Topic: Tech Question - what would cause a plug to not fire at higher RPM's?

[Geeto67]

Here is a riddle: what would cause a plug to not fire at higher RPMs?

Bike is a 1975 CB750F. It has a martek 440 ignition, andrews coils, jeg's 8mm carbon supression core wire that tested out at less than 350 ohms resistance per foot nad had continuity. New NGK caps that tested under 10 ohms resistance. Spark advance works fine, and the martek is timed as best you can time one of those things. Stock coils had the same symptoms.

Cleaned all the plugs last night, fire the bike up and it's idling on all cylinders. Every exhaust pipe is getting scalding hot. So I take it for a 10 mile ride around town including a jaunt on the highway where it felt pretty good. Get back to the garage and #1 exhaust pipe is cold. Check the float bowl and it's full of fuel. Pulled the plug and it's fouled but functioning. wire wheeled off the fuel and carbon and hooked it up, nice fat spark. Decided to rev it and noticed that the spark goes out at higher revs (between 2.5K and redline). It's just that cylinder, and it's fine at idle and just above idle but as soon as it revs up it doesn't fire. What would cause this? I'm thinking it might be a defective plug wire but let's hear from the experts.

[Deltarider]

How wide have you gapped the plug(s)? Carbon core HT wires are not recommended for our bikes. If the gaps are within limits, I'd experiment with a non carbon HT wire.

[Geeto67]

plugs are gapped at .035

[madmtnmotors]

Well, you have definitely narrowed it down to spark (not fuel), and you have isolated it to one cylinder only, so that rules out the electronic trigger. The fact that you have the same condition with two different sets of coils would likely rule out the coil, even more so that the issue occurs on a single output.

The only things left that I can imagine are:
>Poor connection between the plug wire and the coil or plug? (least likely IMHO, but still possible)
>Bad spark plug boot. When you say "under 10 ohms" I'm guessing you meant "under 10K ohms"? I had a spark plug boot whose resistance would spike when it got hot. Ran fine when cold, but resistance would go over 20K ohms when hot. They are a PITA to check when hot, and you have to work fast! Of course, when the resistance started to rise the offending boot would start arcing against the head instead of passing the spark to the plug. This made it very easy to diagnose in a dimly lit garage (along with the associated "ticking" noise generated by the arc)...
>Bad wire
>Bad plug. I have seen reports of counterfeit NGK D8EA spark plugs that exhibit poor reliability.

Deltarider's post brings up a question of my own. I have seen a couple of posts lately where resistor wires are used with resistor plug boots. I was under the impression this was an either/or, in that you would use either one but not both. I have used resistor wires with plain old rubber boots with Dyna 3ohm coils and Dyna S ignition without issue. I have the same setup (Dyna 3ohm coils, resistor wires, plain rubber spark plug boots) with a Dyna III ignition that has been in place for 30,000 miles.

Are resistor boots required (or recommended) when using resistor wires? I've never combined the two...

[scottly]

Get back to the garage and #1 exhaust pipe is cold. Check the float bowl and it's full of fuel. Pulled the plug and it's fouled but functioning. wire wheeled off the fuel and carbon and hooked it up, nice fat spark.

Carbon means too much fuel. Once the plug is fouled, it weakens or kills the spark altogether. Using a wire wheel is not the best way to clean a fuel fouled plug; I've had much better luck using a propane torch to burn the carbon off.

[Deltarider]

plugs are gapped at .035

That's a bit wide. Try gap within Honda specs. Impedance HTlead and plugcap should not exceed 10Kohm.
But bear in mind that carbon core cables can give problems.

[madmtnmotors]

Get back to the garage and #1 exhaust pipe is cold. Check the float bowl and it's full of fuel. Pulled the plug and it's fouled but functioning. wire wheeled off the fuel and carbon and hooked it up, nice fat spark.

Carbon means too much fuel. Once the plug is fouled, it weakens or kills the spark altogether. Using a wire wheel is not the best way to clean a fuel fouled plug; I've had much better luck using a propane torch to burn the carbon off.

You cut the quote short. He continues with:

Decided to rev it and noticed that the spark goes out at higher revs (between 2.5K and redline). It's just that cylinder, and it's fine at idle and just above idle but as soon as it revs up it doesn't fire.

[Geeto67]

Carbon means too much fuel. Once the plug is fouled, it weakens or kills the spark altogether. Using a wire wheel is not the best way to clean a fuel fouled plug; I've had much better luck using a propane torch to burn the carbon off.

It can also mean weak spark. Thanks for the tip re propane. Will have to try the heat wrench on it. To be fair hitting it with carb cleaner takes most of it off before I wire wheel it.

[Geeto67]

plugs are gapped at .035

That's a bit wide. Try gap within Honda specs.

Its a force of habit, I think I have gap-ed every plug in every vehicle I have ever owned at .035. I'll give it a try, but I think I might be leaning toward the resistance wires and the NGK caps not being compatible. I have a set of old accel solid core wires I have run on my cb750K for 20 years, might try swapping them and see what happens.

[scottly]

No, a weak spark is caused by carbon fouling. If the spark is the issue, you won't light the fire, so the plug will be wet but clean. You may want to try a new plug.

[scottly]

Get back to the garage and #1 exhaust pipe is cold. Check the float bowl and it's full of fuel. Pulled the plug and it's fouled but functioning. wire wheeled off the fuel and carbon and hooked it up, nice fat spark.

Carbon means too much fuel. Once the plug is fouled, it weakens or kills the spark altogether. Using a wire wheel is not the best way to clean a fuel fouled plug; I've had much better luck using a propane torch to burn the carbon off.

You cut the quote short. He continues with:

Decided to rev it and noticed that the spark goes out at higher revs (between 2.5K and redline). It's just that cylinder, and it's fine at idle and just above idle but as soon as it revs up it doesn't fire.

This might be an indication of a plugged up emulsion tube on that carb. If you want to rule out the ignition system, swap the #1 and 4 plug wires: if the problem moves to 4, it's ignition, and if it stays on 1, it's the carb. Bear in mind that that plug may be damaged beyond cleaning, so it wouldn't hurt to replace it.

[madmtnmotors]

Get back to the garage and #1 exhaust pipe is cold. Check the float bowl and it's full of fuel. Pulled the plug and it's fouled but functioning. wire wheeled off the fuel and carbon and hooked it up, nice fat spark.

Carbon means too much fuel. Once the plug is fouled, it weakens or kills the spark altogether. Using a wire wheel is not the best way to clean a fuel fouled plug; I've had much better luck using a propane torch to burn the carbon off.

You cut the quote short. He continues with:

Decided to rev it and noticed that the spark goes out at higher revs (between 2.5K and redline). It's just that cylinder, and it's fine at idle and just above idle but as soon as it revs up it doesn't fire.

This might be an indication of a plugged up emulsion tube on that carb. If you want to rule out the ignition system, swap the #1 and 4 plug wires: if the problem moves to 4, it's ignition, and if it stays on 1, it's the carb. Bear in mind that that plug may be damaged beyond cleaning, so it wouldn't hurt to replace it.

I was under the impression that the spark plug was removed for the testing and that the spark was observed visually (effectively running on three cyls) at both low and higher rpms. This would take fuel metering out of the equation and reduce it primarily to wire, boot, wire/boot combo, or plug, or was an inductive timing light used? Now, if the plug is fouling to the point of being "damaged beyond cleaning" that could very well be a contributing cause, but the fact that the plug will spark at low rpm kind of makes me think this is not the case.

But I've been wrong before!     

[madmtnmotors]

To clarify, if an inductive timing light were used to determine spark, then at higher rpms the main fuel circuit would begin to feed fuel. If the main circuit were overly rich, could it be richened enough to the point of snuffing out the spark? I know nitro fuelers have this issue, but they are effectively filling the combustion chambers almost to capacity with almost pure liquid...

[Geeto67]

I was under the impression that the spark plug was removed for the testing and that the spark was observed visually (effectively running on three cyls) at both low and higher rpms. This would take fuel metering out of the equation and reduce it primarily to wire, boot, wire/boot combo, or plug, or was an inductive timing light used? Now, if the plug is fouling to the point of being "damaged beyond cleaning" that could very well be a contributing cause, but the fact that the plug will spark at low rpm kind of makes me think this is not the case.

But I've been wrong before!     

you are correct, the plug was removed for testing and when I rev'ed  it up the spark disappeared. I was holding the plug and wire grounded against the head so if I broke the ground I would have known really quickly via shock.

[Don R]

 Spark leak to ground or excessive ohms? Just for fun, measure the ohms from plug cap to plug cap on that coil. When a plug can't fire the charge seeks a route to ground. The more voltage, the greater the need to ground it's self.
 My buddy cranked his drag car engine with a plug wire off and the MSD burned a hole through the new rotor. Extreme case, but illustrates my point. Those are my thoughts, I'd double check float level in case it's a double cause.
 Don't forget, this is fun.

[grcamna2]

I'm with Wilbur on a faulty wire plus I think it's recommended to have a wide gap for hot coils.
Those suppression wires have a tough time making contact if you mess with them.

[madmtnmotors]

if I broke the ground I would have known really quickly via shock.

AMEN! I hate it when THAT happens...

[madmtnmotors]

Don't forget, this is fun.

    

[grcamna2]

Wilbur,
I know off-topic a little but how's that HondaMan ignition box running for you ?

[scottly]

Regarding my statement about damaged beyond cleaning, I once chased my tail for about a week due to bad plugs. It turned out that I had corrected the carb problem that caused the original issue, but the symptoms remained. After hearing the motor run, a friend said "Your plugs are fouled." "I just cleaned them!!??" (using a wire brush and carb cleaner) "Your plugs are fouled", he stated again. Since there was nothing left to try, I put in a set of new plugs, and the damn thing was fixed.

[madmtnmotors]

Wilbur,
I know off-topic a little but how's that HondaMan ignition box running for you ?

HA! I'll know more once I actually get it installed... 

Hoping to get another "project" underway that I can use it on. Stay tuned! Geeto's "Patina Flip Special" that we are troubleshooting here has got me thinking about a budget oriented build... 

[Jerry Rxman Griffin aka MuthaF'er]

So you swapped plug wires between cylinders, did you swap plugs too?

[madmtnmotors]

Regarding my statement about damaged beyond cleaning, I once chased my tail for about a week due to bad plugs. It turned out that I had corrected the carb problem that caused the original issue, but the symptoms remained. After hearing the motor run, a friend said "Your plugs are fouled." "I just cleaned them!!??" (using a wire brush and carb cleaner) "Your plugs are fouled", he stated again. Since there was nothing left to try, I put in a set of new plugs, and the damn thing was fixed.

No doubt about that. That's why I also mentioned the counterfeit plugs as a known issue as well. Plugs can indeed fail but still "look" good. I've learned to keep new plugs on hand, in stock.

Make sure to only change one thing at a time! You mentioned replacing the wires, as well as replacing the plugs. If you replace them both at the same time you won't know which item was at fault. 

[Geeto67]

So I swapped out the plug, and the high rpm no fire went away. So I guess it was just a defective plug.

I figured out the running rich problem, the carbs on the bike are 657B units running 120 main jets. The original carbs on the bike (which were unusable) are 064As running 105 jets and by the way apparently the slow jets, the emulsion tubes, the needles, the jets, and the settings are all different for these two different carbs. I found another set of 064A carbs in my stash and am going to rebuild those.

[Deltarider]

So I swapped out the plug, and the high rpm no fire went away. So I guess it was just a defective plug.

Just curious, how wide did you gap your new plug?