74 auto stick - eratic idle, control valve leaks?

[andk5591]

Been tearing my hair out and got this car drivable and sent home, BUT I need to resolve what is going on when i comes back

74 Super.  1600 DP, correct carb 34 PICT3 with extra port and distributor (kind of - I forget the model, but SVDA ending in 205P I think).  New points, cap, plugs, valve adjustements, etc.  Carb cleaned, adjusted etc, also swapped

Initial problems were linked to leak in intake center section - stalling at idle.  Replaced that and started making progress.  All through this there has never been a shifting problem or is there ever. 

So here is where I am now.  Idle mix adjustment not even close to where it should be.  Probably 6 or more turns out (also dropped idle jet from 60 to 55 with very little change).  When you rev it - the idle stays around 1200 or more for about 10 seconds then drops to 900.  Plus it stumbles when pulling out and almost feels like a slight surging at lower speeds.   You have to almost baby it when pulling out and it then drives OK. 

I have looked for vac leaks everywhere - nothing on the carb, intakes, hoses.  Have also replaced the small and large vac hoses. I even swapped the carb with a rebuilt by Volzbitz a couple times.  Acts a little different, but not much. 

It still acts like a vac leak to me and if I understand the operation of the control valve properly, a problem in the vac can or the servo would only happen when shifting.  So, I think I have a prob with the control valve. Something that we have seen is when revving with the control valve electrical connection disconnected, the idle comes back down much faster (couple seconds)  Any insight would be greatly appreciated.

[bhartwell59]

Have you cleaned/unclogged the heat risers of the intake manifold?

I had a stalling out issue that seemed to defy logic... monkeyed w/the idle and bypass, timing, etc. Wasn't until I cleaned out the intake manifold that the idle stabilized and I could properly get my 34pict mixed right.

[68autobug]

YIKES..... very odd problem when disconnecting the control valve....
I would try this - put a new wire from the battery via a relay connected to the ignition... to see if that fixes the problem...  or just connect the control valve to the battery while You do this test.  and check Your ignition , points & distributor rotor button....  which can all cause problems...  check to see if You have 12-14 volts...
I would also adjust the carburetor choke so it doesn't work... wide open.-----  for this test. [also take the wire off]
until You get everything running correctly ---
You can seal all the exhaust ports using silastic... silicon... which will take the very high exhaust temperatures.
replace the carburetor inlet gasket & inlet gaskets etc...

A leaking inlet anywhere on a VW engine really makes a huge difference...
A good investment is an electronic ignition like pertronix....  so You set and forget... and never have to replace points again...
or the condensor which can also cause problems...

with the engine turned off, and the key turned on, can You change gears about 10+ times... by moving the gear shifter slightly forwards or backwards....??   there should be enough vacuum in the tank to do this at any time....

Now the gear shifter works by grounding out, when You move the shifter knob... so, the gearbox needs to be grounded with a ground strap which originally was at the nose of the gearbox - front gearbox mount... Make sure that is still there and still grounded... clean both ends etc.... or replace it.... otherwise the ground is the shaft from the gearshifter base to the gearbox.

oil everything that moves on the carburetor to stop any wear and to stop leaks...
have you checked the timing with a timing light or meter to see if it is correct... and not moving about...


Lee in Australia
40C degrees today  42C tomorrow... YIKES that is hot.... Lee   68Autobug

[tmea]

ANDK5591:

Do a search on this forum on my username. I had the same problem sometime ago. With vacuum leaks. One was a cracked intake manifold (under the aluminum jacket). The other was a vacuum leak in the vacuum control valve. Posted some pictures and also a test you can do to see if the leak is in the VCS.

VCS leaks are not covered in the service manual but in my case was significant enough that it caused an unstable idle and excessive engine heat even though it was working perfectly!

Tom

[sb001]

ANDK5591:

Do a search on this forum on my username. I had the same problem sometime ago. With vacuum leaks. One was a cracked intake manifold (under the aluminum jacket). The other was a vacuum leak in the vacuum control valve. Posted some pictures and also a test you can do to see if the leak is in the VCS.

VCS leaks are not covered in the service manual but in my case was significant enough that it caused an unstable idle and excessive engine heat even though it was working perfectly!

Tom

Tom, I remember your issue with the crack in the intake manifold but I didn't realize you also had a problem with the control valve. Can you briefly recount what the issue was and how you diagnosed/ fixed it? Just curious more than anything--thx

[tmea]

Cannot figure out how to paste a link to the write up I did but I wrote it on 29 April 2018. That might help you find it. Check it out. My engine had been running a little hot for sometime and I could not ever figure it out. Having eliminated everything else I discovered the VCV was leaking a significant amount of air into the intakemanifold due to bad seals and gaskets. I included pictures of where I found the leaks.

I though the write up would be helpful because there are no checks or procedures for checking for vacuum leaks on it anywhere. The valve can function perfectly for shifting while also causing erratic idle, lean burning/hot engine. Think I covered everything in the post. Let me know if you cannot locate it and I'll try another way to post it again.

Tom

[sb001]

Thanks Tom I found it and re-read it... I even was the first to comment in it after your post! Just didn't remember in my getting-to-be-old age. 

One quick question regarding your diagnosis- you say you plugged the reservoir tap and then applied vacuum to the intake tap to test the control valve and check for free flow of air-- with the reservoir tap plugged how could you tell that there was air flowing through??

[tmea]

The most simple and quick test is to disconnect the resivor tap and plug the spigot with your finger. With the vacuum hose disconnected at the manifold you can suck on the hose or use a hand vacuum pump. In my case, I also disconnected and plugged the servo spigot on the valve and still had a significant vacuum leak. Disassembling and cleaning the valve together with the application of sealant where I indicated solved the problem. No more erratic idle or hot running engine. A significant vacuum leak all while the valve is functioning for shifting perfectly.

The biggest indicator if a leak problem does not require any test at all. If, after not haven driven for a few days or a week or so you turn on the ignition (without starting the engine) and do not hear the release of stored vacuum from the resivor being released into the system then you definitely have a vacuum leak which in my case was the cause of the erratic idle and hot running engine.

It really is quite a significant source of trouble not accounted for in any manual that will definitely shorten the life if your engine and cause never ending and needless carburator trouble shooting.

Hope that made sense.

Tom

[68autobug]

Yes, I can shift about 10 times or more after months of NOT driving the car.... 
My reservoir holds the vacuum perfectly... all New hoses and clamps etc....




Lee in Australia

[sb001]

The most simple and quick test is to disconnect the resivor tap and plug the spigot with your finger. With the vacuum hose disconnected at the manifold you can suck on the hose or use a hand vacuum pump.

So if I do the "suck on the manifold hose" method with my finger plugging the reservoir spigot are you saying I should feel my finger get sucked into the spigot? Or I shouldn't feel anything if it's working properly?

[tmea]

Yes to both. By plugging the spigot to the vacuum tank you are eliminating it from the test. You should have complete vacuum between the intake manifold and vacuum control valve with the spigot to the vacuum tank plugged. You can see from the diagram where my VCV was leaking (severely).

Back to the primary symptom, if after a few days of not driving you are not getting a vacuum surge when you turn the ignition switch to the on position you have a vacuum leak. The possibilities for the source of the leak are: Hoses and connections, leak in vacuum tank (rust hole etc..) or, in my case numerous leaks in the vacuum control solenoid.

Bottom line, with the system at rest and the VCV spigot to the vac tank plugged there cannot be any free flow of air to the manifold connection or you have a vacuum leak. In my case, worse than the usual carb-manifold flange leak. If you really want to confirm the leak you can spray some starting fluid around the VCV with the engine running. Depending on the severity of the leak, your RPM will change.

Tom