Off-idle stumble

[conradk]

I'm back! So, the bug is back from the 4th mechanic to try and figure out the off-idle stumble. He was unsuccessful in figuring it out.

1. Starts up right away.
1. I have tried 3 carbs, 1 rebuilt, 2 new (no change in symptom)
> On latest carb have added proper vacuum port, now shifts smooth, latest mechanic increased jet size, checked everything else
2. I have switched to an SVDA from aircooled with proper advance (though we have it advanced a little more than we should just to keep it from dying off-idle)
3. New coil, new spark plugs, new spark plug wires, new vacuum hoses
4. This is a rebuilt 1600 that performs well, accelerates fast after stumble (probably because it is advanced so far)

The last mechanic suggested I may have a vacuum leak in the clutch servo, that the stumble could be due to a drop in vacuum - could this be the issue. Anyone replaced the servo to resolve this problem?  

I have read about sealing the hole in the venturi flap, but have also read that it makes too much vacuum for the autostick. So I haven't done that.

My wife is ready to sell as she is tired of seeing it sit around for the last 18 months - I just want to get it running right for the new owner. (I also can't get it inspected with the current problem.)

Thanks,
Conrad

[volkenstein]

Conrad,
          I can't see how the servo would be a problem on acceleration as the vacuum release should have been done, the main CV check valve closed, and the reducer valve in the CV should be shut. Theoretically possible to have the CV diaphragm providing a huge leakage, but then you'd have shifting problems.

Until the solenoid is actuated, your manifold is feeding the CV and the reservoir. I'd be looking there first rather than the servo. You've replaced a whole pile of hoses and such, have you tested for leaks?

Back to the stumble.
Have you done the 34 Pict 3 adjustment as per www.vw-resource.com ? Idle speed around 900-950 rpm?
What type of aircleaner do you run? Paper or oilbath?
Have you played with the accelerator pump stroke?
Have you checked the acc pump volume?
Have you adjusted the acc pump spray nozzle position?
Got a degree pulley on it?
If it is an OG VW one, I would verify TDC location and then set the timing accordingly.
Electric choke opening fully?
Manifold rubbers haven't sprung a leak?

Two sore points on this carb are the passage that feeds fuel from the acc pump housing to the spray nozzle and the passage that the auxilliary air jet is in front of. Both can be blocked or partially blocked and are very hard to clean out.

I'll think hard and see if I can come up with anything else.


Regards
Sean

[Crasher]

There could be a leak on the feed to the tank (such as a rusty tank) and up to the valve; this would produce a constant air leak into the manifold. What distributor are you running? Have any of your mechanics measured the temperature of the inlet manifold heater pipes to see if they are blocked?