Pierburg Secondary Throttle Diaphragm Fault

[1_eyed_jim]

I've taken the van to the garage today as I'm fed up with the lack of power it's giving me. He has spent an hour on it tinkering with the timing and checking things over and he has come to the conclusion that 2 things need to be looked at.

1. He could not get the secondary throttle diaphragm the open by it's own mechanical way. There is plenty of vacuum produced on the other pipes but upon removing the pipe from the STD there was no vacuum to be found. If he revved the van up and manually opened the STD the van sounded much better. He also added it was hard to replicate road conditions in a garage and I must do some stunt work to confirm the STD is the lack of power but he feels it is that.

2. He said the timing was so advanced that my van is surely going to kill itself soon! But when he but the timing to it's correct position the van would not run or have a severe lack of power! He has asked me to find a donor distributor to confirm this.

Question

How could the carb not be putting a vacuum out for the STD? We tried blowing and sucking the pipe from the carb but there was no blockage...

[itchyfeet]

The secondary barrel opens by vacuum once the primary is suffiiently open
So if you take the air trunking off so you can see down the carb and open the throttle ( brick on the accelerator will do this) then suck on the vac pipe to the rear vac unit ( you may need to borrow the vac pipe from the dizzy to extend it enough to suck on it) you should see the second choke open

If you can suck fresh air its perforated

Then try sucking on the dizzy vac pipe it may also be perforated you should not be able to suck fresh air

[Ian Hulley]

Contact a known breaker like Billy numbers or Mick (VW_NW) Crowe and get a formerly appreciated dizzy. I have a feeling I gave my spare spare away ... as usual.

[1_eyed_jim]

The secondary barrel opens by vacuum once the primary is suffiiently open
So if you take the air trunking off so you can see down the carb and open the throttle ( brick on the accelerator will do this) then suck on the vac pipe to the rear vac unit ( you may need to borrow the vac pipe from the dizzy to extend it enough to suck on it) you should see the second choke open

If you can suck fresh air its perforated

Then try sucking on the dizzy vac pipe it may also be perforated you should not be able to suck fresh air

Thanks for the reply. Both those tests where carried on on the std and vac advance and we're found to be in good working order. The fault lies with the carb not putting out the vacuum to the std asking for the secondary throttle to open thus lacking power.

Think I need to strip the carb down and check for blockages (again) but concentrating more on the vacuum rather than jets etc.

Thanks for the tip Ian I'll get in touch with them.

[itchyfeet]

Vacuum is created by the engine not the carb
If there isn't any vacuum its because there is an air leak downstream and the engine is pulling it in or perhaps you have very poor compression

I suppose the carb vac pipe connections have been checked for blockages?

Suspect lower gasket? This just below the vac pipe connections

Worn spindle is another culprit as is a broken pipe to the air filter mechanism

[1_eyed_jim]

Thanks I'll check those out

[itchyfeet]

Vacuum is created by the engine not the carb
If there isn't any vacuum its because there is an air leak downstream and the engine is pulling it in or perhaps you have very poor compression

I suppose the carb vac pipe connections have been checked for blockages?

Suspect lower gasket? This just below the vac pipe connections

Worn spindle is another culprit as is a broken pipe to the air filter mechanism

I have just remembered the second throttle vac feed is above the throttles so could well be a blockage as its a complex internal route


If you can take the carb top off you can check it

There s a drilling into each barrel stick your finger over each in turn and blow through the external vac connection I think the second choke drilling is smaller so has a little more resistance

holes are just above and to the right of the numbers in each barrel

[MidLifeCrisis]

Like your mechanic said I think you'll struggle to replicate the operation of the secondary throttle in the garage.
The throttle needs to be quite a long way open before the secondary will function and without the load of the van in gear you run out of revs pretty quickly. Also, I'm not sure that you'll get the correct vacuum that you need without load on the engine.
I had the same niggling doubt that the secondary wasn't working so I ended up zip-tie'ing a little camera in the engine bay looking at the secondary throttle diaphragm and recorded it while I drove along (and it worked fine). That was the only way I could think to prove that it was working (short of putting it on a rolling road ....)

(You could also get a helpful friend to sit in the back with the engine cover off and watch what happens as you drive along - but at your own risk especially re engine fumes etc ... be careful!!)

[phil300161]

I've just had the same fault on mine had the carb apart a few times tried to get some mig wire in the pipe to the secondary choke it was blocked, so I drilled it out with a tiny drill, when I rev up the engine now you can see the second choke open ok, drove it on the motorway yesterday and made it up a long hill in 5th gear

[1_eyed_jim]

I've just had the same fault on mine had the carb apart a few times tried to get some mig wire in the pipe to the secondary choke it was blocked, so I drilled it out with a tiny drill, when I rev up the engine now you can see the second choke open ok, drove it on the motorway yesterday and made it up a long hill in 5th gear

That was going to be my plan next weekend. Glad to hear someone's had the same problem as me! I'll let you know how I get on. Could you see it open when you revved it when the van was stationary?

[phil300161]

Yes reved it with my head over the carb you can see the choke open, I'm sure from the way mine drove back from Germany slowing down up hills that it has never worked before I drilled out the carb

[MidLifeCrisis]

Just note that the vacuum port goes to two separate holes (one in the primary venturi and one in the secondary venturi) as seen in itchyfeets photo from an earlier post - so you'd need to confirm that both were clear