E type 4.2 triple webers swap for triple su carbs

[Badey]

Hi
I have triple webers in excellent order on my standard 4.2 engine and I just cant get on with the things .
I dont generally drive flat out and that's what webers are good at but not so good at lower revs.
It was on Peter Baldwins dyno last week at 235bhp which is pretty good for a standard motor.
They're much more suitable for a fast road engine, high lift cams , high comp , flowed head etc..
I want to swap them for triple su carbs with manifold and linkages as my webers come off with manifold and linkages .
I dont mind if they need a clean up and service , my webers dont by the way.
I'm interested in offers with negotiation.
If you're interested let me know .

[Tom W]

Whilst there’s nothing wrong with SUs, I wouldn’t give up on the Webers. They can be made to work just as well as SUs. The problem is, there’s no book settings to work to. A lot of it is left for the owner to figure out. With SUs, Jaguar did all the hard work, and published the settings in the workshop manual.

The perception that Webers are a race only carb, and don’t or can’t work around town isn’t correct, but often seems to be perpetuated. I wouldn’t swap mine for a set of SUs.

[Badey]

I've spent a fortune getting the things set up twice by people who know them and I'm just not happy with them.
I get a constant judder, missfire and spitting back at around 50 to 60 mph , acceleration is great and below those speeds its smooth.
Most of the time I'm driving in the range where they play up and its driving me nuts.
I cant help but feel su`s are a bit more forgiving.
It's been on rolling roads and they've got them as good as they can .

[Tom W]

I tried specialists to set them up when I first got my car as I thought Webers were a dark art and I’d never get there myself. When I learned a bit more, and had a go myself, I got things much better. I found there were basic errors the specialists hadn’t addressed, having given the car back saying that was as good as it was going to be. It’s not just adjusting the carbs that influence things, ignition timing makes a big difference.

[christopher storey]

Webers. They can be made to work just as well as SUs. The problem is, there’s no book settings to work to. A lot of it is left for the owner to figure out. With SUs, Jaguar did all the hard work, and published the settings in the workshop manual.

The perception that Webers are a race only carb, and don’t or can’t work around town isn’t correct, but often seems to be perpetuated. I wouldn’t swap mine for a set of SUs.

Whilst I agree that Webers can be made to work up to a point , I disagree strongly that they can be made to work as well for road use as SUs. Apart from anything else, there are no settings on SUs that need to be worked out. The only variable is the needle and for virtually all Es, UM will work as well if not better than any other needle. The problem with DCOEs is that essentially they are a full throttle carburetter, designed for everything to come together at full throttle. Anything below that is a matter of compromise, and that is where the difficulties start. First there is the choice of main and secondary venturis, and even more critical is the choice of emulsion tubes which are very temperamental creatures, and that is before you start jetting them. SUs by comparison are fit and forget.

I accept that sometimes it is possible to come to a point where everything works in harmony for quite a significant part of the running range in road use, but my experience ( which stems back more than 60 years with SUs and more than 50 years with DCOEson quite a range of cars ) is that , even with John Passini's wonderful books to guide one , good progression in part throttle running is more a matter of good luck, and this I'm afraid is what Badey is finding.

It might be helpful if Tom were prepared to let us know the full specification of his DCOEs, which Badey could then compare with the spec on his car

PS One aspect where Tom and I are in full agreement is that ignition timing is critical on DCOEs - my own experience is that an appreciable amount of extra advance over and above the factory settings is helpful on the lower part of the running range i.e. up to about 3500 rpm, but one needs to be careful to avoid detonation which cannot always be heard

[mgcjag]

Hi...just make sure that the manifold you get is for an E type....there are plenty of tripple SU,s out there on manifolds but it needs to be for an E type...if not the bonnet hits the carb tops.....Steve

[Joes66]

Are you sure you have a bog standard engine?
235hp on an un modified engine at wheels seems rather high
Even with webbers

[Badey]

As far as I know its unmodified apart from the webers.
The rolling road people were surprised as well.
The head was skimmed during the engine rebuild but it was a tiny amount , maybe it had been skimmed before in the past , so I have higher compression.
It's actually a series 1 xj6 lump, the original engine was wrecked by a so called "specialist" who rebuilt it twice , both times lasting no more than 500 miles dropping oil at a vast rate and eating its main bearings , then refused any form of recompense.
I stripped it down and it was totally filthy inside , grit and swarfe everywhere .Blocked cam oil pipe so that wrecked a cam and the oil bypass valve was floating around the housing so unfiltered oil was being pumped around .
He gave up trying to remove the crank plugs after chewing one up .I removed them , completely blocked.
You live and learn

I rebuilt the xj6 lump myself needless to say.

[Tom W]

I have 45 DCOE 9s. Different models of 45 will have different progression drillings, so may need different components inside. I think the manifold I have is from Webcon. It came with the car, as did the carbs.

The internals, I think, follow the specs recommend in Des Hammill’s XK engine book, except for the main jets, which were reduced a size when the car was tested on the rolling road. That’s usually what the ones marketed as jetted for Jag’s come with. I’d have to pull one apart to confirm exactly what’s inside. Apart from the main jets, I’ve not had to change the starting point settings the carbs were supplied with.

I was expecting to have to change the internals to get things to run right, but so far the biggest improvements I’ve made have been through basic setup.

Things to check would be as follows:

Float level - are they all the same (most important), and what level are they set to? Slight changes in level can be used to tune the behavior of the carb slightly.

Air leaks - usually where the carb meets the manifold. There’s a fine line between bolted too tight so there’s no movement between the carbs and manifold, and too loose so there’s air leaks. Check too for leaks on the vacuum circuit going to the reservac tank. Leaks effect lower throttle openings more.

Carb balance - do all the carbs draw the same at idle? Do all the butterflies open at exactly the same time? This can be tricky to achieve depending on the linkage you have. As with air leaks, errors here manifest themselves more at lower throttle openings.

Idle mixture adjustment - As well as idle quality, this also influences low speed running.

Ignition advance - mine responded to more advance low down, but no more at the top end.

My car has always gone well at wide open throttle, but when I got it the performance below about 2000 rpm was poor, with stumbling and spitting back. The above checks have made the biggest difference in curing the low speed stumble, and now the car’s very driveable. There’s still more development for me to do, but it feels more now like fine tuning than anything fundamentally wrong with the carburettor choice.

[Tom W]

I should also add to the above list of checks, fuel delivery pressure and fuel flow rate.

[Joes66]

You say you re built the xj6 engine yourself
Did you replace everything or just strip and re build
Pistons,cams,gas flowed head or anything?
Where did you get it from?
Are you sure its not been breathed on by someone previously?
Because at that horse power figure it sounds like it has been played with
It may have uprated cams or any other modification which may well cause flat spots and other issues
At 235hp at the wheels that would put it over jaguars original boast of 265hp I am fairly sure
And thats really not possible from what I have experienced which admittedly is not much other than going through dyno reports of other cars but its suspiciously high
Others may know if a bog standard car can get to 235hp just by bolting on webbers

[malcolm]

I don't think he claimed 235 BHP at the rear wheels. Rolling road operators use an assumed algorhythm to convert the figures they read from the wheels to an assumed figure at the flywheel, then quote a flywheel BHP

[Tom W]

My car made 216bhp at the crank when tested on a proper rolling road. I bought myself one of those Road Dyno boxes, and that showed 226bhp when using the weight values and aerodynamic losses I’d found on the web. When I put the same power run through the software using the figures David (Heuer) arrived at for calibrating his Road Dyno based on the proper dyno runs he’s had done, I got 257bhp! Nothing’s changed on my engine, it’s all down to how each dyno is calibrated. I think you can only really make an accurate comparison doing back to back testing on the same dyno.

As far as I know, my engine is standard. My graphs are uploaded into the rolling road thread.

http://forum.etypeuk.com/viewtopic.php ... es#p130772

[Joes66]

I admit I was assuming he meant rear wheel bhp
Whenever i used to take a car to the rr, bhp was measured at the rear wheels but its been a long time since I have done that
Maybe things have changed now
Even so 235hp is still high
From what I have seen standard engines range from 150 to 200 bhp
I was only questioning it in relation to his problem however as a possible explanation to his spluttering
Joe

[Tom W]

Have you seen this Badey?

viewtopic.php?f=7&t=16260

[tinworm]

I find Webers can be made to work well but as said before it takes patience and experimentation .It took me a good few months of playing to get the quadruple set of twin choke downdrafts on my Ferrari 308 to work properly. The problem was hesitation during progression , I had eventually to go a size up on the idle jets to fix it. The problem for most people is not realising a carburettor can both be rich and lean at different sections of the rev range - you just have to think about it and work through... If I had a set of webers for either of my Jags I would not swap them out for SU's . Although saying that I would not buy a set of Webers to replace a set of SU's - hope you understand what I am getting at.

Barrie