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Mon Jun 04, 2018 7:58 am

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

Mon Jun 04, 2018 8:15 am

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.

Mon Jun 04, 2018 8:55 am

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?

Mon Jun 04, 2018 9:15 am

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

Mon Jun 04, 2018 9:15 am

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.

Mon Jun 04, 2018 10:46 am

Great suggestions Tcash and SGKent.

I will check those tonight after work and post findings.

Mon Jun 04, 2018 11:13 am

Finding vacuum leaks with smoke (smoke test, tester)

Mon Jun 04, 2018 11:42 am

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 .

Tue Jun 05, 2018 12:09 pm

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

Tue Jun 05, 2018 12:41 pm

asiab3 wrote: 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

When the setting is unknown those screws are often set to lightly bottom so they have no effect and then the idle balance checked. Then they are opened a set amount (e.g. 1/2 turn, 3/4 turn, 1 turn etc.) as specified for the vehicle/engine. Did the last guy tamper with them? The bypass is most prominent at idle speed. Once the idle balance is set then the mid-range is confirmed as to the linkage. He is at 2000 RPM so he is checking linkage but both should be the same unless someone has already tampered with those bypass or they were wrong from the factory which brings up another point. If they are new he should be talking with the party that sold them to him to see how they were set up. Tinkering with carbs is an art form ESPECIALLY because they get blamed when a lot of other things are at fault. A smoke test and a compression test are really in order first I think. Once they rule out other possibilities then he can look at the balance screws - unless he already knows that one has been tampered with ...

Weber Tech Manual by Bob Tomlinson is suggested as a source to read for those carbs. I don't know if the light green or blue books from Weber are still available.

Tue Jun 05, 2018 2:17 pm

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

Robbie

Tue Jun 05, 2018 2:55 pm

asiab3 wrote: Thanks for the info Steve- I misread the idle/2000rpm chart.

Robbie

Robbie I know you are really brilliant at all these VW buses. Included you as well as many others in my thoughts when "it is an art form' was said. Kind of off the subject but those who will carry the torch forward are emerging as we old guys drop out. I hate carbs anymore because the casting parts are inferior to what was available 40 - 50 years ago - before smog carbs and laws that regulated the foundries.

Tue Jun 05, 2018 6:20 pm

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.

Wed Jun 06, 2018 7:07 am

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.

Sat Nov 03, 2018 2:12 am

.5 difference is nothing to sweat.

Sat Nov 03, 2018 6:54 am

I sure wouldn’t sweat .5

Sun Nov 04, 2018 10:48 am

Lil Lulu wrote: I sure wouldn’t sweat .5 Precisely my thoughts, could just be a difference in the fit of the synchrometer.

Mon Nov 05, 2018 12:20 am

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.

Mon Nov 05, 2018 3:57 am

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