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Bike Review - Posted 2nd October 2015
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Harley-Davidson XL1200S Misfire - Part 2

Half of all carb misfires are caused by ignition faults. Or is it the other way round? Martin Gelder's search for Sportster Sport enlightenment continues, with copious notes...

Part One is here...

Accepted wisdom tells us that you set carb jetting by doing a plug chop. This involves accelerating at full throttle in a high gear until you reach the redline, and then killing the motor, whipping the clutch in and inspecting the sparkplugs after you've coasted to a stop.

There are a few difficulties in actually using this method. Firstly, at the redline in top (and in fourth, and probably in third) my Sportster is breaking all the UK speed limits. Secondly, this only tells you about the correctness of the main jet, and on a 1200cc high geared twin fitted with a CV carb, you're hardly ever running on the main jet unless you're breaking all the UK speed limits. Thirdly, with modern fuel it's much harder to 'read' the mixture based on the colour of the plugs; too rich is easy to spot, too lean much less so.

The answer is either to stick the bike on a dynamometer (a surprising number of small indy bike shops have them now) and set the carb by adjusting for a good mixture at various throttle settings, or to do it the old fashioned way by making one adjustment at a time, riding the bike after every change, and keeping copious, meticulous notes.

Harley-Davidson Sportster Sport XL1200S Misfire Change one thing at a time, and keep notes; I used cardboard because it handles petrolly fingerprints better than paper. Box is for keeping parts from different carbs separate, micrometer for measuring shims, optical loupe on left for reading jet sizes

The Keihin CV40 carb fitted to Harleys has five main adjustments; the main jet size, the needle diameter and taper, the needle clip position or height, the pilot jet size and the pilot (or mixture) screw setting. All of these adjustments affect different areas of the carb's operation, but they also overlap quite a bit; any extra fuel added at tickover by the pilot jet is still being added while the bike is running in the midrange using the needle setting, and so on.

The chart hereabouts – which is actually for a slight different type of Keihin carb, but is close enough to be useful – shows how all these adjustments overlap. Note that where it refers to throttle opening, it doesn't mean how far round you've twisted the throttle grip, but how far the slide has opened, which depends on the vacuum or pressure difference in the intake on a CV carb; at low revs, you might have wound the grip wide open but the slide could only have risen halfway up.

Harley-Davidson Sportster Sport XL1200S Misfire

You'll also notice from the chart how much of an influence the needle diameter, taper and clip position (or height) have on the fuelling from 1/8th throttle to about 7/8th throttle. Or “normal riding” as the rest of us call it. We'll come back to that shortly.

Getting the pilot jet size and pilot screw settings right involved spending a Sunday afternoon riding round the local science park (like an industrial estate but with geeky glasses and a worried frown) tweaking the pilot screw to get the right balance of good throttle response at low speed and smooth running on a just-off-closed throttle. Internet advice will tell you that you can set this by tinkering with the bike at tickover, but I found there was no substitute for riding the bike; the pilot screw setting and to a lesser extent the pilot jet size make a *huge* difference to how the bike feels when you're riding round town. Copious notes help here, as changing the jet size (a back at base operation) can also affect the screw position. The two aren't as closely related as the internet advice thinks though, and I ended up with a bigger jet than I'm told I should have. I'll list all the settings I ended up with at the end of this article.

The main jet only comes into play at big throttle openings and higher revs; I went for the standard XL1200S size, despite being told by some people that it was too big. Copious notes made while out riding told me the bike ran better with the bigger jet, and the 'S” has different carb requirements than most other 1200 Sportsters, which is what most people base their secondhand advice on.

Harley-Davidson Sportster Sport XL1200S Misfire Two different needles. Different lengths are obvious, but thickness and taper are also different

Which just left the various needle adjustments. As the throttle slide lifts, it moves the needle upwards through the needle jet. The needle is tapered, so the higher it lifts, the more fuel flows through the jet. You can alter the amount of fuel let through by changing the needle's taper, changing its diameter, or by lifting it relative to the slide (by using shims under the needle head, or moving the clip on an adjustable needle). You can also change the speed at which the (vacuum operated, remember) slide lifts, by fitting different springs or drilling the slide air hole to a bigger size. H-D use the same needle jet across all their models, so at least I didn't have to worry about that.

Needle adjustments are critical to the riding experience on a big, low-revving twin. Various tapers are available and I started with two that were recommended by various people specifically for the S model (in part one, we learned that the standard needle is no longer available). Despite this, riding the bike showed a mid range, on the throttle, misfire. Swapping needles made a difference, and putting shims under the needle improved things further. Keeping accurate notes is vital as it's too easy to lose track of which setting was better and you end up going from better to worse quite often while trying to find the optimum setting.

Harley-Davidson Sportster Sport XL1200S Misfire Change something, write down how it feels

I also swapped slide springs, as the Dynojet stage one kit uses a lighter than standard spring, and I tried drilled and undrilled throttle slides as I had examples of both.

Eventually I reached the point where the mid range hiccup was dialled out. I had a creamy throttle response from tickover in the lower gears to wide open in the upper gears, the bike started easily without choke when it was warm and with choke when it was cold, and ticked over steadily. The plugs were a light biscuity brown and the same on both cylinders. Lovely.

What better way to celebrate successfully setting your bike's carb than to ride to a pub car park in Leicestershire for one of Mad Mike's lunchtime meets? Getting there in time for lunch meant setting off quite early, and a slightly misty morning greeted me as I rolled onto the dual carriageway a mile or so from home. Accelerating into the outside lane, the bike coughed, then coughed again. On a steady throttle it was fine, but wind it on to overtake and the bike would misfire. I put it down to the colder and damper than usual weather and gave the bike another five miles to warm up. It made no difference; in fact, if anything it was getting worse. I rode the Harley home, dug the BMW out of the shed – the BMW with four times the milage and twice the age – and went on that instead. It ran faultlessly.

Harley-Davidson Sportster Sport XL1200S Misfire 110,000 mile BMW to the rescue....
1200 Sportsters on Now...

Back to the needle adjustments. More shims improved things. Changing the needle didn't. And then fewer shims improved things. That can't be right. The only part of the carb I hadn't touched was the accelerator pump, which squirts an extra dollop of fuel into the intake when you open the throttle. All the 883s and 1200s use the same pump mechanism and nozzle, except the 1200S which seems to have a different one; the pump is part of the floatbowl and the Sportster Sport's has a different part number. The 1200S specific Dynojet stage one kit comes with a different pump nozzle and I'd been using that so far. I swapped it for the standard non-1200S bowl; no change.

Harley-Davidson Sportster Sport XL1200S Misfire MAP sensor, tucked up under the tank

The only other component connected to the fuel system is the MAP (manifold absolute pressure) sensor, which takes an air bleed off the inlet manifold and uses the intake vacuum to select different ignition timing curves depending on the amount of throttle you're using. This is another XL1200S specific part – other Sportsters use a VOES (vacuum operated electrical switch) to do a similar job – but the actual MAP sensor is a widely used and relatively cheap automotive part so I replaced it anyway. No change.

Eventually, I got to the point where I could make it misfire at will, and tried doing a plug-chop while it was coughing and spluttering. The front plug was fine. I tried again. The rear plug was sooty and black; not wet from rich running, but sooty black and dry. Harleys only have one carb; it wasn't possible that the two cylinders could fuel so vastly differently. I fitted new plugs, jiggled the HT leads, burnt an offering, checked the sparks with the plugs out. No change.

Most Harleys also only have one coil. The XL1200S has two, one for each cylinder, cunningly moulded into one lump to look like a great big single coil. This lump sits close above the front cylinder head, getting heated from below by the engine and cooled from the front by the airflow. Could it be breaking down?

Harley-Davidson Sportster Sport XL1200S Misfire

Time for some more internet research, but at least by now I'd learned which sources to trust. On the “S Model Specific Q&A” area of the XLForum I found someone else who had suffered a very similar problem. I posted a question about my bike's specific behaviour and in reply it turned out that not only did it sound like my coils were packing up, but the person I was talking to on this US based forum lived about 60 miles away from me. He had a spare standard coil I could try, and if it worked I could keep it; he'd changed to an aftermarket ignition system so it was surplus to requirements. Thank you, Steelworker. What a nice man. Another one. People are great, you know. We sometimes forget.

I put the carb back to the settings which had worked well just before the coil started giving up the ghost – copious notes, remember - fitted the new, old, replacement coil, fired the bike up... And the transformation was remarkable. It was now probably running better than it ever had. I was back to that creamy but crisp throttle response, smooth running at steady speeds, even-tickover happiness.

Harley-Davidson Sportster Sport XL1200S Misfire

What had started as a split diaphragm cough and become a failing coil splutter had taken me on a journey though internet advice - both useful and utterly useless - to a point where the bike was running much than before. I could probably improve things with a shim to the needle here and a tweak of the mixture screw there, but it's so close now I haven't been motivated to make any changes.

I've learned more than I ever wanted to about the Sportster Sport's intake and ignition systems, found some great sources of advice, some great suppliers of parts, and some truly generous people.

The End Result:

Here's the set-up I've ended up with.

  • Carb body: Standard Keihin CV40
  • Main jet: 200
  • Pilot jet: 48
  • Pilot screw: 2.5 turns out
  • Needle: “N65C” (actually a copy of an N65C, marked D578)
  • Needle shims: 0
  • Throttle slide: Standard
  • Floatbowl: XL1200S with Dynojet accelerator pump
  • Air filter: K&N

    Apart from the bigger pilot jet and the substitute needle, it's pretty close to standard despite me using a freer flowing K&N airfilter and much less restrictive exhausts.

    Dynojet sell a stage one kit that consists of a replacement needle, emulsion tube, selection of main jets, selection of pilot jets, lighter slide spring and a drill to modify the slide, but my experiences suggest that all you need is a different pilot jet and adjustment of the pilot screw to richen up the bottom end.

    Anyone online who tells you that something that suits all other 883 and 1200 Sportsters will suit an XL1200S is probably wrong. And vice-versa; the jets that work on my bike almost certainly won't be right for a single plug Sportster.

    Parts Suppliers (in no particular order):

    Black Bear Harley-Davidson in Newmarket: My local main dealer, stock all the standard stuff like diaphragms and carb screws when you need them in a hurry.

    Jersey HD: Mail order supplier of standard parts at a discount, they have an online 'parts book' you can use for cross referencing; invaluable.

    Krazy Horse in Bury St Edmunds: Stock a lot of the parts from the big catalogues of aftermarket stuff for Harleys, like different sized jets. Good cafe too; you don't feel so stupid riding 60 miles to buy a two quid jet if there's coffee and cake involved as well.

    Ian Marshall at Marshall Speed and Custom, Newport, South Wales: Stocks all the stuff I needed but couldn't find anywhere else – needles, springs, emulsion tubes, etc. Fast mail order and useful advice over the phone. Find him on Facebook or 01633 276976

    And thanks again to Nick and Steelworker!

    Online Advice:

    The HDRCGB Forum is really good for general Sportster advice, but the XLForum's XL1200S Specific area is unbeatable for Sportster Sport specific knowledge.

    How to Tell If You're Running Rich or Lean

    Most of the “How To Set Up Your Carb” guides I've found – be they online or on paper – assume that you already know if your bike is running too rich or too lean. Oh, that it was so easy. In reality, you probably know it's not right, but in which way is it not right? The following hints particularly apply to the pilot jet and screw settings, which make a big difference to the ride-ability of a lumpy air cooled twin.

    Runs better when cold, worse when hot: Too rich. If you think you've fixed it for the first two or three miles of your test run, but then the bike feels fluffy in traffic as it warms through, it's probably too rich.

    Runs better if you ease the choke out a little once it's warmed up: Too lean. Riding along at town speeds, try easing the choke out slightly. If running at slow speeds improves, you're probably too weak. This works for H-Ds with Keihin carbs; other bikes might use different types of cold-running enrichment which might not have the same effect.

    Bike seems very touchy to small amounts of throttle input: Probably too lean. I found that as I went too lean the bike got harder to ride smoothly at town speeds.

    Smell of petrol immediately after you stop: Probably too rich. Sometimes the obvious things are easy to miss.

    Also keep an eye on fuel consumption (too rich?) and how hot the engine is getting (too lean?) during normal use. A too big main jet won't have much effect until you're flat out at the top of the rev range, but a too small main jet can upset the workings of the needle circuit.

    Words and Photos: Martin Gelder

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