Carb Sync Weber IDF 40s disparity

[superpickle]

Need some advice...

77 Westie, 2.0L, Dual Weber IDF 40 carbs with velocity stacks.

I have the snail type syncrometer (German) from aircooled.net and am balancing the carbs. Very fun tool BTW.

I have them balanced but need to know if something is wrong with #3.
Linkage disconnected.
The Idle is at 820-840. These are numbers I am getting as I place the syncrometer in each velocity stack at idle and then at 2000rpm.

idle / 2000rpm (all values in kg/H)
------------------------------------------------------------------------
#1 - 4.0 / 10
#2 - 4.0 / 10
#3 - 3.5 / 9.5
#4 - 4.0 / 10

By the instructions I know you're supposed to really just balance between two of the stacks. For me, that was easiest between #4 and #2. I only stuck the sync tool on #3 and #1 just to see what they read. I was surprised that #3 is lower by 0.5kg/H throughout the range. For fun, adjusting the idle mixture screw did not affect the reading.

Since you adjust the idle mixture screws for each cylinder independent of these readings, then I assume #3 is not running rich.
Does this disparity in readings mean anything significant?
Should they all be equal?
Could I have a vacuum leak into #3 manifold that I can't find (yet) that would account for the 0.5 difference?
What else could do this, outflow blockage?

Thanks, Craig

[SGKent]

are there any exhaust leak? Have you tried a smoke test to see if there are any intake leaks on that cylinder - a loose head, bad manifold gasket, open port you missed etc.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[superpickle]

Just replaced the manifold gasket.
Have not done a smoke test.
So I am looking at youtube on how to DIY.

So your thought is that something is wrong, that those values should all be the same?

[Tcash]

With the throttle valves closed. Measure the gap with a feeler gauge on 3-4.

[SGKent]

each cylinder and venturi should be the same size creating the same draw. A small port that is open could account for it. Also be 100% sure you are getting a good seal on that barrel.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[superpickle]

Great suggestions Tcash and SGKent.

I will check those tonight after work and post findings.

[Tcash]

Finding vacuum leaks with smoke (smoke test, tester)

[SGKent]

There is a guy on fleabay who sells a nice home made one with a regulator and 12V heater in it for $75 shipped. Gary (Aeromech) turned me on to it. Works really well. Tried it on the mercury sable a few months ago because it had a small miss at idle I'd been chasing for awhile. Found a failing O-ring on the intake manifold up underneath where I had no idea there was anything. Replaced the $3 O-ring and an aging hose - problem solved. When I have time I'll go back and spend more time looking.

Plan to do the bus in a few weeks when it is out. Guy is xfordguyx .
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[airschooled]

Isn’t this about the time you would start “expert tuning” with the air bypass screws to bring all the idle airflow into equilibrium?

Robbie
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[SGKent]

[airschooled]

Thanks for the info Steve- I misread the idle/2000rpm chart.

Robbie
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[SGKent]

[superpickle]

Work is getting in the way of my life. Thanks for the advice, will get to it soon.

In order to check the throttle valve clearance, I need to break down the carb. It's my daily driver, so this will come a bit later.

Gotta build a smoke machine first, was looking at those threads and videos to see how it's done and do that ASAP.

The Bob Tomlinson book looks good, but I saw a picture of the index and IDF 40's are not covered. Maybe those came later than the publish date. Is it still worth getting? ....probably.

[riverside66]

The whole "Webers bolt em on" section starting at p. 21 uses the 40 IDF. I used this book, and the Dellorto Superformance book when learning about and tuning my 40's.
_________________
1977 Standard Beetle -Sold-
1979 Transporter - "Wheatbread" -Sold-

[udidwht]

.5 difference is nothing to sweat.
_________________
1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires

[Lil Lulu]

I sure wouldn’t sweat .5
_________________
'65 Beetle "Lil' Lulu"- Ruby Red
1600 stock from '71 bus

'72 Deluxe - Niagara Blue w/pastelwiess Camper Special 2L dual 40 Webers 002

'74 Hightop Weekender "Dixie" 1800 34 Del singles

[TomWesty]

[udidwht]

What can cause this...

1. Twisted throttle shaft

2. Vac pull from cylinder for brake assist (lower signature)

3. Vac leak

4. Improperly set up carb

5. An eager cylinder vs another.

Don't lose sleep for a reading difference of up to (1) between the opposite barrel.
_________________
1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires

[airschooled]

6. I had a set of duals this summer with base gaskets so poorly made they affected airflow differently on each cylinder.

7. Consistent valve adjustments between cylinders is mandatory. On high-milage engines, check valve adjuster screw AND valve stem tips for wear every 10,000 miles or so. As the screw 'pecks' into the valve stem, it can create a crater, and while a .006" feeler gauge may feel good, the actual clearance could be more, since the flat metal can't detect a low spot on the valve stem.

8. Velocity stack nuts should be snugged a little at a time. Tightening one then the other could warp the stack, especially since the newer stacks seem to be flimsier than vintage stacks. This could show up on a sync gauge but be unnoticeable to humans.

9. If you have a stock Type 1 distributor, cylinder #3 has a 3° timing retard, which would slow the airflow down. (Some aftermarket distributors are so poorly made that they unintentionally have this too…)

OK, now we're getting into some ridiculous stuff… Good luck!
Robbie
_________________
Learn how your vintage VW works. And why it doesn't!
One-on-one tech help for your Volkswagen:
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