Lean Misfire

[Bill Blue]

In short, what contributes to it? I was running with narrow band oxygen sensor reading in the double digits (that's way lean) with no misfire under light acceleration. I now will have misfire whenever the readings get down 200-300 range. Like a fool, I made several changes at once to cause this, wondering which ones I should change back to. The changes are:
Points to electronic ignition
Plugs two heat ranges colder
Resistor to non resistor plugs
Timing bumped a few degrees

I'm especially interested in the effect of going from cold resistor plugs to normal non resistor plugs. If I buy new plugs, should they be resistor? I'd go out and change them, but it is a bear of a job with the turbo and it is hotter than a two dollar pistol.

Bill

Bill

 

[RootesRacer]

Ok leaner mixtures are tougher to light off, if you went to colder plugs, the ionization voltage will go up, which can make a marginal spark weaker yet.

I would not use non-resistor plugs, there is no good reason to do so, same with wires, you want suppression wires.

To make the colder plugs work (assuming there was a reason you went to them), you may need to reduce the plug gap.

 

[Bill Blue]

Got that backwards, went to a hotter plug. Non resistor was an "added benefit". Asked the guy for the same plug, only hotter as the old ones looked pretty brown. While installing them I noticed they were non resistor.

Sort of the same thing on the plug wires. They are Accel spiral wound wires, the box says 7,000 Ohms/ft, they test at about 2 Ohms. They are what the engine was assembled with and have not changed them. Should I? What am I giving up by using these wires?

Bill

 

[RootesRacer]

Non resister plugs are a no-no with just about any electronic ignition since they radiate more EMI.

Non suppressing wires are a definite no-no for the same reason, and becuase they are the radiators for the EMI.

I'd still close the gap on those plugs and see if the miss goes away.

 

[Bill Blue]

I'd still close the gap on those plugs and see if the miss goes away.

Not a chance in the world. If those babies come out, they get replaced with resistor plugs. Way, way to much trouble as they need to be replaced anyway. In addition, they are not gapped that wide. The cold resistor plugs, used with the points ignition were gapped at .032". I gapped these at about .035", figuring to get some advantage of the hotter spark.

Thanks
Bill

 

[RootesRacer]

Not a chance in the world. If those babies come out, they get replaced with resistor plugs. Way, way to much trouble as they need to be replaced anyway. In addition, they are not gapped that wide. The cold resistor plugs, used with the points ignition were gapped at .032". I gapped these at about .035", figuring to get some advantage of the hotter spark.

Thanks
Bill

Heres the thing with boost, the higher the cyl compression pressure, the higher the voltage required to arc (ionization voltage).

When it comes to ignition misfire, and the ignition is not lighting off reliably, there are 2 things you can do:

1) Put more energy to the plug.

2) Reduce the plug gap.


#2 is a band-aid but the simplest and cheapest way to go.

Also you can help things out by making sure your plug boots are tight to the insulator and to use a liberal amount of dielectric grease on the inside of the boot. This helps keep the arc on the inside of the plug instead of arcing on the outside of the insulator.

 

[Bill Blue]

Jarrid, all this is without boost. I decide to make some changes to the throttle plate and was tuning the carb without the carb bonnet when I ran into what I have to consider excessive lean misfire. I am running slightly richer than before, with more misfire.

I went to electronic ignition to ensure I have enough spark for the boost I will run.

I will stick with the .032" plug gap and will increase the amount of dielectric grease, but am at a loss as to what I can do about plug boot tightness. The plug boots seem a little bit loose, but stay on.

Bill

 

[RootesRacer]

Electronic ignition alone will not insure an adequate spark.

Its more of the coil and the dwell strategy than anything else.

A simple check of how good of spark you have is how far will it arc, and how hot does the arc look (fat and blue).

If you are running duraspark II with its correct "blue" coil and series "ballast" resistance wire, this has proven to be a very adequate setup IMO.

Along with boost, arc voltage increases as a function (as I said before) cyl compression pressure, which means that its harder to arc a plug at WOT naturally aspirated than it is to arc at 20 inches of idle vacuum.


The reason the plug boots should be tight around the insulator is to deny the high voltage an opportunity to arc across the insulator, it needs air to do this.
If your boots are not tight, replace your wires or make a custom set with boots that are tight.

 

[V_Mad]

I would rule out plug heat range. Plug gap is important, and there will be a range outside which you will get misfire. Timing may affect it too if way out.

Resistors in both the plug AND leads may reduce the spark energy; I would use supressor (resistor) leads with no-resistor plugs (or vice versa as second choice).

Going from points to electronic ignition may be your problem if the electronic system is not correcly optimised or is faulty. Try reverting back.

 

[Bill Blue]

It appears the problem was a bad ground on the spark box. Was sort of a stupid move on my part. The spark box came with a nice big ring on the ground wire, so I bolted it to the chassis. There is a ground wire I could have hooked it to three inches away. Really stupid part: When I wired the car, I put the ground wire there to ground a spark box.

Duh.

Bill

 

[skywords]

It appears the problem was a bad ground on the spark box. Was sort of a stupid move on my part. The spark box came with a nice big ring on the ground wire, so I bolted it to the chassis. There is a ground wire I could have hooked it to three inches away. Really stupid part: When I wired the car, I put the ground wire there to ground a spark box.

Duh.

Bill

Bill
Your story reminds me of my last Saturday's experience. I was called to my shop by a customer with a bad mag on her Cessna 182. So I proceeded in the usual manner of troubleshooting by un cowling the engine and asked her to start the engine and switch to the bad mag and run it for a short period to find the cold cylinder.

When she started cranking the engine I noticed smoke coming from the P-lead on the Left mag. Now the P leads ground the mags shutting them off. The p leads also are shielded wires for noise suppression. It had melted the insulation on the wire shorting it to ground.

The root of the problem was who ever installed the engine omitted the grounding straps on the mounts so the starter current was going through the P leads. Funny how many times it is a bad ground that causes so much fuss.

 

[V_Mad]

Glad you solved it Bill, but puzzled as to why bolting it to the chassis didnt work. Also I wonder where your other earth wire (3 inches away) goes to that works so well.

 

[Bill Blue]

Glad you solved it Bill, but puzzled as to why bolting it to the chassis didnt work. Also I wonder where your other earth wire (3 inches away) goes to that works so well.

Bolting to the chassis did not work because of paint. I thought there would be enough contact area through the bolt and threads. Not so. The other wire is part of the ground system I wired into the car. There are seven ground wires and a ground cable that meet at a stud.

Bill

 

[Bill Blue]

I'm sure glad I said it "appears" the problem was a bad ground. The next time I took the car out for a spin, the misfire came back. To make a very long story short, I am now running points ignition. Think the electronic ignition "box" has an intermittent problem that is getting worse.