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Author Topic: 79 yz 125 won't start  (Read 9058 times)

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Offline ridered125

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79 yz 125 won't start
« on: June 23, 2010, 07:14:27 PM »
I have a 1979 YZ 125. Has spark. Float bowl fills with gas, carb is clean, pilot jet open, has compression,  motor just rebuilt. The spark is blue and thick, has a new stator. When I put the motor back in and hooked it all back up, it ran for about 30 second and now it won't do anything. Not even pop. Someone said it sounds electrical but the bike sparks.

Any ideas?
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Offline TMKIWI

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Re: 79 yz 125 won't start
« Reply #1 on: June 23, 2010, 07:20:12 PM »
Maybe the ignition timing is out ?
Has the carby still got fuel ? Could be a faulty pet cock.
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Offline Coop

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Re: 79 yz 125 won't start
« Reply #2 on: June 23, 2010, 07:27:56 PM »
Did you check the reeds?
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Offline ridered125

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Re: 79 yz 125 won't start
« Reply #3 on: June 23, 2010, 08:02:10 PM »
The timing is a possibility. Reeds are new and the carb fills with gas.

Never checked the timing before. Is this done with a timing light (gun)? Can you check the timing when it won't run?
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Offline riffraff

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Re: 79 yz 125 won't start
« Reply #4 on: June 23, 2010, 08:03:53 PM »
Did you check for spark after the 30 second run, if it does make sure it sparks at the right time (check timing like TM says)
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Offline riffraff

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Re: 79 yz 125 won't start
« Reply #5 on: June 23, 2010, 08:05:02 PM »
Do you have a manual for it?
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Offline ridered125

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Re: 79 yz 125 won't start
« Reply #6 on: June 23, 2010, 08:11:47 PM »
Got a Clymer manual. Their troubleshooting section is pretty basic.
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Offline TMKIWI

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Re: 79 yz 125 won't start
« Reply #7 on: June 23, 2010, 09:11:11 PM »
Most ignition plates have 2 marks on them to line up the timing.
You line up the mark with a casting on the housing.

Check your plate is tight and has not moved.
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Offline SachsGS

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Re: 79 yz 125 won't start
« Reply #8 on: June 23, 2010, 09:31:08 PM »
Check to see that your flywheel key has not sheared.As well, the spark should be blue not white (make sure the sparkplug has not fouled).Pull the intake boot off and spray a little gas in the carb.
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Offline riffraff

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Re: 79 yz 125 won't start
« Reply #9 on: June 23, 2010, 10:20:54 PM »
Go to the ignition timing section of the Clymer book, you'll need a dial indicator.
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Offline G-MONEY

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Re: 79 yz 125 won't start
« Reply #10 on: June 24, 2010, 05:52:35 PM »
Check the coil out, plus the kill swicth contact & wiring, Here is a odd ball request pull the exspansion chamber off your exhaust could be obstructed somehow.
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Offline ridered125

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Re: 79 yz 125 won't start
« Reply #11 on: June 24, 2010, 08:54:35 PM »
Got it running. A vintage guy I know said turn the bike upside down, take out the plug, crank it over a few times. Gas poured out of the spark plug. The crank was full of gas. It fired up after that.
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Offline riffraff

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Re: 79 yz 125 won't start
« Reply #12 on: June 24, 2010, 10:00:07 PM »
And it continued to run? Why was the crank full of fuel in the first place?
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Offline ridered125

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Re: 79 yz 125 won't start
« Reply #13 on: June 25, 2010, 01:53:39 PM »
Not sure why it was full of gas. I squirted gas into the carb when I was trouble shooting it.

But it starts easily now. Still need some tweaking on the jetting but it's close.
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Offline SachsGS

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Re: 79 yz 125 won't start
« Reply #14 on: June 25, 2010, 05:21:52 PM »
If the carb needle and seat are worn gas will continually run into the crankcase (and out the exhaust port). Always turn your gas petcock off when the bike isn't running.
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