125H runs for 5 minutes then dies

[DJRacing]

This is a tough one 250H. I will talk to a few friends over here and discribe what you have said and ask there advice. Do you think the motor is getting excessively hot? and have you done the timing? What jetting is in the carb?

I have a question for my mechanic friend about your bike, but before I ask you about it, I will ask him about an idea/thought I have just in case it is a stupid thought, then I will only get laughed at by 1 person and not the whole wide world

[BONEHEAD]

have you tried kicking the bastard after it dies with a new spark plug in the cap or screw driver to see if you still have spark? you need to eliminate a temperature affected magneto coils , and maybe try a different cdi unit if you have one,is the coil lead secure in the coil, you need to check spark as soon as she dies, this will ensure everything electrical is good, then you can move on   will the coil lead unscrew out of the coil?, if it is long enough cut it where the coil lead screws in so its nice and clean then screw it back into the coil, this was my problem with my 125h run for 5 mins and die due to resistance change at the ignition coil  lead not being a good contact in the coil screw, hopefully your coil lead screws out and you can see what i mean  , let me know cheers (check spark when the bastard dies)

[gorby]

had a bike doing similar thing to yours and found that the two vent pipes on carby were connected together instead of open to the outside,if you only have one vent pipe it may be blocked?

[DJRacing]

Talked to my mechanic friend and he is saying what Bonehead is saying. You need to check the spark the moment she dies.

[Stewart Allen]

Could simular to electrical probblems that were common in VL C/dore 6 Cyl's, Diode or transitor in the dizzy would die car would start & run for about 5 mins then stop. 2 ways to check - check spark at time of stopping or through bucket of water over dizzy.

[YZ250H]

I reckon you blokes are onto something here.  I had a closer look at the plug after it stopped last time the core was dry, but the outer still had some liquid on it.  I cleaned out the carb and blew it out with the compressor - clean as a whistle (toobs too thanks Gorby - that's how I got stuff in me eye  )
I've been running 20:1 Valvoline 'cause I just put her back together - I'll change to 33 to 1 next batch.  I don't imagine it would cause this, but you never know.
So.... after going from suspecting electrical back to fuel, I'm now suspecting electrical again.
DJ - I thought the motor might be getting too hot due to the gurgling and boiling, but since I fixed a couple of hoses and bled the air out of the head it seems to have settled.
The way I see it there are only a few elements to this bloody electrical thing.  Stators, coil, lead(s) and spark plug.  I will as Boney suggests get a new spark plug and try the spark straight after she stops.
Does anyone know if the spark plug lead screws out of the coil as I don't want to break it    I noticed during my wiring inspection the coil lead does have a rub mark on it from the throttle cable.  That may cause the change of resistance thing Boney (??)  I assume you can replace the  high tension lead 
I took a photo of the plug for you sickoes that are into that sort of thing !!!
Is the colour OK considering I'm running 20:1 (for about another litre) it is actually a bit lighter grey tahn the photo shows, but hopefully you get the idea (I have none).
The saga continues (on the week-end)
Cheers

[matcho mick]

it's not running a resistor plug cap,they(old ones) tend to build up resistance when running a while from cold to operating temp,do the resistance thing with your multimeter,(over 5kohms, binit!!),if it's not resistor ignore this

[yzhilly]

Hey Tony wear ya safety glasses. Have you tried the stator off your 250 ?. I have 2 dodgy stators that i have to get checked and maybe rewound. Both were on engines i rebuilt ,both started up but after initial run  they would only start once or just fire once,i had very weak yellow spark  .The high resistance coil was open cicuit  so i now have the YZ100 one in the 80K and the 125K in the 100. both have not missed a beat since i changed just the coils over off the backing plate . You need to check the resistance in the lead from the stator coils to the cdi the spec is in yuor book There is 3 wires cant remeber the colours there is a common wire between the coils that also has a soldered join near the coils this can cause drama's too. The upside to all this your second bike is goin to be a piece of piss  cause you will know them inside out . Just dont go switching brands or you have to start all over again. good luck Hilly.

[BONEHEAD]

yes the coil lead should screw out of the coil , is it the original coil , what happens is the anchor screw for the coil lead either corrodes away or due to vibration looses contact with the coil lead sporadically, just unscrew the coil lead and take a look in the coil and you should see the anchor screw if this is not there then i would suggest a new coil , if all is ok cut a little bit off your coil lead you will see what i mean, let me know cheers bonehead. ( you still need to check spark when she dies)

[YZ250H]

OK,

On Boney's and DJ's advice I pulled the plug after she died.  It was almost dark by then and it was much easier to see the the spark (yes it was there).  SOOO we are back to fuel again which has me really stuffed .
Will start on the kick start with choke on and wide open throttle.  Symptoms have changed slightly now she only runs for a minute as though it's got about a teaspoon full of fuel in it then dies.  Runs longer at lower revs.   Leave it a few minutes same thing - jump starts easily, runs for a minute then dies.
I think we are finally on a winner here dudes .  I'll pull the whole fuel system off clean it and start again - starting from the fuel cap (which seems to vent OK).
I was thinking the other day that maybe Mr Yamaha sold me the wrong needle and seat .  You wouldn't think so.

Hilly, I was going to take the video, but 1 minute of riding would be nothing more than embarrassing 

This weekend - fuel system again.  New fuel hoses, new filter, flush the tank, clean the vent holes in the cap.  Clean the carb (again), set the float levels (again - maybe I'm doing this wrong).  Change fuel mixture to 33:1.
Any other suggestions  ??  You blokes sick of talking about this yet ??  Mind you for a newbie would be a great trouble shooting guide as you blokes have almost exhausted the list of things that can go wrong and given of much of your experiences - THANKS .

[DJRacing]

250H what pilot jet are you running? There isnt an obstruction in the exhaust pipe ?

[Hoony]

good thought DJ.

is the airbox and airbox manofold also clear to breathe ?

many years ago a work mate had a XL185 and it would only run for 5 mins before stopping then when the plug was pulled out it was sooty.

ended up being he had spare spark plugs stored in the air breather holes along with some rags,
no air  = no go!

[BONEHEAD]

hey what pilot jet are you running , was the plug wet when you pulled it out, how good was the spark, it is possible that the pilot jet is  too large and your running to rich on the pilot and killing it with fuel, had a 250 h do that and also sucking in oil from the gearbox due to crankseals not sealing, fouling the spark plug, these are just another combination of things to look for , does it blow a lot of smoke when it fires back up after bump starting it, i dont believe you have a fuel flow problem unless you are blind and cant see the obstruction maybe also your oil to fuel ratio is incorrect , are you measuring it correctly this can cause fouling of the plug, you still havent answered my question as whether you have checked your coil lead as i have explained , just holding it at a different angle could still give you a spark , was it a fat blue spark, does the lead screw out of the coil,you really need to check this for good continuity to totally eliminate this concern, if it was a fuel concern like a blockage you would have spied it by know, if it was a blocked exhauast it possiblly would not even start or rev, if pain persists see a doctor (i may need to see this bike)CHEERS BONEHEAD

[YZ250H]

Geez you blokes ask a lot of hard questions  !!

I don't know what pilot jet I am running - how do I tell ?
Airbox and airbox manifold is clear - filter element is new unipart oiled not too much
The spark was good and fat (the sun had gone down) and blue, all other times have been in the daylight when maybe I couldn't see it as well.  The core of the plug was dry, but there was an oily residue on the outside part.  It will jump start now quite easily with the choke out (without cleaning the plug in between).
It does blow a tiny bit of darker smoke, then white smoke when it first starts up then clears very quickly.  I'm with you boney, I'm pretty sure I don't have a fuel obstruction, but the way I'm going I'm not too sure of anything any more  .  I did just get my first set of reading glasses (old age is setting in), so going blind could be a possibility 
Oil to fuel is 20:1 (fuel to oil), but I will change that now that it has run for a bit with the richer mixture after being put back together.
I didn't try the coil lead  but will this weekend.  I tried the one of the 250 and I'm pretty sure it doesn't screw out.  I didn't want to get too ham fisted with it for fear of buggering it.  The cap that goes on the plug on the 125 does screw out, but seems to have a pretty good connection with the copper inner on the lead.  Exhaust seems to flow out pretty well and makes a hell of a racket, but there is an oily residue sort of dripping out.  It did rev out very well, but doesn't now.  My patience is running out fast, so i might buy a new coil just to rule that out - where would I get one ?  If I can't get this bloody thing sorted soon my startegy will be to start replacing bits with new ones (at the cost of my other restos :'().
My other thought was burn it and buy a Honda 

[cyclegod]

Heres a coil...
http://cgi.ebay.com/ebaymotors/Yamaha-YZ-125-Ignition-Coil-74-89-OEM-Replacement-NEW_W0QQcmdZViewItemQQcategoryZ35594QQihZ012QQitemZ220151685893QQrdZ1QQsspagenameZWDVW