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Troubleshooting a bike that will crank but won't start

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  • Troubleshooting a bike that will crank but won't start

    Troubleshooting a bike that will crank but won't start
    by Mike Hart

    If your bike has sat for some time, check What to look for on a neglected XS/XJ 1100 .

    This tip is meant to help you with a bike that will not start at all but has run well recently, not to help you correct a bike that starts but runs badly, although some of the information here does apply to that situation.

    Make sure your battery is fully charged and the connections are in good condition (tight and clean). Somewhere not far below 11.7 volts, the ignition will no longer fire. If the engine cranks vigorously, you're ok, if not, find out why - fuses, connections, switches, battery condition - and correct the situation before continuing.

    > > > > - AIR - FUEL - SPARK - < < < <

    The easiest place to start is with the air filter. If a lot of gas has got in the crankcase it can easily pump an oily mess into your airbox through the crankcase breather. Or maybe some rodent or insects have set up house in there. If you do find an oily mess, see Where Does the Gas Go? and the Carburetor sections of the Repairs and Maintenance forums for information on how to correct this problem with the petcocks and/or carbs. Obviously you'll need to change your oil and filter, and your plugs might be fouled as well.

    Okay, we have air, right?

    Which next? Do you smell gas after cranking? If so, check for spark next. Keeping in mind to check simple/easy things first, check for petcock fuel flow and check for a good fat spark at the plugs before digging deeper into either system.


    There is fresh gas in the tank, yes? Is the petcock in the On or Res position? Try the Prime position too.

    Confirm you have fuel flow from the petcocks. Checking this depends on whether you have a Standard or Special (including XJ), and if you have a Special or XJ, whether or not you have the stock octopus ("Diaphragm Assembly") . If you have the octopus, see the testing the octopus tip. If you have a Standard, the petcock will not flow with no engine vacuum - if the bike is cranking good, there should be enough vacuum to allow fuel to flow. Remove fuel line from petcock (standards have the vacuum line going to one petcock tap; other models flow from both taps). Crank the engine and see if fuel flows from the tap. Alternatively, remove the vacuum line as well (from the vacuum nipple) and give it some mouth vacuum action. If no fuel flows, you need to remove and fix whatever the problem is with the petcocks. (see links above) If you have a Special or XJ with no octopus, fuel should flow freely from either tap in any but the Off position. Check the fuel hoses for kinks or blockages.

    Either your petcocks are okay, or you've fixed them, and the bike still won't start! - now what? You could check for spark now, or verify fuel flow - you should be able to reach the float bowl drain screws on the bottom of each carburetor. Loosen each screw, and use a small clean glass container to catch what comes out, you are looking for water or crud in the bowls. You can just use a rag to catch the gas, looking for crud. (Early in my bike's life - less than 1000 miles on it - it wouldn't start one morning - the XJ has a convenient float drain screw with nipple, easier to catch what comes out, and there was a lot of water from a gas station I never revisited. One flush of the bowls got rid of the water, and it fired right up.) Turn the petcocks to Prime and try again.

    If the bowls fill again, you are likely getting fuel flow, but it's not out of the question to still suspect some carb or fuel delivery problems. Confirm spark before digging deeper into the carbs.

    Okay, you have confirmed good spark and the bike still won't start? It isn't all that likely that all the carburetors are clogged, but if you've done everything else, then it's carb cleaning time. See the Carburetor sections of the Repairs and Maintenance forums.


    If you have an ignition tester, you know what to do. Otherwise...

    Remove each spark plug, ground it against the engine, see if you have a bright healthy spark. (Don't crank the engine with spark plug caps not attached to the plugs, or with the plugs not grounded to the engine, you could damage the coils.)

    For those still with us, with weak or intermittent spark, or no spark at all, here is a simple ignition circuit diagram:

    Click image for larger version

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    Ballast resistor (all models 78-80; 81 and on use different coils that don't require it) – check terminals; resistance should be 1.6ohms +/- 10%. The coils should have 12 volts with ignition on (coil side of ballast resistor red/white wire vs ground). The TCI cuts off current to the coils after a few seconds of the ignition being on but engine not cranking or running. When the motor cranks 180 degrees, the pickup coil signal turns the voltage back on, so you might have to crank the motor briefly to measure this voltage. If no voltage, you probably have a pickup coil or TCI problem. See the Battery & Electrical Repair forum.

    You can measure some resistances by pulling the plugs off the TCI unit.

    All models have the same wire colors for the ignition coil primary windings:
    Orange --> Red/White <-- Gray

    Ignition coils: (Primary measured from plug at TCI, Secondary measured at spark plug wires - the caps are resistive, remove the cap to get a true reading)
    1978 -1980 Primary 1.5 ohms +/- 10%, Secondary 15K ohms +/- 20%
    1981 and later Primary 2.5 ohms +/- 10%, Secondary 11K ohms +/- 20%

    Pickup coil wire colors:
    E, F, SF: two pairs of Red/White <--> Green/Yellow
    G,SG, H, SH, XJ: Orange --> Red/White <-- Gray

    Pickup coils: (measured at TCI plug)
    XJ 120 ohms +/- 20%
    All other models - 720 ohms +/- 20%

    TCI: The pickup coils produce a small current signal when the reluctor (a projection on the timing plate) passes by each pickup coil. If there is no signal from the pickup coils, the coils won't fire. If the signal is present and the coils don't fire (and the ballast resistor is not open-circuit) it is possible the TCI is defective. Don't suspect the TCI until you've thoroughly exhausted all other electrical possibilities. Of course, if you have a spare lying around, it's worth a try to swap it in.

    At this point, if you have air - fuel - spark and the bike won't start, it's hard to imagine losing compression 'overnight', but a compression check is in order.

    I hope this has helped. Please Email me with any corrections, suggestions, or comments.
    Last edited by tplank; 11-22-2020, 06:19 PM.
    Mike * Seattle * 82 F'n'XJ1100 *