YZ250 back fire issues...

DrSweden
Posts
6767
Joined
8/30/2008
Location
Stockholm SE
8/22/2016 3:49pm
Bike runs great when it rides: on idle, half or full gas. It is crisp, no misfire or bogs, can run full motos. But I can't kick start it because it either kills my foot, or it breaks the kick axle (has happened twice). I have owned many bikes (another YZ250), no problem with my kick skills.

Imagine kicking it and halfway through, it hits back hard in the soil of the foot, to the degree that the kick axle breaks. I have rotated the stator plate in all possible ways. Right now full retarded. It is so mellow its mad. If I hold down the kill switch, or pull the spark plug I can kick it like a normal two stroke (but it doesn't fire up of course). I have OEM piston, cylinder and head. Hotrod crank. Bought a new CDI, new reeds, tested a new cylinder head nothing changed.

Previous owner repaired the stator, one coil was bad. He put new wirings in it.

I had high hopes in a new CDI, but noooooo. It's like I get an extra spark before TDC when I kick it, but as it fires up, no issues.

It's a bitch to jump start it as well. This back fire seems to add extra compression, makes the back wheel lock, even on tarmac. I need to really jump on the saddle in third.

I have an extra coil and spark cap. Also extra wiring. Well of course test that...
|
8/22/2016 4:03pm
Are you sure that you have the correct crankshaft? What kind of compression do you have? Is the power valve adjusted correctly and clean?
pete24
Posts
2733
Joined
10/20/2011
Location
Marlborough, MA US
8/22/2016 4:25pm
repaired stators do not work never ever, buy a new oem stator
FGR01
Posts
5092
Joined
10/1/2006
Location
AZ US
Fantasy
1334th
8/22/2016 5:40pm
1st thing I would do is measure the squish with some solder and the compression with a gauge to make sure you don't have some ridiculously high compression going on there.
DrSweden
Posts
6767
Joined
8/30/2008
Location
Stockholm SE
8/23/2016 1:23pm
I don't think I have to high compression. I bought a gauge, figure the bike has to be warm when testing though. Tried to get a value when it was cold, kicked about ten times, but got really low values. like 100 psi, instead of the 220 psi some say is about in the right ball park. Bad gauge? Compression without spark plug cap feels good though. I have kicked bikes with to little and to much compression. Also, if relevant, the bike feels ok to kick until you feel the crank fills up with gas, then you get the kick back.

The squish thing I don't get? Would it end up with backfire if faulty?

Would a bad wired stator be able to give extra sparks?

Power valve, I have no idea? What would it do if it was faulty? Bikes runs great as soon as it fires up?
I have no idea about the crankshaft, previous owner bought it. But would another bikes crank even fit and make it run?

So strange...

The Shop

Paw Paw
Posts
679
Joined
12/24/2011
Location
Benton, LA US
8/23/2016 1:32pm
Yes the squish is very important as these bikes have a history of the squish not being correct right from the factory.
RE: Compression....100psi is very low, so the guage must be incorrect. At that pressure it should not even start.
RE: Stator....Yes a bad stator can do all sorts of strange things to the spark timing .
RE: Power valve....If dirty it can be sticking and not allowing full power.
RE: Kick back...Timing is not correct.

Paw Paw
BobPA
Posts
8023
Joined
10/31/2013
Location
PA US
8/24/2016 5:23am
100 PSI is way not sufficient

220 PSI is in race has territory

Compression gauges do vary from gauge to gauge, but not that much

DrSweden
Posts
6767
Joined
8/30/2008
Location
Stockholm SE
8/24/2016 3:37pm
Paw Paw wrote:
Yes the squish is very important as these bikes have a history of the squish not being correct right from the factory. RE: Compression....100psi is very...
Yes the squish is very important as these bikes have a history of the squish not being correct right from the factory.
RE: Compression....100psi is very low, so the guage must be incorrect. At that pressure it should not even start.
RE: Stator....Yes a bad stator can do all sorts of strange things to the spark timing .
RE: Power valve....If dirty it can be sticking and not allowing full power.
RE: Kick back...Timing is not correct.

Paw Paw
Bike runs great all the time. Good grunt and response very well to different timing setting. Still get kick back. So can't be power valve. Yeah, figure gauge is incorrect.

Picture you kick down, you get this extra spark before TDC, it kicks back, but you are already committed so bang in your foot, or kick axle breaks. While jump starting you get some serious tire marks on the asphalt, despite third gear. But as soon as you overcome this back firing, it starts right away.

So if I had a bulky kick shaft made of titanium, and a foot worthy an terminator, I would start, no problem.

DrSweden
Posts
6767
Joined
8/30/2008
Location
Stockholm SE
8/24/2016 3:38pm
I will test the gauge again tomorrow. You guys think I would get the value after one kick, ten kicks? Warm or cold?
mark1960
Posts
240
Joined
8/8/2016
Location
AU
8/24/2016 4:02pm
Sounds like ignition, not sure what, but ignition.
Position of the stator,
Installed incorect, coil backwards, wire not insulated????????????
lumpy790
Posts
9147
Joined
9/18/2007
Location
York, SC US
8/24/2016 7:30pm
DrSweden wrote:
I will test the gauge again tomorrow. You guys think I would get the value after one kick, ten kicks? Warm or cold?
To do a compression test Hold the throttle WFO. And kick till the gage stops going up.
BobPA
Posts
8023
Joined
10/31/2013
Location
PA US
8/24/2016 9:08pm
DrSweden wrote:
I will test the gauge again tomorrow. You guys think I would get the value after one kick, ten kicks? Warm or cold?
Usually they are done cold...but you do not see that big of a variance. Hold it wide open and kick it until the gauge stops.
adam8781
Posts
670
Joined
12/22/2015
Location
CA
8/24/2016 10:18pm
check your timing, .007 inch clockwise from tdc is stock timing.

.004 is 1 degree retared
8/25/2016 2:14am
Had an old Honda XR500. There were two blue wires in the ignition wiring. One had a very light green stripe on it. I had the two wires reversed, striped wire into the solid wire connector.

That thing kicked back so hard, I thought I had broken my foot / ankle. It had me on the ground for about 20 minutes. Reversed the wires to the correct positions, and all was fine.

Would bet it's an ignition problem.
DrSweden
Posts
6767
Joined
8/30/2008
Location
Stockholm SE
8/25/2016 8:14am
adam8781 wrote:
check your timing, .007 inch clockwise from tdc is stock timing.

.004 is 1 degree retared
I'm in tune with timing. Went full retard (yeah lol) and full advance. And stock...
DrSweden
Posts
6767
Joined
8/30/2008
Location
Stockholm SE
8/25/2016 8:16am
Had an old Honda XR500. There were two blue wires in the ignition wiring. One had a very light green stripe on it. I had the...
Had an old Honda XR500. There were two blue wires in the ignition wiring. One had a very light green stripe on it. I had the two wires reversed, striped wire into the solid wire connector.

That thing kicked back so hard, I thought I had broken my foot / ankle. It had me on the ground for about 20 minutes. Reversed the wires to the correct positions, and all was fine.

Would bet it's an ignition problem.
I like your story, the previous owner did repair the stator himself. One of the coils had bad wiring, maybe he screwed up the wiring there, hence the spark or some spark at the wrong time?
EEE299
Posts
418
Joined
8/21/2014
Location
NJ US
8/25/2016 11:45am Edited Date/Time 8/25/2016 11:46am
BobPA wrote:
100 PSI is way not sufficient

220 PSI is in race has territory

Compression gauges do vary from gauge to gauge, but not that much

Just rebuilt my yz250, it had 235 psi after 3 hrs all oem top end. All depends on the gauge, temperature, altitude Unsure

It definitely sounds like ignition though.
DrSweden
Posts
6767
Joined
8/30/2008
Location
Stockholm SE
8/25/2016 1:24pm
mark1960 wrote:
Sounds like ignition, not sure what, but ignition.
Position of the stator,
Installed incorect, coil backwards, wire not insulated????????????
Coil backwards????
pete24
Posts
2733
Joined
10/20/2011
Location
Marlborough, MA US
8/25/2016 4:38pm
you need to throw that stator away and start with a good one i have a good 99 yz250 stator, but im a parts hoarder i dont want to sell it i want to trade it , what do you have ?
DrSweden
Posts
6767
Joined
8/30/2008
Location
Stockholm SE
8/27/2016 2:33pm Edited Date/Time 8/28/2016 5:19pm
pete24 wrote:
you need to throw that stator away and start with a good one i have a good 99 yz250 stator, but im a parts hoarder i...
you need to throw that stator away and start with a good one i have a good 99 yz250 stator, but im a parts hoarder i dont want to sell it i want to trade it , what do you have ?
2004 CDI, coil, harness, spark plug cap and cord. OEM Replica graphics 2007. New silicone hoses in red and blue. A used and POS gearbox in pieces, Just gave away a box with 50 lbs of YZ parts for nothing. So I'm pretty stripped What do you need?
DrSweden
Posts
6767
Joined
8/30/2008
Location
Stockholm SE
9/8/2016 3:28pm
Had an old Honda XR500. There were two blue wires in the ignition wiring. One had a very light green stripe on it. I had the...
Had an old Honda XR500. There were two blue wires in the ignition wiring. One had a very light green stripe on it. I had the two wires reversed, striped wire into the solid wire connector.

That thing kicked back so hard, I thought I had broken my foot / ankle. It had me on the ground for about 20 minutes. Reversed the wires to the correct positions, and all was fine.

Would bet it's an ignition problem.
New stator fixed the problem, so you were spot on! Tnx!!!

Post a reply to: YZ250 back fire issues...

The Latest