Jaguar XK140 Coughing

[jjbradders]

Hi All

Restoring a 1955 Jag XK140 for a friend and have fitted triple Weber 45DCOEs in a hope to get a few more horses (plus they look amazing!)

However the rear two cylinders are coughing very frequently at idle and touching the throttle just makes the whole situation much worse. This never happened when it had twin SUs fitted. All I've seen are yellow flames around the throttle butterflies on the odd fire. I'm pretty sure it's not the cam timing as I've set this twice, plus it's only on the rear cylinders.

Can anyone diagnose this? I've fitted a new coil and have an electronic ignition set up on order, but that's more just to compliment the carbs.

Any help much appreciated
Cheers

James

[eldar]

Triple 45DCOE, great fun, and a dark art to tune, balance and properly fettle.

Would 40DCOE have been a little more appropriate size, 45s might be a little large unless cams, porting and the like have been done?

[jjbradders]

Well that wasn't an option in the kits! Apparently they work well but I could be wrong. The main reason they were swapped because the old SUs were too worn (the car hasn't run since 1992, does now but badly)

I've had another fettle with the ignition timing and it's running a lot better so I think I'm getting somewhere.

Cheers

James

[TonyRPH]

When I previously had flames / fire in the throat of the carb (also DCOE) it was an over rich mixture.

Are all your main and idle jets the same size, and emulsion tubes the same type?

Also - if the butterfly is held open ever so slightly (usually by poor linkage) and the mixture is on the rich side, this can also cause a similar issue.

The above assumes ignition timing is spot on.

Are the carbs in good condition and the butterflies nice and clean around the edges?

In the past when having trouble tuning multi carb setups, I have found it beneficial to isolate all the throttle mechanisms, just to eliminate linkage issues. Obviously it'll be tricky to rev it like this, but it should idle perfectly.

EDIT: Forgot to say - check the float level on the offending carb before anything else.

Edited by TonyRPH on Sunday 24th April 16:36

[lowdrag]

These were on the car, along with Lumenition, when I bought it 16 years ago and apart from the usual spitting back (especially when cold) I've had no problems. The engine is rally specification with different cams, pistons, con-rods etc and more bhp which would account for some spitting back I guess. I would mention that instead of mesh filters I put foam ones on and they lasted no time at all, the intake eating them and leaving no filter. So back to the mesh ones.



And yes, they are DCOE 45's.

[jjbradders]

Thanks for the help guys, tried everything you said and it now runs a hell of a lot better.

Still spitting slightly but at least it revs properly AND runs on 6 now!

Car looks fantastic! Sadly I'm the wrong side of the voting age so haven't been around long enough to identify cars from their engine bays, so what is it? Looks E-Type esque?

Cheers
James

[lowdrag]

Just a Jaguar-based replica, so you were nearly right. It uses Jaguar mechanicals. Good observation!

[TonyRPH]

Lowdrag, did you ever live near Apperley Bridge (Leeds / Bradford area), alongside the Leeds / Bradford canal?

I was once walking along the canal, and saw one of those (or very similar) in a garage and I went and had a brief chat with the owner.

I subsequently saw it in the Guiseley (Leeds) area, but haven't seen it for several years now.

This was around 2002 / 2003 I think.

[jjbradders]

XKSS?

About a year ago I was taken for a spin in a replica XKSS that looked extremely similar at the NEC?

[lowdrag]

No, a southern boy who never got further north than Leicester, and living in France since 1999 now. Hope the restoration of the XK finished well. A lovely car the XK140, although I've always preferred the sleeker lines, especially the bumpers, of the 120.

[jith]

lowdrag said:

No, a southern boy who never got further north than Leicester, and living in France since 1999 now. Hope the restoration of the XK finished well. A lovely car the XK140, although I've always preferred the sleeker lines, especially the bumpers, of the 120.

Lowdrag it would appear from what you say that you have never taken your lovely motors over the Rannoch Moor or soared along the gorgeous roads on the Isle of Skye. You don't know what you're missing!

Get up here and we'll go for a drive and I'll show you some of the most beautiful places on the planet; roads that were built for proper motor cars.

J

[lowdrag]

Sorry I was talking about living. Back in the day we got as far north as Dornoch in the midget. We are planning a trip of three weeks in September next year in Scotland using the E-type. about 3,000 miles all in I reckon.

[jjbradders]

Sounds like a decent trip that.

I'm trying to cobble together a 1979 Fiat X1/9 for myself, which is my start in the classic car world! This Jag is the first time I've ever worked on carbs actually so it's a learning curve...

Sadly the Fiat doesn't look quite as majestic as your replica lowdrag!

That was from a few moths back but not much has changed!

[a8hex]

How much of the inner wing did you have remove on the XK140 to get the Webers in?
Even the triple SU conversion needs things doctoring to make enough room.

Pictures would be nice. There are never enough pictures on XKs
Oh and which flavour of XK140?
Is it listed on xkdata, it's always nice to keep track of the XKs which are still around. If the owner doesn't want it publicly listed then there are a couple of archivists who track XKs who'd like to know about it without listing the chassis number publicly.

Lowdrag, All XKs are beautiful :-)

[jjbradders]

Pretty small section actually, the rear webers fit fine without modification and I only had to cut the area to fit the trumpets. Does however mean the front carb must be mostly disassembled to fit it in.

It's a LHD roadster that was converted to RHD in the early 90s. From what I know: imported 1990, driven until 1992 then garaged ever since.




I'll try and get more pictures soon.

[jith]

lowdrag said:

Sorry I was talking about living. Back in the day we got as far north as Dornoch in the midget. We are planning a trip of three weeks in September next year in Scotland using the E-type. about 3,000 miles all in I reckon.

That's great. Pop in and say hello, I'm 5 minutes from Glasgow Airport.

J

[jjbradders]

Well the coughing has pretty much gone!

Sadly it now struggles to rev, just dies down if you open the throttle quickish and only revs if you gently open the throttles. Any more ideas?

Cheers

James

[TonyRPH]

jjbradders said:

Well the coughing has pretty much gone!

Sadly it now struggles to rev, just dies down if you open the throttle quickish and only revs if you gently open the throttles. Any more ideas?

Cheers

James

Ideally you need to get a CO tester on it, to see what the idle mixture is like at least.

Weber carburettors use the idle jets post idle - e.g. on part throttle just prior to the main jets coming in to play. It depends on several factors, but the idle jets can be operative (under no load) up to as high as 2000 - 2500 RPM).

The symptoms you describe could be one of several things; (in order of possibility)

Carburation
Air leaks.
Float levels incorrect (either too high or too low) - this is always the first thing to check.
Accelerator pump jets too small
Accelerator pump jets too large
Accelerator pump diapraghm failure
Venturis too large
Idle jets too large
Idle jets too small
Incorrect emulsion tubes (although this would usually only be noticeable under actual driving conditions except in extreme cases - likewise with air correction jets)

Ignition
Check timing, is the vacuum advance working?
Are the plugs gapped correctly?
Are the points gapped correctly (if present)

IIRC Haynes used to do a manual on side draft Webers - and it contained a guide to optimum jetting / emulsion tubes and venturis.

For a CO tester something like this Gunson is ideal for initial setting up, and then once you have the basic initial setup, take it to a rolling road.

ETA: Here's a Weber sidedraught manual on Ebay.

And another of the same manual

Edited by TonyRPH on Monday 25th April 22:26

[jjbradders]

That's fantastic information John, thanks!

The carbs are all new and allegedly came jetted suitable for the Jag, just to get it to a rolling road. I imagine that it's the dizzy potentially screwing things over. I have to retard the timing to start it then advance again to run, which would be problematic for normal driving to say the least.

I'm going to fit one of these 123 electronic ignition distributors instead but I'm waiting on that.

Has anyone actually driven an XK, and what are the things to watch out for?

Cheers

James

[TonyRPH]

Is the vacuum advance (if present) pipe connected to the right place, on not straight into the manifold?

It sounds as though you might be getting full advance as soon as the engine starts.

In case you're interested, (although this may be difficult in your case) you can readily see the carburettors working when revving the engine, as looking into the choke tubes, you should see the emulsified fuel being drawn from the auxiliary venturis (the smaller round bits in the throat of the carburettors).

The aux venturi is 17 in the diagram below.