Still a bit of a missfire...

[1396midget]

Evening All

P6 is going like a rocket at the moment (so rockety that I accidentally got it into second in the wet about 2 yards from where I'd started..), but I'm still hunting a slightly elusive missfire.

New plugs (NGK BP6ES), new rotor arm, new cap, new correctly gapped points (with the protective coating removed ), timed to perfection and mixture set blue at idle. And Magnecores.

It goes really well, but occasionally has a higher speed miss that sometimes results in popping, banging and fire from the tailpipe (4000 rpm upwards), enough fire to light up the surrounding areas in the dark

which is funny, but not ideal.

Any ideas? It's a lot better than it was with cheapo leads on, now its an occasional irritation rather than a barrier to speed, and the car remains totally usable, but I'd like to know what it is...

Might be the needles being very rich at the top end? (they're KK) Don't think it's the fuel pump because of the explosions...

[chrisyork]

Well the conventional answer would be a tad too weak and a tad too advanced.... But I really do think only a rolling road will sort that out. Sorry

Chris

[harveyp6]

Fire from the tailpipe sounds very exciting! It can only be unburnt fuel igniting for whatever reason (yet to be discovered) but I would just be re-checking all the normal engine tuning suspects and I'd be very disappointed with myself if I couldn't sort that out without resorting to putting the car on a rolling road.

[corazon]

Rob, you're not using the ignition amp still are you?
What timing have you arrived at with your mallory?
I've enriched my carbs recently and running at 6 btdc, infinitely better than it was.
Really flies now, and mpg is better too. Crazy as it may sound i made 30 mpg on a long run last week
I too though have that high speed miss occasionally now.
Next on my list is that msd ignition box, so i'll let you know if that helps mine -you could get a more complete burn with the multi spark i guess.
Oh what coil are you using?
Jim

[Demetris]

It can only be unburnt fuel igniting for whatever reason ...

... at the wrong place, usually because the ignition is switched off momentarily for some reason.
Perhaps some connection along the feed to the coil gets rather overexcited at high revs and wants to relax for a second.
Just to rule out (or verify!) this, you could wire a temporary warning light somewhere in the car, feeding it straight from the coil, and see what happens at the moment of the misfire.

[1396midget]

It's ever so slightly on the rich side at idle, and the profile of the KKs makes me think that weak at high rpm is unlikely, particularly the fire... I've had lean backfires before and it's a different sound, more a pop than a sharp crack like this is (and the Kmidget also, when it has it's 'choke' on)

currently set at 4 degrees BTDC. Guess a full re-check of everything is in order, more tinkering (yay!) It's not running the ignition amp yet, as I thought adding more unknowns into the mix without it running flawlessly first was potentially not clever.

it's running one of those standard silver intermotor coils, a ballasted one.

I'll check the coil feeds, the misfire is evident on the rev counter as a little 'blip', if it's bad enough to make fire there's quite a waver on the needle. A little light is a good idea though!

does sound exactly like it's giving up one of the 8 sparks at higher rpm, then burning that5 cylinder's fuel when it gets to the end of the pipe. It's only running the front silencer (the rear one was comically restrictive) so a lot easier for that lot to get out.

Cheers guys, some things to try, helpful as always

[harveyp6]

does sound exactly like it's giving up one of the 8 sparks at higher rpm, then burning that5 cylinder's fuel when it gets to the end of the pipe.

Check for crossfiring.

[1396midget]

Check for crossfiring.

The Magnecores (red ones )are set out as per the original diagram in the manual, so I was hoping that crossfiring wouldn't be occurring. Still possible however - how would you check to see if it was doing that? All is fine at all rpms not under load. Perhaps I should tape my camera into the engine bay and go for a drive in the dark..

[chrisyork]

Ahh, so the misfire is rather larger than I understood! To make the rev counter shimmer is a big one! That implies an LT intermittent fault surely? And more of a very momentary multi cylinder hesitation? That really would set up the conditions for a backfire!

Why not find an old 12V coil from the tool box and jury rig it with a feed off the washer bottle or similar. That ought to eliminate an ignition LT generated problem. Still leaves you with a discussion between a faulty coil and an intermittent open circuit in the ignition feed So why not rig up a small bulb in parallel with the feed to the coil that you can run into the car? If the interruption in feed is big enough to register on the tacho you ought to be able to spot the bulb flicker.

My thoughts from this would be to eliminate the coil and also to eliminate a wiring fault on the LT, most likely an internal derangement of the ignition switch!

Then we could get back to proper engine set up issues.

Chris

[V8P6B]

There's 2 things that spring to mind with me. I found out by accident that my car wouldn't start with the rev counter removed, so could a fault with the rev counter or it's wiring, cause your symptoms? (You mentioned a rev counter shimmy)

The other thing is, with a points setup, could contact breaker bounce at high revs cause a misfire (4000rpm's quite high...)

Just my (random) thoughts...

[1396midget]

so far nothing done to attack this - having more fun with a free rotating alternator pulley...

My thoughts are possibly with coil / LT side - points bounce isn't something I'd considered however, I'll look into it.

The rev counter has been added by me, not that long ago, but will also check it's wiring. Certainly that missfire is a lot better than some of the TVRs and one of the BV8s at harewood hillclimb yesterday.

[1396midget]

was a dodgy coil. Swapped the Intermotor one out for an old AC delco spare and we're all the way to 5K without a hint of a miss.

going propely like stink now, quicker than dad's subaru (in a straight line..)

[corazon]

That's very good news Rob!
Jim