Apologies for crossposting; also listed in Yamaha posts.


Hi all,

This one has me absolutely stumped

I bought a good condition 1986 PW50 for my 7 year old's birthday as it would be a easy learning curve for him. He got one good run out of it and then it had trouble revving.

Start's OK, but won't rev and Start/Run switch makes no difference to either starting or running. On these bikes, the Start position should stop the bike from revving and it should only start in Start, not Run.

So, reasonably handy with bike engine I:

* Stripped the carb and clean it out (lots of varnish; is it only me that turns of the fuel taps at ride's end?)
* Took muffler and exhaust off, scraped carbon off and burned it out with a blow torch
* Took the head off and rebored to 2nd over (some scratches but also just worn out of spec), new piston, rings top end bearing
* Tested coils and replaced ignition coil (10 times out of spec. for resistance)
* Took apart all bullet plugs cleaned with contact cleaner
* Took apart Start/Run switch, cleaned and tested
* Disconnected Start/Run switch completely and tried switching with a piece of wire instead.
* Disconnected Ignition Control Unit completely
* Replaced plug (of course! )
* Ran out of ideas and replaced CDI http://www.pwonly.co...products_id=134
* Loosened flywheel and checked key; looks good


My arm's broken after burying a mate's 2011 CRF450 into a sand berm (I ride a '93 KDX200 normally; probably vintage class almost?) so really frustrating trying to fix it with a broken right arm and having these probs. still going on!

My only remaining option is to make the wiring as basic as possible (Stator to CDI to Ignition Coil to Spark Plug) to troubleshoot further.


Any ideas greatly appreciated!


~easykiwi-rider (Western Australia)

I don't know if these came with Stop/Start/Run switches everywhere or for how long. But a common problem is for water or another source to cause a short in the start circuit which keeps the bike from revving due to severely retarded timing. Since the bike is always in gear, starting it at WOT would cause potentially disastrous results without this feature.
IIRC, there's several wires coming off the switch, the one you want to disconnect is white. It should still start, but you'll need to keep the throttle closed and of course, turning it to off should kill the bike. If this solves the problem, you can either better insulate the white wire or leave it disconnected.
If this fails, trace the white wire as far as you can and make sure it's not grounding out anywhere.
Seven years old is pretty big for a PW50, I would start looking for a PW80 immediately.
Then come back and PM me or read the "PW 80 Questions" thread for lots of info on the 50's big brother.

Edited by Smacaroni, 25 July 2012 - 04:35 AM.

Thanks Smacaroni.

I had already disconnected the switch, so I'm sure it is not shorting out there (looking at the inside design, the points to short to Start are the closest; I can see why water in there can play havoc with these little beasts...)

I will try tracing the white wire tonight; an earth leak along that would make a lot of sense.

chrs.
easykiwi_rider.

I'm pretty sure it's white. Since you opened the switch, if another wire color made more sense, check that one as well.

OK, came home from work, neighbour came down with a bottle of red and we had another go at it.

We completely by-passed the switches and Ignition Control Unit and just ran the stator to the CDI to the Ignition Coil to the Spark Plug.  Couldn't be simpler.  Exactly the same problem; even though all those parts have now been replaced.  So either switch, loom or ICU (unfortunately).

It must be a dodgy stator...  Neighbour brought down flywheel puller for us to check, but no luck getting the flywheel off; PW's are left handed threads!!!

Anyone know the specs for the coil?  I checked it once before and found the ignition coil out of spec by 10x (now replaced) but thought it worth while checking again...

Sorry, I don't. The CDIs on these bikes are extremely reliable. But if you had a bad stator before, maybe it partially fried something and it just took a while to notice?
Know anyone else who has a PW50? Might be time to take it over there and start swapping parts back and forth.

As far as flywheel pullers, I bought a 10pc set off eBay and it's had every puller I ever needed. Only ran me like $40 US. I bet there's an Australia seller with a similar kit so you don't have to pay overseas shipping.

Kiwi,

I don't think it is the  ES control unit. I think it is more likely that you have lost your way and the devil has led you into a life of sin.
Let Jesus Christ into your heart, renounce the glamour of evil and all of your earthly problems will be resolved by the power of the holy ghost. Rejoice!

But just in case this doesn't work you should replace the stator coils and buy your neighbour an entire case of Barossa Shiraz for his spiritual devotion in assisting you.

God Bless.

Yeah thanks Christian Biker. you wouldn't happen to know the neighbour that's been helping by any chance? You certainly share his taste in South Australian wine -

I had another go at the carb and dropped the needle down a bit as it sounded like a mixture issue almost; way too rich maybe or air somehow blocked…? Leaning it right out like that got it revving better and I could now hear the difference between Start and Run; suggesting to me it was not electrical but some over-arching problem instead; pointing to carb again which I completely stripped and cleaned twice.

I got peeved in the end and took it to a local scooter shop. With all the motox 4-strokes nowadays I wanted someone who knows 2-strokes peculiarities to look over it with a clean slate approach.

The verdict is in; blocked exhaust

As always, it all makes sense in the end.  The annoying thing is that I think the electrics were at fault o begin with (water in switch or similar) which I why I started tidying up other parts in the bike while I was waiting for it to dry.  Including taking the exhaust of and cleaning it out with a blow torch; bit of rust/creosote/ash doesn’t take much to stop these little guys as I’ve hear in the past.

Classic 2-stroke problem though eh?

Took the bike out on the weekend; goes like stick!  Could jump better (bit nose heavy!) but no shortage of power there at the moment for the little fella.  My cast’s off now too; as soon as my wrist has more movement than a fence pole I’ll be riding next to him to.


Many thanks for your help Smacaroni; you were on the money with the CDIs and electrics being solid; I reckon a lot of these spares get bought by people not able to find some of the more basic 2-stroke quirky faults that are really causing the problems.

Glad to hear it, but you probably don't want to jump a PW50, nose heavy or not, the PW series suspension is not one of it's strong points.