ST1100 spark plug cap resistor

TMF

TMF

Joined
Oct 27, 2021
Messages
110
Age
67
Location
Brookeville, MD
Bike
1992 Honda ST1100
I've been reading up on the OEM spark plug cap and how the resistors can go out of range. I measure two and get 4.7K ohms and 5.8 ohms. Replacement resistors don't seem to be a thing. I was thinking of wrapping copper wire around a thin bolt to a length of roughly 17mm the same as the resistor length and inserting that over the spring. Has anyone tried something like that or alternatively know where to source the resistors?resistor.jpg
 
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Neither do I - but the problem is probably not with the resistor, so replacing it won't help.

I think the wrong resistance is a symptom rather than a faulty resistor - a change in resistance can indicate poor conductivity or poor insulation in the cap itself. My 1300 developed a fault in the ignition circuit - which turned out to be a combination of corroding copper core in the HT leads, and issues with the plug caps. 2 of the caps proved to be faulty. One had a resistance of 4.5K ohms, the other a resistance of 5.5k ohms. The resistance was a symptom of the actual fault - ie a faulty cap. I doubt that it was a faulty resistor, though I never checked that.

I verified that the caps with the wrong resistance were indeed faulty by refitting them once I had eliminated the fault with brand new caps and new leads. (I didn't want to throw away good caps if the fault had been with the HT lead). The fault returned.

All of the new caps were 5K +/- 0.05k

The write-up of that problem is here.

(5) ST1300 Poor Running - HT Leads & Plugs - Solved | ST1300 Tech | ST-Owners.com
 
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In the CX500/650 etc. community (and others) the solution is to toss the resistors and either replace them with brass rod cut to length or too the caps and use non-resistor versions.

The consensus is that the original intent of the resistors (not affecting your neighbours radio) doesn’t really apply any more so, similar to bypassing the 1100 vacuum fuel valve, toss them and move on.
 
I measured the resistance of the wound copper wire in my photo and there is no measurable resistance! I'm shooting from the hip but everything else being equal that would drop the circuit ohms from 25000 to 20000, get rid of a rare but known point of failure and generate a slightly hotter spark. Would there be any downside?
 
It really depends on the electronic noise thrown out by your ignition system and if it (the noise) can bother other electronic hardware on your bike. The short answer is probably not. If it does you will be the first to find out. Why are you even checking the resistance of your plug connectors? Do you have a rough running engine? If so, why not just replace the caps as John suggested? If everything is working ok, are you looking for something to fix?
 
SMSW said:
It really depends on the electronic noise thrown out by your ignition system and if it (the noise) can bother other electronic hardware on your bike. The short answer is probably not. If it does you will be the first to find out. Why are you even checking the resistance of your plug connectors? Do you have a rough running engine? If so, why not just replace the caps as John suggested? If everything is working ok, are you looking for something to fix?
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When I picked up the bike it had a weak battery and stale gas. It lost a cylinder or maybe two the first time it idled for a couple of minutes. I was able to clear the misfire by revving the motor a bit. With fresh gas and a better charge on the battery the problem did go away and it ran well thereafter. I got the ST late in December 2021 and only put 150 miles on it before taking it apart for the winter. The plugs had too much carbon for my taste which could be either ignition or fueling. Without knowing the gas mileage I have to guess and am looking at everything. I like to make a bike perfect, document and then go four or five seasons with minimal maintenance.
 
I think it's fuel/carbs. Run that fresh fuel through with a cleaner additive and I think Robert will be your father's brother. If you want to change the HT leads, change em, it can't hurt but might be dollars down the drain.
Upt'North.
 
TMF said:
I got the ST late in December 2021 and only put 150 miles on it before taking it apart for the winter.
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If you want to make it perfect, replace the whole plug cap. Did you read this:

Article - Maintenance Items Needed for a New (old) Bike | Articles

This article is the result of collaboration by Andrew Shadow, CYYJ, jfheath, Max Pete, SMSW, and SupraSabre. Igofar’s list was copied from the first link listed above. Click on the attached file. This one is V.11; in the future, any changes will be noted in the title on the first page as the...
www.st-owners.com www.st-owners.com
 
I think the coiled wire is probably there to eliminate high frequency interference to other electronic devices. (It is on my model trains anyway. It's called a choke coil I think. Please - someone that knows what they are talking about help me out here !!!)

Issues with the ignition circuit will not go away with riding. Initially they go away as higher revs boosts the quality of the spark. I was answering your first question about the spark plug cap.

I'm with Upt'North on the problem that you describe now. Look at the fuel. The fuel filter is an easy clean on the ST1100 - check it out or replace it.

If water has got into the fuel at any time - it sinks to the bottom. The fuel filter can pool water at the bottom. Normally the fuel just flows over it or maybe causes a splutter or two. But right now, it is probably freezing and the water turns to long thin ice crystals in the filter, which will stop the flow of fuel. I've not had it happen to me on an ST1100, but I have on a different bike.

Put some fuel cleaner in your fuel and get some of the higher octane, quality fuel in the tank - without ethanol if you can. Take it for a good long ride and keep the gear low and the revs high - in the 4000 to 5000 rev range for at least half an hour (this has nothing to do with the spark quality). You will know when you have done it enough - suddenly the engine feels as though it is spinning really easily. A nice bright sunny day at this time of year when the air pressure is high and there is more oxygen per lungful is ideal.
 
Upt' North said:
I think it's fuel/carbs. Run that fresh fuel through with a cleaner additive and I think Robert will be your father's brother. If you want to change the HT leads, change em, it can't hurt but might be dollars down the drain.
Upt'North.
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OA carb float may have been hanging up and because my friend who
jfheath said:
I think the coiled wire is probably there to eliminate high frequency interference to other electronic devices. (It is on my model trains anyway. It's called a choke coil I think. Please - someone that knows what they are talking about help me out here !!!)

Issues with the ignition circuit will not go away with riding. Initially they go away as higher revs boosts the quality of the spark. I was answering your first question about the spark plug cap.

I'm with Upt'North on the problem that you describe now. Look at the fuel. The fuel filter is an easy clean on the ST1100 - check it out or replace it.

If water has got into the fuel at any time - it sinks to the bottom. The fuel filter can pool water at the bottom. Normally the fuel just flows over it or maybe causes a splutter or two. But right now, it is probably freezing and the water turns to long thin ice crystals in the filter, which will stop the flow of fuel. I've not had it happen to me on an ST1100, but I have on a different bike.

Put some fuel cleaner in your fuel and get some of the higher octane, quality fuel in the tank - without ethanol if you can. Take it for a good long ride and keep the gear low and the revs high - in the 4000 to 5000 rev range for at least half an hour (this has nothing to do with the spark quality). You will know when you have done it enough - suddenly the engine feels as though it is spinning really easily. A nice bright sunny day at this time of year when the air pressure is high and there is more oxygen per lungful is ideal.
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Good ideas all. My running issues all went away with fresh fuel and some run time. Idle became stable, throttle response good, smooth pulls to redline. It was running well enough that I'm leaning towards skipping a carb sync unless very easy. All that improvement came before any maintenance. It's got to be even better in the spring with the fresh iridium's, valve adjustment and Fram G3 I've done. I run Startron year round on all motorcycles, mowers, generators for extra insurance.
 
TMF said:
Good ideas all. My running issues all went away with fresh fuel and some run time. Idle became stable, throttle response good, smooth pulls to redline. It was running well enough that I'm leaning towards skipping a carb sync unless very easy.
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That is good news.

And if it is running that well, and idle is steady, I would leave the carbs alone. What more do you want ?
If you ever change your mind, do not use instructions from a Haynes manual. Ask for help on here.
 
I have not removed the resistors from my ST but I have removed them on some of my old Honda Trailbikes. Being that I run resistor plugs the resistor really is not needed and it knocks down the plug voltage a bit.
 
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bdalameda said:
I have not removed the resistors from my ST but I have removed them on some of my old Honda Trailbikes. Being that I run resistor plugs the resistor really is not needed and it knocks down the plug voltage a bit.
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That is interesting. Resistor plugs take the place of resistors in the spark plug cap!! Eureka. I can remove the occasionally problematic resistors after all?
 
I've heard some grumbling about Haynes. Sounds like we need a new thread - Egregious Haynes Manual Mistakes and How They Sabotage ST Mechanics.
 
I continued to read tech articles and decided that a single resistor in the form of resistor plugs is sufficient. In a nutshell, the resistor slows the transfer of energy from the coil to the spark plug electrodes (slightly). One experimenter put a million ohms of resistance onto a test circuit and stated the spark was visibly weaker and longer in duration but still there. That weaker spark has trouble crossing the gap of a wet or oil fouled plug and that is why nonresistor plugs and wires were better in the old days of points ignition and 2 strokes. Resistor's clip and smooth the energy burst between coil and electrodes enough to minimize EMI. Again, the duration of the spark with a resistor is slightly longer but weaker. It's possible and even likely that a single resistor configuration is optimum and the resistor cap was specified by Honda purely out of caution for when nonresistor plugs are installed. Eliminating 5K ohms from the circuit seems relatively insignificant and is partially offset by a theoretical increase of resistance in the rest of the circuit due to age. Right now, I'm going with my homemade copper spacers and losing the OEM spark plug cap resistors. I don't expect any running improvement, just an infinitesimally more reliable system without the resistors as a point of failure.
 
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I bought a new NGK plug cap as I had a badly corroded resistor in my VFR750; the new NGK cap contained the exact same resistor so I fitted that into the old OEM plug cap, bought the plug lead resistance right back to the required spec.
 
TerryS said:
I bought a new NGK plug cap as I had a badly corroded resistor in my VFR750; the new NGK cap contained the exact same resistor so I fitted that into the old OEM plug cap, bought the plug lead resistance right back to the required spec.
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I found the identical resistor, spring and screw assembly that are in the OEM Honda cap on a Yamaha forum. Maybe NGK is the OEM. It would be nice to source alternative plug wires equivalent to the expensive Honda part.
 
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