Hello Everybody,

I'm considering replacing the H6 carbs on my XK140 MC with H8's. SNG Barratt sells a pair of H8 sandcast SU's made by Burlen Fuel. XKs Unlimited also sells a pair, but I'm not sure who makes it.

Any opinions on the benefits of such a swap? Thanks.

 

I'm no expert, but if I were, the first questions I'd ask are: Why? What do you hope to accomplish? Is there a problem with your current set of carbs?


I think only then can you determine if the swap makes sense.


Just my 2 pence.

 

Thanks, George, for the input. My current set of carbs need rebuilding, which has prompted my consideration of an alternative. I spoke to a friend in Phoenix who works on Jags and he told me that the H8's make a world of difference over the standard H6's. My car has the C-Type head, which he said would benefit in performance with the larger carbs.

Just thought I'd see if anyone had a similar experience. Thanks again.

 

Understood. I hope you'll let us all know what happens.

Thanks.

 

I absolutely will. I'll include pictures as well, if I decide to move forward. Thanks again for your responses.

 

hello,

You can't just replace the H6 by H8.
The inlet On the manifold has to be enlarged.
The H8 is 2" the H6 is 13/4".
But the only thing you will accomplish is a higher fuel consumption.
May be a little bit of extra torq if you are lucky.

regards,
Peter Jan

 

Thank you, Peter, for your response. I was aware that I would have to enlarge the intake manifold to accommodate the larger H8. I would hope that this type swap would yield better results than just increased fuel consumption and a bit more torque. I guess I had thought that the higher performance XK's and racing engines used the H8's because of superior performance.

Best regards,
Borron

 

The real induction problem on the XK120 is the "log" type intake manifold. The H8s are marginally better, but nothing will be noticed except at full throttle and high speed. You also need to install a manual choke cable.

The big advantage of the H8s is they look pretty neat.

Installing an E Type head with tri-carbs really makes a huge difference.

 

No point putting H8 on a 120 unless you are racing. They are the carb from the C type and made to work at the top end. HD8 are good as the extra bore give you more air over 4500 rpm.

chris120

 

Folks,

Just want to point out that the car in question is a 140MC - not a 120, and *with* the C-Type head. I think we're all in the same chapter, just not quite the same page. Not sure if this alters other responses or not.....

My other thought is if other changes would be necessary to really take full advantage of larger carbs and get the performance bump which it sounds like he's looking for. I'm thinking specifically of cams here (though the C-Type head may already have the appropriate cams....).

Finally, I would think that, if the car is running well (carb rebuild need aside), then rebuilding the carbs would be significantly cheaper (again, I *think*). So there's a cost/benefit question which only he (OP) can answer.

Thanks.

 

Good afternoon,
I appreciate all the responses. I think I've decided to stay with the H6's, since the engine runs very well.
Borron O.

 

Hello again, Everybody,

Well, I guess the passage of time, and the fact that I ended up having to pull the engine out of my XK140MC, caused me to rethink this question one more time. Of course, it didn't hurt that I got a pretty good package deal on a set of new H8 sand cast carbs with all gaskets, studs, washers and nuts, plus a set of period correct air filters with bolts and gaskets. With all that said, I took a breath and purchased the package deal.

The new stuff arrived today, and I have to say that the H8's look really cool! Truth be told, I think the way they look is what pushed me over the edge to buy them. I realize I'll have to do a little work on my intake, but that's ok.

I'm still a ways off in getting my 140 back on the road, so I can't report how the H8's are different, but I will.

My engine is back from the machine shop and I've got the short block mostly completed (crank & pistons installed) except for a few additional tasks on the timing components. I still need to re-assemble the head with new valve springs and cam bearings, along with a new intake valve and a new exhaust valve on the #1 cylinder. Both those valves got bent, and the cylinder wall dinged up a bit (had to bore to .030" over) when a piece of metal went through the cylinder. At least, that's what I have deduced.

I bought the car in early 2014 and had been driving it a bit for 6 months or so, when, on a bright, clear, cool Sunday morning drive in the fall of 2014, the engine developed a dead miss. I limped home and did a compression test. Cylinders 2 thru 5 were fine. #1 was dead. No compression. Course, I knew what that meant.

What I found inside the intake manifold as I was taking the head apart were 2 round pieces of metal (magnet picks them up) that looked like smooth welding slag. They were just sitting on the "floor" of the intake between the 2 carbs waiting, I guess, to get sucked thru my engine like at least one other piece did when it went thru the #1 cylinder.

I have no idea how they got there, but I suspect the last shop who rebuilt the engine (I was told the last rebuild was about 15 years ago in Southern California) wasn't careful and somehow those pieces of metal got inside the intake. Whatever. All I really know is the result, and what it has cost me in time and money. The primary consolation is that I really enjoy building engines, especially old Jag engines.

Thanks for letting me vent a bit.
Borron

 

I had triple HD8's on my 1964 Aston Martin DB5. They were certainly very effective at sucking in fuel.

As I recall, my 1966 E-Type returned around 12 mpg around town but the Aston struggled at 7 mpg.

Graham