The Triumph Dolomite Club - Discussion Forum

I had an issue at the track this weekend with a misfire at high revs (6000 plus) under load with full throttle. It disappears when feathering the throttle but revs don’t rise. I end up changing up to a higher gear or overdrive. Tends to happen when engine is hot, not on the warm up lap and interestingly most apparent on third and third overdrive (hardly at all in second so load does seem to be part of the issue). I also had a random cutting out and difficulty starting but that I’ve put down to a faulty centre contact in a new distributor cap.

So far I’ve done the following:

Checked the resistance of the primary and secondary windings on the coil. I have three coils of the same brand and the readings are all very similar and in the expected range. Coil was getting hot but not too hot to hold.

Checked for blocked jets and correct operation of the pump jets (Dellorto 45s). Floats okay. Sponge air filters cleaned and re-oiled.

Spark plugs (NGK BP7EFS – type used for a decade) are relatively new and electrode colour is perfect. 33 thou gap as usual. Timing maxes at 32 degrees at 3500rpm, which relates back to about 10 degrees at tickover.

Checked the electric fuel pump and even swapped to a new spare one in desperation. Both appear fine. Good flow reaching the carbs so filters(2) not blocked. Fuel is fresh (Avgas)

Feed to the coil from starter motor is 8.5v, but only while turning over, so functioning correctly. I have a separate ballast resistor which feeds 6.2v when connected to the coil (12v when disconnected and under no load). Feed to Magnetronic unit is 12v.

I have another meeting next Saturday, but as this is a 6-7 hour round trip away and the last this season, I want to do everything possible to ensure I don’t have the same issue. As the fault is really a high speed under load one, I don’t have any opportunity to test on public roads.

So, I have also changed the distributor rotor, cap and leads. Engine starts easily and revs fine in neutral. I've also disconnected the rev limiter, change light and tacho, wiring back to the standard in dash tachometer. If the fault has disappeared I can progressively add those back in.

I can’t find any detail anywhere on testing the Magnetronic unit. Anyone any ideas? Is there anything else I’ve overlooked that anyone can suggest?

I’ll go with spares of everything I can think of, but I still have no real idea of what is going wrong, so not knowing where to start if the problem persists is frustrating

Geoff.

So many choices! But load missfires are ALMOST always HT related, so coil, cap, rotor, leads and plugs are all under suspicion. Don't ignore the plugs just because they are newish and look OK, a hairline crack in the porcelain is enough to give you a problem.

Steve

Seems you did check all the usual suspects already. The only thing I can think of but probably not you would have noticed yourself is an alternator what is faulty sometimes. Under load you need all the voltage at the ignition system/coil. When 14v is just enough and 12v just too less in an extreme situation. I never experienced such a fault but when you replaced all there is not much left.

Jeroen

I had this fault myself when using Dellorto 45's after a long period of tickover while waiting to join the track I just put another new set of NGK's in and all was well, there after I always had a new set if the fault returned.
I suspect low voltage as Jeroen suggests is the likely culprit.

It did turn out to be a low voltage issue of sorts, but from an interesting cause.

I had recently replaced the distributor cap, rotor (both a reputable German brand) and plugs. The former are the top entry 'Mini' type rather than the standard Sprint as the Dellorto manifold allows these to fit. I had taken the cap off a couple of times and done a visual check, nothing appeared amiss. When I got the car home and took everything off, this time the centre carbon contact had broken off and part was lying in the distributor.

Replaced with an old side entry cap and rotor and all the problems had gone away.

So intermittent / poor contact between the broken parts of the carbon contact were causing a voltage drop at that crucial time of trying to start, resulting in poor starting. Similarly at high revs the lower part of the contact started spinning and losing contact, hence the symptom of misfire. Eventually the two parts separate enough for the end to fall off.

Not a problem I've ever seen before an hope I never do again, although I'll remember to look. There is some truth in the old adage 'if it ain't broke, don't fix it'.

At least the day's racing at Taupo last Saturday went with a hitch. Interesting to race with the only other Dolomite racing at present in NZ, not a Sprint but a Leyland P76 powered 4.4L V8. Nigel has had the car about 10 years now and it is beautifully presented (he also has a very nice 2 door Toledo and TR7 3.5V8 in the same matt black finish). Not that much faster than me which made for a lot of fun when they let me off 5 seconds before him. Took a few laps before he could pass even after he'd caught me up. There was also a heavily modified TR7 (5.0L TVR motor) which did leave me for dead in a straight line!
Geoff