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Bike Review - Posted 25th September 2015
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Harley-Davidson XL1200S Misfire - Part 1

Half of all carb misfires are caused by ignition faults. Or is it the other way round? Martin Gelder searches for enlightenment by taking a coughing Sportster Sport back to standard...

It started with a cough.

Halfway through one of those early spring rides when the weather is better than you could have hoped, just as I pulled away after filling up with petrol, the bike coughed. It did it again when I went to overtake a lorry pulling up a long climb. Setting off after leaving the lunchtime stop (full English and a black coffee please), it coughed again, several times. By the time I was home overtaking had become a problem, the bike refusing to respond to any meaningful opening of the throttle. Something was wrong.

Harley-Davidson Sportster Sport XL1200S Misfire You can almost hear the coughing

My first thought was dodgy petrol, either from the pump that I'd used just before the coughing started, or from the bottom of the tank as I'd run onto reserve before filling up. For a bike with a carb that hangs out in the breeze, the Sportster's is fiddly to access; the airfilter cover has to come off, then the airfilter, then the airfilter backing plate; 11 bolts needing four different tools... maybe I've just been spoilt by airhead BMWs and their clip off floatbowls.

There was no sign of grit, muck or water in the floatbowl so I gave all the obvious holes a squirt of carb cleaner, put everything back together and started the bike. No different, still coughing. Everything off again (11 bolts, four tools) and it was time to attack the top of the carb, and there was my problem. The CV carb's diaphragm had a couple of small splits at its edge where it was held between the carb body and carb top.

Harley-Davidson Sportster Sport XL1200S Misfire Old diaphragm on the left, splits arrowed. New stock one on the right; note smaller bleed hole, above centre

On conventional carbs the throttle slide is raised by the rider twisting the throttle, allowing more fuel and air into the engine. CV (or constant velocity) carburettors work differently, using a butterfly throttle worked by the rider to control the amount of air flowing into the engine, and then using the flow of air through the carb to raise the throttle slide. This means that you can't give the engine more air and fuel mixture than it can cope with, but it relies on there being a difference in air pressure between the static air in the carb above the slide and the moving air below the slide, rushing into the engine. This is where the diaphragm comes in; it seals the gap between still air above and moving air below, creating the pressure difference that will lift the slide. If the diaphragm doesn't seal, air will leak past it and there'll be no pressure difference; the slide won't lift and the bike will cough when you open the throttle.

Harley-Davidson used the Keihin CV40 carb across most of their range for many years, so parts are easily available. Armed with a new diaphragm (and attached slide) the next day, I replaced the carb and set off round the block to revel in the glorious achievement of fixing the misfire.

No, of course it wasn't fixed. Well, it was, but now there was a different misfire; more of a bottom end hesitation and then mid range stumble. Carb off again. I'd obviously disturbed something while cleaning it first time round, so now it was time to do the job properly, remove all the jets, blow through all the passages, check all the settings, double check the reassembly, and so on. Basically, time to open a can of worms that would take me the best part of the summer to close again.

Harley-Davidson Sportster Sport XL1200S Misfire Jets shouldn't look like this
1200 Sportsters on Now...

At some point in its fifteen year history, my Sportster had had a “stage one” kit fitted; loud exhausts, free flowing airfilter, tweaked carburettor jetting. Most Harleys go through this as they are bit strangled by emissions and noise controls as standard. It quickly became apparent that whoever had “stage one'd” my bike had been a gorilla armed with a hammer, a chisel and a complete lack of mechanical sympathy. The main and pilot jets had been screwed in so tightly that they were burred and, in the case of the main jet, looked distorted out of round. The mixture screw which should have been between one and three turns out from fully seated was eight turns out and barely clinging to its threads. The slide had been drilled out (a performance mod) but not deburred. About half of the screws holding the floatbowl and carb top in place were chewed and one was too long, preventing it seating properly. The slide spring was bent (how?), the needle was worn and it had been “tuned” using shims even though it had an adjustable clip.

Harley-Davidson Sportster Sport XL1200S Misfire One of those shims is a split washer. What was that doing in there?

None the less, it had run well before the diaphragm split so I carefully cleaned everything and put it all back together, replacing the chewed screws with the correct ones. With the mixture screw in the middle of the right range of adjustment, the bike started better than before but it was a pig to ride, lurching, coughing, spitting and banging. Whatever delicate balance of butchered parts had made it work before had been disturbed by my fettling. With a sinking heart I realised there was only one thing for it; undo all the bodges of the previous “tuner” and start from scratch.

A nice man called Nick offered me a standard CV40 carb if I went to collect it, which I did. Opening it up it turned out to have been stage one tuned with a Dynojet kit, but at least the work on this carb had been done by someone with the right screwdrivers for the job; I now had a solid platform to start my rebuild from.

Harley-Davidson Sportster Sport XL1200S Misfire Off to the Harley shop on the bike that works. A BMW

I went online and attempted to order the right – standard – parts to return the carb to stock specification for a Sportster Sport. The XL1200S differs from other Sportsters in having twin-plug heads with higher compression, different cams and a different ignition system. All of these affect carburetion settings, and the 'S' has its own unique jets and needle. The jets were available off the shelf, but the needle was no longer manufactured. Many other needles are available, and there's quite a lot of information online describing the differences between various needles.

There's a wealth of information about setting up, tuning and fixing H-Ds on the internet. Some of it is really good, carefully researched and based on evidence and experience. A lot of it, however, is based on people regurgitating other people's advice without having tried or tested what is being said; it's nonsense, in short. I'm sure people are trying to be helpful when they recommend solutions to problems they've never encountered themselves but have read about on an internet forum and then slightly mis-remembered, but the end result is worse than useless. Choose your sources wisely, is my advice.

Harley-Davidson Sportster Sport XL1200S Misfire Looks easy to get at. Isn't.

Eventually, I ended up with a good carb body (thanks again, Nick) a set of jets to suit a standard XL1200S, a standard but new emulsion tube (the bit the main jet screws into), a good needle jet, an untampered with slide with a good diaphragm, and a needle recommended to be close to the standard XL1200S one. I bought a new K&N airfilter to replace the one that came with the stage one kit, and researched and bought the correct pilot jet to suit. Nothing could stop me now. Pass me those four tools and eleven fasteners.

And you know what? It was better. Not right, but better. A bit stumbly at the bottom end when I first cracked the throttle, a bit hesitant when rolling the throttle on in the higher gears, but this was progress. Surely now I was within a few adjustments of carburettor nirvana?

Coming in part two; more than a few adjustments...

Words and Photos: Martin Gelder

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