Backfiring & carboned plugs - resolved

A follow-up on an earlier post where I had backfiring through the carbs.

When I first got the bike I replaced the original carboned plugs with NGK Iridium plugs. During a carb rebuild I'd found #2 carb was missing the #40 jet. The jet was later found hiding between the float bowl and the top of the carb. Must have either been wedged in there from the PO or was loose in the bottom and ended up there when I turned the carbs over. I never heard it rattling around though.

After rebuilding the carbs and before replacing the carb boots, I drove it to see how it acted. I wasn't happy with the way it ran & there were backfires coming through the carbs. I looked at the plugs and they were carboned, just like the original plugs were.

I adjusted with a colortune and took it for another run. This time it bogged down and would only idle, any challenge to the engine and it would stall. I ended up pushing it back to the house. I put in the original plugs and it fired right up and ran well, some but little backfiring. It was here I was told about needing to have the air filter on (which I normally would always do but I was still working on this & wanted the space to get my big hands in there.

I did though follow the advice and put the boots in and it stopped 99% of the backfiring through the carbs. then back assward I checked the valve settings and found I needed to replace three shims. One of the shims turned out to have some definite wear & looked for all the world like an etching of a bush. I couldn't detect any wear on the cam lobe but replaced that shim with another that needed exchanged and now it miced correctly so I only needed two shims, not three (lucky I found that problem).

Synced the carbs with a homebrewed mercury manometer and re-colortuned now the filter was in place and it ran beautifully. BUt...

I had these pricey new Iridium plugs and I wanted to use them, not throw them out but they were still blackened even after cleaning them a few days back. The bike wouldn't run with them before (that was when I had to push it back) but I put them in figuring the tune-up would be the cure but again, the bike wouldn't start without backfiring and stalling.

So I had an idea & that was the plugs must still have some carbon tracing left between the cathode & anode so I put three of the original plugs in & left the one Iridium. The bire ran poorly and there wassome backfiring but I took it out anyway and after 3-4 miles the backfiring stopped. I took it for maybe 10 miles, came back & checked the iridium plug and sure enough, it was white on the tip.

I put another Iridium in and brought the other two & the wrench with me & took a trip, stopping to put another Iridium in & then lastly, the other one. The plugs all got all the darkness burned off and the bike runs like a top with these plugs.

I'm still though not convinced it's prudent to run a resistor plug with resistor caps. I'm thinking of just taking some small alligator clips and soldering some tiny brass screws the size of the ones in the plug caps to the end, screwing those into the end of the plug wires and clipping them to the top of the plug & then wrap with plumbers teflon tape to prevent arcing.

The plugs have 5K ohms resistance & that I believe is the proper resistance to make the TCI happy.

SO... it was the carbonization on the plugs that was the issue with the Iridium plugs, apparently they do play nice with a properly tuned XJ.

 

These bikes tend to "eat" plugs if you ride them at extended high speeds (freeway commuting, for instance) so expensive plugs are kind of a waste.

The book calls for NGK D8EA in the 550 and BP7ES in the 650 so that's what I run with no problems. I just buy them by the box and replace them once the electrodes start to lose their shape. Neither are resistor plugs.

 

"I'm still though not convinced it's prudent to run a resistor plug with resistor caps".......why?

 

If you run resistors in series you add the resistances. If you run resistors in parallel you reduce the resistance.

With a resistor cap which is 5K ohms & then series a resistor plug which is also 5K ohms you get 10K ohms & that's 5K ohms more resistance that the TCI apparently was designed to work with.

From what I've learned here, part of the resistive effect is to delay the spark although I don't see how that happens enough to have an effect but what a resistor does do, is to reduce the voltage in the circuit. Too much resistance and you can whittle your spark to a minuscule amount. How much that is or if there's a negative effect of merely doubling the expected resistance to the plug, I don't know. That's why I figure it may not be prudent to double the resistance.

 

Thanks for the update, KA1J. Are the iridium plugs worth the price, IYO?

 

First off - the TCI sees the resistance of the coils, not of the plugs.

Second - 5K or 10K is nothing compared to the resistance of the plug gap.

The resistor increases the spark voltage, but decreases the duration. Here's what happens:

The primary in the coil is energized, building up a magnetic field. When the TCI cuts the power to the primary the magnetic field collapses. That collapsing field induces a high voltage potential in the secondary. At this point it's very similar to a battery.

As the voltage increases in the coil (and the attached spark plug wire) it ionizes the gas between the electrodes. When the gas is sufficiently ionized to conduct electricity (at that voltage) a spark jumps the gap - draining the coil. The extra resistance of a resistor or resistor plug increases the resistance in the circuit, requiring more ionization in the gap before current will flow. So the voltage with a resister will have more opportunity to peak before it's bled off by the plug firing.

You can see this in operation with a fuel fouled plug (like your iridiums were). The fouling conducts some electricity, draining the coil before enough voltage builds to jump the plug gap. If you loosen the cap so it doesn't contact the conductor on the plug the coil can't be drained through the shorted plug. The voltage in the secondary will build until it can jump that extra gap (resistor) you created. At that point there is enough voltage to also fire the plug. From time to time I've managed to salvage a fuel fouled plug doing this until it heated up and cleaned itself. Seldom works and way more trouble than it's worth though.

Which brings me to why I don't bother with the iridium plugs. Fuel fouling is quite common on these motorcycles. The iridium will foul just as easily as a conventional plug, and both are usually ruined when it happens. Much less painful to throw away a $2.75 plug.

 

Hi Micarl,

Perhaps the majority of what you say is true, sounds right to me. But, no, there is not more voltage with a resistor in line. A resistor does just that: it resists electricity. If you measure your voltage with a decent multimeter, you will read less voltage after the resistor and you will also have less current. That's just the way it works. If you see one of those power resistors, the ones with inductors in ceramic/sand you will find them getting hot. The reason they get hot is that is where the differential energy is being dissipated. YOu'll find them in watt ratings where voltage times current = watts. They're rated in their dissipation value ie: 5W, 10W, 20W & so on.

But a resistor does not increase voltage. It will... lengthen the duration of the energy passing through it.

 

gee, i use non resistor plugs and non resistor caps, it's a wonder it runs at all

 

i just jam the wires on the end of my plugs. (well, not really, but it works . . .)

 

Hey Fitz, How ya doin'?
How many miles do you get from your plugs, before replacing them?
Thanks! Bill

 

And that is a KEY point when you're trying to set light to a combustible mixture....especially if it is very rich, or very lean.

 

LOL do I detect a bit of facetiousness? I surely am not the one to know these bikes, I'm just learning from the ground floor. I do understand electronics though and all I've said is the reality of how resistors work in a circuit.

I have read here, in XJbikes, that 5K ohms is the resistance the output of the coils needs to see. I am not questioning that nor am I thinking a different method is better.

My statement about prudence relates to my not trying to re-invent the wheel but rather maintaining the expected design parameters.

Personally, it seems to me that there should be zero issue with running non resistor plugs and non resistor caps. To my thinking, the the ignition system should not be so fragile as to be damaged by such non-resistive terminations but if the wizened people on the forum feel it is important to maintain this resistance, I'm surely not going to be the one to try to outguess them.