Pre-Cat engine cutting out only after prolonged high revs?

[JamesK]

I seemingly have an odd and annoying symptom in my pre-cat BV. It has quite a few non standard parts including an additional ECU so I appreciate a specific diagnosis isn't likely - I just wondered if anyone had suffered the same general problem and could give me an idea of where to look next.

After a prolonged period at high revs, somewhere around 4.5-5.5k but no specific point (I've replicated the problem at under and over 5k), the rev counter stutters, dies and the engine cuts out/stops responding.
All other electrics work normally at all times.
If I stop the car the engine restarts first time and drives normally straight away.
I can go past 4-5k under full load fine and go through the gears under accelaration etc. The engine only cuts out after a period of 10 seconds or so at constant load above 4-5k.

In normal road use this never happens as I don't hold that sort of rev range for any period of time. It has only happened 3 times in 2 years but sadly 2 of them have been at an expensive day at Goodwood where there are fast flowing corners where holding a gear at these revs is efficient. I am very mechanically sympathetic so it's not just a case of being driven to the point of failure (I hope!).

It seems electrical where the engine suddenly loses spark or something? So far I've tried changing the coil, the king lead and generally having a poke about but I can always replicate the cut out by going down a quiet A-road in 2nd gear afterwards. I do have a crank sensor which I am tempted to look at next but I don't seem to have a 100 amp fuse as I gather earlier cars didn't.

Any ideas humbly and gratefully received...

[MPoxon]

Hi James, iirc you have the Omex 150 running with the 14CUX setup by Mark Adams. The Omex systems are very reliable so I doubt it is that but you can eliminate it from your fault finding by unplugging it and re-connecting the ignition amplifier and going back to the dizzy to control the ignition advance via the vacuum. Give Mark a shout if you are unsure.

Also might be worth having a look at the engine earth strap.

[blitzracing]

The big clue is what the rev conter does- if it drops faster than the engine, then its loosing its primary ignition trigger, be it omex or amp.

[JamesK]

Thanks for the thoughts. The rev counter drops instantly to zero like a switch being flicked which is what I notice first. A split second after that I realise the throttle is non responsive and what's happened. The engine is obviously still turning over in gear at this point so it's like a total electrical failure but only of the ignition as wipers, lights etc all function normally?

[blitzracing]

JamesK said:

Thanks for the thoughts. The rev counter drops instantly to zero like a switch being flicked which is what I notice first. A split second after that I realise the throttle is non responsive and what's happened. The engine is obviously still turning over in gear at this point so it's like a total electrical failure but only of the ignition as wipers, lights etc all function normally?

Id do some really basic tests- get yourself a small low wattage (important) 12 volt lamp on the dash, and connect the positive side to the coil, so its on when the ignition is on. Then see if this goes out when the fault occurs. If its still on when the tacho dies then the supply is ok. The coil feed is ballasted, so you get 12 volts with the coil switch open, and about 9 volts when it closed to the lamp will flicker a bit, but should not not go out. As the tacho is dropping you know its supply or switching circuity, so this will eliminate supply.

Edited by blitzracing on Tuesday 29th April 18:13

[Rob_the_Sparky]

May not be related but just in case it helps:

I had a fault on my 95 500 where when driving it long distances it would start to cut out for a short period then come back to life. Would start to notice it as a mis-fire at particular revs, then developed into increasingly long periods of no power (rev counter also dead) that really got quite unnerving!

TVR power tried replacing the fuel pump relays wen they did my Taraka upgrade but it still did it, but only after 20 to 30 minutes on the road, and worse on faster roads. I replaced the throttle pot and since then it has not repeated the issue. Maybe worth a try as it is a quick and easy replacement, although I am running the stock system so this may all be irrelevant!

Rob