Jaguar mkII 3.8 Carb/Head swap 3.8S & Triple carbs?

Saloons
1

Hi, I have a 3.8 which has a sticking valve. On close inspection I can see that it is actually a 3.8l block but with the code on the head indicating that that was from a 3.4

The question I have is is there any difference between a 3.8 and 3.4 head anyway? Also, I’m thinking that I might like to fit a triple carb setup. To look original it would be nice to have triple SU’s, but I’m not adversed to the idea of it being triple Webers. Do I need to have a 3.8S head for this and indeed what is the actual difference between a 3.8 and 3.8S in the first place?

Any advice much appreciated before I send my cash into an ill advised upgrade.

2

3.4’s and 3.8’s in MK2s and S Types are all "B"type heads and the same. E Type 3.8s and 4.2s have a straight port head and can be identified by a spark plug in the front position instead of a core plug. Either head can be adapted, but the intake stud spacing is different. See the archives.

3

Thanks for the comments. Always helpful.

I have seen the archives but they all seem to refer to E-Types and XK’s which I can’t be sure is the same as a mkII of course.

4

it is possible to fit the triple su carbs on the mark 2 but requires some alterations in engine bay. webers would be more difficult. unless you are sure engine is good on bottom end you might want to avoid ordering parts until head is off.

5

Hi, thanks for the note. The engine is virtually new, but unfortunately it has a sticky valve guide. Just a chance in a million. So the heads got to come off anyway and I have the chance of a 3.8S head from an acquaintance. Since mine doesn’t actually have a 3.8 head in the first place, I wondered what the differences were.
Thanks

6

Very early 3.8 heads have a chamfer round the combustion chamber,
other than that, and the colour difference 3.4/3.8, 3.4/3.8 MK2 and 3.4/3.8 SType are the same. All are termed B type.
To fit triple SUs you need one of the following- 3.8/4.2 E type, XK150S
3.8/4.2 MK10, 420, 240 or any XJ6. Check the thread re,
Interchangeable cylinder heads XK to XJ.
Peter B.

7

Hi Peter, that’s great information thanks. I’ll track down the thread you’re referring to. I’ve found a head off of a 3.8s, just need to decide on carbs now. Webers or SU’s?
Thanks again

8

SUs: Webers are neat, fine, and dandy…for high performance.

They are also tricky and expensive to set up correctly, FAR more so than SUs, and the latter, when adjusted properly, work just fine.

1 Like
9

I’ve spent some considerable time collating all the information I’ve found whilst looking into these upgrades, so I’ve made up a considerable list of models (mk1, 2 7, 8, 9, 10, XK’s, E-Type, 240, 340 etc etc…), engine codes, head codes, head colours, port and waterway differences, valve angles, valve sizes, carb options and numbers and cam lifts (along with a diagram showing differences on heads. Unfortunately, this website won’t let me upload it so if someone else feels they might find handy (or not), feel free to message me and I’ll send you a copy.
Cheers chaps

10

Now Uploaded - Ironically the post I put up saying I couldn’t list it made me go past the minimum number of posts you need to upload something. Natch.

11

Hi - Is your stuck valve stuck after sitting for a while? I’m asking because my resurrected 3.8 had a stuck valve that I was able to loosen up without pulling the head - penetrating oils, vibration, and patience. Mind you, this is an engine with lots of miles, and probly lots of carbon buildup. Just thought I’d ask…David

12

Hi,
that’s certainly a worth while question to ask. Sadly it was nipping up on the guide even though in theory it was rebuilt just before I had it a year ago. Now there is no compression to speak of on Nº5 which I take to mean it got a slight kiss one time when it was sticking and now is a tad on the bent side. Thanks