340 ENGINE SNATCHING

[PARKIE007]

I have a 1968 340 which runs well apart from occasional 'snatching' or 'coughing'. This seems to get worse when the engine warms up. The car has a Powerspark electronic ignition and the HT leads and coil were replaced when this was fitted some years ago. I believe it to be a fuel-related problem as there is always a strong smell of petrol immediately after it happens. I recently removed and cleaned the solenoid in the starting carburettor because the engine was idling too fast some of the time and this is now cured. I do wonder however whether the solenoid may be linked to this intermittent coughing? I have treated the petrol with a carburettor cleaner but this has had no effect. The insides of the carburettors seem clean and the fuel filter was replaced recently. I have also emptied the fuel tank to ensure there is no 'crud' coming through. The petrol pump was replaced a few years ago and the breather pipe is clean. Ideas please? Chris.

[B87UL]

Chris

I would check out the continuity of the HT leads and caps initially, it is not unknown for an initial break in a HT wire to lead to a wider gap as the spark erodes the broken wire further.

Although the spark plug gap controls the HT voltage, as the broken wire gap widens the coil output will increase but eventually be overcome at higher engine RPM. At that point a misfire will develop.

You need also to be aware of a similar issue developing in the distributor rotor arm.

Norman

[PPV]

I have a similar problem sometimes. I installed a Powerspark system to try and cure it together with new coil and leads etc but it still prevails sometime. The petrol smell you have is probably when it 'coughs' and sprays a bit out of the carb. I likewise have checked my carbs, fitted a service kit which helped with smoother running but I still get the 'cough' occasionally. I have checked the timing with a timing light and all seems Ok.
One way to check if it is the starting aux carb is to get the engine up to temp and disconnect it at the carb (easy to do I found) and blank it and the connection to the inlet manifold off. Seemed again to make no difference to mine though. Let us know if you find a solution as I will if I do!

[davidr]

It strikes me that the petrol smell will be unburnt fuel as a result of the occasional misfire which perhaps points to the carbs working fine but an occasional loss of spark?

[PARKIE007]

Thank you all for your replies. Loss of spark is not something I had considered, but the HT leads and caps are more or less new, having been replaced just 13000 miles ago, as was the coil. That was when the electronic ignition was fitted in 2014. I am obviously getting some unburnt fuel as on a trailing exhaust, ie: when not accelerating, there are consistent but small backfires. I understand this to be unburnt fuel exploding in the exhaust pipe. When I say small, they can hardly be heard but they are there. Perhaps if I could pinpoint which exhaust pipe it is, I could then look more closely at the relevant carb? I shall also take your other comments on board and let you know how I get on. Thanks again. Chris.

[davidr]

Another possibility is that the carbs are over-fuelling? That might explain the misfire as maybe the mixture becomes too rich and the backfire on the overrun?
Have you pulled a spark plug to see if it's soothed up?

[B87UL]

Chris

If you are 100% sure you have perfect ignition, then I suggest you should look for vacuum leaks. A leak in the heater or brake tank vacuum circuit will result in no1 cylinder running weak. Don’t assume the ignition components are in working condition because they were renewed only a short time ago.

It is a mistake, made by many owners that a higher voltage coil is a “must have”. Spark voltage is a result of plug gap, or gap in the HT circuit including the rotor arm. If a poor connection or gap exists in an HT lead for instance a high voltage coil will really go to work and increase the gap, before long the engine is experiencing irregular misfires.

If a vacuum leak has persisted for a while you will find someone has attempted to tune out the problem and the jet drop position in each carburettor will be different. Check this by removing each carburettor dash pot and accurately measure the jet drop below the bridge on each carburettor; you are looking for a jet drop of 0.065 inch on each unit. If you find the rear carburettor is set further down suspect a vacuum leak and find the source before resetting the jets. It is worthwhile temporary blanking off the vacuum feed to see if the idle speed alters.

Next, whilst you have the dashpot removed, switch on the ignition and check by sight that the fuel level is about 0.125 inch below each jet bridge and is not overflowing the top of the jet.

Note. Either an exhaust leak or weak mixture (vacuum leak) will cause popping back in the exhaust in overrun conditions as will tight tappet clearances.

Another likely problem is restricted float chamber breathing, ensure you have TWO special internally serrated fibre washers in the CORRECT position on each float chamber overflow pipe.(S.U part AUC 1928). These special washers need stacking in twos as on higher powered large engines S.U found breathing restricted after tightening down on just a single washer.

…………….and yes I spent a lot of time at S.U in my younger days!!

[PARKIE007]

Thanks again all. I have cleaned and gapped the plugs, which although a bit sooty were correctly gapped anyway. This appears to have helped already and the spark appears strong and bluish, although the colour is difficult to see properly. The ignition timing is about 28 degrees BTDC at 2000rpm with the vacuum off and about 22 degrees BTDC at 1500rpm with the vacuum off, which I think in each case is about 4 degrees more advanced than it should be. The engine is not pinking so I feel inclined to leave the timing alone for now and see how the car runs. I did check the tappet clearances recently and changed the shims as necessary to widen any gaps that were a bit too tight, so I can at least rule that out as a cause. However, when I have the time I shall go through all the points you have put forward and post the results here. Thanks again - and it is particularly helpful to hear from someone who knows SU carbs so well - I do indeed only appear to have one serrated washer on each chamber overflow pipe! Should these be just below the cap or underneath the overflow pipe?

[B87UL]

Chris

The serrated AUC 1928 special fibre washers are located adjacent to the float chamber lid, in other words the first items positioned after the lid is placed in position. The serrated washer is followed by the overflow pipe, then the alloy washer and finally the cap nut.

Warning........Watch that assembly sequence because the illustrations in some Jaguar parts manuals are incorrect but the description in the text usually reads correct as "adjacent to the lid".

The problem was possibly caused by S.U illustrations showing correct numbering / positioning but their actual illustration showing a serrated washer instead of an alloy one under the cap nut, although this requires their drawing to be enlarged to see clearly. Jaguar appear to have used the problematic S.U drawing in their later parts manuals, although early E type manuals are correct !!

[piman]

Hello Chris,

can you describe the symptoms with a bit more detail, ie, when does this 'snatching' occur, revs, gear, engine hot or cold for example?

Alec

[PARKIE007]

Hi B87UL. Thanks, I have ordered new washers and will fit them in the next couple of days.

Hi Alec. It is difficult to be specific as the snatching can occur at any time. However, it tends to happen most when I am accelerating hard and when the engine is warm. I intend to check the fuel flow rate as another possible cause.

I am planning to fit a Broquet fuel catalyst to improve combustion - does anyone have any experience of these?

The car is going to the garage on Tuesday for a final finish after a full respray, so I shall be unable to do much more before heading back to the UK with her on 30th May. I shall be at the CMC event on 1st June to celebrate the 60th anniversary of the Mark II launch and shall have the 340 with me, so if any of you will be there and want to have a chat please let me know. Best regards, Chris.

[piman]

Hello Chris,

I would say that a fuel supply problem would give a longer duration problem than just a hiccup or so, more of a lack of power which is only restored by closing the throttle. What oil do you put in the dashpots, it should be an SAE20 grade, i.e not a thin 3 in 1 type oil. This can give a momentary flat spot when openeing the throttle if the level is low or the oil is the wrong grade?
This sort of problem can be tricky to resolve especially if you can't replicate it while stationary and monitoring various parameters. I had a misfire problem with car one time that I was convinced was dirt in the carburettor and it was only by using my strobe light that I found it was an ignition problem (using the strobe to monitor the presence of a spark) which turned out to be a weak \intermittent capacitor in the distributor. I'm afraid it's a bit of a process of elimination.

Alec

[PARKIE007]

Just to let you all know that I have finally resolved this problem. I discovered several tiny rust holes in the brake servo vacuum reserve tank which were obviously letting air into the system. Just enough to make the engine run very slightly rough and then snatch occasionally. The reserve tank has been sand-blasted, coated with resin and then given four coats of protective paint and the car now runs as smooth as silk. Incredible that such tiny holes can affect the whole running of the engine! Anyway thank you all for your ideas and I'm delighted that this is now sorted. Hopefully this post will be useful to someone else who has the same problem one day.