I truely have struggled with this bike. Had it for 2 1/2 years, "re-did" it in first 6 months and have had this issue for 2 years. The bikes been ridden 30 minutes max by me due to run issue. Sad.
Backfires frequently while attempting start. Once running acts rich midrange on up but plug doesn't indicate that. It *sounds* like it has a rev limiter that kicks in way before it should and of course makes essentially no additional power at that point. Power/ running off idle on up to mid seems normal for a vintage mx'er.
Points to consider;
- Replacing parts has been mostly a "used off eBay" deal except for seals, brg's, gaskets, Wiseco piston, and Boysen reeds.
- No OEM ignition parts still dealer available. Yes, a new *generic* hi-tension coil works fine but I mean like CDI box, stator exciter or trigger coils.
- I own 3 used CDI boxes, (2 bought from different eBay folks), all *suposedly* from a 80 yz250. The Hitachi # on all 3 boxes match, but it's not a Yamaha part # and *could* be for something else...who knows? ......*Understandably,this is one thing that bothers me*......
- I also bought a complete stator plate, "used but good" off Ebay. I cannot say it looks identical to old one.... because, (this is a killer... I lost the origional shortly after getting the bike. Yes ... I did)! But, ... I can say both coils the right ohms ressistance value as stated in Clymers service book. Futhermore, while it could breakdown with running vibration or heat, the bike still has a bigtime starting backfiring issues too.
- Kill buttons disconnected right now, though I've had it connected and disconnected many times in past while troubleshooting.
- Hi tension coil.... I have 2 origionals ... AND in past I've swapped-in a coil from a well running 2007 Husky I also own. (Of course new spark plug tried on different occasions.)
- Carb: yes it was a mess when I got it. I do know how to properly clean carbs,( I'm a 58y/o, 6years a m/c mech, then last 35 years an industrial mechanic). All jets are cleaned, all passages pass compressed air as they should. Jets are sized as Clymer suggests and air screw set to typical "good starting place" 1 1/2 turns out. Float height, 18mm as per Clymer and seems right as fuel never spews out over flows. While all looked good after cleaning, since the bike ran crappy, (and you never know what some P/O may have done to a jet), I replaced the main jet, needle jet, and *needle*. (Also later, the fuel delivery float needle and seat). IDLE jet's unchanged and the SLIDES unchanged. The slide shows a lot of wear in that chrome plating is worn away in places exposing brass make-up. But it isn't like super sloppy in carb bore, nor does it hang up. Also, keyed to locating pin in carb bore properly so as to not rotate, ... duh. I did notice chrome missing from EDGE of slide that refers to it's "cut-away" dimention. (maybe a P/O filed down the slides cut-away to alter fuel/air mix.) I'd like to think otherwise.
- More later about smoking, but to describe pipe comdition, I removed dents via acetylene torch/compressed air, so the pipes essentially free of any old built-up carbon and the silencer got complete rebuild too, decarboned with torch and fresh packing.
- Motor bottom-end is fresh, (crank big-end brg was good), but replaced main brg's and seals of course. Tranny/clutch/engine case halves all good.
- Motor *top-end* is worth discussion. Cylinder coming with bike had seen boring several times and was needing again. So I buy a worn std bore Ebay cylinder suposedly from a '80 250. Identical EXCEPT a port araingment slightly different. Feeling it may have been a mid-year Yam change, or a '81 cyl I used it. Following realization bikes not right, I bought another Ebay cyl. and it *is* identical to one bike came to me with. It's also rebored to 1st OS with Wiseco.9 Results: bike runs exactly the same.)
- Bike came to me with what looked like fine "one-piece" fiberglass reeds which I replaced with new "piggy-back" style Boyensen's way back. Troubleshooting included going back to the old reeds. It ran/ starts same. (Boyensens now back in there).
- Used to using BelRay @ 50:1 on all my bikes,smoke was *maybe* more ? ? Sop, I pressure tested crank chamber/intake/cyl by plugging exhaust and intake manifolds with big rubber plugs and putting 6 psi to it. Valving out air supplyline after building pressure, it holds for about an hour. (did same test at 10psi with about same results). To be completely asured there's no issue here I equipped same set-up with vacuum generator set to 6psi vacuum. It held essentially the same.
- The wiring harness on stator and Hitachi CDI box all match up both in color and # of conductors...that is, there aren't any extra unused wires.
- As far as I can see, no one ever punched some matchmarks on engine cases and stator plate to "save" origional factory set timing advance. And, the stator was off the engine when I got it. Button rotor has a factory hash mark on it. Oh, BTW, I have 2 rotors, look same, both have strong magnetic attraction.
I've put a induction type timing light on bike to set.... AND to witness advanse. From Clymer book, ( and advise from other forums), timings to be set at 1.6mm BTDC.... FOR the 1980 yz250. The previous years and following years are different... some quite different! Anyway, I cannot quite set the timing to 1.6mm as seen with light, AT FAST IDLE, slots not long enough in stator plate... fully retarded. AS I rev bike up it will reach 1.6 mm punchmark, (and go past it).
Now I'm gonna mention a real concern. The following makes no sense to me, contradicts what I was "taught".
As it revs up, the liming light shows the spark timing retards, not advances. That is to say, as revs increase, the spark takes place CLOSER to top dead center.( In plain words, spark is retarded by increased RPM). That is opposite what I expect to see.
Lots of info, maybe long winded too, but don't say you didn't ask me. Ha! Please ask away for any more info, and if you see a flaw in my efforts , SAY SO ! Please. I gotta end this and maybe you now can see where I'd just love to substitute a known good ignition.
BTW, efforts to replace with a modern new system like PVL or Electrix World in England), were fruitless. Found no one who makes specifically for YZ250 in that era.
Dave
