1972 411 2 DR. WITH 1.7 F.I. FLAT SPOT OR BURP AT CRUISING

[rayray1]

have a peculiar drivability issue with the 72 411.
the car runs well mostly. we put all new bosch parts for cap,rotor,plugs,wires,condenser.
The car has a mini 'burp' at cruising or when coming off gas and a flat spot(mini) at increase. The pump has been changed twice and eaxactly the same issue exists.

it idles perfect. no blip in there.
the car has is fuel injected and runs often with 1.7 litre.
high test gas and frequent fuel filter changes.
On heavy acceleration, NO problems of 'burp'.

i have sorta triggered the 'burp' by cruising in second and rolling 'slightly off the gas. The car - appears to hunt or have a flat spot that it ''lives' with at light pedal pressure, but disappers under throttle. It is at all ranges of cruising and appears to be only with light or reduced load.
No backfires or pops heard. The intake was sealed up a few years back. this problem showed up 2 years ago and has s l o w l y got more invasive in overall usage.
I'm really miffed at this -
The car had a overall remake mecahnically several years ago and has not had a valve adj in a few years.
We had distibutor out and cleaned, inspected.
nothing obvious so far. help ! in st.pete florida.

[raygreenwood]

have a peculiar drivability issue with the 72 411.
the car runs well mostly. we put all new bosch parts for cap,rotor,plugs,wires,condenser.
The car has a mini 'burp' at cruising or when coming off gas and a flat spot(mini) at increase. The pump has been changed twice and eaxactly the same issue exists.

it idles perfect. no blip in there.
the car has is fuel injected and runs often with 1.7 litre.
high test gas and frequent fuel filter changes.
On heavy acceleration, NO problems of 'burp'.

i have sorta triggered the 'burp' by cruising in second and rolling 'slightly off the gas. The car - appears to hunt or have a flat spot that it ''lives' with at light pedal pressure, but disappers under throttle. It is at all ranges of cruising and appears to be only with light or reduced load.
No backfires or pops heard. The intake was sealed up a few years back. this problem showed up 2 years ago and has s l o w l y got more invasive in overall usage.
I'm really miffed at this -
The car had a overall remake mecahnically several years ago and has not had a valve adj in a few years.
We had distibutor out and cleaned, inspected.
nothing obvious so far. help ! in st.pete florida.

Hello rayray1! sorry...I found your remarks in so many other threads that I had not realized that you had started your own thread. Ok..a couple of definitions for you:

TVS: Throtle valve switch. this is the black box under the intake butterfly with 4-5 wires on its plug. This device add enrichment upon opening the throttle and on some models shuts off all fuel above 1200-1400 rpm when the throttle is closed.

MPS: Manifold pressure sensor. Its the size of your fist, has a single 8mm vaccum hose and a 4 pin plug and is usually bolted to the firewall on the forward drivers side of the engine bay. It sets the load condition for enrichment for your fuel injection

CHT: This is the cylinder head temp sensor. it has a single wire and is screwed into the driver side head just above #3 cylinder. This helps to set background/baseline fuel mixture and issensitive to temperature. it helps by giving enrichment at warm-up and leans teh mixture out as it heats up. Some call it TS-2...which is really only a useful term for some type 3's and busses that swapped sensor positions over the years. If its in the head...iot measure cylinder head temperature. Call it what it does so no one gets mixed up.

AAR: Auxiliary air regulator. Its a round thing with two 12mm vaccum hoses coming off of it and a single red wire. usually on the right rear corner of the center intake manifold (which is called the plenum). It is effectively the choke. It has a heater element in it and slowly closes in about 8 minutes as the car warms up.

IAT: Intake air temperature sensor. It is a two wire screw in briwn colored sensor on the forward left corner of the center intake manifold. It changes background/baseline fuel mixture as the outside air temperature changes.

FPR:Fuel pressure regulator. it is on the firewall on the forward drivers side of the engine bay...near the MPS.

There are lots of other parts. So...you have a two door? wagon or sedan?
If it is a two door sedan does it have a manual transmission?

There are more than a handful of things that can cause your symptoms. First guess with a four speed...would be a TVS that needs adjusting or is worn.
But....Because the TVS is and enrichment device.....it could be several issues.

We need to get a few things out of the way before we can actually say it is the TVS.

(1) Your valves cannot go several years without adjusting. All air cooled VW's are senitive to valve adjustment. It can literally destroy the engine if ignored. The D-jet injection on type 4's is even more sensitive...because it affects the running of the injection. The valves should be checked at every oil change....3000-5000 miles maximum intervals.
(2) You need to check your fuel pressure. Check it while running...it should be 28-30 psi. At idle the gauge should not fluctuate more than 1 psi in either direction. When you rev it...the gauge should not drop more than 2 psi and for more than a split second.
You need to leave the gauge on after shut down. it should drop to 18-23 psi and stay that way slowly dropping for a couple of hours. Overnight it is acceptable to have only about 8-12 psi reading on the gauge when you come out in the morning. If it leaks down fast or drops all the way to 0....you have a leak.
You will need to pull the injectors out after shutdown...and check for dripping injectors. This is not hard..and you need new injector seals every 2-3 years anyway. use CIS seals from an 82 Saab 900...cheaper and better.
(3) You need to check every single vaccum lin for leaks. Every line should be clamped. You need to draw a vaccum on both the vaccum advance unit on the distributor and on the MPS. they should not leak at all. The intake runner boots should be clamped. They should be tight enough that you cannot turn them with your fingers. there is an o-ring between the throttle body and the center intake. Chances are it has never been replaced and is a suspect vacuum leak part.
(4) The cork gasket under the square box on the oil filler should be replaced about every 2-3 years. There is a rubber elbow on this box that has a 12mm hose going to the intake. under that elbow...is a PcV valve. These are almost always bad. The better way is to crack the small plastic disc out of this pCV...and then plug the 12mm line with a 13mm plastic or aluminum plug with a 3mm hole drilled through the center. This makes a constant PCV source that is easier to adjust for.
(5) what is your timing adjusted to? With vaccum hoses off of the vacuum can....It should read about 8-10* BTDC at 850-900 rpm at idle and should read 27* BTDC at 3500 rpm (the red mark on the fan). It must be set this way or the injection timing will be off.
If the idle timing reading is unsteady...jumping around....you have some small issues in your centrifugal advance springs.
(6) Make sure your throttle closes all the way and adjust the throttle cable.

Start with these and then we can get to adjusting your TVS. Ray
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