Posts
160
Joined
8/16/2009
Location
Shenandoah, VA
US
Edited Date/Time
10/16/2020 2:50pm
I have an 05 pw 50 that i bought used for my three yr old son. It was always a little hard to start ( 10-15 kicks when not warm). It gradually got to the point where it took me 30-40 kicks, till not starting at all. I have cleaned the carb numerous times (including replacing the pilot jet which i had been told was hard to tell if it was fully unclogged). I've rebuilt the top end, and checked the reeds. I pulled the exhaust off to make sure it was not clogged. The bike has spark, and gas through the carb but will not start. However, if i pour gas into the sparkplug hole or spray starting fluid the bike will fire right up but will only run for a few moments until the fuel is burned up. I've tried everything I can think of and my lil boy is gettin pretty tired of waiting!! Hoping for some help guys.
Have you done a compression test? Putting starter fluid in the cylinder and it runs some times indicates low compression as starter fluid will fire under lower compression than gas.
Also a weak spark can cause it to be hard to start. Check to make sure your are getting a good blue spark at the plug. A yellow spark will ignite starter fluid, but not gas. If the spark is yellow in color, clean all the coil connections and the plug cap. You can also do an ohms test of the coil on the primary and secondary sides. I don't have the exact specs for that bike, but the primary side should be above 20 ohms and the secondary should be below 16 ohms.
The Shop
Make sure the head is sealing good at the gasket. Spray some soapy water around the head gasket area as you kick the engine over. Make sure there are no bubbles squeezing out.
Another area on those little bikes to check is the ignition side crank seal. They sometimes will actually work their way out of the cases, and the bike will never run until you pull the flywheel and stator plate, and replace the seal. It is a small seal, and they are very easily distorted over time.
If the seal is out of it, no matter how fresh the top end is, all of your compression /cranking pressure is lost through the crank seal. I would bet this may be your problem.
I smear a little Yamabond on the outer edge of the seal, where it comes in contact with the cases to help it stay put.
You can also disconnect the white connector from the start / run switch behind the number plate to make sure the switch is not going bad. Leave the single black wire connected. It will still operate as a kill switch, but you will no longer need to place it on Start to get it to run. Just click it to Run. Careful though, as it will be ready to take off as soon as the throttle is opened, as this bypasses the little control module under the seat that retards the spark on the Start setting.
Do the free ,cheap stuff first. Pull the fuel line from the carb, and turn the petcock on. If gas flows, scratch a clogged petcock.
Loosen the little drain screw on the bowl of the carb. If fuel flows out of the drain hose, it is filling the bowl.
It should fire with a clean carb , fuel in the bowl, and a reasonable compression reading.
If it is not getting the plug wet after several kicks, low vacuum would be suspect. It relates to a well sealed engine. If the crank seal is out of it, or worn out, It will never draw enough fuel through the carb to run on its own ,as any crank case pressure is lost right out of the seal.
The crank case /cylinder is where sealing is critical to hold a vacuum, and keep cranking pressure /compression within an operable range.
A crank seal is about $4. I have had this happen on PW's , Suzuki JR 50's -LT 50 quads, and Eton models.
Other than a bowl gasket or pilot jet, have never had to replace a complete carburetor or any ignition parts on any of them.
Bought the old white 87 pictured above for $100, someone had tried to pry the cylinder off and broke the RH case half.
Replacing the case half on that one was the most engine work I ever had to do on one of those little bikes.
It fired off and ran well with the old carb that came with the bike, once the cases were back together, new top end , crank seals, and a good cleaning of the carb.
You only have to pull the flywheel and stator plate to access the seal.
Remove the little plastic cover that is held on by three little screws. This is on the LH side of the bike as you sit on it.
You will see the flywheel staring at you. You will need to remove the nut on the end of the crank in the center of the flywheel.
The flywheel will want to turn as you try to remove the nut. Hold it with your hand , give your ratchet a few bumps with your other hand. The nut will come loose. Or if you have an air ratchet or little butterfly ratchet,this will make removing the nut easier.
DO NOT, use a hand impact that you strike with a hammer to remove the nut.
Then , you are going to need the correct flywheel puller. Again, do not pry or beat on the flywheel to try and get it off.
If you have a local shop close by, they should have a puller, or can pull it for a small fee. Don't lose the flywheel key when you pull things apart.
Once the flywheel is off, you need to use an impact screw driver to loosen the stator screws. Get a tight fitting bit, as the screws will round the Phillips slots easily. Give the impact a fairly good rap with a hammer, and loosen the screws.
Pull the stator plate out of the case cavity. The seal is right there centered over the crank end. Before you go any further, look for any signs of oil / premix in the cases and around the seal. If there are signs of this, the seal is leaking.
Get an angled pick, and carefully remove to old seal. Try not to knick the crank end. The seal is not in there that tight on those bikes.
Clean things up, get a new seal, oil the inside edges of the new seal, and carefully slide the seal over the crank. Do not nick the seal as it goes over the crank. A little Yamabond or Loctite on the outer edge of the seal where it touches the cases will keep it from moving.
Replace the stator plate, tighten the screws, slide the key way into the crank slot, and carefully slide the flywheel back on.
Torque the crank nut down, give it a few kicks ,and see what happens.
It is a simple job. The correct puller is a must, though.
http://www.yamahapartshouse.com/oemparts/a/yam/50045219f8700209bc792233…
Pit Row
The primary side seal stays pretty well lubricated from the gearbox oil, and does not suffer the effects of engine heat as much as the stator side seal. Even in some of the bone yard PW's I've had, I replaced one primary side crank seal ,and that was only because the cases were being replaced.
Make it easy on yourself, put a stator side seal in it, and go from there. If the primary side needs replacing, you have not lost anything by doing the stator side seal.
If it fires up and runs with a stator side seal, call it good, and let the kid ride it.
For what it's worth, I have seen many PW's that would not run due to a bad stator side crank seal. I can not recall ever seeing one not run due to a bad primary side seal. Not saying it could not be possible, but the critical, most commonly replaced seal on those is the stator side.
I have pulled the covers on more than one PW, only to see the seal totally out of it's boss / holder in the cases, sitting out on the crank end. That is where a little Loctite or Yamabond on the outer edge of the new seal is helpful in keeping the seal in place.
These are not F1,NHRA, or 250F engines here. It's a moped engine.
Don't push the seal in too far. Flush with the cases or a tick more. They will go in deeper, however. You can usually press them in with your fingers- thumb. Try to get it in as straight as possible.
Make sure the key is removed from the crank before you install the seal.
Be ready though. There is a real spaghetti mess of wiring on those PW's once you get the seat off, and start unplugging things.
If you see what appears to be grease around the seal, it is more than likely a bit of fuel / oil mixture that is getting past the seal. If you want to do a poor man pressure check, get a little spray bottle of soapy water, squirt the seal /crank area as you roll the engine over a few times with the kick starter. Make sure to leave the spark plug in the cylinder so the engine will build a bit of compression /pressure.
This will allow enough pressure to build in the cases to determine a leak as you spray the seal.
If you see any bubbles forming around the crank seal as you spray while kicking the engine over, it is leaking.
Or just carefully install the new seal, button it back up, and see what happens.
There is really not much to these little engines. Keep us posted.
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