Good Day All,

I have a MK2 fitted with a 4.2 engine and a pair of 2" HD8 carbs (ex - 420 Jaguar). Needles are UE (found the UO's too rich). I'm having problems synchronizing the SU's using a Gunson Carbalancer - the reamost unit seems to suck much more that the front one. I've turned the idle screw all the way down but it still sucks away merrily. Using a torch and mirror the butterfly disc seems to be completely closed, as is the front one. I understand that, at idle, the idle screw is the only supply to keep the engine running. The carbs were rebuilt by myself prior to fitment and I've tested for leaks around the spindle using WD40 and all seems airtight. The inlet manifold is also from a 420 and so has two vacuum take-offs (one for the brake servo and the other for the heater air supply). I blanked off the heater one with a piece of rubber hose and jubilee clips so I don't think that is leaking (but I may be wrong ??). If that were leaking, would or could that be the cause of the problem. If not, does anyone else have any suggestions - I'd be very grateful for any help/ideas. Many thanks.

 

Would air leakage via a worn spindle or bush cause this?

 

Hello Paddy. Carbs were rebuilt with new seals etc and I’ve checked for any leakage at spindles using WD40 with engine running (no increase in speed). I’ve ordered new seals for the idle screws just in case air is getting past the screws (they look ok). Will change those tomorrow but I’m not holding my breath - lol).

 

When the carbs were off the engine did you hold them too the light and make sure there was a minimal gap around the disk?
Might also be work loosening off the throttle shaft to make sure it turns freely, then loosen the screws on the throttle disk and push the throttle shaft fully closed, then tighten the screws. The disk may not be exactly centred in the bore.

If you drilled holes in the throttle shafts for the stop arms it's easy to get the position slightly out and the throttle shaft won't fully close before the stop arm hits the body of the carb.

 

G'day Captain Q
It sounds like you may have a vacuum leak into the inlet manifold. Have you tried clamping off the vacuum line to the brake servo? You may have a a leaky servo diaphragm Also it may be worth getting a length of 1/4 inch flexible pvc tubing (in OZ we call it brewers tube), put one end to your ear and go around the manifold and carbies listening for the sound of air "hissing" with a manifold leak.
Does your distributor have a vacuum advance? Another possible source of a leak.
Cheers
Bill Mac
MK1
MK2
XJS3
X300
About 14 previous Jags.

 

Yep!, Air Leak, False Air, Butterfly not closing completely likely culprits.

 

Thank you everyone 👍👍👍. It’s been around five years ago since I rebuilt the carbs and I seem to recall that I did have to drill a hole in the new shaft to mount the rear carb stop, so that is a strong possibility - I’ll certainly check that out later. I currently have the servo pipe disconnected and have replaced it with a short ‘bunged’ hose, so that’s not the source of an air leak, nor is the advance pipe (I previously ‘bunged’ that as well). Other than a leak at the manifold to head or carb to manifold, for which I’ll try as suggested with a length of hose, the only other possible source is the currently ‘bunged’ take-off on the manifold for a 420 heater. Only trouble is that it’s situated in the most inaccessible place, right behind the linkage (If I take the linkage off I may be able to check it out). I’m thinking the drilled shaft may be the most likely culprit however. Will check it later and report back. Thanks again everyone !!

 

Hi Everyone,

PROBLEM SOLVED I'M REALLY PLEASED TO SAY. Having tried all the obvious possibilities (drop test of both pistons, swapping dashpot/pistons front to back, double checking float bowl settings, linkage adjustments, jet centralisation, sticking pistons, checking for leakage at butterfly spindles, new seals on idle screws, correct oil in dashpots, checking for vacuum leaks at manifold take-offs) I finally concluded that the problem was confined to the body of the rear carburettor in some way. With the engine idling (and the linkage disconnected and the pancake filters removed) I could see that the piston on the rear carb was noticeably higher than the front one (swapping dahpots/pistons over front to back didn't move the discrepancy). As stated at the start of this thread, my MK2 is fitted with a 4.2 XJ engine (large valve head) with 2" HD8's and pancake filters - the carbs originally came off a 2.8 Series 1 XJ6 and between the carbs and the manifold is an aluminium spacer. The XJ (and the MK10, which also uses HD8's) utilise a rubber circular seal each side of the spacer and the four nuts that fix the carb to the manifold, sandwiching the spacer, have small springs that compress as you tighten the nuts, squashing the rubber seals to form an airtight seal. After 50 years of use these springs have obviously lost some of their strength and that allows them to squash more than originally, meaning that the nuts bottomed out on the stud (which has a wider, shouldered, unthreaded section) before the seals were completely sealed and airtight. Those springs, unfortunately are no longer available so I used similar, but stronger and slightly longer, springs used to secure brake shoes. Once fitted and tightened I started the car and 'Hey Presto' car now idles as it should, with equal heights on both pistons. It may also result in better performance overall, as it was likely sucking too much air in during acceleration as well. Hopefully, the fuel consumption will also be better (finger crossed).

 

Good catch!

There is a good variety of Thackeray washers available that could also do the trick instead of a standard spring washer.

 

Thanks Glyn.

Having got it running OK (still need to fine-tune it however) I'm leaving well alone, as the brake springs seem perfect for the job. Time to look at the next job on the list I think (you know - that list that gets longer every time you take a classic car out - lol !!).

 

Agree ~ leave well alone. Just file away in you memory if you face a similar problem in the future.

 

Those nuts under the carb (particularly the rear one) are a literal pain to get to, let alone put a spanner on, so it's onto the exhaust mounts now as it hangs a little low and keeps catching on local speed bumps - lol !!

 

Those nuts under the carbs have pushed many to use language that would blister paint.

 

I've got quite the selection of modified sockets and bent wrenches to get into places where no man has gone before !

 

Hi JeffR1. Yes, but have you got the fingers of a contortionist to put those spanners/sockets on the nuts - lol !!