I just purchased a 1965 Mk2 and it came with a box of parts including another head, and a couple of intakes, one of which has a set of HD8 carbs. My current head is given decent dry compression after sitting for many years (compression from 115 to 130 psi - uncorrected for elevation) so I won't be changing the head soon. But if I needed to is there a noticeable benefit of upgrading to the larger carburators and the straight through intake?

Extra intakes

Extra intakes

Extra head

Extra Head

 

Hi Chris, you would not see much if any difference upgrading to HD8 carbs with a B-type head, there is some difference with the straight port head, but more difference with an XJ6 head fitted simply due to the intake tracts. XJ6 head has larger inlet valves and larger ports.

To make the gains to get the benefit from the HD8's you will need to fit higher lift cams, port the B-type head (not loads, but it can be improved) some porting to a straight port, but this is more of a tidy up than much else. For a fast road setup you only want the cam lift a little greater otherwise you will suffer torque loss at lower revs and rough idling.

I haven't checked the part number to see which head you have spare, but it looks like you may have the head, inlet and carbs from a later XJ6 series 1. These are a good combination to use if you decide to rebuild the engine for fast road use.

So to answer your question, leave the carbs in a box with the head and inlet and either save them for further down the line or sell them as a complete set.

 

"I just purchased a 1965 Mk2 and it came with a box of parts including another head, and a couple of intakes, one of which has a set of HD8 carbs. .......... so I won't be changing the head soon. But if I needed to is there a noticeable benefit of upgrading to the larger carburetors and the straight through intake?"

The MK2 would have delivered with a "B" cylinder head with a two carburetor manifold. To get to the 3 SUs, you will need matching cylinder head and tri-manifold. The inlet ports do not line up between the two heads. "B" Head would have been Blue and the Straight Port Head would have been Gold.

Not sure what jaguar model the 3 carburetors and Straight Port Cylinder Head came from, but you could use the specs for the 3 carburetor XK 150S, for set up of carb needles, hardware, & etc. You will also want a XK-150S distributor curve to get best performance. Factory marketing would show a 220 Gross HP jump to 265 HP.

I had a 1965 3.8S and spent a lot of time figuring out what could be done, but in the end stayed with original setup.

Rgds David

 

Thanks, I plan to do as you suggested for now.

 

One advantage about the HD8 Carbs, at least the ones in your photo, is that they appear to have a manual choke on them.
That set up IMO, is miles ahead of the putzy automatic choke arrangement with it's starting carburetor and Otter switch.

 

I'm fighting with the auto choke on my HD6's right now as everything is running way to rich to the point the engine will not start. Will have to remove and disassemble again, I'll be an expert soon.

 

This assumes that you know your way around SU's

Remove the dash pots and piston assembly on each carb.
Adjust the mixture screw on each until the main jet is level with the jet bridge.
Now adjust each mixture screw 2.5 turns so the main jets are now below the jet bridge.
Assemble the dash pots and fill the damper tubes with red transmission oil. (there is special oil for this, but auto trans oil works well enough for now)

Be sure that both float and needle valve assemblies are working properly and that the float level is correct, bad leaky needle valves and mis-adjustment are a common causes of flooding; especially the float bowl that feeds the starting carburetor.

You may also need to "lean" out your starting carb, even more so if your garage is heated.
The nut that controls the starting carb jet needle needs to be turned in to lean the choke and screwed out to enrichen.
There should be small keeper to hold the nut in place.

At this point you may also want by-pass the Otter switch for now so the choke does not go off too soon or come on when the engine is warm enough to run with out the choke.
You can install a toggle switch for now or just ground the starting solenoid to activate the choke. (or turn it off)
Be sure NOT to ground the live wire that runs to the starting solenoid, the one that grounds through the Otter switch is the one to connect ground.

Nut #48 in the second link photo is the one to adjust the choke, screw in for lean, out for rich.

Technical - SU Carburetters

HD Diaphragm Jet Type Carburetter and Auxiliary Enrichment Carburetter - SU Carburetters

 

Just for the record, the carbs in your top photo look like HS8s to me, rather than HD8s. HS8s were fitted to some XJ6 Series 1s, so that may be the origin of the head and manifold.

 

The picture of the cylinder head shows a drilled oil hole on bottom of cam lobes indicating it should be from an Pre-XJ6 era engine. ( XKE, 420 or Mark 10)

.........."the Series 1 XKE’s used the same camshafts whether 3.8 or 4.2. They were identified by the front mounting to the chain drive by two bolts. These also have oil holes drilled in the bottom of the lobes." In 1968, Jaguar changed the lobe pattern of the camshafts. This was not an emission change but rather a change to smooth out the tappet noise. The lobe engaged the tappet more gently rather than slapping. These cams are identified by the two bolt front and a groove cut around the front flange. These cams adjust to a .012-.014 clearance ".......

Rgds David

 

Thanks for the nice summary, I have the choke "off" and its still flooding, so looks like I will need to pull the carb assembly and give a good cleaning and properly set everything.

 

Thanks, it does have a different inlet to the Mk2 so you are likely correct.

 

If you have the choke off and it's still flooding, then the needled valves and float levels are the next thing to check.
Check the floats themselves for cracks, there should be no fuel in them of course.

The red arrow points to a diaphragm that can crack, this allows fuel to leak internally inside the carb, this can also be a source of flooding.

 

Thanks, the diaphragms are in poor shape, rebuild kit on the way...

 

When things are running well, probably down the line a bit when the car is restored, these work well to synchronize the carbs.

https://www.britishtoolworks.com/pro...-kit-for-hd8-s

Despite the shortcomings of these carbs, it's much easier to adjust the mixture and to balance them.

Screw "18" in the diagram is used to balance the carbs and set the idle.
Both butterfly valves are shut off tight eliminating the use of them to achieve proper balance and idle.
Screw 18 has a taper and is a very precise way to achieve this compared to using the butterfly valves.

 

Hi David,
the drilled lobes in the camshafts were present in MK2's S-Type, XK150 and XJ6 among others.

 

Screw 18 controls to flow of air through a bypass around the butterflies, and as Jeff says are used to adjust the idle and balance.

 

Just a note to say this is obviously when the engine is cold and this also may be of some help https://sucarb.co.uk/technical-auxil...nt-carburetter

If you are getting a rebuild kit, follow the instructions on the SU site, and start from scratch with all adjustments. Good Luck, and as Jeff says, SU's are far simpler than many think, if you can do some research and work out how these constant depression carbs operate you will find your way round them fairly easily. If you understand the fundamentals of constant depression carbs then it will all fall into place.

 

Yes, I was just looking at straight port heads (SP), where cams had drilled lobes. Looking at other picture of the spare cylinder head, the cooling hole pattern, without extra holes between cylinders would also indicate a pre-XJ6 head??? My guess is that it is a (SP) head and a 2 carb, 2" SU's manifold from a 420???

As long as this is a future project, I would be on the lookout for a set (3) of 3.8 (SP) intake manifolds and go all the way to 150S configuration.

Rgds David

 

It is difficult to tell exactly which head it is, but it appears to me that it is an XJ6 head, exhaust valves look like they are larger, but not easy to see for sure due to the shadows, would need a pic of the complete block side to confirm that.

HS8 carbs were also fitted to later XJ6 S1's and the intake manifold is the same, so I would put money on the whole lot being from a later XJ6 S1. I could be wrong, but that's my best guess.

The XJ6 intake manifold has pretty good air flow, the larger valve head needs very little porting to make the most of it, and the 2" SU's with correct jetting would perform almost as well as a triple set up. Jaguar cars with triples are pretty well over-carbed in my opinion, more for the look of power than actual power. A set of triples would set you back well in excess of £1500, and the OP has these for free, not sure he would get any benefit. ?