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Old 09-03-2008, 05:46 AM   #1
Flyingleek
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Default Help 2 Stroke sparks, doesn't run but backfires?

Hello everyone,

I bought a non-runner Yamaha RT100 offroad bike. Initially there was no spark which I traced to the pickup coil on the Stator. The previous owner had attempted to lock the magneto flywheel by shoving a screwdriver through the magneto to undo the nut! I continuity checked the pickup coil which had signs of physical damage caused by the screwdriver, it was open circuit so I unwound it, resoldered the break and rewound it, the impedence is now 12ohms against a limit of 9ohms in the manual but IT SPARKS now All other electrical components have the correct impedences.
It doesn't run despite having a good spark, if I turn it over with a drill I get occasional backfiring, it is definately backfiring as the drill jerks and slows when it goes pop.

I have removed and cleaned all jets in the carb, put in a new piston and honed the bore, it has good compression, new correct specification spark plug.

I have bought a new CDI on eBay America and will try that when it arrives.

Can anyone with electrical knowledge please help? I'm wondering how the ignition timing can be wrong, could it be the pickup coil impedence affecting it? Possibly the CDI unit is at fault? Is the coil to blame? Any help would be much appreciated. I'm more of a mechanical guy, electrics baffle me

Thanks in advance,

Pete
Wales
United Kingdom
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Old 09-03-2008, 07:54 AM   #2
mscuddy
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What kind of spark? A big fat blue spark, on an anemic yellow flicker? The flicker will cause the motor to act like you discribed, or...

Since it's going PFATH...and PHUHT...PFFFFFT...POP it sounds like the plug is firing way out of time, like when the piston just clears the exhaust port.

Time to buy a manual and get the bike timed to specs before moving on to other areas. A process of elimination. I sold my dial indicator so another MOridian, so you'll have to figure out where 2 degrees BTDC is...

Good luck. Two strokes are great, you should have no problem getting it running.
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Old 09-03-2008, 08:22 AM   #3
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Compression,spark,ignition,fuel.
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Old 09-03-2008, 08:23 AM   #4
Flyingleek
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Thanks for replying, it's a nice blue spark. The timing doesn't seem to be adjustable as the magneto flywheel is on a keyway and the stator plate cannot be moved as it only has two screw holes that aren't slotted. I have marked the flywheel with TDC and keyway position. The North South point of the magneto appears to pass the pickup coil about 25 degrees AFTER TDC <?<scratch head>?>
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Old 09-03-2008, 08:26 AM   #5
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You forgot timing, which of course, is everything.
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Old 09-05-2008, 02:27 AM   #6
Flyingleek
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Every post mentions timing, it doesn't seem to be adjustable- magneto flywheel on a keyway and stator plate is bolted with no adjustment possible. That's why I'm thinking something else must be wrong, like the CDI Unit or the repaired Pickup coil (see first post) being slightly higher resistance?

Any help appreciated,

Pete
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Old 09-05-2008, 05:07 AM   #7
seruzawa
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Quote:
Originally Posted by Flyingleek View Post
Every post mentions timing, it doesn't seem to be adjustable- magneto flywheel on a keyway and stator plate is bolted with no adjustment possible. That's why I'm thinking something else must be wrong, like the CDI Unit or the repaired Pickup coil (see first post) being slightly higher resistance?

Any help appreciated,

Pete
I'm thinking that the idiot who used a screwdriver to lock the flywheel may have bent something slightly. It only takes a couple of degrees to ruin the timing. Can you hook up a timing light? If the new CDI unit doesn't fix it there's not much else to check ignitionwise.
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Old 09-06-2008, 03:42 PM   #8
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You can turn it over with an electric dril? Any decent running 100cc two stroke would stop that drill in it's tracks (unless you're using a really BEEG drill). If the timing is spot on, has fat blue spark, time to check the compression $ carb-reed valve assembly. With a dinky little 100 there's not much to go wrong...like a model airplane engine.
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Old 09-11-2008, 06:28 AM   #9
Flyingleek
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Thanks mscuddy, I use the same drill to start my 150cc Four Stroke mower and that has alot more compression, gives a helluva kick though

The CDI unit should have arrived when I get back from travelling at the start of October.

If that doesn't cure it I'll get hold of a compression tester to check, but it has a new standard size Wiseco piston and the bore was honed and is within the limits in the manual.

Thanks for the replies everyone, I'll keep you posted in October how it goes.

Pete
Currently paragliding and hang gliding in Czech Republic
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Old 09-28-2008, 08:51 AM   #10
Flyingleek
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Default Update:

Hello again, thanks for having the patience to read through the saga.

The latest attempt was to swap the CDI for a new one- guess what? Exactly the same. So wile I was playing with it I looked at an old 4 stroke 150cc engine I have in the garage. I looked at the position of TDC in relation to the Pick Up coil (it tells the CDI when to Fire). The RT100 is about 90 degrees Before TDC(!) and the 150CC engine is about 18 degrees Before TDC. I took out the key in the keyway on the shaft and put the flywheel in the approximate position to give 18 degrees Before TDC, and guess what again! IT RUNS!! I say it runs but it'll only tickover and won't rev, it just bogs down. There must be something very wrong with it as the keyway is now about 30 degrees out from lining up.

Sorry this is a convoluted description, the bottom line is-

It runs if I move the flyweel to approx 18' BTDC.
The Ignition Timing seems to be the problem.
The is no way of adjusting the ignition timing.

My next step options-

Check the flywheel is the correct one for this model.
Take it to an Auto Electrician.
Scratch my head and hide it in the shed

As usual, any help/suggestions appreciated,

Pete
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