Troubleshooting Starting Problems

Discussion in '2-Stroke Engines' started by jaguar, Mar 24, 2015.

  1. jaguar

    jaguar Well-Known Member

    What to do when your Grubee engine is hard to start,
    runs terribly, or won't start at all


    A two stroke engine needs 3 things to run: 1) spark, 2) compression, 3) gasoline mixed with air
    SPARK
    The spark is essential and additionally it has to happen at the right time (within a generous margin of error) and the stronger it is, the easier the engine will start and the more power it will make. Before getting into checking for spark there are two things you have to do. First is look and see if the magneto woodruff key is positioned at 1 o'clock on the crankshaft when the piston is at the top of its stroke. If not, then the timing will be off. On these Grubee engines there is an inherent error straight from the factory. They designed the current ground path to depend on a clean metal to metal contact between the heads of the four screws and the chassis of the stator coil. But they didn't clean off the shellac from the chassis. You have to use sandpaper or something to leave nothing but raw metal for those screw heads to contact. After doing that you can proceed troubleshooting if the bike still won't start. To check for spark just remove the spark plug from the engine, clean and gap it around .025" (.6mm), connect it to the spark plug wire, and use a rag to press the plugs threads onto the cylinder head (or use a jumper wire to connect the two together electrically). Crank the engine and look for spark. Choose a dark location if that is possible so you can be sure to see the spark. The kicker to this is that even if you can see spark, if it is weak then it may not occur inside the engine because compressed air makes it harder for spark to happen. So this test is not the final say-so. If you don't have spark then try a different spark plug. Make sure the spark plugs connector matches the spark plug boot connector. Some connectors are made for spark plugs without the nipple, and some are made for use with the nipple. A spark plug without a nipple stuck into a boot with a connector made for use with a nipple will not work. Other than that there are just 3 other things to check; 1) spark wire, 2) CDI, 3) stator coil
    Spark wire- just make sure it is firmly screwed into the CDI and the spark plug boot.
    CDI- this CDI is a combination CDI and high voltage coil, and it is prone to failure. If you replace it then get a good high performance CDI and high voltage coil which will be more reliable and give more engine power. If it is receiving the right voltage from the stator coil and there is still no spark then you can assume that it is faulty.
    Stator coil- disconnect the blue wire going to the CDI and use an analog multimeter such as #22-109 ($15) from Radio Shack to test the coils output voltage. Strip the wires enough for alligator clips to attach to them. Their other ends should be connected to the multimeter. Set the multimeter at 50v AC. While pushing the bike (with spark plug removed) you should be able to see that there is at least 30 volts being output from the coil. If not then it needs to be replaced. It is terribly unreliable and I recommend that you buy two extras so you'll have an extra spare. Reconnect the wires only using solder because crimp connections lose good connection due to vibration and moisture.
    COMPRESSION
    This is always a problem with the Grubee engine because it comes from the factory with too much ring end gap and a head mating surface that is not completely flat. You have to use a thick plate of glass and some sandpaper to correct it. Or have it lathed down at your local machine shop just enough to make it flat. You can also lath off 1mm to increase engine compression even more (read more at http://www.dragonfly75.com/motorbike/Compress.html). But to do it yourself just put the glass on the floor and the sandpaper (rough side up) on top of it. Step on one corner of the sandpaper and put your thumb on the other. Place the head on the sandpaper and move it in a circular pattern to sand down the high spots on the head. Afterwards use a new head gasket (because aluminum ones are meant to be thrown away because they conform to the irregular surfaces and can't correctly do so two times) or make your own from gasket material (#9614 from Autozone). An engine with good compression is noted when you dump the clutch while pedaling. You should be able to feel its resistance to turning over.
    GASOLINE/AIR MIXTURE
    If the spark plug tip is wet after trying to start the engine then you can rightly assume that there exists a fuel/air mixture. The only problem here is that if the ratio is far off from 1:15 then it will be resistant to being ignited. If there is spark and the compression is good then you can use starting fluid to help you start it. Once started and you're going down the road then you can easily tell if the mixture is too rich (too much gas) or too lean (too little gas or an air leak). Too rich will cause irregular running and too lean will cause consistent running but will lower engine power. At http://www.dragonfly75.com/motorbike/jetting.html I tell how to correctly adjust the float bowl height (which affects the fuel mixture) and change the main jet orifice size. If running too lean then make sure there are no air leaks between the carb and the cylinder, or at the cylinder base gasket, or at the crankshaft seals. There is a lip on the crankshaft that can catch the right side seal when everything is being assembled which can leave an air leak there. The factory never starts these engines to make sure they run, believe it or not. You can check the crank seals by removing everything that prevents you from seeing the seal. Then spray soapy water onto the seal and turn the engine over a few times and watch for air bubbles (which indicates a leak). The seals can be removed with a jewelers screwdriver without taking the cases apart. It is very easy for there to be an air leak where the carb attaches to the intake manifold. Trash inside the fuel in the float bowl can also cause a lean condition by restricting fuel flow into the bowl or from the bowl at the main jet. Also of consideration is the air filter. If it is restrictive to air flow then that causes an overly rich mixture of fuel/air. If you take it off and blow thru it then you shouldn't feel any restriction. If you do then buy a better filter. I made my own out of a foam lawnmower filter that I cut and glued together using silicone sealant. It never came apart even after years of use. I lubed it with engine oil and squeezed out all excess oil. The Grubee engine comes with a carburetor that doesn't have its own idle mixture circuit (which is normally there on all good carbs). To adjust the idle mixture you can raise/lower the needle or solder onto the end of the needle to make it fatter and allow less gas to mix with the air. That procedure is described in more detail on my jetting page that I listed above. The adjustment screw on the carburetor is the slide stop. Set it for 1-2mm vertical opening when the throttle is closed. Once you get the engine running right then adjust it for correct idle speed. If you can start the engine but after a while it starts to run weak and then die, it may be that the filter for the gas inside the gas tank is clogged, allowing too small a flow of gasoline to the carb. Also the same condition can be caused by an air vacuum inside the tank if the cap is not breathing right. I had to drill a small hole into mine to allow air into the tank as the gas level reduced.
     
    Last edited by a moderator: Dec 18, 2015

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