XK 140 MC upgrade to HD8 SU carbs

(John Burke) #1

So the weather finally cleared up around here and I took the XK140 MC OTS I acquired last fall out for a good run. I’m considering running it in the Copper State 1000 here in the US this fall so I wanted to start getting comfortable with it over long periods at speed, which is basically how I drive anyways. I had it on a highway and was motoring along at about 100 mph, top down of course. It was running great and I came upon a nice open stretch so I decided to stretch its legs to test its boundaries. I got it up to about 120 mph when it started losing power, smoking blue smoke out the back, and knocking a little. I immediately eased off the throttle which produced even more blue smoke initially, then decreased. Oil pressure declined from 60-80 psi to about 20. Temps remained about 70c. It was a cool day so that seemed appropriate. I limped home another 5 miles or so while watching the oil pressure and temps carefully. Both stayed the same. Once home I popped the hood and smoke / vapor was coming out of the rear of the engine, but there was no apparent coolant or oil leak. I felt the engine was hot so I measured the block with my pyrometer. It was 270F! I knew I had trouble. I’m currently having Bill Terry of TT Race Engines build the 4.2 in my '66 OTS at the moment so I contacted him about a rebuild on my XK140’s 3.4 as well. As I tend to run my cars hard, I queried Bill on what we should do for a rebuild on my 3.4 MC to maximize performance while keeping the original block and head. He suggested larger valves, a bigger cam, and 2" SU carbs, which all sounds reasonable to me. That leads me down the road to putting a set of HD8’s properly on an XK140 MC. I think I’ll be needing HD8 carbs, manifolds, linkages, air cleaners, and coil brackets. Does anyone know where I can source these? Can I just modify my current manifolds for 1 3/4" SU’s to work??

1 Like

(John Burke) #2

By the way I suspect a blown head gasket and probably a spun connecting rod bearing as my immediate problem.


(peder) #3

More like an Italian tuning job. I have only done 110 in a 150 dhc, with the top up, on the Autobahn at 6 inthe morning.
That car has the best tyres for such driving, the Michelin Pilote X (a modern radial tyre which looks very period correct in shape and pattern (185/80).
What tyres does your car have?
Your suggested changes sound good. HD8 carbs on a 420 intake manifold, or you can enlarge the vurrent holes from 1,75 to 2”. That’s what cost conscious Jaguar did when they offered the 2 » sand cast carbs. Same part number for both installations. How about disc brakes from the 150?, if you are planning more speed records. Or go all in as did Jaguar on the Belgian motorway in1952 with Norman Dewis clocking 172 in a 120 roadster😂


(Rob Caveney) #4

I usually back off when the air coming up through the floor boards starts blowing the dirt up around my face. I have a bored out manifold and a couple HD-8’s on my 3.8 XK-140 FH. One caution…be really careful not to widen it out too much and accidentally open up the water passage. An easy way to make some scrap aluminum. I also use a “vintage” race cam offering a little extra duration. The problem is on the other side of the engine. It’s easy to get more air in…but harder to get it out using the same old stock exhaust manifolds. I also installed the rear axil from a doner 150, but didn’t use the brake “plumbing” (servo/diaphram whatever else). It is just pedal pressure like the drums I had before the swap. In hind sight, the drums work pretty well. The only reason I did it was to improve the rear axil ratio and to return the car to spoke wheels. An electronic ignition improved things a lot too.


(peder) #5

EDIS Megajolt or similar with a trigger wheel?
Or the much simpler 123, perhaps the programmable one?
I like your crude speed limit system😂


(John Burke) #6

Thanks for the input guys. I’m not really going for speed records, but I do enjoy running my cars hard on occasion and I like them to be up to the task. The car has a new set of Michelin Pilote X tires on it. They run nicely at speed. I find the drum brakes quite adequate, it brakes strong and straight, although they do require some pressure on the brake pedal.


(Morris Barnett) #7

Go XK Data to 830933 see my manifold mod for HD8.


(John Burke) #8

Thanks Morris, that looks fairly straightforward.


(Morris Barnett) #9

John . After removing studs l used a router with a 3/4 straight bit and went round and around until l got 2 inches


(peder) #10

Just curious about your choice of electronic ignition.


(Pat Cashman) #11

Copper State is in April and the most recent one was just a week or so ago. Ran one in the Keeble in the 90s.


(John Burke) #12

Oops, my bad. I meant the Colorado Grand, which is in September, not the Copper State 1000. Similar concept, different state.


(Mike Spoelker) #13

I would think that Bill Terry & company knows as much about what makes these XK engines run as does any other living person. I can not imagine they couldn’t set up the H8’s when they dyno the engine.