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Looking to swap triple Weber's for triple SU's

E type ( XK-E ) 1961 - 1975

Looking to swap triple Weber's for triple SU's

 
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Old 04-12-2019, 09:42 AM
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Default Looking to swap triple Weber's for triple SU's

Hello! I'm new to this forum. I'm currently working on a customer's '66 E-type. We are looking for a triple SU HD8 setup. Ideally, we would like to trade our nearly new Weber DCOE 45 complete setup for a triple SU HD8 setup. Anyone interested?
 
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Old 07-07-2019, 08:33 AM
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Sorry to ask a question that is not enabling your intended trade, but why would you want to mess up a nice Weber set-up like you presently have for the grief and pain of installing S.U.'s with their higher-maintenance (don't forget to oil the dash-pots) and relatively crude low-speed (tending to run rich/fowl plugs, etc.) adjustment problems?
That combined with no accelerator pump made them a pleasure to part with.
Did you know that if you floor the triple S.U.'s from a dead stop, the throttle spring/linkage set up they provide are not strong enough to prevent your Jag from running off out of control up to around 40-45 MPH?
If you are not quick with the key ignition switch, to switch the ignition off as soon as it starts off, you will have an unwanted thrill ride that can end up with a tow-truck to the shop/rental car day.
That was one main reason why so many older Jag "Bonnets" (front ends) needed to be replaced.
Remember that with the S.U.'s you will want the special manifold as well.
Since the new S.U. sets can cost as much as a good transportation car (and the newly made copies do have issues with quality control, like fuel leaks from not pressure testing to discover factory machining defects) as an alternative.
Also, I would first look into buying an old set someone took off their old Jag and has laying around before outright "swapping" which if nothing else puts your car out of service while your old Webers are gone, and puts unnecessary pressure on you to make the others work, and they might need re-jetting, etc.
Once you have your own set in you hands, then you will be better able to make an objective decision.
 

Last edited by 67ECoupe; 07-07-2019 at 10:48 PM. Reason: Spelling
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Old 07-07-2019, 11:21 AM
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Originally Posted by 67ECoupe View Post
Did you know that if you floor the triple S.U.'s from a dead stop, the throttle spring/linkage set up they provide are not strong enough to prevent your Jag from running off out of control up to around 40-45 MPH?
If you are not quick with the key ignition switch, to switch the ignition off as soon as it starts off, you will have an unwanted thrill ride that can end up with a tow-truck to the shop/rental car day.
That was one main reason why so many older Jag "Bonnets" (front ends) needed to be replaced.
.
Hi 67ECOUPe,
The 3 carb 4.2 E-type, if the engine mounts are worn (squished) or if the engine is not level when installed the throttle linkage may jam against the frame rail when you jam the throttle to the floor. But In my opinion this isn't a defect of the SU Carb, it is improper maintenance, defective engine mounts or improper installation of the engine. There were some poorly made engine mounts that would be worn (squished) after a short period of time. So one work around was to install a 2mm shim under the RH engine mount to prevent this from occurring. I haven't heard of this problem with the heavy duty engine mounts. This wasn't a problem with the 3.8L E-type to my knowledge.

If you are overly rich and fouling plugs at low speed you need to run a different tapered needles. There are many different tapered needles available for SU carbs.

Classic66,
Nice Weber set-up.
 

Last edited by Bob_S; 07-07-2019 at 11:29 AM.
  #4  
Old 07-07-2019, 11:56 AM
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Wink Those Naughty SU's

Originally Posted by Bob_S View Post
Hi 67ECOUPe,
The 3 carb 4.2 E-type, if the engine mounts are worn (squished) or if the engine is not level when installed the throttle linkage may jam against the frame rail when you jam the throttle to the floor. But In my opinion this isn't a defect of the SU Carb, it is improper maintenance, defective engine mounts or improper installation of the engine. There were some poorly made engine mounts that would be worn (squished) after a short period of time. So one work around was to install a 2mm shim under the RH engine mount to prevent this from occurring. I haven't heard of this problem with the heavy duty engine mounts. This wasn't a problem with the 3.8L E-type to my knowledge.

If you are overly rich and fouling plugs at low speed you need to run a different tapered needles. There are many different tapered needles available for SU carbs.

Classic66,
Nice Weber set-up.
Thank you for your opinion Bob, but in fact around Los Angeles at the time, I had heard that others were having the same exact "run-away" problem, but mine was a "well-worn" 4.2, so that might have had something to do with it, but since the carbs were directly attached to the manifold that mounted on the head, I do not see how they could be affected by a worn engine mount.
But now remember this was back in the '70's before car owners started freaking out and started suing manufacturer's for making cars that they had to learn to operate intelligently in order to be safe.
As an engineering designer, personally, I attribute it to the poorly designed geometry involving the spring, it's mount while at a wide-open throttle position.
At the flat-open position, the center-line of the spring is in nearly direct alignment with the center of the throttle shaft, with the return-spring hole barely being enough above center to allow the spring to exert enough leverage to close the butterflies against the volume of air rushing in at that position.
So, now we have two opinions-maybe there should be a string dedicated to this topic, eh?
While we are at it, has anyone mentioned Jaguar's fetish with using cotter pins in the older 3.8 connecting rod bearing cap nuts?
And remember to check the oil in your SU dash-pots before going for a Sunday afternoon drive.
 

Last edited by 67ECoupe; 07-07-2019 at 02:00 PM. Reason: Additional Information
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Old 07-09-2019, 10:07 AM
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Originally Posted by 67ECoupe View Post
but mine was a "well-worn" 4.2, so that might have had something to do with it, but since the carbs were directly attached to the manifold that mounted on the head, I do not see how they could be affected by a worn engine mount.
.
Well you almost answered your own question but you stopped short. System engineering teaches you to look at all interactions that may affect your component/design.

So to continue where you left off ---- the cylinder head is connected to the engine block, the engine block is connected to the engine mount bracket assembly (C15849/C15850), the engine mount bracket assembly is connected to the engine mount and finally the engine mount is connected to the frame rail. So, as the engine mount ages or is made from the wrong material it get squished (compressed) and since everything rests on (supported by) the frame rail, the carbs and carb linkage moves closer to the frame rail. That is when the carb linkage arm can come in contact with the frame rail and jam. There have been people who have confirmed that there were wittness marks in the paint of the frame rail from the carb linkage arm.

Then there is the torque of the engine on the engine mounts. The RH mount is compressed while the LH mount is under tension (stretched), Over time this causes the RH mount to compress (squish) as they age (maintenance item) or due to cheap aftermarket parts (compress in a short time frame).

Originally Posted by 67ECoupe View Post
the flat-open position, the center-line of the spring is in nearly direct alignment with the center of the throttle shaft, with the return-spring hole barely being enough above center to allow the spring to exert enough leverage to close the butterflies against the volume of air rushing in at that position..
One of the beauties of the butterfly valve , i.E. the throttle valve, is that the airflow forces through it is counterbalanced (one side of the valve the air flow is trying to push it open, while the other side the air flow is trying to push it closed). As long as the valve shaft is in the middle of the valve plate the forces due to the air flowing through it has a minimum effect on the torque required to turn it.

That's all I have to say on the subject.


Sorry Classic66, for derailing your post about the Weber carbs. They look great on the E-type, if I was building a high HP engine I would make a deal for them. You may want to post on the XKSS forum, someone there may want them.
 
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Old 07-09-2019, 12:50 PM
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Originally Posted by Bob_S View Post

That is when the carb linkage arm can come in contact with the frame rail and jam. There have been people who have confirmed that there were wittness marks in the paint of the frame rail from the carb linkage arm.

Then there is the torque of the engine on the engine mounts. The RH mount is compressed while the LH mount is under tension (stretched), Over time this causes the RH mount to compress (squish) as they age (maintenance item) or due to cheap aftermarket parts (compress in a short time frame).

.
Well Bob, I wish you had told me this while I still had the car!
Back then it seemed like everybody who owned a Jag and had such problems kept it a secret!

Thank you for the eye-opening explanation.

Still, even though all three cars I owned at the time has S.U. carbs on them, and I got used to their idiosyncrasies, I still would not trade a good set of Webers for them!

My advice to Classic66 remains the same:
Keep the Webers, buy a used set of S.U's and play with them, having the Webers as a known, working set.
 
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Old 07-10-2019, 11:18 AM
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I traded my webbers for SUs on my old '64 and regret it to this day. (At the time I had perished gaskets on the intake and couldn't source replacements, but I could have looked harder (pre-internet)). The SUs, off sedan, if I recall correctly with an electric choke, worked OK but I really loved the Webbers. I lost power and the result didn't look as good.
 
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Old 07-11-2019, 07:32 AM
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Hi Enderie,
So did you put the 1 3/4" carbs on from a sedan or 2" carbs? What manifold did you use? Was your engine highly modified? Did you spend time tuning the SU to your engine? Just want to understand your statement of losing power when switched over to SU's from webers.

Thanks
 
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Old 07-12-2019, 11:39 AM
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This was back in the 1970s but, I believe, the carbs and manifold came off a Mark X (as did my front brakes on that car). We did tune the carbs to the engine (this was when I was working in a Jaguar shop). The engine had a race cam in it, the head ported and polished, and it was fully race prepped by the prior owner. But going from 3 Webber 45s to the SUs really had an adverse impact on performance (the car was a ton more drivable but that was because I no longer had a leaking gasket).
 
 
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