Hello all,
Is there a relationship between these two things? I'm fairly sure that the membrane on my accelerator pump is torn (Weber 34 DTMR), since I get a big pause on acceleration, especially if the engine revs drop too low (eg. turning into a junction in 2nd gear, or pulling away from standstill in 1st). But the idling is also very poor -- it doesn't stall, but although the tach shows 900-1000 rpm the engine sounds like it's having real trouble keeping going. When the engine is cold, I have to help with the pedal for the first minute or so, which was never needed before, and even with the choke fully closed (ie lever pulled out) the revs won't go above 1000 at idle. Used to go to at least 1500 rpm with the choke out.
I think I read somewhere that when the accelerator pump membrane is torn, that causes a substantial air leak into the carb, which throws the idle settings out. Sounds logical to me, but is it true?
The jets, fuel bowl and filter are clean, and I put some carb cleaner through everything I could get to, reassembled and got exactly the same result as before. The idle solenoid is working, and I've changed the plugs, which were basically black. The points and condenser are also new. Above 2500 rpm the car goes very well, with no hesitation or misfires.
Unfortunately, there are several other possible causes I can think of:
1- The carb is mounted to the manifold without any gasket (I didn't have one and the mechanic said it shouldn't be necessary. Wrong?)
2- The head was recently refitted after valve work, and since it had been skimmed slightly the timing marks on the cams didn't line up exactly as before. Took a bit of experimenting with an extra tooth this way or that to get the car running again, but everything seemed fine on start-up and initial testing.
3- Could it be coil-related? I'm running the original 30 year-old coil, but could the car run well at speed if that were failing?
As I wait for my carb overhaul kit to arrive (4 weeks already... think I'll have to change supplier), I'm trying to rule out the other possibilities and hoping that a new pump diaphragm and some new carb gaskets will cure this, but I'd be grateful for others' views on my diagnosis.
Regards,
Andy
Poor idling / accelerator pump diaphragm
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leo capaldi
Re: Poor idling / accelerator pump diaphragm
Hello Andy,
I am no expert but I reckon that the acceleration pump is only affected by the mechanical movement of the throttle linkage and not by vacuum so I don't think that there is a connection from the diaphragm area to the venturi. However if the diaphragm is leaking then the extra squirt of fuel may be happening too easily so the engine is "flooding" but then again maybe the effect is the opposite! Can you see the fuel squirted in when you open the butterfly quickly? Or is there a constant trickle from the little nozzle (with the engine running)? The carburettor should have a rubber "thermal insulator" between it and the manifold but the main thing would be that there is no air leakage that would definitely affect the idle.
I think that it may still be a camshaft timing thing on the basis that the choke makes little difference (ie not a fuel problem). Like you I don't think it could be the coil, but you probably have a spare and it would be easy to fit temporarily.
Not sure what you can do about the camshaft timing but I believe that the unofficial Lancia mechanic rule was to get the inlet as close to the mark as possible and have the exhaust "backwards" a tooth.
Best regards, Leo (even though you are not joining us for the Great Northern Weekend!)
I am no expert but I reckon that the acceleration pump is only affected by the mechanical movement of the throttle linkage and not by vacuum so I don't think that there is a connection from the diaphragm area to the venturi. However if the diaphragm is leaking then the extra squirt of fuel may be happening too easily so the engine is "flooding" but then again maybe the effect is the opposite! Can you see the fuel squirted in when you open the butterfly quickly? Or is there a constant trickle from the little nozzle (with the engine running)? The carburettor should have a rubber "thermal insulator" between it and the manifold but the main thing would be that there is no air leakage that would definitely affect the idle.
I think that it may still be a camshaft timing thing on the basis that the choke makes little difference (ie not a fuel problem). Like you I don't think it could be the coil, but you probably have a spare and it would be easy to fit temporarily.
Not sure what you can do about the camshaft timing but I believe that the unofficial Lancia mechanic rule was to get the inlet as close to the mark as possible and have the exhaust "backwards" a tooth.
Best regards, Leo (even though you are not joining us for the Great Northern Weekend!)
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andy b
Re: Poor idling / accelerator pump diaphragm
Ahh, the Great Northern Weekend! Quarter Scots, but sad to say I haven't been north of the wall for about thirty years now... Besides, that's not a journey for me to do while my Beta is doing 10 mpg.
I think I'll try to rule out the various carb issues before touching the timing belt. The idea of dispensing with the carb/manifold gasket is nice in theory, but the chances of both flanges being totally plane seem remote, so it's probably taking in large gulps of air through there. Still waiting for the parts to arrive...
Thanks for the advice anyway.
Andy
I think I'll try to rule out the various carb issues before touching the timing belt. The idea of dispensing with the carb/manifold gasket is nice in theory, but the chances of both flanges being totally plane seem remote, so it's probably taking in large gulps of air through there. Still waiting for the parts to arrive...
Thanks for the advice anyway.
Andy