High RPM miss-fire

[steveww]

Just want to check my reasoning here.

I have checked the mixture with a wide band Lambda probe and the engine is running typically at 12.7 AFR and goes to 13.8 AFR at aprox 5000rpm. So it is leaning out a little bit at the top but is still well within accepted levels. At bit rich but OK but max power is at 12.6 AFR. However I get a slight miss-fire above 5000 rpm. My guess that it must be ignition related. Any guess on too much or too little advance?

I will check the static timing and advance with a strobe next. I have a spare dizzy which I really must get round to sending off for a rebuild.

[type26owner]

Steve,
Try closing the spark plug gaps to .020" (0.5mm) and see if that makes it go away. That's the condition that the cylinder pressure is highest and the peak voltage in order to jump the plug gap is the greatest. Assuming you're not using points anymore you most likely have some dielectric insulator giving up the ghost somewhere. It can even be internal to the coil were there is a short between the coils.

If it were a racecar with no charging system then I'd immediately suspect the battery voltage is dropping too low. We always run just a .020" gap on our race engine's spark plugs to delay the onset of the misfire from a flat battery.

[steveww]

Just pulled the plugs to check the gap etc. Noticed that 1&2 are rather sooty and 3&4 looked OK. Hmm time to double check a few things......

These are Stromberg carbs. Checked the needle seating. Checked the jet height. All OK so far.... Started checking all the linkages and mountings. Noticed that the choke was not being pushed fully home. Adjusted the cable and normal operation resorted. Now the choke body is on 1&2 with a cross bleed pipe to 3&4. My guess is that this is what was causing the over richness on 1&2. I do not have time for a test drive tonight to confirm this

I will be able to give it a run in the next day or so and I will post up my results. I will also double check the timing at the same time. I have checked the static setting and all is OK.

[type26owner]

Steve,
I've already explored how lean it needs to get before one can expect a lean misfire up above 5000 rpms. It does not happen until you're above 17:1 mixture value. You're probably okay with the mixture you've stated. Seek out an ignition related issue instead.

However, the wide-band O2 sensor does kinda averages out lean and fat cylinders so just looking at the AFR is not really enough analysis of the info. Send me a logfile and I'll have a look at the scatter pattern and look for other carburetion problems.

[steveww]

I was thinking that the over rich mixture on 1&2 along with the sooted up plugs could be causing the missfire. I will also go through the ignition system. Checking out the leads, cap, rotor etc. At least I spotted the choke thing and that can only make things better.

As I said it is only happening at high rpm so it must be a weakness in the system rather than something that is actually broken. These tricky little problems are always a PITA

[type26owner]

You're correct that could be the problem but only if the spark plugs are too cold. Do any of the plugs look like the ideal self-cleaning one does here? If none of them do then install hotter plugs. Sooted plugs will short out to ground through the carbon.
http://www.ngksparkplugs.com/techinfo/s ... country=US

The only time I managed to wet foul spark plugs that were the proper heat range was when the mixture accidently got below 9:1. Err, it also emits clouds of black smoke at that point.

Close down the plug gap to help make it more tolerant to all these potential problems.

[steveww]

Plugs 3&4 were ideal. Nice and clean with a brownish colour. 1&2 were very black and sooty but dry. I think normally the plug temp is fine. I am using Champion RN7YCC. I should have chance for a quick run out tonight to see how it goes. Will check the gap and go for 0.5mm

[type26owner]

Hey Steve,
Finally found a spec on those Champion spark plugs. The 'R' at the beginning of the part number means those are resistor type. The center electrode has a 5k to 10k ohm resistor built-in and buried inside the ceramic body. Those types of Champion plugs are famous for having the resistant go up to 1 or 2 mega ohms and causing a miss. Check the resistance with a volt/ohm meter on the carbon fouled plugs. Best to check the resistance of any new plug you plan on installing. I use Autolite 64 resistor plugs because they are cheap, provide noise suppression, are gentle on the bakelite insulator parts and still allows me to use stranded copper plug wires.

Closing down the plug gap won't cure the resistor going wonky though.

[steveww]

Just installed a new set of plugs, just to be safe. Double checked that the choke was shutting off properly then went for a blast round one of my local loops.

Missfire has gone, she now pulls cleanly all the way to 6500

I have not pulled the plugs yet to check the colour as she is still rather hot. However my gut feel is that this has cured the problem. The engine is pulling better and the sound is crisper. I will pull the plugs tomorrow morning and check their colour.

Having just come back from some rather rapid motoring it make you realise what a fantastic machine the Elan is. I blew the doors off a couple of modern hot hatches

I will check the resistance of the old plugs I just removed. I will post an update tomorrow.

Thanks for your advice.

[steveww]

Checked the resitance of the old plugs and all was OK.

Pulled out the new plugs after thing had cooled down from the run. All of them a nice even brownish colour

Looks like problem solved. I guess it was the soot build up that made the spark weak leading to the missfire.

[type26owner]

Steve,
Are you being careful to tighten the spark plugs by the same amount which is normally 90 degrees more than finger-tight? They are heat exchangers afterall and if the thermal connection to the head differs much then they will tend to cause that kind of trouble. If need be you might have to run a hotter plug just on the cylinders that are fouling unless you're willing to find and fix the root cause. From your PITA comment I doubt that will happen soon. We all share the pain.

As far as the color of the insulator goes I prefer a hotter plug. Want the exposed part of the ceramic insulator to be a whitish-gray and only the portion recessed well down into the cavity to be brownish. If you have a darker brown stripe that shadows underneath the grounding tang down the insulator then for sure the plug is too cold. May I suggest you try running a hotter plug. Typically the plugs differ by 80C increments as you step through the heat range. When the plug is too hot and it causes auto-ignition to happen, it's easy to hear the pinging/pinking. You'd have to be a dunce not to notice the change immediately if your hearing is still acute.

Only wish the spark plug was not so much off center in the combustion chamber. Bet having dual spark plugs and dual ignition systems would have been the ideal setup for this design of head.

[steveww]

I always put the sparkplugs in with a torque wrench. Aluminium heads don't take too kindly to overtightening things.

I will give your idea of a hotter plug a test. Can't remember which way the temp numbers go for champion? Do I need a 6 or 8

[Hamish Coutts]

If my memory serves me correctly with NGK the lower the number the hotter. Champion is the opposite the higher the hotter.

Regards,

Hamish.

[M100]

>type26owner
>Bet having dual spark plugs and dual ignition systems would have been the ideal setup for this design of head.

There were pics of a twincam dual plug conversion floating round the other week (probably the main yahoo elan list)