98 G16 starter runs, have spark, motor won't fire

1989Marathon · 07-27-2013, 09:42 AM

It's normally done with a leak-down test. Remove the valve cover and make sure both valves are closed(piston at TDC). Remove the spark plug and pressurize the cylinder to about 10-30psi. Usually the hose for the compression tester can be used to do this, although the schrader valve will need to be removed from the hose if it has one. Screw the tester hose into the spark plug hole, and then hook the other end to the air supply. Be careful when you do this because the crank will want to turn because of the air pressure(piston wants to go to the bottom of the cylinder). If a valve isn't closing completely, you'll know right away because the air will escape. If it comes out of the muffler, it's the exhaust valve. If it comes out through the carb, it's the intake valve. If it is escaping, but not through the muffler or carb, it is likely the piston/rings or head gasket.

The other option is to just remove the head and see what is going on.

raypa · 07-27-2013, 10:47 AM

Will try it now. Thanks! I'll report back.
]

raypa · 07-27-2013, 11:06 AM

I have air coming out the exhaust and carb at TDC.
Can you remove the head without pulling the motor from the cart?

1989Marathon · 07-27-2013, 11:12 AM

You may be on the exhaust-intake overlap(wrong stroke). Turn the crankshaft 360° and try it again.

raypa · 07-27-2013, 11:40 AM

Here are some pics. The piston is tight. I thought the piston would travel up into the head, I guess not. The valves were a little carboned up, but not real bad. It doesn't look too bad, Now I'm real confused.

raypa · 07-27-2013, 11:41 AM

Here is a wiggle test of the piston. Apparently .mov from my phone doesn't upload.

I tore it apart before I saw you post. I'll reassemble and turn 360 deg and try again. (but I'll clean the valves first)

1989Marathon · 07-27-2013, 11:44 AM

Are you saying that the piston doesn't move when you rotate the crankshaft/clutch? What happened to the other valve?

raypa · 07-27-2013, 12:10 PM

I was wiggling the piston to see if there was play if the rings were broken/missing. there is no slop or scoring on the cylinder wall. it goes up and down freely. That photo I pulled the intake valve to see the carbon build up. there was a decent amount. I cleaned it all up and was going to re assemble. Do you see anything as a glaring issue? Is there anything else I should look for. I've never pulled a 4 stroke apart before, but it all looks normal, but I don't know what I'm looking at. Should I tear it down more to see something else?

now would be a good time for a big block..lol

1989Marathon · 07-27-2013, 12:17 PM

I would clean up the valves, reassemble the motor, adjust the valves, then try the leak-down test again. Nothing really sticks out as being wrong.
Remove the exhaust valve and clean it up too. Put the valves/springs back in and make sure the valves are fully closed before you put the head back on.

raypa · 07-27-2013, 12:23 PM

When i was trying to adjust the valves earlier, it seemed as thought I could tighten down the set screw almost all the way and I still have more the .004. It never really got tight.
is there a trick to setting them. Going to reassemble...

07-27-2013, 09:42 AM	#11
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730	Re: 98 G16 starter runs, have spark, motor won't fire It's normally done with a leak-down test. Remove the valve cover and make sure both valves are closed(piston at TDC). Remove the spark plug and pressurize the cylinder to about 10-30psi. Usually the hose for the compression tester can be used to do this, although the schrader valve will need to be removed from the hose if it has one. Screw the tester hose into the spark plug hole, and then hook the other end to the air supply. Be careful when you do this because the crank will want to turn because of the air pressure(piston wants to go to the bottom of the cylinder). If a valve isn't closing completely, you'll know right away because the air will escape. If it comes out of the muffler, it's the exhaust valve. If it comes out through the carb, it's the intake valve. If it is escaping, but not through the muffler or carb, it is likely the piston/rings or head gasket. The other option is to just remove the head and see what is going on.
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730

07-27-2013, 10:47 AM	#12
raypa Getting Wild Join Date: Jul 2010 Posts: 119	Re: 98 G16 starter runs, have spark, motor won't fire Will try it now. Thanks! I'll report back. ]
raypa Getting Wild Join Date: Jul 2010 Posts: 119

07-27-2013, 11:06 AM	#13
raypa Getting Wild Join Date: Jul 2010 Posts: 119	Re: 98 G16 starter runs, have spark, motor won't fire I have air coming out the exhaust and carb at TDC. Can you remove the head without pulling the motor from the cart?
raypa Getting Wild Join Date: Jul 2010 Posts: 119

07-27-2013, 11:12 AM	#14
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730	Re: 98 G16 starter runs, have spark, motor won't fire You may be on the exhaust-intake overlap(wrong stroke). Turn the crankshaft 360° and try it again.
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730

07-27-2013, 11:44 AM	#17
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730	Re: 98 G16 starter runs, have spark, motor won't fire Are you saying that the piston doesn't move when you rotate the crankshaft/clutch? What happened to the other valve?
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730

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07-27-2013, 12:10 PM	#18
raypa Getting Wild Join Date: Jul 2010 Posts: 119	Re: 98 G16 starter runs, have spark, motor won't fire I was wiggling the piston to see if there was play if the rings were broken/missing. there is no slop or scoring on the cylinder wall. it goes up and down freely. That photo I pulled the intake valve to see the carbon build up. there was a decent amount. I cleaned it all up and was going to re assemble. Do you see anything as a glaring issue? Is there anything else I should look for. I've never pulled a 4 stroke apart before, but it all looks normal, but I don't know what I'm looking at. Should I tear it down more to see something else? now would be a good time for a big block..lol
raypa Getting Wild Join Date: Jul 2010 Posts: 119

07-27-2013, 12:17 PM	#19
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730	Re: 98 G16 starter runs, have spark, motor won't fire I would clean up the valves, reassemble the motor, adjust the valves, then try the leak-down test again. Nothing really sticks out as being wrong. Remove the exhaust valve and clean it up too. Put the valves/springs back in and make sure the valves are fully closed before you put the head back on.
1989Marathon Gone Wild Join Date: Oct 2010 Posts: 6,730

07-27-2013, 12:23 PM	#20
raypa Getting Wild Join Date: Jul 2010 Posts: 119	Re: 98 G16 starter runs, have spark, motor won't fire When i was trying to adjust the valves earlier, it seemed as thought I could tighten down the set screw almost all the way and I still have more the .004. It never really got tight. is there a trick to setting them. Going to reassemble...
raypa Getting Wild Join Date: Jul 2010 Posts: 119

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