[fatdarryl]

I have an intermittent part throttle miss on my twinspark, I have checked the ignition system generally to no avail, swapped out injectors and all sorts. It happens usually when I'm cruising along for a way with the throttle reasonably constant, then accellerate. It's a constant, stuttering misfire, if I hold the throttle steady when it's missing it carries on, if I let off then press the throttle again it goes. It seemed to me to be when theres a substantial change in vacuum, so Ive checked the vacuum hoses and started to look to the AFM, but cleaning the track inside etc has not changed things at all. I have tried with 2 different fuel pressure regulators, as well as using 2 different ECUs and I'm running out of ideas. Not checked compression yet, but I dont lose any fluids or burn oil at all, so I'm confident that is OK.

um..

HELP?

 

[Mound Dawg]

Are you still running the aluminium cover plate over the plugs? If so, try taking it off. It doesn't do anything other than sometimes cause the spark plug leads to arc out and lose the odd spark.

I always remove these after getting a misfire on a 164TS about 15 years ago.

 

[fatdarryl]

I have run it both with and without, I actually bought the car with it missing, it has been off for at least the last 6 months, it misses either way. Also, i noticed a similar miss when running a chipped ECU a while ago, but at that time it always worked fine on the standard chip, so effectively, it was worse with more ignition advance and I guess has worsened so that now it does it on the standard advance too, so i have checked and it misses on 2 separate stock ECUs and a chipped one. Maybe I'll swap out the leads, Although they are reasonably new.

 

[fatdarryl]

one thing is coming to mind, Jim you have started mne thinking on the whole cover thing, I also don't have the little plastic part that the ht leads sit on as they go over the cam cover, I need to check the leads are not arcing here, this would surely account for it being worse on the chipped ECU, as more advance = more comp at ignition = arcing looks good to a HT lead compared to firing a plug... I also run sporty coils so higher voltages in the leads...hmmmm.... you think that good leads are 'likely' to arc if they are touching each other?

 

[fatdarryl]

I've temporarily insulated the HT leads from the engine with rubber, will see how it goes!

 

[v6junky]

I heard a recommendation some time ago of revving the engine in pitch darkness. If you see a spark, you know where your problem lies. I would also look into the possibility of bad grounding somewhere.

 

[fatdarryl]

So.. after isolating the HT leads, no change :-(
one of the problems with revving in darknes, is it only happens when driving, I cannot get it to miss while the car is stationary, but I guess it could still be arcing, just it's not enough to stutter the engine with no load?
grounding could be impossible to find, all the main grounds are fine and no other electrical issues ( clear ARC lights and everything!)

I'm going to try a load of different coils next. Does anyone know how to electrically test the coil modules?

 

[fatdarryl]

no visible sparks in the dark!

 

[dpberner]

Have you checked the distributor "timing" recently? Rotation of the distributor body doesn't change the actual spark timing, since that comes from the ECU. But, if the distributor body were rotated a bit, it could cause loss of ignition at max (or min) advance. This would happen since the rotor would no longer be pointing directly at a spark-lead at the desired time of ignition.

If the distributor body were not clamped tightly, the body could conceivably rotate over time due to friction with the shaft. Rotation in this direction would eventually cause dropout at max advance, or poor functioning with the chipped ECU, which might use more ignition advance.

Anyway, just a guess. I suppose since there are two distributors it is not very likely that they are both mis-timed.

 

[fatdarryl]

no, you are right, it can happen, but mine are bang on where they should be.

I have tried some different coils, and at the moment that seems to be a big improvement... I wont say fix..yet! (touching wood right now)
So that may be the problem

 

[fatdarryl]

Looking like it was the coil for sure, just drove about for over an hour with no hint of a miss.