[Healeys] SU Dashpot puzzle

[Perry]

Simon 
I thought I had written to you about the “shoulder” issue previously but I don’t see it in my mailbox. 

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.  

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.

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.  

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. 

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. 
Perry

Sent from Mail for Windows 10

From: simon.lachlan at alexarevel.plus.com
Sent: Sunday, October 6, 2019 1:33 PM
To: 'Healeys'
Subject: [Healeys] SU Dashpot puzzle

Here’s one for the technoctats:-
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.
So, I swapped the offending two and that gave me three HD6s, all with one shoulder.
Car started on the choke, same as normal. Seemingly!
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.

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.
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. 
Adjusted down quarter of a turn and all seems well.
Questions:- beyond the obvious – ie that changing dashpots does make a difference after all – what is going on? Why was there a change?
(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).
Any ideas?
Simon

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://autox.team.net/pipermail/healeys/attachments/20191006/c1beca7a/attachment.html>