ST1300 Poor Running - HT Leads & Plugs - Solved

jfheath

John Heath
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Describing an issue with poor running on my ST1300. Symptoms were engine seemed to be running fine except with little low end power and a sensation of always riding on a rough surface. Revs had to be kept up in order to set off safely. It turned out to be HT leads and plug caps. Details below, or downloadable in the attached pdf.


Experiences with an Old Bike Problem - and a Honda Dealer.

In my case, the engine was running very badly at low revs - as if it was missing a cylinder - but it would pick up when the revs were increased. Keeping a steady 20mph was impossible, and setting off at road junctions was downright dangerous, as there was no reliable power to balance the turn onto the main road. High revs and lots of clutch slipping was the only answer.

I looked at the HT leads, and #3 lead (left rear) pulled away from the plug cap as I moved it. The cable was longer than it should be, and it had a tight bend in it. I had owned the bike from brand new but closer inspection revealed that this was not the original Honda lead. It was too long and was routed incorrectly (under the main harness instead of over). There was also a bite out of the rubber cover which prevent debris from falling into the spark plug recess, but that is something that happens unless the silver cover is modified. I replaced the lead with standard copper cored HT lead, and the bike ran much better.

But it wasn’t right. It still stuttered at low revs, and felt like it was riding across a bumpy road surface at low speeds. I took it to a local official Honda dealer, told him exactly how to make it display the fault and hoped they would plug it into their computer to track down the fault. They didn’t do that - apparently the computer is only any good if the FI light is showing a fault. Instead, they took it for a ride, and told me the bike was running perfectly, and added that it was an old bike and they do that sort of thing as parts start to wear out !

I won’t be going there again then ! So I was left to fix it myself.

What I found


▪ The original faulty #3 lead
▪ The earth contacts for the coils were rusted under the mounting bolts.
▪ One spark plug cap was faulty (#3 cap)
▪ Two plug caps registered the wrong resistance. It should be 5K ohms.
▪ One HT lead / plug cap had verdigris (copper oxide) - ie it was corroded inside the core of the lead.

So much for the experts.

Here’s what I did.

The fairing was off, all loose leads tidied away, and the mirrors taped into position for a road testing. I replaced the suspect leads with HT lead from an Auto Electrician at £1 per metre. I bought the correct Honda cap and lead for cylinder #3, and went for a slow ride up a steep hill on a naked Pan.
Perfect. Absolutely perfect.

I had no evidence that the cap for cylinder #3 had been faulty, so I used that to replace the cap that had the corroded pin. The fault returned. So Cap #3 was definitely duff. I measured the resistance for this faulty cap, it was 4.5K ohms - too low.
So I measured the others. 2 were spot on at 5K. One was 0.5k too high. So as well as the faulty one, I had another with a wrong reading and a third with a corroded pin. I decided that I would replace the lot, and got them ordered. In the meantime, I ran the bike for a couple of weeks with home made leads, and the original caps. Cylinder #3 had its brand new plug and lead. It ran much better and although my imagination suggested a niggly fault, this disappeared when I replaced the other leads and caps.

P1010618.jpgSnippets worth mentioning.

The Honda OEM leads comes cut to length with an extra protective sheath, a cap to screw onto the coil and a rubber grommet. The grommet and sheath are glued firmly in place.

Connection with the Coil

The lead is a push fit inside the coil body, a pin inside the recess inserts itself into the strands of the copper core. It needs to be twisted back and forth before it seats properly. The rubber grommet sits up against the threaded recess, and the screw cap clamps down on the grommet to hold the lead in place. Without the grommet, the lead can work out - which is what had happened to my non-original lead #3.

P1010622.jpgConnection with the Plug Cap


The other end of the HT lead is a screw fit into the plug cap. It is not a push fit. The depth of the hole is about an inch, and it is a tight fit. I measured the depth of the hole, and marked this onto the HT lead with a pencil, so that I knew how far into the cap I had screwed the lead.
On reassembly, apply a smear of rubber / silicone grease to the end of the HT lead. It makes it much easier to screw it onto the cap.

The lead can be screwed out of the cap in order to inspect the state of the copper core, but doing this too many times will cut the copper strands inside the HT lead. It will also do this if you pull the lead off the cap, rather than unscrew it. If you suspect this lead, it may be possible to snip a small section of lead of the end - 4mm or so, and still have enough length to route the HT lead properly.

You can see in the photo how far the end of the lead has been twisted into the cap. It is about an inch (2.5cm)

HT Lead Routing

On each side of the engine, there is a long HT lead to the front cylinder and a shorter one to the rear. The longer lead is routed under the main wiring harness. The shorter lead must go over the harness, otherwise it is not long enough to remove the plug cap from the spark plug.
P1010619.jpg

Measuring the resistance of the Plug Cap

Its easier if you get a length of HT lead and just push it into the plug so that the pin touches the copper core. A twist doesn't harm, but it shouldn't be necessary for this. Then stick one multi-meter probe in the other end of the wire, and the other probe into the bit that pushes onto the spark plug.

See the photo on the right.

P1010623.jpgYour multimeter should be set to ohms. The reading should be around 5KOhms, so the dial on your multimeter needs to be set to allow 5K to be displayed. My multimeter has 200, 2K, 20K, 2M, 20M and 200M. To read 5K, I need to set it to 20K (ie it can read up to 20K).

The image on the left, shows the reading from my defective #3 plug cap. The brand new plug cap from Honda read 5.02K. Two of mine were 0.5K out, and one of these was definitely faulty. The third had verdigris on the pin. It made sense to replace the lot.




What about the bite out of the #3 plug cap ?

It seems that this is caused by the moulding on the underside of the plastic cover. One of the mouldings has large circular grommet to clip the cover down. The other moulding does not, and seems to be there to nibble holes in the plug cap each time the cover is removed.
I’ve now snipped the plastic on the underside of the cover to stop it from taking bites out of my plug caps.

And How About using Standard HT leads ?

It seems to be perfectly Ok - up to a point. The HT lead screws firmly into the end of the plug cap. The OEM leads have a thick rubber protective sleeve - which is a good idea. I have had HT leads on other bikes develop weak spots in the insulation. The first touch of salt water from the roads, and these arc to earth through the HT lead wall, stopping the engine dead.
The HT lead is a push fit into the ignition coil. The grommit glued onto the OEM lead ensures that the lead is held firmly in place in the coil. Without some similar mechanism, the HT lead could easily work its way off the coil. Normally an easy roadside repair normally, but on the ST1300 you have to remove the fairing to get to it !

(This thread was originally posted Link Here - and I accidentally deleted my first post, so resubmitted it in this thread. The questions and answers there are also informative).
 

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Joined
Sep 18, 2018
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36
Location
Shawnee, KS (Kansas City Metro)
Bike
2008 ST1300
STOC #
9057
Where do you find OEM plug caps and/or boots? My RR cap is cracked (from the infamous unused molding inside the cover - now cut away). I'm still new to ST1300, and my old favorites for parts for GL1800 are not good with the ST. Any good sources for parts would be appreciated.
 

SupraSabre

48 Years of SoCal Lane Splitting/Commuting-Retired
Site Supporter
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Nov 20, 2005
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9,498
Location
Cedar City, Utah
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12/04 ST 1300s
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5901
Where do you find OEM plug caps and/or boots? My RR cap is cracked (from the infamous unused molding inside the cover - now cut away). I'm still new to ST1300, and my old favorites for parts for GL1800 are not good with the ST. Any good sources for parts would be appreciated.
Try Partzilla, which is where I get most of my parts!
 
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