Poor idling / accelerator pump diaphragm
Posted: 02 Apr 2004, 10:59
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
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