SU minutiae

Technical advice Q&A

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jag68
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#1 SU minutiae

Post by jag68 » Sat Oct 20, 2018 11:38 pm

When I give my engine a bit of quick throttle at low rpm there is an immediate but short flat spot, which can become quite irritating when driving at slow city speeds. It typically happens when you back off the throttle to zero, then try to accelerate back to the speed you were going. Typical close city traffic driving in other words. These are triple SU's on a 4.2l modified engine - cams, porting, big valves. My question concerns the construction of the damper. It has a loose fitting brass sleeve with a washer on top.This permits a quick rise in the piston of about 1/8 to 3/32 of an inch before the sleeve is forced up to the top stop and partially seals the damper to delay rising further. You can easily demonstrate this quick rise by using a screwdriver to raise the piston. It seems to me that this undamped quick rise could create a flat spot at low throttle, as it would likely kill the vacuum over the bridge. Does anybody have any insight on why the damper is built this way?

Terry
1967 E Type coupe
1968 E Type OTS
2007 XKR

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PeterCrespin
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#2 Re: SU minutiae

Post by PeterCrespin » Sun Oct 21, 2018 9:49 am

“Does anybody have any insight on why the damper is built this way?”

No special ‘insights’ - AKA opinions - sorry, but a few facts might be more germane:

1: Every HD/HS carb is made that way.
2: They were standard fitment to all manner of XK engines.
3. The XK in standard form is famous for torque from low revs.
4: Apart from the very first examples, every XK used effectively the same cam.
5: You have changed the cam, which involves pushing torque up the rev range.
6: You now have a low rev tuning issue.

The last thing I would concern myself with is why SU made their carb that way, as it was obviously a very good and ultra-versatile design; handsome is as handsome does. You could try thicker oil to richen the transition, or start the sort of induction optimisation program required by the changes you have made, or you could put the Jag cam back in. I’d be interested to read cam spec, carb jetting and other changes, if any.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 79 S2 XJ12L; 97 XJ6L

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#3 Re: SU minutiae

Post by jag68 » Sun Oct 21, 2018 5:10 pm

Thanks Peter. I've owned this car since 1972. For most of the time it's run with the stock engine. It has always had this problem. I've also owned another 4 E Types, driven many more, non of which had this issue. I'm not convinced that the state of tune of the engine has anything to do with this. It seems peculiar to this set of carbs. They have been rebuilt, using new dampers. They have 200,000 miles plus on them, and I believe the inner bores of the steel tubes that contain the dampers are worn. I say this because I need gear oil in them to generate the resistance to lifting them that I see in my '67 which has far fewer miles and light weight oil in the dampers. Going to the heavier oil helped as has adding an additional washer to the damper to reduce the "free play". It's almost eliminated the problem. I'm going to fit another washer sufficient to make the damper unmovable and see what happens. My query was only directed towards a reason why the free play? I'm quite familiar with how SU's work, but I've not been able to find what the "loose" damper has to do with it.
1967 E Type coupe
1968 E Type OTS
2007 XKR

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PeterCrespin
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#4 Re: SU minutiae

Post by PeterCrespin » Tue Oct 23, 2018 10:51 am

OK, lots of new info that changes the focus entirely and closes off many lines of enquiry.

Sounds like worn carbs but I’d suspect the soft metals that run largely dry before I worried about brass on steel running in oil.

Non-engineer opinion follows, based on the empirical evidence and WYSIWYG principle:

The damper is obviously designed to restrict movement, not eliminate it. If the piston were fixed to the rod there would be hydraulic lock depending on closeness of fit.

Instead the piston movement on the rod makes it into a shuttle valve that permits oil to flow past the piston in a controlled manner, thus damping the relative motion of the moving air valve along the stationary damper rod. Depending on the gaps/spaces within or through/around the piston or a reed-type valve etc, the damping can be one way, two way, fixed or variable, as in telescopic suspension dampers.

So the answer to your original ‘why’ question is ‘because form follows function’.

As for a solution, you could replace worn parts or you could spring for a new set of carbs and hang the old ones on the wall over a plaque “They died with their boots on.”
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 79 S2 XJ12L; 97 XJ6L

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#5 Re: SU minutiae

Post by jag68 » Tue Oct 23, 2018 10:38 pm

Thanks Peter, but my question is why the free play - why is is designed to have 1/8 to 3/32 of upward movement free of dampening.
1967 E Type coupe
1968 E Type OTS
2007 XKR

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#6 Re: SU minutiae

Post by jag68 » Tue Oct 23, 2018 11:58 pm

Perhaps I should add more to my question. All carburetors need some method of increasing fuel flow when there is a demand for acceleration. That is the function of the damper in a constant depression carburetor. By slowing the rise of the piston in the face of a opening throttle, and thereby temporarily restricting the gap between the bridge and piston bottom the damper causes an increase in the speed of the incoming air creating a greater depression across the bridge, which in turn lifts more fuel into the air stream. At low throttle openings a sudden jump in the piston of 1/8 of an inch or so, caused by free play in the damper may literally double the volume of the gap, initially slowing the air and reducing the depression. Intuitively it would strike me that this is inimical to the primary function of the damper. I'm in the process of shimming the damper to reduce free play, to see if that will reduce or eliminate the gasp. So far it seems to help. Perhaps I should end the inquiry with that, but I had hoped somebody would have an explanation of why there is free play in the first place.
1967 E Type coupe
1968 E Type OTS
2007 XKR

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christopher storey
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#7 Re: SU minutiae

Post by christopher storey » Wed Oct 24, 2018 8:50 am

I am inclined to think that this is a classic case of the very old saying that "99% of SU problems are in fact ignition related". The symptoms you describe sound suspiciously to me like retarded ignition at low rpm . Only a few degrees or retard are needed to produce this stutter. I should try 2 or 3 degrees increase in the static BTDC setting and see what happens

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#8 Re: SU minutiae

Post by PeterCrespin » Wed Oct 24, 2018 4:45 pm

jag68 wrote:
Tue Oct 23, 2018 10:38 pm
Thanks Peter, but my question is why the free play - why is is designed to have 1/8 to 3/32 of upward movement free of dampening.
Because it’s inherent in shuttle valve design?

Again, if there were no free play there’d be no shuttle valve. I.e. no passage of oil and no damping - just hydraulic lock. The loose brass piston and usually one or more smaller brass rings above it, is restrained between a fixed top collar (tapered & self-centering to enter and block the orifice) and an external C clip in a groove.

This involves extra parts, extra machining and hand assembly. A fixed piston could have just been pressed on and over-peened. That option might have pleased the SU bean-counters by generating extra profit, reducing parts inventory and cutting labour costs, but even they would have conceded that removing all free play was not desirable, due to the no-longer-able-to-shuttle valve having the minor niggle of a carb that wouldn’t run beyong idle.

Given that some axial play is therefore required in this design, I’d posit that slight air slide free movement is a consequence of, rather than design goal of, this damping systrm. I daresay a carb maker, familiar with fine drilling technology, could have designed a fixed piston with a very small hole or holes to control its movement through oil, thus obviating all slack at a stroke (‘scuse pun). However, such a system of narrow passage(s) would be unavoidably more sensitive to oil viscosity, courtesy of Doctor Poiseuille. I’m pretty certain it would be less reliable in service, given that fine passges block more easily many owners, me included, occasionally service their carbs in camp grounds or petrol stations on long trips, where absolute cleanliness or airborne dust can’t be controlled.

Meanwhile, since nobody else here reports the same issue with a well set up engine, there remains the real possibility that SU deliberately built in a specific amount of free initial movement because it helped matters.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 79 S2 XJ12L; 97 XJ6L

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#9 Re: SU minutiae

Post by jag68 » Wed Oct 24, 2018 7:36 pm

Thank you gentlemen.
Chris. I agree with the old saying - it has certainly been my experience. I have an EDIS system in the car so I can make those changes simply - just need to borrow an old lap top to do it. The Mega Jolt is very old technology.

Peter. I think that the piston locking theory is a good one, I hadn't thought if that. However I think it works in reverse of your suggestion. As the piston rises it pushes the "shuttle" up to seal against the washer and the taper of the post, restricting oil flow and slowing the piston rise. I suspect the design is actually to effect a quick piston drop when the throttle is closed or reduced. When the piston is descending the shuttle will be down and away from the taper, facilitating greater oil flow around it, or more properly through it. The point is that you need the free play for proper operation which is what my question was about. I appreciate that it was there for a reason but I still needed to know. Thanks
1967 E Type coupe
1968 E Type OTS
2007 XKR

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#10 Re: SU minutiae

Post by PeterCrespin » Wed Oct 24, 2018 9:56 pm

jag68 wrote:
Wed Oct 24, 2018 7:36 pm
Peter. I think that the piston locking theory is a good one, I hadn't thought if that. However I think it works in reverse of your suggestion.
Hello Terry. I am honored to be competing against Terence Sturgeon Esq., the artful Gentleman Racer of Victoria, that jewel in the North Pacific.

However, whilst I agree with your agreement, I'm too old and too crafty to buy your dummy manoeuvre, deep into the braking zone for La Source... :swerve:

Since I didn't really go into SU directionality, I'm not sure how you can claim your idea is the reverse of mine (largely because it isn't). I mentioned in post #4 that a shuttle valve could be engineered to provide damping in one or other direction, or both. Because of the inherent viscosity of oil, the SU damper works in both directions to stop air slide flutter, albeit more in rising than falling (which is how you'd want it, or it would tend to 'ratchet' the slide down, like suspension with soft springs and too much rebound damping). If in doubt, try unscrewing and pulling a damper out quickly from a dry carb and again from an oil-filled damper.

The second mention came in post #8, referring to the piston movement being restrained between a fixed tapered collar and a circlip. Fixed means immovable, so they come together only on the upstroke when the piston slides up the damper rod to meet - and be blocked by - the fixed collar. There is some residual downstroke damping as oil runs past the open fixed collar and down the inside of the piston past the circlip.

All together now, to the tune of 'Dem Dry Bones': "The head is connected to the carb body; the carb body's connected to the dashpot; the dashpot's connected to the damper, now hear de word of de lord!" :dance:

Well, it beats oil pressure and tyre threads...
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 79 S2 XJ12L; 97 XJ6L

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Series1 Stu
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#11 Re: SU minutiae

Post by Series1 Stu » Thu Oct 25, 2018 5:25 pm

Well, it beats oil pressure and tyre threads...

Brilliant!

:swerve:
Stuart

If you can't make it work, make it complicated!

'62 FHC - Work In (slow) Progress
'69 Daimler 420 Sovereign
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#12 Re: SU minutiae

Post by jag68 » Fri Oct 26, 2018 10:15 pm

Wow Pete. You must have been talking to my publicist. I had to fire him!

Terry
1967 E Type coupe
1968 E Type OTS
2007 XKR

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#13 Re: SU minutiae

Post by Hugo » Mon Oct 29, 2018 1:46 pm

jag68 wrote:
Wed Oct 24, 2018 7:36 pm
..........you need the free play for proper operation which is what my question was about. I appreciate that it was there for a reason but I still needed to know. Thanks
A very interesting question - I too am intrigued. Here's a guess; when you snap open the throttle, the airflow will speed up more quickly than the fuel flow, due to the relative density of the materials. The 'disadvantage' of SU's over accelerator pump carbs is that they overcome this issue by slowing down the airflow rather than speeding up the fuel flow by giving it a squirt. This makes the throttle response less sharp. My suggestion is that during the initial un-damped rise of the piston, there may be enough fuel sitting in the top of the jet tube that the engine can just slurp it up. Thereafter, the fuel will be struggling to squeeze through the jet tube to maintain the fuel level in the jet tube and keep up with the airflow, so they had to slow down the increase in rate of airflow to compensate.
Having said all that, I wouldn't expect your symptoms even if there were no oil at all in the dampers. Many SU's didn't even have them.
Hugo Miller - rebuilding an imported Series II OTS & converting to RHD

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#14 Re: SU minutiae

Post by christopher storey » Mon Oct 29, 2018 3:00 pm

Hugo wrote:
Mon Oct 29, 2018 1:46 pm
The 'disadvantage' of SU's over accelerator pump carbs is that they overcome this issue by slowing down the airflow rather than speeding up the fuel flow by giving it a squirt. This makes the throttle response less sharp.
I don't think this proposition is correct , superficially attractive though it may seem. Because the piston rise is damped, it actually speeds up the gas passing through the ( variable ) venturi and thus maintains the suction on the jet . This is why these are called constant depression carburetters . On a fixed venturi carburetter, if the throttle is snapped open, there is an alteration of depression until such time as the inertia of the required increase in airflow is overcome, thus weakening the mixture until this has occurred . This is the whole reason for accelerator pumps

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#15 Re: SU minutiae

Post by Hugo » Mon Oct 29, 2018 3:36 pm

Fair point, the speed of the air may be maintained, but the volume of mixture is reduced, giving a less 'snappy' response.
Hugo Miller - rebuilding an imported Series II OTS & converting to RHD

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#16 Re: SU minutiae

Post by jag68 » Mon Oct 29, 2018 6:36 pm

Bernoulli's principle dictates that an increase in airspeed (a fluid) through a fixed, or in the SU a slowly expanding venturi results in an increase in the depression in the venturi. It's the principle that all carburetors operate on. I think that the less than "snappy" response is probably in comparison to an accelerator pump which dumps a lot of fuel into the throat immediately. A Weber in comparison to an SU is positively explosive, but it may not actually be any faster over the long run.
1967 E Type coupe
1968 E Type OTS
2007 XKR

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#17 Re: SU minutiae

Post by Hugo » Mon Oct 29, 2018 7:04 pm

That is exactly what I said ;)
Hugo Miller - rebuilding an imported Series II OTS & converting to RHD

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#18 Re: SU minutiae

Post by MarkRado » Tue Oct 30, 2018 5:43 am

Hi Terry,
I'd start with the test given in the SU manual - remove dome/piston from carb, remove spring, drain oil, refit damper, plug the two holes at bottom of piston, fit a washer to one of the dome lugs to prevent piston falling out, hold in the normal upright position, push piston upwards (in), release and count seconds the piston needs to come down/ starts falling out (hitting the washer). Should be 7 to 5 seconds, not less.
I assume that the rest of the carbs, the distributor, ignition leads and timing are as they should be (let alone cam timing and valve clearance). Next I'd check the play in the accelerator control from pedal to carbs, try to minimize just to ensure throttle discs fully close and make sure that the spindles move freely.
I'd make sure there are no leaks on the brake vacuum side and then tune carbs at idle and 3000 rpm using a synchrometer and gas analyzer (lambda 0,87). Removing the air plenum, you can check visually the pistons moving with the engine running; move throttle linkage by hand, the pistons should go up and down equally (hopefully without hesitation)
Hope that helps,
Mark
Mark
1963 OTS 880436 - since 1988

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