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Marty
Unregistered guest
Posted From: 211.30.122.24
Posted on Tuesday, 21 February, 2006 - 08:35 pm:   Edit PostDelete PostView Post/Check IP

Gidday all,

Got myself a new book on Bentley's today. In it they talk about changing the needles from SC at B2BH, then to SP at B478NZ, to improve engine power and cooling.

A richer needle? Anyone comment on this? I haven't checked yet, but assume mine, (B256MD) has SC, so a quick trip down to midel to acquire some SP needles is in order.

CHeers

Marty

(Message approved by david_gore)
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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 961
Registered: 4-2003
Posted on Tuesday, 28 February, 2006 - 04:03 am:   Edit PostDelete PostView Post/Check IP

It depends on a few things, mainly carburettor size and air cleaner type.

Early cars have a pair of 1 1/2" SU carburettors. The camshaft changed markedly at B2BH, changing from a high lift to a long duration type, and that requires a change from an LB1 needle with a Bentley air cleaner to a richer SC with an AC air cleaner. The AC air cleaner along with the low-lift camshaft means that air flow losses are reduced. The consequently lower vacuum generated at the needles makes the mixture too weak, hence the richer SC needles.

For the larger 1 3/4" carburettors on cars from B83HP, air flow losses are again reduced, so the needles went again up to a richer SJ with an AC air cleaner. However, with the new oil bath air cleaner for some export cars, all Australian cars included, air flow is more restricted so the much leaner SF is employed in those cars.

With the introduction of the 4 1/2 litre motor, AC air cleaners determine an even richer SP needle. York Motors always fitted SFs with the oil bath air cleaner on all these motors, 4 1/4 and 4 1/2 alike. I have, however, switched to intermediate SN needles (richer that SF but weaker than SJ or SC) on my R-Type with an oil bath air cleaner, believing that they are kinder to the valves.

Anyone needing a chart of all the 0.1" jet needles and profiles at 14 points in the shank may email me and I'll send a 441kb pdf file with all the information.
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Marty
Unregistered guest
Posted From: 211.30.122.24
Posted on Monday, 27 February, 2006 - 07:06 pm:   Edit PostDelete PostView Post/Check IP

Okay, the soup thickens. My car has SN needles. Factory? Comments?


Marty

(Message approved by david_gore)
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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 962
Registered: 4-2003
Posted on Tuesday, 28 February, 2006 - 09:55 pm:   Edit PostDelete PostView Post/Check IP

Marty,

The difference between SN and SP is only in the high-demand range, the SN being slightly leaner. Unless you are driving at 100mph to a wedding, there will be no effect whatsoever. See the table below, the dimensions being inches as you measure down the needle shank. Your car probably would originally have had SP needles, being a 4.5 with an AC air cleaner.

Unless the needles are worn, leave well alone I suggest. Just keep them tuned. If replacing the needles, I would go to SP, ST or SU needles, but the differences are very small. Other factors have a far greater effect than these very slight differences.

RT.

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Marty
Unregistered guest
Posted From: 211.30.122.24
Posted on Tuesday, 28 February, 2006 - 03:49 pm:   Edit PostDelete PostView Post/Check IP

Thanks Richard. I will stay with the SN's for now, maybe try and do a plug chop and see what I get. I have adjusted the position of the needles minutely, as they where standing a little proud of the piston, meaning they would poke a little further into the jet, only half a mill or so. Have relocated them flush, will take it out on the weekend and see if any difference.

Marty

(Message approved by david_gore)
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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 965
Registered: 4-2003
Posted on Thursday, 02 March, 2006 - 02:30 am:   Edit PostDelete PostView Post/Check IP

Great, Marty.

As you have so many cars with SUs, I expect you know precicely how to set the balance and mixture.

Anyhow, for completeness, after balancing the butterflies (I use the method with a plastic hose and water in a jar or by listening since a friend swiped my flow meter), adjust the mixture leaner until the idle just becomes rough. Then richen it by 1/2 turn. Repeat this twice on each carburettor in turn.

Let me know if you have some more tips short of using a CO monitor for fine tuning. That's an exercise demanded by some authorities, but useless in my opinion as it is irrelevant apart from when idling.

The idling CO level says nothing about the emissions when driving, but I must pay out $100 for that stupid statutory test each year for each car I own.

RT.
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Great48 - Ron Lewis
Unregistered guest
Posted From: 1.144.97.77
Posted on Tuesday, 13 December, 2016 - 06:02 am:   Edit PostDelete PostView Post/Check IP

Hi, everyone:

I have fitted a paper air filter to my standard Mark VI air cleaner/silencer on B146BH in lieu of the normal steel mesh element (the paper element OD is just slightly bigger, but it sandwiches nicely - and air-tight - between the air silencer body and the removable end cover).

My query is does anyone know of a satisfactory SU needle type for use with a paper air cleaner element on the 4.25 Litre engine? My motor has started running rich (exhaust is now a bit sooty) since the swap, which I assume is due to the lower manifold pressure created when dragging intake air through the paper element, as compared to drawing air through the more open (i.e. less resistance) steel mesh supplied as standard.
Any help would be appreciated, as the car doesn’t always accelerate as cleanly as it did before altering the air cleaner – it isn’t a definite misfiring situation - just runs a bit “piggish” when pulling away sometimes. Ignition is electronic and timing is spot on the mark, so I don’t believe the electrics are to blame.

Ron

Posted by Moderator in response to request from member unable to post direct.

(Message approved by david_gore)
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 451
Registered: 4-2015
Posted on Tuesday, 13 December, 2016 - 10:06 am:   Edit PostDelete PostView Post/Check IP

Perhaps my recollection is defective since it has been years since fussing with the twin SU carbs on a '55 Silver Dawn...same set up as your Bentley presumably...but is there not a adjustment screw to simply fine tune the mixture?
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Bob Reynolds
Grand Master
Username: bobreynolds

Post Number: 438
Registered: 8-2012
Posted on Tuesday, 13 December, 2016 - 08:20 pm:   Edit PostDelete PostView Post/Check IP

That screw is only to set the mixture at idle.

In order to change the mixture over the entire throttle range, you need to change the needle profile.

Use the SU Needle Charts to select a needle slightly different from your current one, over the range you want to adjust.

Or put the original air filter back!
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Christian S. Hansen
Grand Master
Username: enquiring_mind

Post Number: 452
Registered: 4-2015
Posted on Tuesday, 13 December, 2016 - 08:55 pm:   Edit PostDelete PostView Post/Check IP

Ron...
The mixture issue aside, and you seemed to imply that the problem was noticeable only when pulling away from a stop which is for all intents and purposes at the "idle" end of the curve and thus maybe the idle mixture could have an effect, so worth at least playing with, I am surprised that the paper air filter (K+H?) restricts the air flow to the extent as to enrichen the mixture. Have you confirmed this by removing and replacing the paper filter a few times to confirm the filter as the correlating cause of the issue?
At one point I was going to switch to the K+H pleated paper filter simply due to the perception that the original metal gauze was not the least bit efficient with filtering dust particles, certainly not to the extent that modern filters do and thus the benefit to the longevity of the engine if operating in dusty conditions. However, at the time I was blown down by the cost (circa $75 as I recall) and never got around to it but also rotated away from the Dawn anyway so it is moot point until the day comes that I use it again. Given that the use of a paper filter is probably desirable, I will be interested to hear how you address the mixture issue. Thanks!!

.
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Great48 - Ron Lewis
Unregistered guest
Posted From: 1.144.97.102
Posted on Wednesday, 14 December, 2016 - 06:21 am:   Edit PostDelete PostView Post/Check IP

Thanks Christian and Bob for the comments.

Will try removing/refitting the air cleaner when running the motor and will check the needle chart dimensions to find a needle, at least as a good starting point.

Will advise the outcome when done.

Ron

(Message approved by david_gore)

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