Spark, fuel, compression, but no start!! By mossy173 (Read 17,303 times)

[mossy173]

Here's a puzzle: I have a Jonway 50cc I'm trying to upgrade to 100cc, and after a seemingly uneventful install, the thing no longer runs. Here's what I've checked:

I gapped my rings correctly
the cam sprocket is lined up correctly
the valve lash is set to .002 intake and .003 exhaust
there's spark
there's compression (judging by placing a finger over the sparkplug hole)
and there's fuel entering the cylinder.

I've even tried starter fluid sprayed into the vacuum port and straight into the carb port, and that didn't help. The starter cranks the engine OK, but I haven't heard the slightest hint of a bang--just absolutely no firing at all!

Any ideas?

[mossy173]

I should add that it's a brand-new Jonway yy50qt with barely any mileage to speak of, and it worked great before I went and borked it all up.

[Alleyoop]

1. How did you check for spark?
2. After you crank it take the plug out and see how the plug is WET or DRY.
3. Have you checked the timing since you put in a BBK you would of had to take the Cam chain off.
A. So line up the Cam Sprocket correctly as if your going to adjust the Valves and tell me where the Case Pointer is pointing at on the Flywheel, on the "T" After the "T" Before the "T" etc..
Alleyoop

[mossy173]

1. I checked for spark by pulling the plug, reinserting it in its boot, and holding it to the head cover while cranking the engine with the electric starter.

2. I thought the plug was wet, but after consulting with someone who knows how wet it should have been, the plug is dry. I do feel suction when I put my hand over the carb boot (with the carb removed) and crank it, and there's definitely gas coming through the carb, but maybe it's not getting into the cylinder somehow? I tried opening up the air filter to shoot some starter fluid in where the filter element had been (didn't do anything) and while I was in there I noticed that the filter element had some gasoline soaked into it. If it is just a fuel problem, I'd expect the starter fluid to work, no?

3&4. The timing was off by a tooth! But fixing that didn't help. There's still no hint of ignition on either gasoline or starter fluid.

[Alleyoop]

Nov 12, 2012 10:59:49 GMT -6 mossy173 said:

1. I checked for spark by pulling the plug, reinserting it in its boot, and holding it to the head cover while cranking the engine with the electric starter.

2. I thought the plug was wet, but after consulting with someone who knows how wet it should have been, the plug is dry. I do feel suction when I put my hand over the carb boot (with the carb removed) and crank it, and there's definitely gas coming through the carb, but maybe it's not getting into the cylinder somehow? I tried opening up the air filter to shoot some starter fluid in where the filter element had been (didn't do anything) and while I was in there I noticed that the filter element had some gasoline soaked into it. If it is just a fuel problem, I'd expect the starter fluid to work, no?

3&4. The timing was off by a tooth! But fixing that didn't help. There's still no hint of ignition on either gasoline or starter fluid.

Good on #1 checking for Spark.

2. If the plug is dry after cranking and NO FIREING WHATSOEVER that means NO GAS is being sucked into the motor.

3. The Timing, I still question I would like to see a picture or Video of the CAM SPROCKET lined up and a shot of where the timing is on the FLYWHEEL.


******
Forget about the starter fluid for a minute and actually squirt some gas directly down the INTAKE MANIFOLD.

Take a cup, sygrine or something that you can squirt some GAS into the VACUMM port on the INTAKE MANIFOLD. So take the VACUUM hose coming OFF the INTAKE MANIFOLD where it connects to a "T" or "Y" pour just a tiny bit of gas into a cup and bend the cup to make a point and pour it down the tube.
That gas will go directly into the motor that way and bypass the carb incase the problem is with the Carb.

Now doing this IF IT HAS SPARK IT will FIRE at least once and it may even start and die.
Let me know if it fires.

If it fires then the problem is NO FUEL is being drawn in to the motor.
The fuel may be getting to the carb BUT not to the motor.
Alleyoop

[mossy173]

Nov 12, 2012 12:29:25 GMT -6 Alleyoop said:

Nov 12, 2012 10:59:49 GMT -6 mossy173 said:

Good on #1 checking for Spark.

2. If the plug is dry after cranking and NO FIREING WHATSOEVER that means NO GAS is being sucked into the motor.

3. The Timing, I still question I would like to see a picture or Video of the CAM SPROCKET lined up and a shot of where the timing is on the FLYWHEEL.


******
Forget about the starter fluid for a minute and actually squirt some gas directly down the INTAKE MANIFOLD.

Take a cup, sygrine or something that you can squirt some GAS into the VACUMM port on the INTAKE MANIFOLD. So take the VACUUM hose coming OFF the INTAKE MANIFOLD where it connects to a "T" or "Y" pour just a tiny bit of gas into a cup and bend the cup to make a point and pour it down the tube.
That gas will go directly into the motor that way and bypass the carb incase the problem is with the Carb.

Now doing this IF IT HAS SPARK IT will FIRE at least once and it may even start and die.
Let me know if it fires.

If it fires then the problem is NO FUEL is being drawn in to the motor.
The fuel may be getting to the carb BUT not to the motor.
Alleyoop

Well, I've done what you said, and gas got sucked right into the piston and soaked the plug. No ignition whatsoever. I doublechecked the compression with the finger over the sparkplug hole trick and it feels good, even at full throttle. There's a real puff of air being forced out the tailpipe by the engine, and some gasoline smell, but no bang.

As for the cam sprocket, I can verify that the smaller holes are level with the head when the flywheel's T mark is level with the cast-in indicator. here's a somewhat cruddy picture taken of the sprocket and the flywheel without touching it between shots:




Just to be on the safe side I've reset the valve lash to 0.004" on both intake and exhaust with no discernible effect.

[Alleyoop]

What I think part of the problem is IT IS TO RETARDED The plug fires around the "F" mark.
So I would try moveing the CHAIN on the sprocket 1 tooth over. The Case marking should then point BEFORE the "T" mark. On the Flywheel you have the-- T and F .. The pointer should point around the v--- V T and F.

Also before you move the chain mark the current Chain Link and tooth like this:


Then move it to the left one tooth.


Alleyoop

[mossy173]

Nov 12, 2012 17:52:17 GMT -6 Alleyoop said:

What I think part of the problem is IT IS TO RETARDED The plug fires around the "F" mark.
So I would try moveing the CHAIN on the sprocket 1 tooth over. The Case marking should then point BEFORE the "T" mark. On the Flywheel you have the-- T and F .. The pointer should point around the v--- V T and F.

Also before you move the chain mark the current Chain Link and tooth like this:

Then move it to the left one tooth.

Alleyoop

No luck. No change that I could see. I tried starting it normally as well as putting gas in the vacuum port on the intake manifold, nothing. The plug came out wet, but not even a cough.

Thanks for taking the time, by the way! I really appreciate your expertise.

[Alleyoop]

Well then I would have to say YOUR NOT GETTING SPARK or you do not have COMPRESSION.
Especially if the SPARK PLUG IS WET.

Are you sure you have a GROUND WIRE FROM the FRAME to the MOTOR? If no ground wire The Spark Plug will NOT FIRE while on the motor.

Also the COMPRESSION you said you tested using the Finger Trick. When you did this DID you hold the THROTTLE WIDE OPEN and did it FORCE YOUR FINGER OFF THE HOLE? If you just felt pressure that would NOT be enough Compression for the motor to run.
Alleyoop

[mossy173]

Nov 12, 2012 18:51:05 GMT -6 Alleyoop said:

Well then I would have to say YOUR NOT GETTING SPARK or you do not have COMPRESSION.
Especially if the SPARK PLUG IS WET.

Are you sure you have a GROUND WIRE FROM the FRAME to the MOTOR? If no ground wire The Spark Plug will NOT FIRE while on the motor.

Also the COMPRESSION you said you tested using the Finger Trick. When you did this DID you hold the THROTTLE WIDE OPEN and did it FORCE YOUR FINGER OFF THE HOLE? If you just felt pressure that would NOT be enough Compression for the motor to run.
Alleyoop

Sure, the ground wire is connected to one of the head cover bolts. I checked the spark again while I was advancing the timing just to be sure, and it's still nice and strong and blue. If I get a spark with the plug pulled against the cover, it should still be sparking inside the cylinder, right?

When I did the finger compression trick I did it with the throttle wide open and it blew my finger right off the hole, no matter how hard I pushed.

It's a real puzzler!

[Alleyoop]

The Plug are you sure it is the RIGHT LENGTH Some are shorter than others and they will NOT Stick out of the head and ground out on the head.

WOW!! Ok one more test take the plug out and put a straw in the hole and turn the motor over by hand slowly and watch the straw and see when the straw comes out, you will probably need to do this a couple of times because on one of those strokes it is the compression and one on the EXHAUST stroke. And just about when it starts back down again CHECK the VALVES with your FINGERS and SEE if you can wiggle them and at that point the CAM should be aligned and the pointer on the flywheel should be pretty close to the "T" mark. If the HOLES on the cam are not visible at this point it is on the wrong stroke.
Alleyoop

[Alleyoop]

Got to say Mossey, my brain is starting to HURT(HAHA).
Alleyoop

[mossy173]

Nov 12, 2012 19:10:02 GMT -6 Alleyoop said:

The Plug are you sure it is the RIGHT LENGTH Some are shorter than others and they will NOT Stick out of the head and ground out on the head.

WOW!! Ok one more test take the plug out and put a straw in the hole and turn the motor over by hand slowly and watch the straw and see when the straw comes out, you will probably need to do this a couple of times because on one of those strokes it is the compression and one on the EXHAUST stroke. And just about when it starts back down again CHECK the VALVES with your FINGERS and SEE if you can wiggle them and at that point the CAM should be aligned and the pointer on the flywheel should be pretty close to the "T" mark. If the HOLES on the cam are not visible at this point it is on the wrong stroke.
Alleyoop

Sounds like a plan. I'll be giving it a try tomorrow after work, but in the meantime a question--wouldn't it be ok if the sprocket were 180 degrees off? I'd think (based on limited knowledge) that it would still run, it would just be one revolution off from where it would have been otherwise--running a bit later in the cycle, so to speak. I say this because I actually had it with the sprocket 180 off from where it is now, and reversing it didn't help. After I reversed it I spun the motor a few times with my hand and found that the sprocket was back to right-side up with the flywheel at TDC due to a 2:1 gearing between the cam sprocket and the crankshaft. If I reverse it again, wouldn't it be equivalent to spinning the crankshaft once?

[Alleyoop]

If the sprocket is 180 degress off, the WRONG valves are opening and closeing at the wrong time. Example the INTAKE VALVE would open on the exhaust stroke and the EXHAUST would open on the INTAKE STROKE. That would get the PLUG wet and not fireing because it sucked in the gas at the wrong time and would push the fuel out the exhaust on the exhaust stroke.

TO be very sure on the timing bring up the PISTON to TDC and check the timing marks it should be CLOSE to the "T" mark on the FLYWHEEL.
THEN put your cam on and align the holes with the edge of the top of the head and then adjust your valves. This way you are sure at least it is TOP TDC and the CAM AND LOBES are on the RIGHT STROKE for opening and closeing them.
Alleyoop


Alleyoop

[Alleyoop]

Also to check that you can take the Air Filter off and put your hand over the carb and crank it IF you get air hitting your hand the intake is open on the EXHAUST STROKE.
Alleyoop