There is nothing more hateful than bad advice.

—Sophocles

Here’s the problem with deciding that one particular make and model of motorcycle is the only one you need for the rest of your life: they get old, develop problems, and at some point you’re pretty much on your own to figure out how to fix them. My 2001 Suzuki SV650 is the best motorcycle ever made, at least according to me, but that’s assuming it runs on both cylinders, which it currently doesn’t.

What is it about old motorcycles? Like cats, as soon as they hit the decade mark, they lose that like-new patina and start developing weird issues. Not only has my cat developed bad breath, he also feels the need to sit on my chest and yawn into my face at three a.m. Dude: if you’re nocturnal, why are you yawning? Similarly, my SV isn’t running right at high rpm, and the reason isn’t obvious. Like the cat, if my 16-year-old SV disappeared tomorrow, almost nobody would notice, but I’m going to try to fix it anyway.

It’s that most elusive of problems, the one disguised as no problem at all. The bike starts fine, idles cleanly, pulls okay and cruises along without a care in the world. But open the throttle to the stop in sixth, and it won’t pull redline, petering out well before the century mark (on a closed-circuit course, with a professional rider aboard, of course, and certainly not on a rainy Wednesday afternoon on the crowded I-580 freeway).

This exploded diagram of a Mikuni carburetor also doubles as Motorcycle.com's organizational flowchart, except Sean is the needle and Kevin is the clip (currently 2 grooves from the bottom). Evans is the pilot jet, and depending on day of the week, Tom is either the main jet or mixture screw.

This exploded diagram of a Mikuni carburetor also doubles as Motorcycle.com‘s organizational flowchart, except Sean is the needle and Kevin is the clip (currently 2 grooves from the bottom). Evans is the pilot jet, and depending on day of the week, Tom is either the main jet or mixture screw.

If you want the gory details, here they are: it’s a 2001 SV650 with a full M4 exhaust system. There’s an EBC filter, the airbox snorkel has been removed, there are spacers under the tank to allow more airflow, and a 4% ignition advancer replaces the stock timing bolt. The carburetors, God bless their little constant-velocity souls, have been modified with a period-correct Factory Pro jet kit, with pilot jets, main jets and needles replaced and set to the recommended specs.

The problem is that the rear sparkplug – yes, just the rear plug – is fouling, dry fouling if that helps. At least I assume whatever is causing the plug to foul is the problem. After 30 years of riding motorcycles with varying degrees of shittiness and un-diagnosable conditions, I have finally learned to doubt that a totally obvious symptom is actually causing a problem. So this time, instead of trying to do things the old way, trying out free Internet advice until I give up, I’m going to try to methodically figure it out myself.

So far, I’ve discovered that the valves are likely in spec, and yes, the plugs are clean and gapped correctly, at least before my last test ride. They also deliver a fat spark when I test them on both coils. The fuel pump’s working because the front cylinder is running fine (I suppose), and would the bike even start or idle if the cam timing was off a tooth or an intake valve wasn’t fully closing? Everything obvious has been checked, so I’ll start with the not-so-obvious things the troubleshooting guide in the shop manual suggests (What? That’s not how we do things on the Internet!). I’ll swap out coils, the fuel pump, the black box, and yes, check the cam timing and valve clearance, as I haven’t yet spilled enough coolant on my shoes. At some point I will fix the problem or put the bike on Craigslist, hopefully attracting the attention of somebody with no desire to exceed 100 mph.

I have also learned, in my 20 years of swimming in the World Wide Web, to realistically value free advice. Because judging from what I discovered when I tried to search the Googles for other accounts of a similar problem, checking obvious and simple stuff is for suckers. Going up (or down) a heat range on the plug is popular advice, and it always solves the problem of plug fouling like magic, said no motorcycle owner ever in human history. Neither does moving needle clips, going up a jet size, balancing the carbs (carb sticks are a wonderful way to spray mercury into your eye, in case you need a good method for that), cutting a coil out of your carb slide spring, switching to flat slide racing carbs, getting high-output coils or even, as one forum wunderkind recommends, titanium emulsion tubes. I have no idea where to buy these, but I haven’t looked in a medical supply catalog yet.

Titanium emulsion tubes. You gotta get yourself some of these titanium emulsion tubes. They'll fix ya right up.

Titanium emulsion tubes. You gotta get yourself some of these titanium emulsion tubes. They’ll fix ya right up.

When humans realize there’s no obvious solution for a problem, the crazies comes out in full force, leaping to the most tenuous and unlikely solutions. A bunch of ships and airplanes disappearing near Bermuda? Must be caused by aliens. Pyramids built by ancient people with no heavy machinery? Again, aliens. Donald Trump being elected president? Russians. Or aliens. Maybe even Russian space aliens.

When it comes to motorcycles, it’s no different. Within a few hours of this column posting, there will be some tasty examples of diagnostic overreaching. There may also be some good advice, but how to separate the wheat from the chaff? I know (now) to not grant credibility just because somebody sounds like they know what they’re talking about. If you don’t know what somebody’s talking about, it’s possible, even likely they don’t know what they’re talking about either. And if somebody tells you they know what they’re talking about because they’re an engineer of some kind, drop everything and run. Even engineers don’t listen to other engineers. If, however, he or she tells you they’re a software engineer, offer to sell them your motorcycle.

I wonder what medicine would be like if this sort of diagnostic science was applied to the human body. “Mr. Ets-Hokin, we’ve discovered a distinct heart murmur, and after going on the HeartSurgeon.com advice forum, I think the best course of action is to install the fuel pump from a ’63 Chevy Impala, because you need the extra flow.” Or maybe infrastructure projects? “My fellow citizens of the great state of California, to insure a steady supply of clean, reliable water for our children and future generations, we will replace our water system with a very long garden hose we bought on Amazon. Arizona tried it and said it worked great.”

"Okay, the guys on the WebMD discussion forum said this would burn off warts really quickly."

“Okay, the guys on the WebMD discussion forum said this would burn off warts really quickly.”

Anyway, I better go. I have to buy a lathe so I can mill tungsten boots for my new 52mm flat-slide carbs I took off an ultralight airplane. Does anybody know how to upgrade my apartment’s circuit breakers? My blast furnace keeps tripping the fuses.


Gabe Ets-Hokin is an American medical researcher and virologist. He discovered and developed one of the first successful polio vaccines and plays jazz accordion with Paula Abdul’s pet llama, Sparky.

Note: This article holds the record for most carburetor jokes in a 1,000-word column.

  • JMDGT

    A lot of food for thought here. Since the seventies I have managed to keep bikes ten years or so on average. Because the bikes are so much better than they used to be, sometimes better than just a few years ago I am wondering if it might not be better to get a new bike every few years. Sell off the old one and get a new model of the same type. Providing of course one can afford to do so. I still have my 2004 Roadster and I enjoy riding it but I can’t help but thinking that a new replacement may be better. Using this strategy you don’t have to worry about diagnosing anything other than the financials.

    • I think you are a very smart man. Life’s too short to ride shitty motorcycles!

      • Eric

        However, if push comes to shove, it’s as the old saying goes: It’s better to have ridden and angrily fixed an old shitty motorcycle than to never have ridden at all.

        Great article, and good luck.

      • Starmag

        Don’t give up too easily. You’ll miss the great satisfaction that comes from sticking with and finally overcoming a problem. Few things are more rewarding in life for me.

        • Jon Jones

          That’s the spirit!

        • Truer words were never spoken! I found the problem (CV diaphragm) and I wasn’t depressed for at least an hour.

          • Starmag

            The reason I said that of course is that I’ve given up too soon and regretted it when I found out it was something simple. CV diaphragm. This all turned out well. You fixed your bike and I learned something new. Now stored for future reference.

  • john burns

    en fuego today, rare form! Why are you yawning if you’re nocturnal?

    • Thanks JB! Not as good as the goldfish in a bag while you eat your lunch at the Woolworth’s…

      • john burns

        the float or needle/seat deal is jacked on whichever cylinder won’t fire. Your welcome.

  • Gruf Rude

    My 1969 Honda CB750 still runs great, but it is dirt simple. So does my 11 year old KLR650, but it is simpler than dirt. Maybe stone-age motorcycles are too simple to quit.

    • Just like their owners, lol

      • Gruf Rude

        Yep.
        Check float level on your SV. Good advice earlier about swapping coils to see if problem moves.

        • We did! They were out of adjustment. It didn’t solve the fouling. But even if the levels were off, why would it cause fouling and not starvation symptoms?

          • Gruf Rude

            Too high, too rich.

          • Me or the float levels?

          • Gruf Rude

            As a California motoscribe, you might be high but you surely are not rich . . .

          • Insert Cheech and Chong joke here…

          • azicat

            I think it’s time to consult the expertise of Ichiban Moto.

          • Lol! You know I actually met the guy?

          • azicat

            Yes, I really enjoyed your interview from three years ago. Sometimes you just have to call those connections in for a favour!

          • therr850

            he really does exist???

          • For sure! A very nice, mild-mannered retired guy. Awesome vintage-bike skills and wicked dry sense of humor.

          • a small tube of stage III cylinder grade ointment should clear that right up

  • Starmag

    1. voltage at both coils near 12V?
    2. coils resistance?
    3. bad coil lead?
    4. Fuel psi?
    5. Plugged gas line?
    6. Bad ign module?
    7. swap coils to see if problem moves?
    Seems like a relatively small problem to get rid of such a great bike. Good luck.

    • JohnnyS

      This is an excellent starting list.

      Also check your grounds and electrical connections. You may also want to check the voltage at speed.

      If the fuel filter is partly clogged, this can cause fuel starvation at speed.

      Do you oil the air filter? If over-oiled, that can cause a rich condition.

      • Did not oil it! But I’m thinking I’ll replace it with a stock paper filter just to see. But if it was the filter, why would it only foul the rear plug?

        • JohnnyS

          If one cylinder is behaving differently, then there must be a difference somewhere.

          With an newer bike, you have a good chance that “one” thing is wrong, like a bad plug wire, coil, etc. With an older bike, there’s a larger probability that multiple things are wrong, and it could be the case that items that would not cause a problem singly may cause a problem together.

          You have a bike with a few changes from stock and a few years on it. That makes it tricky to sort out.

          Is the compression right in both cylinders?
          Are you certain the valves are all set OK and the timing chain is right?
          One more item to check is if there’s an air leak in the carb boot: With the engine idling, spray a little contact cleaner around the rubber carb boot: If the engine revs up the boot is leaking and it’ll never run right until it’s fixed.
          Are the carbs synched OK? Does each carb go to “full” open when the throttle is WFO, or is one of them not quite all the way open?

    • So far, I’ve swapped coils (from my good-running 1999 SV), tried 3 different rear jets. checked the air cleaner, checked float-bowl levels, replaced plugs,replaced the fuel line. Today, I’m going to check valve clearences and cam timing and change the rear jet (only doing one thing at a time, of course) and see if that helps.

      • Starmag

        Are you sure about 12v at the coils? The power makes a run though the kill switch and a voltage drop would produce the high miss result you have. It’s quite common. I had it on my 900F. A shop owner friend rigs a relay for bad cases of it. Sometimes that voltage can be 9 or less.

        • How do you check that?

          • Starmag

            Volt meter at the leads with the ignition key on and the kill switch on.

  • Old MOron

    “If, however, he or she tells you they’re a software engineer, offer to sell them your motorcycle.”

    Bwa-haha! Best skid mark you’ve posted in a while. Keep the faith, Gabe.

  • i_am_the_koi

    I go with majority rule. If more people are saying to check for apples than oranges, I’m going to listen for zebras, not horses.

  • edbob

    The ethanol now in fuels is hell on carbed anything. It will eat up your fuel lines, floats, filters, and put particles of all the above in your jets, slides, etc. If you did replace the fuel line make sure it’s ethanol proof. Good luck.

  • kenneth_moore

    IIRC, Suzuki ultimately wound up putting 2 sparking plugs in each head of their 90-degree 650cc V-Twin. At least for the Wee Strom.

    So, you’re going to need a drill, a thread tap, an extra sparking plug and wire, and lots of duct tape.

  • Jon Jones

    The emulsion tube might not be indexed correctly due to shearing of the index boss.

    • There’s no insulation, sorry boss. I refuse to buy new emulsion tubes.

  • Chris Kallfelz

    Hah! Great stuff, Gabe, just perfect.

  • azicat

    This is a classic case of the solid state rectifier clipping the voltage peaks into square waves as the current is delivered to the rear ignition coil, resulting in unpleasant distorted spark. Swapping this for a custom rectifier using the 12AX7 valve will result in a smoother warm spark with the higher gain provided at high rpm.

  • Donnie

    The answer is as obvious as Sparky the llama. Old motorcycles are crap!

  • novemberjulius

    Low octane gas…JK

    “If you don’t know what somebody’s talking about, it’s possible, even likely they don’t know what they’re talking about either.”
    The problem is I don’t know what I’m talking about most of the time. I gotta lean on others experience.

  • Boris Roberts

    Pop the gas cap next time it happens. I know you said it is fouling out, but it did that on my Ducati. A new $900 gas cap fixed it. Check for vacuum leaks (yeah, yeah, rich fouling, but check the obvious things first). What about a damaged plug wire, higher RPMs, more resistance, spark stops sparking so well, eats a spark plug.

    Your welcome. Send my free susbcription to me, Corndog67. You know how to get ahold of me.

  • RE: Org Chart. DON’T MAKE ME MOVE KEVIN UP A GROOVE!!!

  • Jon Jones

    Thought about this article and Gabe’s SV650 issue, and came up with the following:

    I really do appreciate and respect do-it-yourselfers. I try to repair all of my power equipment and vehicles myself and am mostly successful. That said, sometimes it’s best to cut your losses and have a professional mechanic look at a tricky problem.

    I understand the fear of paying big bucks only to have the new kid in the shop butcher your baby. I get that. Do some homework here. Ask around for The One who really knows their stuff. At a big shop you can usually request a particular mechanic. Yes, hourly shop rates are scary these days. But you can waste endless time and throw expensive, non-returnable parts at your bike without coming close to fixing it.

    Disclosure: I’m a wrench at a busy dealership. Been at it for over thirty miserable, thankless years. I’m not the best, but I’m pretty damn good. I take notes of interesting, tricky problems that go back for decades. Sometimes it’s better to pay a mechanic, independent or otherwise, an hour or two of labor to pinpoint the exact problem. An hour on my lift, sometimes less, will often get the job done. Again, ask around and find Mr. Greatwrench.

    Want just the troubleshooting done and will do the repair yourself? Fine by me. I can tell you a main circuit is plugged on a particular carb, or a stick coil is malfunctioning on your #3 cylinder. Then you can go forth from there and do-it-yourself. Customers can ask a bunch of good questions, and I’m happy to take some time to help a decent home mechanic. But until the motorbike is actually in front of me with the clock ticking, it’s all conjecture and best guesses.

    I’ve had nice customers spend an enormous amount of time (and a bunch of my unpaid time) trying to repair a troublesome bike that just remains troublesome. I finally convince them to haul it in. Not to brag—well, OK, I’m bragging—but I’ve sometimes fixed it in minutes. Now I can clearly see that the points lead is grounded. Or that a pin is pushed out of a ECM, Or that their “carb-cleaning” didn’t include the INSIDE of the carb. Makes sense?

    Just some thoughts from a Wrench in the Trench.

    • You are awesome! I had a very good professional mechanic working with me for a long time trying to find the issue. I wonder if you would have been able to figure it out.

      Ready for it?

      I need new CV diaphragms!

      • Jon Jones

        Of course I knew it!!

  • Diaphragms! That’s what the issue was. Diagphrams. Amazing, huh? That old CV diagphrams that have lost their stretch, especially at high rpm, could cause fouling and poor-running?

    The thing is, this isn’t the first time old diaphragms on an old bike have tricked me. It also happened on my BMW R100/7.

    If your bike is over 10 years old and has CV carbs, replace them diaphragms! They’re not just for birth control, you know…

    • JohnnyS

      Awesome! Good to know it’s fixed now.

      That’s one of the nice things about EFI: No diaphragms!

    • Russ

      Wow, good info. Was hoping to get my ’77 R100RS going on most of the old carb parts, will spring for new O-rings and gaskets but the original CV diaphragms seem buttery soft. Can always try it. This thing was stored for years in 50* F out of the sun, so there is hope for the rubbers I suspect.

      • I think those parts are cheap and available, so why not change them?

        • Russ

          If they were in the bowels of the beast, I would. As it is, they are very accessible on the airhead and the rubber feels very supple, with smooth, free movement of the slider. Gotta pick my battles.