level 3 heads?
Wouldn't they be level 5? or level 4?
level 4.5?

[boondocksdrew]

Had the car running just a few minutes to move it from one end of the driveway to the other and noticed after shutting it off that the driver side intake manifold was already hot while the passenger side was still cool to the touch. It was also the driver side that had the boiling (maybe) sounds coming from the carburetor the other day. Not sure what to make of that.

For now I've pulled the carbs and intakes off to break them down and make sure the jetting and float level is as advertised and to set them up yet again on the bench.

It's pretty hard to see the mating surface between the manifolds and the intakes now to check fora level fit but I did a mockup before the engine was in the car and everything seemed flush. If nothing else solves my issue I've heard spraying some wd40 where the parts meet while the engine is running works as a leak detector?

Now that the carbs are off the car, when I set the idle speed screws to "1/4 to 1/2 turn in after contact with lever" am I trying to confirm that's got the right amount of throttle plate movement somehow? I had also previously tried using a feeler guage as suggested by the Weber Tech Manual but wasn't especially confident in the result with the feeler gauge having to bend/curve a good bit to conform to the barrel walls.

[boondocksdrew]

[boondocksdrew]

So I'm breaking down the carbs to confirm all the jet sizes and make sure nothing is clogged. All seems fine on both counts there. Then I start to set them up again and discover that the #1 and #2 barrels on the passenger side carb are not starting from the same position.

Number 2 barrel with idle screw at zero contact already had some progression hole visible:

Number 1 barrel with idle screw at zero contact has NO progression hole visible:

So then with the idle screw at 1/2 turn in from contact they are still exposing different amounts of progression hole:

Is this enough difference to be causing tuning problems? If so, is there a way to even them out?

[richparker]

Back off the idle screw till it’s no longer touching the cam, this will allow the butterfly to close all the way. Then loosen the 2 Phillips screws and adjust the plates until they are equal. I would also check the float height on each carb. Above they talk about fuel pressure, I run IDFs on both of my ACVW engines and both have Brosol mechanical fuel pumps.

I just installed a set of IDF on the 2109 in my bug. My builder told me that on a ACVW you should start with the mixture screw 2 turns out. Although I do agree with Oprn and every engine is different, that’s whys it’s a screw.
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[boondocksdrew]

I think they're evened out now. Is this a reasonable amount of the first progression ports to be visible when the idle adjustment screw is 1/2 turn in from contact?

[modok]

maybe you can slightly bend the throttle plate to fix that, that would improve things a bit, but it does not have to be totally perfect. if that hole is a .040 then 1/4 of that is .010" that's not a huge difference.
After all that is why the air bypass adjustment and mixture adjustments are adjustable.... but looking at this is helpful, you should have a clue as to which way they may need to be adjusted, then it will make sense.
Use a small flashlght on the OTHER side and look at the gap around the throttle plates to see they are centered in the bore and gap is even on both when they in the idle position, before loctiting the screws back in.
Ideally the plates should not be touching the carburetor bore at idle. if they are touching they can wear, and will wear.
I maybe did not explain clearly enough before.... but I don't feel how far the carbs can close is highly important, because, what I just said. But if you can get them to be totally perfect then go for it. yeah that seems fine, but the screws are not in yet so lets see....if it stays that way