Triumph Stag runs fine then just stops

[Ian]

Please help! I am racking my brains trying to get to the bottom of a running problem on a Triumph stag, it has the original Triumph engine which has had considerable rebuilding work over the past few years by reputable specialists but, over the last few weeks the car has been developing a running fault. It will intermittently drop to seven cylinders and then back to eight. Then when it's running well it will just stop for no apparent reason. It doesn't splutter to a stop, its more like the ignition has been turned off. It will always restart without problems, run for a few seconds (maybe ten or fifteen) and then just stops again, it carries on like this afterwards.

So far, I have checked the ignition timing which is spot on (Was popping back through the carbs and spitting flames from the tail pipe), carried out a compression test which reveals 90,90,90,100,110,100, 95,100 which seem really low to me but surely this wouldn't cause the engine to just stop?
New Distributor cap and rotor arm (Has electronic ignition so no points / condenser etc) new high quality ignition leads, replacement of the original fuel pump with a facet pump, checked the diaphragms in the strombergs which are brand new, carbs have been rebuilt, new high quality ignition coil ( have also tried earthing the coil bracket as had a similar issue with my Scimitar) New fuel filter, new NGK spark plugs ( I believe some classics run better on Champions? )

Today while adjusting the timing I had a zap from the distributor (Dizzy itself is brand new) this happened several times despite being the second new cap, rotor arm and leads, I have tried bypassing the fuel inertia switch too but nothing seems to make a difference, please please please someone HELP

[Luxobarge]

New Distributor cap and rotor arm

My first reaction is that's your problem, right there.

The quality of most after-market ignition parts is absolutely appalling, many on here will tell you they're simply not fit for purpose. It's really important to get good quality items, and most well known parts suppliers (especially the one best known in Triumph circles) sell cheap Chinese-made stuff that's best suited to land-fill.

There may be other suppliers of decent kit out there, but I always get stuff from Distributor Doctor - http://www.distributordoctor.com/ - it's not much more expensive, he's a really nice helpful chap to talk to on the phone if you need to, and then at least you know you've got quality parts.

Of course your fault may not be this, but you won't know until you've at least eliminated the HT parts. And if you've fitted the cheaper parts, if they're not the fault now they soon will be.

Lovely car, the Stag, one of my favourites - how about some photos?

Cheers!

[Ian]

cheers Luxo, would a sh%& quality cap be the cause for being shocked every time I touch the dizzy? Got some pics of the car on my camera will post them up soon, its a black stag with black trim but quite unusual in that it has crome and black wheel trims rather than the five spoke alloys

[Richard Moss]

The black plastic often has a load of carbon in it to give it the colour - and that rather defeats the object of using plastic as an insulator! I had black, Lucas branded rotor arm on my MGC that was faulty right out of the box - and it caused me a lot of head scratching because I had replaced the rotor to try to cure similar problems to the ones described. A second rotor arm fixed it immediately.

[JPB]

For all of the compression figures to be that far below the correct 160-190psi (assuming this is an HC engine at 8.8:1), the valve timing is almost sure to be off. Check the chains and the tensioner. are the camshaft sprockets worn? Is the tensioner rising to the occasion properly, good spring action and plenty of oil coming up?

I believe that you have more than one issue with this, check the valve timing first, and while you're in about at that check the jackshaft for movement as it doesn't take much slack in the distributor drive end to cause odd misfires. Then, once the valve timing or other cause (unlikely) of the poor compressions has been addressed, chase the leaks from the distributor.

[Richard Moss]

For all of the compression figures to be that far below the correct 160-190psi (assuming this is an HC engine at 8.8:1), the valve timing is almost sure to be off.

Or the compression gauge faulty. I had that one myself

[Luxobarge]

Yes, dizzy cap could WELL be why you got a jolt!

Richard's story about the rotor arm above is typical, folk on here could bore you for pages and pages on this thread with similar stories!

As above, I wouldn't trust my compression guage further than I could throw it, but I do trust it implicitly for telling me the relative differences between the cylinders, which 9 times out of 10 is what I really want to know. In this sense, yours sound fine to me, I'd have thought that if the car runs fine with plenty of power except when it's misfiring, then the timing's likely to be OK. Checking it's free though......

Cheers!

[Wicksy]

I tend to agree with the stuff about ignoring the compression results - Did you put oil in the bores first? Dry rings can give a low reading but as yours are consistent I wouldn't worry too much and I don't think that is causing the cutting out.
The voltage leak from the dizzy should not be happening, the insulation of the cap or leads is breaking down intermittently by the sound of things and the easiest way to fix that is fitting known good parts.
Go with Luxo's advice

[JPB]

Valve timing, not ignition timing would explain the cutting out and the low compression figures. On this engine it would also explain the ignition problem - as a result of jackshaft movement - if the cause were poor oil feed to a tensioner or worn teeth in a camshaft sprocket, since ignition settings couldn't then be accurate or consistent. Right, so some cheap distributor caps are terminally leaky these days but mostly for Lucas applications, this Delco distributor is still well served for split new ones and the arm doesn't make direct contact with the spindle so that's a less likely source again.

Yes, a compression tester could give erroneous results if its thread were leaking or air was escaping past a dodgy O ring but presumably a pro engine builder would have eliminated that as a possibility long before admitting to a customer that the engine on which they'd just spent a fortune was low on compression?
It's a fabulous excuse but you'd surely just get them to do it again. Properly.

[Ian]

Finally I sorted it this morning. Changed the cap and rotor arm again for new items but no change but I will post the cause here in case someone else has a similar issue in the future. I think I have already said that I had tried earthing the engine to the body which had made no difference. Anyway, was really ready to take a hammer to it this morning to be honest then found the bolt into the engine which holds the braided earth strap didn't look properly tight. Tried to nip it up with a 1/2 in spanner but it was just going round and round, yeah it was bolted into an alloy casting and some heavy handed monkey had took the thread out meaning the bolt was more or less balancing in the hole so the earth would make and break contact. Having removed and re tapped the bolt I relocated the earth strap to the iron block and hey presto runs like a good un! Thanks to you all for your suggestions, it's nice to know there are some forums where the members actually know what they are talking about. Thanks guys

Now to get the valve timing checked