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--></style></head><body lang=EN-US link=blue vlink="#954F72"><div class=WordSection1><p class=MsoNormal>Simon </p><p class=MsoNormal>I thought I had written to you about the “shoulder” issue previously but I don’t see it in my mailbox. </p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The “shoulder” is really where the dash pot vent is located. Some SU’s have none, but have a vented cap, some have one internal vent and some have two. Since this vent is a connection to suction chamber for the small chamber above the oil damper just under the cap, it would appear that the use of more than one vent has a bearing on the efficient operation of the SU on that engine or application. </p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>When the engine is cranking or running, the flow of air through the SU carb body reduces the pressure (lower than atmospheric, suction) in the suction chamber or dash pot. On most SU’s there is a set of holes above where the carb and air cleaner are joined. Those holes ultimately connect to the inside of the suction chamber. That reduced pressure slightly raises the piston and richens the air fuel mixture. Much like our 100 starts much easier when cold by not only pulling the choke by lightly pressing on the gas pedal.</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>I have searched through many SU manuals and various write-ups on SU carbs and this particular aspect of the SU’s does not seem to be a topic of concern or discussion. Suspect that the vents, both size and number, have a lot to do with meeting the design specs for the application. The piston drop is one that comes to mind. How fast does the piston fall to idle when the driver is off the throttle? The spring size probably is the biggest factor but pressures inside and outside the carb has some bearing. </p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The Engineer in me wants to know the answers but the retired guy in me says I don’t need to know. What I do need to adhere to is the original design of the parts. IE: If there is a gasket on the air cleaner /carb interface originally, is it there and fit correctly? If there are holes on the carb body, either for fuel or air flow are they clean/unobstructed. </p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Whether your HD6’s had a single or double suction chamber vent, I have yet to find the answer. I do suspect that if they are supposed to have single vents but now has double vents, something may not work as designed. </p><p class=MsoNormal>Perry</p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Sent from <a href="https://go.microsoft.com/fwlink/?LinkId=550986">Mail</a> for Windows 10</p><p class=MsoNormal><o:p> </o:p></p><div style='mso-element:para-border-div;border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal style='border:none;padding:0in'><b>From: </b><a href="mailto:simon.lachlan@alexarevel.plus.com">simon.lachlan@alexarevel.plus.com</a><br><b>Sent: </b>Sunday, October 6, 2019 1:33 PM<br><b>To: </b><a href="mailto:healeys@autox.team.net">'Healeys'</a><br><b>Subject: </b>[Healeys] SU Dashpot puzzle</p></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span lang=EN-GB>Here’s one for the technoctats:-<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>You may have noticed my posts about swapping from two-shouldered to single-shouldered dashpots. I did it because having two of one and one of the other was irritating, not because I thought there’d be any changes.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>So, I swapped the offending two and that gave me three HD6s, all with one shoulder.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>Car started on the choke, same as normal. Seemingly!<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>However, doing the garden hose in one ear trick revealed that the rear carb, always single shouldered, was making less intake hiss than the other recently added pair. Easily rectified.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-GB>I took the car out this afternoon. Started fine and ran well. But, if I eased off, the revs dropped and the engine cut out. Also, there was quite a lot of popping and f**ting on the overrun.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>So, on my return, I gave the front two carbs about half a turn up on the slow idle screws. This, quite small adjustment really, seemed to cure it and even to raise the idle rather higher than I like. <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>Adjusted down quarter of a turn and all seems well.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>Questions:- beyond the obvious – ie that changing dashpots does make a difference after all – what is going on? Why was there a change?<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>(Plainly, I swapped needles and springs when I swapped dashpots so I’ve got all the same in the carbs….the ones that I had all along).<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>Any ideas?<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>Simon<o:p></o:p></span></p><p class=MsoNormal><o:p> </o:p></p></div></body></html>