daverclasper Posted July 26, 2018 Report Share Posted July 26, 2018 (edited) Hi. I'm always looking to improve my diagnosing knowledge. No issue at the mo. If a diaphragm fails can this be diagnosed by off taking air filter box and revving engine to compare piston lift (twin carbs). I understand its vacuum above diaphragm that pulls the piston up, if so a piston not lifting properly, could be due to diaphragm?. I this is the case, then I assumed it would cause a weak mixture, though I think I read diaphragm (god, if i have to spell that word yet again, I will lose will to live) failure causes rich mixture. Any advice great, thanks. Dave. Edited July 26, 2018 by daverclasper Link to comment Share on other sites More sharing options...
Guest Posted July 26, 2018 Report Share Posted July 26, 2018 Dave, If / when you suspect or have a diaphragm issue then the only solution is to remove the top of the carb and physically examine the diaphragm. Always worth holding up to the light to see if any pin holes are letting light through or it may be a case of the rubber hardening over the years. You should replace in pairs. Regards. Richard. Link to comment Share on other sites More sharing options...
daverclasper Posted July 26, 2018 Author Report Share Posted July 26, 2018 Thanks Richard. Car is my only transport, so any quick, , idea of what may be the problem is useful for me. So, concluding. The height of needle pistons when revving, does not vary due to failed d.........'s It was just a thought Dave Link to comment Share on other sites More sharing options...
Clive Posted July 27, 2018 Report Share Posted July 27, 2018 The car will run like a dog, very low power etc. Worth keeping a spare/s in the glovebox.... (Bizarre story alert! some years ago my dad bought and dumped a herald on my garden when I was on holiday. He thought it was a cheao car-wrong! anyway, I eventually got it sorted and it was a nice, if a tad scruffy example. An old friend, female, nt car savvy, desperately wanted to have it. She really wanted the car. A fair deal was struck, she used it for about 5 years. However, one day she broke down, and she called me. The reason, some older gentleman had seen her, stopped and just said "phone Clive". Turns out it was my Dad! I popped over to her, diagnosed the diaphragm instantly, and retrieved the spare I had put in the glovebox some years before. 5 mins later off she went.... If she needed to get hold of one it would have needed to be posted out, and her car recovered home.) 1 Link to comment Share on other sites More sharing options...
NonMember Posted July 27, 2018 Report Share Posted July 27, 2018 14 hours ago, daverclasper said: I understand its vacuum above diaphragm that pulls the piston up, if so a piston not lifting properly, could be due to diaphragm?. I this is the case, then I assumed it would cause a weak mixture, though I think I read diaphragm (god, if i have to spell that word yet again, I will lose will to live) failure causes rich mixture. Expecting a weak mixture is a common misconception. If the piston fails to lift, the mixture will be rich. The thinking that leads to the weak assumption is that the needle is lower so the jet aperture is smaller. This is true. However, because the piston is lower for very similar air flow, the depression at the needle (partial vacuum caused by the venturi effect) is greater, which sucks more fuel through. This latter effect is dominant. This should all make sense when you consider that you put oil in the dashpot. The purpose of that oil is to slow the piston's rise, i.e.temporarily do what you expected the split diaphragm to do, on acceleration, in order to provide enrichment. Link to comment Share on other sites More sharing options...
dougbgt6 Posted July 27, 2018 Report Share Posted July 27, 2018 35 minutes ago, clive said: Worth keeping a spare/s in the glovebox Spot on, Clive! Back in the 1980s I bought a pair intent on a carb rebuild. Two days later on the A4 "What the...……..??***!!!!!. A carb fault? New diaphragms still in the glove compartment, worth a try? Swapped them over, fixed! Shortest fault diagnosis ever. Went back to the dealer and bought two more, still got them, probably not much good now. Better go buy some more. Doug Link to comment Share on other sites More sharing options...
Clive Posted July 27, 2018 Report Share Posted July 27, 2018 19 minutes ago, NonMember said: This should all make sense when you consider that you put oil in the dashpot. The purpose of that oil is to slow the piston's rise, i.e.temporarily do what you expected the split diaphragm to do, on acceleration, in order to provide enrichment. And that leads to the other misconception, that thin oil is better for acceleration! Totally incorrect, for the exact reasons above. Thin oil will cause the mixture to be weak during acceleration, not good at all. Link to comment Share on other sites More sharing options...
Pete Lewis Posted July 27, 2018 Report Share Posted July 27, 2018 i keep finding if i reply on my silly tablet it doesnt post the answers,.......... lost in space ??? i agree there will be a sudden change of erratic running progressively gets worse as the diaphragm fails further,changes in piston heights is not conclusive and can vary for other reasons , if you get spares make sure they are reputable and nice and flimsy not made from recycled elephant condoms some are ridiculously tough and wont flex properly pete 1 Link to comment Share on other sites More sharing options...
dougbgt6 Posted July 27, 2018 Report Share Posted July 27, 2018 33 minutes ago, Pete Lewis said: made from recycled elephant condoms That's nothing! The shipping magnate Aristotle Onassis had bar stools on one of his yachts covered with leathered whale's foreskins. What on earth ever attracted Jacquie Kennedy to the multi millionaire? For pictures, see Pete's other web site. db Link to comment Share on other sites More sharing options...
Pete Lewis Posted July 27, 2018 Report Share Posted July 27, 2018 it was quieter when you were in sunny scotland !!!!! I tried to scupper the Jag but you got it fixed Pete Link to comment Share on other sites More sharing options...
daverclasper Posted July 27, 2018 Author Report Share Posted July 27, 2018 Thanks a lot for your replies. I did know about dash pots slowing down the piston to enrich. Must have had a funny moment!. Cheers, Dave Link to comment Share on other sites More sharing options...
Adrian Saunders Posted August 20, 2018 Report Share Posted August 20, 2018 On 7/27/2018 at 9:00 AM, Pete Lewis said: i keep finding if i reply on my silly tablet it doesnt post the answers,.......... lost in space ??? i agree there will be a sudden change of erratic running progressively gets worse as the diaphragm fails further,changes in piston heights is not conclusive and can vary for other reasons , if you get spares make sure they are reputable and nice and flimsy not made from recycled elephant condoms some are ridiculously tough and wont flex properly pete Indeed. I had a pair of dye-her-frams that wouldn’t allow the pistons to return! Link to comment Share on other sites More sharing options...
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