Pdsit twin carbs, struggling to adjust idle speed

With my son on the T25 today, 2l cu engine pdsit carbs, help and advice please.

We are struggling to adjust the engine idle speed to get it below 1500 rpm and stay that way.

Using the Haynes manual procedure we have tried to get the idle down.

The large idle mixture screw on the top of left carb does little to raise or reduce idle.

The throttle stop screws do something. They are both equal.

We didn't touch any other adjustment screws.

Its an Auto Box, so has a spring/rod/locknut at the throttle balance bar. Screwing this in or out has a big effect and we got the idle down from 2500 to 1500.

Even at this idle, putting the box in Drive causes the engine to die.

Previously I adjusted the throttle cable at the pedal end to spec. It was very slack when my son got the van, so someone had been tinkering with it.

 

Have you checked for air leaks? Brake cleaner sprayed carefully at the offending areas is a good check.

 

We did have hose air leaks and replaced with new, servo vacuum was leaking and t pieces split all now replaced. anywhere else we should be checking? Specific trouble spots?

 

Sounds like you have the throttle stop screws too far in.
Screw the big speedscrew on the top right in then out a couple of turns (even if it's doing nothing).
Then slacken off the stop screws and see if the revs now reduce.

 

Thanks @Zed we will try that method.

Its a bit more complicated for us as its an auto with that bent rod from the box ending with the rod end adjuster spring setup at the carbs throttle balance bar. Turning that rod adjuster in or out has an effect on idle speed as it connects to the auto box arm which is connected to the throttle cable to the pedal.

We did slacken off the little stop adjuster screws on both carbs until no touching. It helped. Turning the big speedscrew on the left carb did very little, fully in or winding it out.

When my son got the van, the pedal was a lash up and the cable was very slack, new pedal fixed issues there and I tightened the cable to spec. So full pedal now opened carb butterflies fully.

 

I was just thinking if the big screw didn't do much then the throttle stops must be over adjusted and likewise if the throttle stops didn't do much the big speed screw must be over adjusted. So it makes sense to back off both of them. Never mind driving just yet, you need the carbs set up first before you can adjust that skanky spring loaded rod from the gearbox. Trying to do it all at once has obviously got you going round in circles.

 

You are right there. We will leave that spring loaded rod alone and do as you suggested.

We want the idle to be about 850 rpm which we cannot get yet, its about 1100 at best and 1500 at worst.

But even at the 1500 revs the engine stalls when put in D drive and it drives poorly out on the road with about 50mph down hills and mostly struggles on the flat to do 35mph. going up any incline is even worse as the auto box won't change down and even selecting 2 or 1 manually the engine lugs badly and won't rev properly.

My son thinks its carbs, I am thinking torque converter and worn clutches, the atf was dark brown and smelled burnt. I will start another thread about that. The carbs may need a good clean inside but would dirty carbs cause such poor performance?

Meanwhile we are focussing on the engine and the carbs.

 

There is also a more detailed procedure on ratwell.com https://www.ratwell.com/technical/DualCarbs.html

But you need to understand stuff. Like the five air fuel metering systems fitted to a PDSIT pair.

If throttle linkages are holding throttle butterflies open you have it over adjusted. (Sounds familiar ?)

The idle systems fitted to the main carburettors are not supposed to provide most of the idle fuel/air mix at idle, they begin work as the throttles begin opening..

Its the extra idle system on the left feeding the balance pipes that provides the idle.


I am sure that in this, fitting a Solex either side, VW and Solex discovered the nasty idle characteristics of having dual single barrel carbs (Weber ICTs or the bigger Solexes sold as Kadrons) so they hid a centre mount carburettor in there as the left hand extra idle circuit that feeds from the middle down the balance pipes..

 

Actually it's about 50/50. If you take the wire off the central idling system shut off valve it should idle at about 450rpm.
I know you've never owned one of these systems Mike, I think you should be more careful about offering your absolute advice based on a bit of pratting about at Techenders and reading on the internet. No offense intended but sometimes you have people going around in circles based on your theoretical guesswork.

 

As far as I can tell the starting point is to get the main butterflies set correctly and equally to 0.1 mm. You need the carbs off and some calipers and then adjust the throttle stop screws. Then refit the carbs and linkages ensuring the screws are still touching. If not the threaded adjustment rod will need tweaking. An experienced mechanic probably has ways round this without taking off the carbs.

Providing you’ve not touched the mixture screws on the main body of the carbs, the idle solenoid is off and it’s warmed up and chokes off, this will give an idle of about 450-55o rpm.

Then with the idle solenoid back on, extra fuel/air mix allows the idle to increase, adjusted by the big screw on the LH carb. This isn’t a mixture screw but an adjustable orifice to let air into the idle circuit. The mixture screw is the small one next to it.

 

If it were my van I would take the carbs off, but my son doesn't want to remove them so we are having to do adjustments in situ. The main butterflies look closed but we haven't done any measuring for exact gap. We haven't touched the mixture screws, only that large screw at top of LH carb and those small throttle stop screws. Chokes work correctly.

As its an autobox, the accelerator cable goes from the pedal to the arm on the autobox which is connected by that bendy shape solid steel rod to the threaded spring loaded adjuster at the carbs balance bar.

We adjusted the throttle cable tension as spec so that the autobox arm is at the lowest point, so that pressing the pedal fully pulls the arm to its maximum which is supposed to be the kickdown position. With the pedal to the floor, the carbs butterflies are fully open.

We are new to the autobox and adjusted according to the manuals.

So the pedal going down pulls the arm from bottom to top of its travel and the rod pulls the balance bar.

We seem to be stuck between getting the idle down to 850 as with a manual gearbox with cable from pedal to balance bar, and at the same time stopping the engine stalling when D Drive is engaged.

The autobox seems to apply a drag which forces a higher idle speed.

We are getting a bit despondent to say the least.

 

Its why I also point people at ratwell.. the sequence there is more detailed.

 

I dont know much about autos or that carb but if posable i would disconnect all the auto linkage stuff and just concentrate on getting the engine running right first.

 

^this. Forget the autobox for the time being.

 

That makes sense. Yes we can disconnect the solid rod at the balance bar and we don't need the pedal and cable and that way we won't be tempted to fiddle with the rod spring adjuster thingy.

So the aim is idle at 900rpm.

 

I’ve found on my auto that I like to set the idle speed higher than on a manual. I usually up it a bit so it’s just got a tiny bit of drag, enough to pull the vehicle forward slightly when I take my foot off the brake.

 

You really haven't set any of the mixture screws?
Either give up or read up is my advice.

 

I can't understand why he wouldn't want to take the carbs off, it's a 5 minute job and makes setting the butterflies an easy job.
And as said forget the auto box stuff at the moment and concentrate on the engine only, I really do think it will be clearer if you break it down into simple small steps.

 

^^and this too. These carbs aren't as complicated as people make out but setting them up needs doing in the correct sequence.

 

Ok, but what precisely is the correct sequence? Just a list of steps would help a lot.