Pistons and dashpots were made as matched pairs originally, all these years later there is a good chance that they are no longer mated pairs. Or have wear, corrosion on the inside of the dashpot, a piston has been replaced from a different carb etc, etc.

I recently overhauled a set of E Type HD8's and there was a significant difference between the 3 carbs, I was moving them around to get the best matching set that behaved somewhat similarly. It's worth a check to make sure that the piston moves freely as it is supposed to.

 

"Moving them around" so you were switching out the piston and suction chambers between the 3 carbs ?
I'm surprised you were able to do that given that they are matched pairs and the degree or precision that the parts are machined too.
In all honesty do you think it would have made a difference in performance once the return springs and oil in the damper tubes were in place, whether they were switched or not ?
It's just rather redundant given that any carburetor is generally sort of crude in its functionality, I don't believe that a few seconds difference in drop is going to affect the proper functionality.
I bet if you took the springs and started switching them around, the equal time of drop would change again, this would be affected by how long the springs were, but how long or short; an 1/8th of an inch, 1/4 or 1/16 ?
In the end the flow of atmosphere into the engine is so powerful that these miniscule differences in seconds for drop, isn't going to make any difference.

The best thing for equal rise and fall to happen is to balance the carbs and in the case of an HD6 is to have both butterfly valves closed shut at idle, that's as close as one is going to get.

If you did a before and after test on a dynamometer, I don't think there would be any difference, I also don't believe one would notice a difference in response or top speed out in the wild.

It's at the point where these very slight differences just don't matter, just like the oil in the damper tubes.

 

Yes, on one pair the piston would not fit inside the dashpot at all, other dropped like a rock. Perhaps it's a matter of semantics, but the clearance between the dashpot and the piston is small, the sample to sample tolerance is huge in comparison. The dashpot and piston are precision machined relative to each other, but they are not repeatable machined relative to other samples. I suspect it's much like the pistons in the XK engine. They have a small clearance, but Jaguar had a whole range of grades because they could not hold precise tolerances and needed a range of pistons to fit the bores that achieved the required clearances.

Considering one piston would not even go in the dashpot, yes.


The purpose of the damper is to give acceleration enrichment in transient conditions. On a dyno at steady state you will see no difference, and you're not supposed to. However, I have put an AFR gauge on my car and I can tell you that changes in damper oil weight and the way the piston fits and it's rise speed does have a measurable effect on air-fuel ratio in the transient acceleration conditions. It's designed to prevent a momentary leaning out under application of throttle, and it does work. How much is noticeable depends on lots of things, like how rich your carbs are set to begin with, how fast the throttle is applied, how dirty the air filter is, etc. If you are running richer than stoichemetric, then less accel enrichment is needed. Running rich hides all sorts of sins!

 

Yes it certainly works much as a throttle pump would in a conventional carb. One should never be running at greater than stoichiometric.

 

Without oil in the damper cylinders, my car would suffer major stutter on acceleration. The dampers do the same job as the acceleration pumps in Webers and Dellortos.

 

By "greater" do you mean an AFR of something like 16:1? I.e. lean?

 

No quite the reverse. I did not word well ~ done some editing. Cars don't run well at stoichiometric (approx 14.7 to 1 AF ratio). They need to be richer for ideal running (our old girls). i.e. most engines run best at an AF ratio in the 13 to 1 area/range. Sorry to confuse. Forget the AED which is probably around 9 to 1 AF ratio if not worse.

We are talking conventional engines. Not lean burn engines.

My Jag is probably running at about average 13.5/13.7 to 1 AF Ratio. Slightest wiff of brown smoke out of the exhausts that clears quickly on hard pull away to clear exhausts once moving briskly. (I've had an analyser up the pipes a few times. They are remarkably equal.)

Pale grey/brown pipes internally cruising. (No Cats obviously & no lead ~ excuse buffing compound specs.)

 

Stoichiometric (Greek word with more than one spelling in modern English) or 'chemically correct' is the air fuel mixture where, if combustion were complete, all the fuel and oxygen would be converted to carbon dioxide and water with neither hydrocarbon or oxygen remaining. Other things being equal, this mixture would generate the highest temperature and pressure. Lean or weak mixtures have an excess of oxygen; rich ones have an excess of fuel. The hydrocarbon molecules in gasoline are big and heavy compared with oxygen molecules. As a consequence, they don't get around (diffuse) so easily. To have enough hydrocarbon around, especially in the thin flame front where it is being consumed, it helps to start with excess of it, a rich mixture. This is especially the case at ignition by the spark and immediately after when the flame is small, when it's too small for turbulence to help mixing and accelerate flame propagation. In a lean burn engine such as the HE version of the V12, the turbulence is much more intense and runs to smaller scales allowing weaker mixtures. The combustion chamber is also relatively compact. With a weaker mixture, peak temperatures are lower (than for stoichiometric) so the compression ratio can be raised. Also excess air is an alternative to throttling. Combined, the engine gains efficiency.

The combustion chamber of the XK is often described as quiescent; the intensity of the turbulence is fairly low with little or no squish and relatively clean inlets. To make things worse, the spark plug isn't central, increasing the distance to the far end of the chamber and making the whole process of getting the mixture burnt take even longer. Extra fuel facilitates ignition, speeds combustion and cools the final product. To avoid knock, cycle to cycle variations in combustion and misfires, the XK wants a richer mixture.

It is interesting that some of Jaguar's most powerful XK engines, as fitted in favoured lightweight E types, had a squish area. Instead of arranging it at the usual deck level, the join between head and block, they complicated the machining of the head to put it up close to the spark plug.

Mixing the fuel with air is a complicated and messy process, even more so with carbs than fuel injection. Some of the fuel going into the combustion chamber is coming from the walls of the inlet and was sucked from the carburettor in previous engine cycles. The consequence is that, when accelerating, the fuel delivery may be left behind; it's only sufficient for an engine running more slowly. To make things worse, it's when the requirement is for power rather than economy. The carburettor compensates for all this by a rapid but temporary increase in the fuel supply. This can be done by an accelerator pump as in a Weber. In an SU it's achieved by delaying the lift of the piston. The delay temporarily increases the air speed over the jet and sucks more fuel through it.

 

Excellent post Peter...

 

You should go into publishing Peter. Extremely well put. My only very, very minor gripe is you should explain the word " squish". It is not understood by many.

 

It's always good to leave one jargon word undefined. As for the rest, it was my bread and butter for a while, several decades ago.

 

I had my DS420 set up to be about 14:1 at highway cruise and it seemed happy there. I was actually very impressed with the SU HIF44 carbs it has, they kept the AFR is a fairly small band. For a mechanical device, I was impressed with their ability to deliver fairly accurate fueling. Cold start and warm up has always been a poor area for SU's though, I was glad the Daimler has a manual choke.

I also had fitted an exhaust gas temperature gauge and at steady highway cruise at 110 km/h I was getting about 550°C with the probe in the exhaust manifold, about 3" from the head. Under sustained full throttle when climbing grades in the mountains it was a different story, the AFR went to 12:1 and the EGT pegged the gauge at 1100°C.

 

12:1 is close to optimum for power for many engines.. You could experiment with a little richer. It might gain a little more power, but would cost in mpg.

 

I'm going a slightly different route to obtain more power. Instead of 164hp I'm aiming for 350hp with a 1995 XJR engine. This is turning into a much more complicated job than I originally intended, but it should be a nice modification when I'm finished.

 

Yes 12:1 would probably give you about 12 mpg from my playing around. I'm happier in the 15 to 17.5 mpg area. You can get a 3.8 up to 20 mpg but it requires a very gentle foot. (Imperial Gallons)

 

My search continues and I have now found a company in Litchfield UK called Carb-Care.co.uk which a friend of mine used last year. They only charge £185 per carb so £370 which includes rebuilding the AED. Full strip, Ultrasonic clean, Vapour blast, Parts, rebuild and test for £370 and they do not charge VAT tax. Just waiting for this second hand set of carbs to arrive (today hopefully) and they will be sent of for rebuild.

 

Still fighting a fuel leak on my Mk2-340. There were drips under two of the four screws in the bottom of the rear carb, and what appeared to be leakage at the tee on the fuel feed. Had the fuel feed tested; no leak. Rebuilt the rear carb and hooked the fuel feed up with the rear carb loosely attached. The rear banjo fitting leaked (and dripped down to the tee). I am going to rebuild the front carb. I tightened the rear carb banjo bolt tight with new fiber gaskets... Still leaked. Any thoughts about Gasoila PLS 2 Pipe Thread & Gasket Sealer? When the bolt is tightened down the gaskets all be t disappear. What about double gaskets?

 

Look for cracks in lower housing, sometimes they leak where the screws through, the lower banjo fittings underneath use aluminum washers, if that's what we are talking about.
The float bowl banjo fittings use the fibre washers.
Put a small amount of grease on both the male and female threads and work the grease in to prevent the steel banjo fitting from galling the soft aluminum.
Thread it in and out by hand to work the grease into the threads.
Look for warpage from over tightening too.

I use this product on the aluminum washers as well as the fibre ones, it just takes a nice even coating _ not very much on the washers only.
It also helps to seat the washers.

I find the banjo fittings have to be quite snug, there is a fine line of over tightening and breaking something and not being tight enough.
Gasoline likes to leak, so if the float bowl ones leak, give them just a little snug up while they are leaking, but you should have grease in the threads already.
Snugging it up just a bit more while it's leaking may solve the problem as the fuel is now lubricating the washers.

LOCTITE 515 - Methacrylate gasketing product - Henkel Adhesives (henkel-adhesives.com)

You say the gaskets all but disappear when the bolt is tightened down, are you tightening it down until you crush the gasket ?

Also, when tightening up the four screws on the bottom, lubricate the threads with grease and work it it, you should be able to do this with your bare hands, if not clean the threads until you can.
Snug them up gradually and evenly, that's important.
The aluminum that was used is very soft alloy, so it can be easily damaged.

 

Thank you, JeffR1

Joe Curto suggested loosening the cap on both fuel bowls, starting both banjo bolts evenly until they are snug. Then snug up the fuel bowl. Then tighten the banjo bolts alternating between both sides. The rear fuel connection has had a long history of leaking, so I coated both sides of the fiber washers with the PLS 2. When I turned on the fuel pump, there was a slight leak at the front banjo... I tightened it a little, and the leak stopped. It has been under pressure for an hour with no leaks. I can take down the throttle linkage and get the two bottom rear carb bolts on. I wanted to make sure that the leaks were controlled. Next, I will rebuild the front carb and hopefully get back on the road soon. Joe asked me about aluminum washers for the banjo bolt. Page C.14 shows only two fiber washers.