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Bob Reynolds
Frequent User
Username: bobreynolds

Post Number: 87
Registered: 8-2012
Posted on Sunday, 18 May, 2014 - 01:19 pm:   Edit PostDelete PostView Post/Check IP

Why did Rolls-Royce come up with their complicated system of weakening the fuel mixture, instead of just using a different needle profile in the carburetter? For what it does, this system seems horrendously over-complicated, takes up a fair amount of space and reduces access to various engine components. With a maze of pipes, valves, filters, venturis, solenoid valve, thermal switch, fuel receiver, and at least 16 hose connections; I would think reliability is bound to be significantly compromised. You only need a slight air leak at one of the hose connections to play havoc with the mixture, and it’s a fault you would not easily find.

Up until recently, I wasn’t aware that the weakener system on my car wasn’t working, but I found that the 12v supply to the valve had been disconnected, so the valve never operated. The car seemed to run perfectly. I re-connected the supply and I couldn’t tell any difference. Ok, it could be that there is another fault that is preventing it from working, but it has made me wonder if all this maze of tubing and equipment on top of the engine is really necessary, or is it just Rolls-Royce’s way of overcomplicating things for the sake of it? I have owned and worked on many cars with SU carburetters, and they have always been delightfully simple to work on and rarely give trouble when properly adjusted. I have never come across such a clunky system as this weakener system. I understand how the system works (just) what I don’t understand is why RR didn’t just use a different needle profile to do the same job. Was that not possible for some reason? Have I missed the obvious?

It is interesting to note that if you type ‘fuel weakener’ into Google, you only come up with Rolls-Royce sites. So, as far as I can tell (and I could be wrong) no other car manufacturer used this complicated and clunky system to artificially weaken the fuel mixture.
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 1199
Registered: 6-2006
Posted on Monday, 19 May, 2014 - 02:39 am:   Edit PostDelete PostView Post/Check IP

Because it allows the mixture to be richened when the air temperature is cold when you need it but weaker when you don't and lots of occasions inbetween. Just relying on a choke flap or jet position will not allow that.

Weakener function also decreases under heavy acceleration / open throttle periods Giving better acceleration especially when cold.

It increases it's weakening under shut throttle and engine braking. This gives less emissions on a choke on cold start in a non cold environment. Better fuel economy when cruising. Effectively cuts off the fuel on engine braking.

Don't forget it also cuts fuel as soon as the ignition is turned off. No matter how fast you are revving the engine will die instantly instead of 'dieseling'

There is no jet profile that could do this.

Two shut off valves (can stick), an otter switch (never replaced one), and an adjustable air bleed (can get dirty). Pretty simple really - although they can give problems. They do so rarely.

The BIGGEST problems I ever get with them is human intervention. Same as with the rest of the SU carb set up. 99 times out of a hundred, problems are caused by people tinkering with them!!

Most cars have a manual choke which is something a Rolls-Royce owner should never need. Other Auto choke SU set ups used auxiliary carburettors . . . if you want to look at complicated and unsuccessful systems check them out. More often than not these owners fitted weakeners which just broke the vacuum. Or a manual choke conversion.

Maybe I am just to fond of R-R's and need to get out more ! LOL.

Anybody know the link between shoes and the invention of SU Carbs ?
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 1200
Registered: 6-2006
Posted on Monday, 19 May, 2014 - 02:41 am:   Edit PostDelete PostView Post/Check IP

I hate this sites preview / post functions!! lol
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Bob Reynolds
Frequent User
Username: bobreynolds

Post Number: 89
Registered: 8-2012
Posted on Monday, 19 May, 2014 - 03:21 am:   Edit PostDelete PostView Post/Check IP

Thank you for your comprehensive and detailed response. I am not totaly convinced yet, mainly because everybody else seemed to manage without such a system. But I will not argue with any of your points at the moment.

"Anybody know the link between shoes and the invention of SU Carbs?"

Leather bellows?
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Geoff Wootton
Grand Master
Username: dounraey

Post Number: 401
Registered: 5-2012
Posted on Monday, 19 May, 2014 - 03:22 am:   Edit PostDelete PostView Post/Check IP

I know the Skinner brother were shoemakers before inventing the SU carb. Not sure how the two were linked - did they use leather in the early carbs - just a thought.
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 1201
Registered: 6-2006
Posted on Monday, 19 May, 2014 - 03:42 am:   Edit PostDelete PostView Post/Check IP

Ahhh Bob firstly remember that this is a Rolls-Royce - you can't accept the problems, you need to find away around them. Just because other manufacturers 'made do' doesn't wash at Crewe!

Plus by about that time most manufacturers were working on fuel injection systems or were already using them. Indeed Shadow II's designed for strict emission areas already had FI fitted as standard. What a Joy that is.


Spot on with the Lilley and Skinner link - although I have no idea if shoe leather was used :-)

Some useless info:

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CG8QFjAG&url=http%3A%2F%2Fdloc.org.nz%2Fattachments%2F088_Magazine-2013-Oct-Nov.pdf&ei=i954U5b9I8itO9aogKgF&usg=AFQjCNFt2Uo3s1pWgTkLOjt2_-oYXHiQgQ&sig2=ZhoZGh0Kwm1NR77rPvQ1Hw
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Brian Vogel
Grand Master
Username: guyslp

Post Number: 806
Registered: 6-2009
Posted on Monday, 19 May, 2014 - 03:49 am:   Edit PostDelete PostView Post/Check IP

Paul Yorke wrote: . . . remember that this is a Rolls-Royce - you can't accept the problems, you need to find away around them. Just because other manufacturers 'made do' doesn't wash at Crewe!

That's indeed true. It's also true, though, that quite a few of Crewe's "solutions" were Rube-Goldbergian in nature and induced a raft of problems of their own!!

I love my Rolls-Royces but, now having worked on these, my Jag, and multiple GM products I long ago stopped "worshiping at the Temple of Crewe" and realized that their engineers didn't always succeed in reaching their intended goals. Calling these cars quirky and, in many cases, needlessly overcomplicated is an understatement.

Brian
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Bob Reynolds
Frequent User
Username: bobreynolds

Post Number: 90
Registered: 8-2012
Posted on Monday, 19 May, 2014 - 05:36 am:   Edit PostDelete PostView Post/Check IP

"Calling these cars quirky and, in many cases, needlessly overcomplicated is an understatement"

I fully agree. This is why I still question if this weakener system is really necessary.

There is a well-known saying in the enginering industry, which I'm sure you've all heard: "Keep it simple - stupid" (KISS). Over-complicated engineering is now classed as bad engineering. The simplest and most elegant solutions are always the best.

As Brian says, in trying to solve one problem you can create a whole host of other problems which then need their own solutions, and introduce another level of unreliability. In your quest for perfection, it is very easy to lose the plot and forget about what you were trying to achieve in the first place.

Of course, there is also the opinion that Roll-Royce deliberately made things complicated and unusual in order to deter owners from tinkering with the cars, rather than taking them to a RR dealer who would then be able to charge them a fortune for simple repairs.

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