K&N air filter, definitive for XK140

I think I’ve had a bit of a result with the K&N pancake filter search.
I was thinking that I’d seen something that would work somewhere but couldn’t think where. Then I walked past my Cooper S and remembered!
My Mini’s heavily modded 1340 Swiftune motor runs with a single HIF44 instead of the factory twin 1¼" carbs. I went onto Mini Spares website and there it was: K&N filter 56-9132 - to fit H6, HS6 etc. 1.75" SU carb, dropped version to fit under the Mini bonnet. I ordered a pair, and have just fitted them (almost) straight out of the box in 20 minutes. The extra time was because I just needed to drill two new holes in each baseplate for the air holes, as I want to use the baseplate inverted. Once done, the baseplate bolts straight up the carb with no other work. The nuts and bolts are easy to access with no bulky filter to hold. The now-redundant air holes are covered by the body of the carb and sealed by the K&N cork gasket which was in the box.
I’ve fitted the front one upside-down, to clear the bonnet flange, and the rear one the right way up to allow my arm to get under the carb more easily for other jobs. This is not strictly necessary, and I would very much welcome opinions from those who know more about airflow etc. if it would be more desirable to have them both orientated the same way.
The front cover nut on the front filter is now the one that’s a fiddle to get to, but it’s not as bad as the mounting screw on the original. As the plate is flat it’s very easy to fit to the carb now.
Photos of 56-9132 filters installed:

New holes drilled:

Front installed:

View from rear carb looking forward:

‘Stepped’ view from above:

‘Stepped’ view looking forward:

Much easier access, and massively better filtration.

An engineer’s opinion on having the filters in this stepped position would by very welcome!

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I found here on the forum, that different needles (SM) should be used when going to these pancake filters.

Thanks Peter, I’ve had a search but can’t find that reference - do you have a link to it please?

Thanks for the link Peter, that makes an interesting read. I do see that this thread was discussing later saloon HD6 carbs, so I’m not sure there’s a direct link there.
In my searches, I did find a post from David Lonsdale from 2014 in which he addressed needle choice in an XK, copied and pasted here:

John,
Have read your and other’s posts on this problem.
As I read it, you have standard H6s which have been checked over but not rebuilt. You have RF needles and K&N filters.
Firstly, setting the mixture at idle with worn needles and jets will not help you in setting the mixture correctly at mid- or wide open throttle. Jets and needles can wear due to the abrasive nature of fuel and also this can be exacerbated if the jet is not centred correctly and the needle is rubbing on the jet. IMHO you should have the carbs rebuilt with new jets. Then you can make a choice of needle. This is influenced by the inlet manifold type, head type and air filters.
RF are correct for 120 inlet manifold, A-type head and pancakes. RG for the same but C-type head. WO2s for under-wing trunking type of air filtration.

My guess is that RFs would be OK with those K&N filters. Just fit new jets, centre them correctly, and go from there.

Good luck.
Regards,
David Lonsdale
Classic Carburetters
Cirencester, UK

David was highly respected in the UK by anyone who had an SU carb, or multiples thereof. He rebuilt two sets of H6s for me, which he built with RG needles, for a 3.4 ‘C’ head with stock cams, valve size, CR etc. He advised that these would be best for the standard engine using modern fuels. We hadn’t discussed air filters, but going by the post I have reproduced above, would guess that in his opinion RG needles shouldn’t be far off for a car equipped with K&N pancake filters.

All this is just a starting point, anyway: Unless the engine is run under real-world load conditions there is no way of telling accurately whether the needle taper is correct or not. The car can drive perfectly well in normal use but if, for example, it is weak at one particular throttle opening which you happen to hold it at for 2-3 hours on a French Autoroute, you could do damage. That’s what my cars do, so I always prefer to have them properly setup on a rolling road. Once the weather improves, I’ll get that done and will report the findings for a standard XK 3.4 8:1CR engine with a ‘C’ head running on UK RON98 fuel.

Hi,

It was my understanding that needle choice is unaffected by air filters, so long as the orifice to the bottom of the piston chamber is taken from the same place as the main venturi inlet. That is generally true when they both on the inside of the filter.

Regards,
Clive.

I’m not sure.
My K&N filters came with this installation sheet, which refers to SUs:

Unfortunately that is not true. The 3.4 Litre Sedan (MK 1) was available with either an oil bath air cleaner or a wire mesh air cleaner. The shop manual indicates that an SC needle should be used with the oil bath cleaner and a TL needle with the wire screen. The reason is that the pressure drop across the air filter element has a critical effect on the fuel mixture. That is because SU saw fit to vent the tops of the float chambers to the atmosphere instead of to the air that enters the carburetor. Because of that, when there is a pressure drop across the air filter there is a partial vacuum at the carburetor input with respect to the atmosphere. While the fuel in the float bowl is at atmospheric pressure the fuel at the jet is then at a lower pressure. That pressure difference has a tendency to draw more fuel up through the jet, causing a richer mixture. This explains why the mixture gets progressively richer as the air filter gets dirty, and also why you won’t get a good tune if you adjust your jets with the air cleaner removed. That also explains why other manufacturers of carbs, including Zenith, vent the top of the float bowl to the incoming air rather than to the atmosphere. That way, no matter how much vacuum or pressure is at the carb intake it will be the same as the pressure on the fuel and it won’t affect the mixture one way or the other. I have contemplated connecting the float bowl vents to the carb intakes in order to cancel out any pressure drop across the air filter, but unfortunately I don’t know which needle would be appropriate for a carb that is effectively run with a totally non-restrictive air cleaner.

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Hi Mike,

Thanks, that certainly makes sense.

I suppose it is more complicated when the overflow pipes are placed in the airflow, that may reduce pressure with speed.

The XK150S did not change needle when they switched over from mesh to paper filter, so I imagine they are not perfect!

Regards,
Clive.

For the XK120 OTS with two pancake filters, the factory specified the RF needle for 7:1 & 8:1 compression ratio, and the RB needle for 9:1, but for the FHC and DHC with the remote muffin filter out in front of the radiator, they specified the WO.2 needle. I imagine they determined this by some sort of flow testing. The gas flow expert Harry Weslake might have been involved.
image

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I’ve always asked myself why the factory went to so much trouble creating the very complicated set-up of remote air filters, probably at a serious cost considering the extra number of parts and different materials involved. After all, the flow of air going through the front grille of a roadster or a coupe has to be pretty close, if not identical. It seems to me that the air flow going through the remote air filter could be more restricted than the pancake filters of the roadster that seem to breathe more freely. Or could it be a question of air temperature. The air going through the remote air filter has to be cooler than the pancake filters more confined inside the hot engine compartment. Whatever the reason, your post certainly established that the different air filters impacted the factory choice of needle. To make things even more confusing, there are hundreds of XK120 coupes today that use pancake air filters (my case too, since my remote air filter system is incomplete). Probably running on R.F needles instead of W.O.2
Finally a question, how do you identify metering needles from one another. I fail to see any markings on them, or am I looking in the wrong place?

The needles should be stamped on the upper end shank. Mine are. You have to remove them from the piston to see it.
With the XK120 FHC/DHC remote filter I suspect the reason was improved quietness.
At least this was the reason for the big filters on saloons.

As Rob says, you have to remove the needle from the damper piston to see the markings.
I seriously doubt that many cars are running on drastically incorrect needles these days. I am sure no professional restoration company (or at least none of those I have ever dealt with in the UK) would deliver a restored car to a customer without tuning the engine under load, which means some sort of either dynamometer or rolling road test - and I think it’s unlikely that such a test would suggest the retention of whatever needle profile Jaguar, Austin Healey, Triumph etc. chose nearly seventy years ago. Many engines are not in absolutely original specification, and the fuel that will go into them most certainly isn’t. I’ve had all of my restored cars of any make undergo a final rolling road session before serious road use, and none of them came away with the original factory-recommended needles (for SUs) or jets (for Autolite, Weber or Dell’orto) carburetters. You can’t set a carburetter tune at idle and then assume it will stay at the correct AFR under load throughout the rev range.

Bernard,

If you cannot find any letters stamped in the (thicker) upper end of the needle, it’s possible to measure what needle you have. The profile of every needle has been specified by SU. They used incremental steps of 1/8 of an inch starting from just under the thicker upper end (shank) of the needle.
The H6 carbs use 0.1 " thick needles (smaller carbs use 0.09" and larger carbs even 0.125" needles). The 0.1" needles are a bit longer than 0.09 "needles and therefore have 14 measuring points (instead of 13 for the thinner needles)
If you have some time left… :wink: it’s possible to measure your needle in this way. See the tables below for all (most?) SU needles for the H6 carbs.
These surveys are also helpful in seeing the difference between recommended needles e.g. are they richer in the middle section (cruising & pick up) or only towards the bottom of the needle (top speed).

Bob K.

A good list, Bob! Most carb tuners will take these as a starting point.
The late and very much missed Peter Baldwin, ex of Marshall’s Cambridge and Wilshers rolling roads etc., would try needles from his vast collection until he found a close one which he would then polish until he got the figures he wanted. Colin at Southam Mini Metro did the same with the needle in my 1340 Mini’s HIF44.
I’d say the taper profile has more to do with throttle opening (acceleration) than speed.