1.9DG Hesitation on Pull Away

[keynsham1]

This seems to be an age old problem and I have read a lot about it. I have a 1.9DG in completely standard form. Over a period of time it has developed a hesitation when pulling away. It idles perfectly and once going, runs like a dream. But if you press the throttle pedal down slowly, the engine dies. If you push it hard, it stutters a little then revs normally. It means I have to rev the engine every time before pulling away to stop me from stalling.
I have changed the plugs, the CDI unit, the coil, the leads, I have adjusted the accelerator pump, I have cleaned out the pilot (idle/slow running) jet, and none of this has made any difference at all. I haven't checked the timing yet and plan to do so, but I would be surprised if that was the cause.
There are no obvious air leaks that I can see. My vacuum advance on the distributor seems to be inoperative, but even when blanking this off on the carburettor there is no improvement. My next move is to remove the carburettor and give it a good clean, but I always like to find something definitively wrong when faced with this type of problems so know I have fixed it.

Has anyone who has had a similar problem, actually found what the cause of this could be?

[MGP]

The vac advance unit is there to advance the timing on pull away, get the vac advance unit working and you will most likely find the hesitation disappears

Sent from my SM-G920F using Tapatalk

[keynsham1]

I have blocked the vacuum advance off at the carburettor to avoid air leaks. It doesn't function at very low throttle as there is not enough vacuum, so in theory it shouldn't make any difference. It is a fault though that should be corrected. Anything is worth a try! Thanks.

[keynsham1]

Having done some more reading, I might be completely wrong there!

[marlinowner]

Yes you are back to front, max vacuum is when throttle is closed.

[kevtherev]

Suck on the vac advance pipe. You should be able to maintain vacuum by putting your tongue over the pipe once the full advance is made by sucking.
If you can't then the diaphragm is punctured.
Which would also weaken the mixture, a weak mixture would also stall an engine.

[keynsham1]

The diaphragm is definitely had it. I can constantly blow or suck on the pipe so it must be damaged. Anyone know a good place to get a replacement?

[ajsimmo]

Powerspark

Sent from my B1-830 using Tapatalk

[keynsham1]

I have replaced the vacuum advance unit with a new replacement. There is no change in the hesitation problem. My next move will be to adjust the idle mixture screw. I have read that this can cause the same symptoms. The process as far as I can see is to set the idle to the correct speed and then turn the screw in until the revs drop, then turn it out. The revs should climb and then drop again. The ideal setting is where the revs are highest as this is where the fuel quantity best suits the airflow. At this point the idle needs to be reset. An exhaust analyser can then be used to check the emissions.

[kevtherev]

Ok now that timing hss been ruled out...
The pierburg has a transfer jet.
When you press your foot on the acc. pedal
The accelerator pump delivers a metered amount of fuel
To compensate for acceleration lag as the idle jet gives up to the main jet. And the full throttle stamp that gulps in too much air for the jet to cope with.
This metered amount can be measured and adjusted.
Read CJH post in a search for pierburg carb refurbishment
When depressing the pedal you should see a good healthy squirt of fuel into the main venturi

[keynsham1]

Is the amount of fuel supplied by the accelerator pump critical? I ask as there is very little adjustment available. H ave adjusted it in both directions with no change but maybe I am over-richening it and the same symptoms are occurring?

[kevtherev]

Yes it has a set value
One pump should deliver 1.5ml
If you find the CJH post he eliminated the hesitation you are experiencing

[kevtherev]

Here you go read this its long but very informative

https://club8090.co.uk/forum/viewtopic.php?f=37&t=147770" onclick="window.open(this.href);return false;

[CovKid]

Its also more than possible, being as you replaced vac unit with no improvement, that the plate in the dizzy that vac unit it is attached to is completely solid. If it is, it will have terrible acceleration. It should move in response to engine suction (ie throttle movement). Or, a combination of things.

[keynsham1]

Last night I checked the vacuum take-off for the servo and that was OK. I also took the carb off to give it a good clean with an airline. I discovered that the choke pull-down unit seems to be faulty. I took it off and put a pipe on it. I can blow and suck down the pipe and it acts as if the diaphragm is split ie. nothing happens. However if I push the plunger in a few mm it seems to work as it should. I am guessing that this is wrong and that this is causing an air leak into the carb as the vacuum pipe attachment goes straight on to the carburettor. Could this be my problem? I have ordered a replacement from Brickwerks. I will try just blocking the vacuum pipe off completely later to stop the air leak path and see what happens?