Troubleshooting Your Motor: Why Won’t My Motor Start? pt. 2

Troubleshooting Your Motor: I’m Not Getting a Spark!

In order for your motor to start and run it needs a spark, which comes from both the magneto and the CDI. If one or both of these components aren’t working you won’t be able to start your engine (let alone use it) not matter how well it’s tuned.

I’m getting a weak spark, and my bike is sputtering:

1) Make sure you’re getting spark

To check if you are receiving power to the spark plug, follow these simple steps.

  • Remove the spark plug from the cylinder head.
  • Re-attach the spark plug to the spark plug cap.
  • Lay the spark plug on the cylinder head so that the metal of the plug is touching any metal portion of the cylinder head. DO NOT HOLD THE SPARK PLUG OR BOOT BY HAND.
  • With the clutch out, push the bike forward or turn the rear wheel. A bright spark should be visible.

2) Wiring Issue

If you’re getting a weak spark and the bike doesn’t seem to be firing the way it should, there’s a chance that the cables for the CDI, magneto, and kill switch aren’t connected correctly. Check to make sure that all of the electrical connections leading from the magneto are connected:

Blue leads to blue, black leads to black as the wires enter the CDI box from the magneto. Follow the white wire as it leads from the magneto up to the kill switch. On certain models, the kill switch may be at fault grounding out against the handlebar. Disconnect the kill switch at the handlebar and attempt to restart the bike.

If these components are connected but still don’t get a spark, try taking the stock cable connections off of each wire, peeling the housing back, and wiring the cables wire-to-wire (those cheap metal connections may not be making a full connection)

3) Spark plug installation and gap

Make sure that a proper connection is being made in the spark plug boot. Inspect and/or replace the spark plug. Correct spark plug gap is 0.038″.

I’m not getting a spark at all!

1) Check your wiring connections

Double check the connection of the wires, particularly the blue to blue and black to black.

Check that no particles are lodged between the side and center electrodes of the spark plug.

Try replacing the spark plug or cleaning it by running sand paper through the side and center electrodes and resetting the gap to 0.038″.

Re-test for spark with the kill switch pressed in. Try feathering the kill switch. Disconnect the white wire completely allowing the wire to hang free, then re-test the engine.

If the connections are free and clean but you’re still not getting a spark, it’s time to test your components:

1) Magneto testing

Take a voltmeter or multi-meter and adjust it to the Ohms setting at 20k. Ohms measures the resistance across a circuit. Check voltmeter across the following wires:

  • Blue wire to the White wire of the magneto. The resistance should be 0.25 to 0.40. New magnetos read 0.31.
  • White wire to Black wire should read 0.0.
  • Blue wire to Black wire should read close to 0.31.
  • If your reading is far off from this, then the magneto is to blame for weak/no spark.

If the readings check out on your magneto, test your CDI

2) CDI testing

Run the voltmeter at the same 20k setting. Check voltmeter across the following wires:

  • From the inside of the spark plug boot to the Black wire of the CDI. Your reading should measure 2.0 – 2.7. A new CDI will read 2.3.
  • Spark Plug Boot to the Blue wire there should be no change, or 1.0.
  • Blue wire to Black wire there should be no change, or 1.0.
  • If the resistance is different than specified, this simple test will tell you where your problem lies.

Check out our spark troubleshooting guide video below:

27 thoughts on “Troubleshooting Your Motor: Why Won’t My Motor Start? pt. 2

    • Hey Samuel, I understand the frustration but luckily you have spark so it narrows down what the problem could be. 1. check for compression: make sure there’s no pin holes in the actual cylinder body as sometimes manufacturers don’t do a complete seal all the way around. 2. check head bolt torque: make sure the head bolts are torqued between 6-8in/lbs. 3. Check to make sure there’s no air leaks between the gaskets on your intake and exhaust. 4. check your cylinder head cap: I recommend taking it off your engine and placing it on a flat surface. If it doesn’t sit flat then it’s warped and will cause compression issues. Hope this helps!

  1. Hey I just bought a 80cc engine kit off eBay when I pull the clutch in the bike rolls forward easy but when I let it out and try and push the bike forward the back wheel locks up is it ment to be hard to push the bike forward when the clutch is out

  2. 80cc had spark 4 2 weeks with off switch in lost spark, on second cdi,tired of buying them,any suggestions tried all trouble shooting, ? Please help

    • Hey there, Stephen! Like another rider asked in this section, if your electrical components keep shorting, here are a few ideas of what could be happening:
      1. The way your magneto, kill switch, and CDI are wired is somehow creating a short, which is taking out one of those components
      2. If there’s moisture build up around the bike (for example, if you ride in cold weather and bring the motor in to a warm area, causing condensation around the engine), some of that water could have gotten on to the magneto or CDI, causing a short
      3. Some newer magnetos have a thin layer of resin on them, which- if not scraped off before installation- can melt, effecting the wiring on that magneto, causing a short
      Let us know if either of those ideas are what’s going on. If not, we’ll keep on trying to get you back up and running.

  3. I’ve had trouble with my 80cc electrical ever since I put it on my bike in August. I put new magneto and cdi on two weeks after I got it .six weeks later its out again.I just got new cdi and put on and still no go I’m lost

    • Hey there, Dewayne! If your electrical components keep shorting, here are a few ideas of what could be happening:
      1. The way your magneto, kill switch, and CDI are wired is somehow creating a short, which is taking out one of those components
      2. If there’s moisture build up around the bike (for example, if you ride in cold weather and bring the motor in to a warm area, causing condensation around the engine), some of that water could have gotten on to the magneto or CDI, causing a short
      3. Some newer magnetos have a thin layer of resin on them, which- if not scraped off before installation- can melt, effecting the wiring on that magneto, causing a short
      Let us know if either of those ideas are what’s going on. If not, we’ll keep on trying to get you back up and running.

  4. I have an 80cc kit, i have been riding for a few years, and have figured out every issue i have ever had using youtube etc. A few wks ago, my exhaust bolts stripped out so i bought a new top end. I swear every since i put the new top end on, its been hard starting, lags serious power and tourque if these thing had it…i just put a racing carb on it yesterday, i have had no change in power, it used to hall ass until i replaced that part…any ideas?

    • That’s an interesting issue, Shane!

      It’s good to hear that your motor will run, but it’s a shame that it doesn’t run the way it used to. Now, if you’re able to start the motor and it runs, you spark and air intake are good. If your head studs stripped and you had to replace them I’m thinking you have one of two issues, both related to a possible air leak:
      1. Bicycle engine kits will have different stud sizes from model to model. Though most 2-stroke engine kits look the same, studs (particularly the head studs) will come in two difference sizes: 6mm or 8mm. If the studs you’re working with have a little wiggle room within the cylinder, chances are you have the wrong size for the cylinder. You’ll want to get a bigger size of studs.
      2. If your studs stripped out of the crank case, you might have an air leak. If the studs can’t seal properly it can throw the compression off, and if your compression is off you won’t be able to ride like before. Inspect the motor head stud ports in the crankcase to make sure they’re not stripped. If they are stripped, you may need to either seal the studs inside of the crankcase with Loc-Tite (this is a last resort) or replace the crankcase.

    • That’s a great question, Kevin!

      When it comes to CDI’s and magneto’s giving you a spark there are two things you want to check on them: their installation and the electrical current.
      The way the magneto and CDI are installed in your engine will determine if the connections are wired together properly. For a magneto, you’ll want to make sure that the ground wire for the magneto sits between the back of the magneto and the crankcase, with a stud through the top of the magneto in to the crankcase. For the CDI you’ll first want to make sure all wiring is done wire-on-wire, which means you’ll probably need to take off the connections of your CDI and magneto, peel the plastic housing back, and wire them together. Then you’ll want to make sure the black wires from the CDI, magneto, and kill switch are all ground somewhere on the frame.
      Now if you connections are good, then the issue will lie somewhere with the OHMs reading of your CDI and magneto. With an OHMs meter, test the black and blue wires on the CDI and magneto to make sure they’re giving you a reading. If they’re giving you some sort of output those parts should be working, but if one or both read open then you’ll want to replace the defective parts.

  5. Hey .. Everything was going fine but it stopped running while going and i have no idea why. None of these troubleshooting pages help. What could be up?

    • Hey there, Josh! Sorry to see you can’t find something helpful for you. If your motor suddenly stops while you’re riding there could be one of three things going on with the engine:
      1. The engine isn’t being choked long enough before you open the choke. When it’s wide open you’ll introduce too much air in to your fuel, which will cool the fuel and it can’t combust if it’s too cool.
      2. The electrical components (CDI and Magneto) might have shorted out. This could be the case if you’re not getting spark, so try testing the CDI and magneto with an OHMs meter to see if you’re getting a current out of them.
      3. The compression on your head could be off. This could be due to a busted gasket, so remove the head of your motor and check to see if the top or bottom head gasket are split or broken, as well as the other gaskets around your cylinder.

  6. My 80cc 2 stroke engine for bike won’t start…it ran great for a month…my spark plug fell apart…got a new one..I have spark but won’t start…any suggestions?

    • Good afternoon, Christian! There are a couple of things that could be going on with your motor. Now, if you’re getting spark your electrical components should be in working order, which means your problem lies within either the carburetor or the compression:

      1. If the engine isn’t being choked long enough before you open the choke the motor will not start, and if it does start it will not stay running. When the choke is wide open you’ll introduce too much air in to your fuel, which will cool the fuel and it can’t combust if it’s too cool.
      2. The compression on your head could be off. This could be due to a busted gasket, so remove the head of your motor and check to see if the top or bottom head gasket are split or broken, as well as the other gaskets around your cylinder.

  7. My bike was just working I I go to ride it today not working so I unscrew the cap that leads to the white blue and black wire and I find my blue wire not hooked to anything ware does it need to be?

    • I’m sorry to hear that, Kayla!

      The only loose wire in the magneto assembly would be the black ground wire. The ground wire would be between the magneto and the crankcase, and held in with a screw to keep that in place.

      The blue and white wires are welded directly to the sides of the magneto, so if it is free floating in the case you’ll either want to try to solder the wire back to its connection point, or replace the magneto all together.

  8. Hi all.
    Just put together my bike kit.
    When I wired it up and tested it I was getting a spark.
    When I tried to start it properly it wouldn’t start for me.
    I am now not getting any spark.
    I stripped the spark plug cable, and can feel power is going to the spark plug, but it is still not sparking.
    I tried other spark plugs, which are not sparking either.
    They work fine on my chainsaw, so it’s not the plug.
    I’ve disconnected the kill switch, but still no spark.
    Any ideas on what it could be would be appreciated, I have ordered anew CDI, but I am not sure if that is the problem.
    It will take about 20 days to arrive from China.
    I will order a new magneto too, but am wondering what else it could be.

    Thanks.

    Peter

    • Great question, Peter!

      When you’re dealing with the CDI and magneto there are two main issues that are causing a weak or inconsistent spark: the wiring of your electrical components (the kill switch, magneto, and CDI), and their conductivity:

      For a magneto, you’ll want to make sure that the ground wire for the magneto sits between the back of the magneto and the crankcase, with a stud through the top of the magneto in to the crankcase. For the CDI you’ll first want to make sure all wiring is done wire-on-wire, which means you’ll probably need to take off the connections of your CDI and magneto, peel the plastic housing back, and wire them together. Then you’ll want to make sure the black wires from the CDI, magneto, and kill switch are all ground somewhere on the frame. After that, follow the wiring instructions provided with your bike engine.
      Now if you connections are good, then the issue will lie somewhere with the OHMs reading of your CDI and magneto. With an OHMs meter, test the black and blue wires on the CDI and magneto to make sure they’re giving you a reading. If they’re giving you some sort of output those parts should be working, but if one or both read open then you’ll want to replace the defective parts.

  9. I installed the 80cc and having a problem. I had the motor running for like two min and shut off now can get it started! It’s got a little spark like a light blue. I ordered a new cdi just waiting on that but what could be the issue

    • That’s a good question, Dracy!

      If you’re seeing a spark then chances are your electrical components (i.e. your CDI and your Magneto) are in working shape. But if you’re not able to get the motor started, here are a few things to check:
      1. Make sure you’re getting fuel in to the engine. That means making sure your fuel is flowing from the gas tank through the fuel valve in to the carburetor, then from the carburetor to the engine.
      2. If you’re getting fuel in to the motor check to see if the spark plug is wet. If it’s darker or more clear than usual there’s a possibility your fuel is not being mixed properly. Make sure you’re mixing your fuel at the right ratio for that engine and its break-in.
      3. If you’re mixing your fuel properly and you’re getting fuel in to your motor, you might want to check your motor for air leaks. That means looking around your engine for any pin-holes or cracks in the body, any busted gaskets, and making sure the torque on your motor head studs is at the right pressure.

  10. I give! I have five(5) cdi coils all new never gotten a spark from any one of them. I run the above ohms test and 4 test 6.1 to 7.0 from boot to blk wire no other readings. The mag tests are spot on for both mags I have . I still don’t have any spark. I have had this kit on my bike for over 2 months haven’t been able to ride it yet. One new cdi has no readings what so ever. Help!

    • I’m sorry to hear that, because if you’re getting readings from the CDI(s) and magneto then you should be getting spark. At this point if you’re not getting spark here are a few things that you can check:
      1. Make sure all of the wires are connected together wire-on-wire. Take the connectors off of the tips of the wires so the copper is exposed. Then peel back the plastic housing on those wires so you can twist the copper together. Once they’re all together, wrap each connection with electrical tape.
      2. Assure your wires are all connected properly (you’ll want to refer to the connection guide your engine kit came with)
      3. Make sure all ground wires are ground. On the outside of the motor, your black wires need to be ground somewhere on your frame. Internally, most magnetos will have a free wire that’s not soldered to the magneto- this wire is the ground wire. There should be an eyelet connection on it, and that connection goes between the magneto and the crank case, and is held in the one of the bolts your magneto is held in with.

    • Hey there, Peter!

      You’re correct, the motor is going to take a little bit to start. Assuming your electrical components are connected properly, you’re getting spark, your compression is dialed in, your fuel is mixed right, and you’re getting fuel in to the motor you might want to look around your motor to see if there is anything that might be a little off.
      For example, check for any air leaks (like pin holes in the case or broken head/exhaust/intake gaskets) around the engine to make sure you’ve got the right temperature in your motor for combustion.
      You also might want to see how long you’re choking the motor, or if you’re choking it at all. Not choking the engine will prevent the motor from starting, or if it does start, it will not allow the motor to run for a long time.
      You can also check to make sure your motor is level, because a carburetor that’s not level with the ground will not get fuel in to the engine properly. Not only that, but you also want to make sure the gas tank is above the motor and not parallel with or below the engine. If the fuel cannot flow down in to the motor it will not get in to the motor at all, as these motors are gravity fed. If your motor is mounted above or at the same height as the fuel tank your fuel won’t get in to the engine, and your motor won’t start without fuel.

  11. Bearing and winding failure is the most common motor failure. The fundamental reason is usually excessive heat. preventive maintenance practices often restrict on-line electrical measurements to interpret the current levels.

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