Help w/ son's YZ80 carb or electrical problem.

[sceason]

I posted this message on the jetting forum but only got one response... so I thought I'd come "home" to where the brains are. ? I know we don't discuss "other" buikes but I'm at my wits end here...

Can someone knowledgeable PLEASE help me troubleshoot son's '97 Yamaha ZY80?

We purchased it used from some kids (young adults) and don't know how it was previously cared for. But we suspect it wasn't taken great care of based on appearance. We replaced wheel bearings, kill switch, brake/clutch handles, etc., and in essence, we fixed it up, and did all the required preventative maintenance, including flushing the coolant, lubing the cables, new brakes, front rotor, rebuild calipers, new fork seals/oil, etc. etc.

The bike initially wouldn't run well at high rpms, and smoked way too much when idling (way too rich a mixture). We adjusted the carb's air screw and idling screw to get it ridable, and it hesitated at high rpms still. Finally, even though we had cleaned the plug, we replaced it and then it ran great. But it was still way too rich and smoked a bit much. I figured I'd get to rebuilding the carb later, which was last week...

So, I pulled the carb off last week, cleaned every circuit, and moved the needle from the 4th clip position to the 3rd clip position, stock setting per the manual, and set the fuel fuel/air screw to 3 & 3/4 turns out (stock setting). It cranked it up (it was like 9:00 pm at night), and it ran like a scalded dog!! Great I though. "Good job, Dad's the hero once again! Next day, son took bike to a track nearby, and returned saying it's funning terrible! Gas is fresh, and carb is set up according to manual. We ride at sealever (Virginia). I played w/ fuel/air screw but can't get it running right! Plug is only a couple months old.

Anyone got any ideas as to what else could be wrong? I'm sure the carb work was done right! I'm not a mechanic but I took my time and followed the book and it ran great at first... small amount of smoke (telling me mixture was right), but now... it bogs out and hesitates BAD at full throttle RPM, almost like the spark plug is misfiring when reving high, and it's s relatively new plug! I'm going to get a new plag but have trouble thinking that's the problem. I'm also going to buy a hotter plug to try but I'm at a loss!

__________________

/s/

Stephen

[Wayward Son]

replace the spark plug.2 strokes will go thru them quickly especially with a

inexperienced rider/rich jetting.

"relatively new plug" doesnt cut it.get a few spares.

unless your son is capable of it keeping it on the pipe regularly it will require fresh plugs every now and again.in order for a 2 stroke to respond quickly to the throttle they are rich.

now,this can be minimized with proper jetting

what are the exact settings of the carb?

how far out is the air screw set?

fuel?oil?mix ratio?

[sceason]

I'll get new plugs today.

I checked manual when the carb was apart, and believe (95% confidance) all the jets were stock. I didn't write them down though.

Needle was on 4th clip, and it smoked way too much.

I moved it to the 3rd clip position. Now it smokes much less, and mixture appears good (much improved).

Air screw is 1 3/4 turns out (per book). Don'

t recall what setting was prior to disassembly though.

Could a coil be breaking down?

[Wayward Son]

doubtful.

replace the sprak plug and report back.ithink you wind it will cure your problem.

[sceason]

Cycle World didnt have plug I needed, and that's the 3rd store I tried. They did have one the was a 1-notch hotter then stock, so I bought 2 of them, It rode ok, but not until aaaI turned air screw in 1/4 turns. Bike still sputters or hesitates somewhat though it is better. If the pilot circuit comtrols 0-~25% of throttle response, then polot seems ok, as I dont/ have a prob. in that range. 25-75% throttle controlled by needle, right? I moved the clip up to 3rd position to lean it a bit and now it smokes like I would expect. I'd bet real $ now that the air/fuel mix is right on the money... DEAD right in fact!

Still the hesitation persists. MJ is not adjustable so if it's ok, that makes me think electrical failure, like coil-shorting out. ??

Help me guys. I'm nol longer my kids hero, and now he sees his dad picking fights w/ an object. probably thinking, "poor dad... andtime now he's going to the nut hut to get some help... he's over the edge now".

[Wayward Son]

the main jet most certianly is adjustable.

how far out is the air screw total now?

i would bet real money it still the jetting.

if it had a misfire i might agree on the ignition.but not a hesitation.

my bet is if you close the air screw down farther it will get better and better until the air screw is fully closed.

that indicates a few possibles.needs a bigger pilot jet or has a leaking ignition side crank seal.wich is very possible and usually goes along with worn main bearings.

take the ignition cover off and grab the flywheel.if you can feel any play in the bearing its toast.

also 2 strokes with worn out topends exhibit running issues.i cant tell you how many times ive seen people try to jet in a worn or half seized top end.

pull the exhuast pipe off and look up the exhuast port.check to see if the exhuast side of the piston shows any sign of seizure.

[sceason]

OK, what I meant by the MJ isn't "tunable", is its a static, non dynamic item that can only ne changed; it's certaiinly replacable if that hat you meant!

The air screw was set by the book at 1 3/4 uurns out. now it's 1 1/2 turns out and running better, (Note:, I thought that controle the idle circuit only... right or wrong?) Does richening up the air screw affect the full range of throttle applications acress all circiuts? (I'm still learning down here)

The hesitation is soo bad it almost qualifies as a missfire in my book. ?

Another symption that may be normal/abnormal is the overflow bowl drain seems to drain excessive amounts of raw gas from the small tube running off the backside of the carb. I always wondered if the float got stuck on ossasion, but since tearing down the carb, an cleaning all the circuits, and inspecting all of the moving parts, I didn't find any problems.

[Wayward Son]

the air screw on a 2 stroke has a drastic affect on the throttle response.

the over flowing cabr is not normal.the float level is high or the needle and seat are leaking.

i highly suggest checking the motor condition as a listed in the previous reply.

small 2 strokes require frequent maintianence.

the kx80 flat trackers i tune require pistons/rings every 3 races and crankshaft rebuils/main bearings every 6.other wise they go boom.

[sceason]

As for the bearings, I removed crank case, and pulled on flywheel... it "feels" like it has a 1/2 of a paper thin amount of wear. I can feel a bump when I try and pull it back and forth but it's soo small a movement, I'd think it's just enough for oil to get under the bearing for lubing? Or should there be absolutely NO movement? If the jets need replacing, which ones, and bigger or smaller?

Does the air screw affect the entire throttle response range or just the idle circuit? The problem feels like a high rpm missfire, but after turning in the air screw, the hesitation/surging condition got less pronounced.

I double checked, and gas isn't coming out carb like it was before tearing it down... you have to lean it way over to make it pour out, so I guess I fixed the float level.

I'm very grateful for the help Mr. Burned. ?

Wish I could somehow repay the favor!

[sceason]

Forgot to add this Q, clearly anyways: I'm not sure if I ruled out the crank bearing as OK or not, but after rechecking your previous e-mail, you said "any" movement in flywheel means bearing is toast. There's s VERY very very small amount of movement, like enough for a layer of oil to keep it lubed (literally), so I'm not sure if it's bad or not... I really had to pull/push back and forth hard to "feel" a little "bumping" feeling... if it's ok, and I think it's ok, then I'll try the PJ. Should I also change the MJ? If so, bigger or smaller?

?

[Wayward Son]

ther should be no movement.they may be ok,but definately not new.

did you pull the pipe and look up the exhaust port?

the air screw has a drastic affect on throttle response.it only really controls the metering of the idle to 1/8 throttle,but acts like the accelerator pump as well.

if you have a "boowhaaa" sound when blipping the throttle close the air screw a 1/4 turn at a time till it responds crisply.

if you end up with the ari screw fully closed,go to the next size bigger pilot.

leave the needle and main for now.

does the bike rev cleanly at wide open throttle?

[sceason]

I had 2 yr. old duty last night so didn't have time to pull exhaust... but, I'm not confident what to look for. Can you describe in more detail? Am I looking for scratches? Burn marks, carbon buildup? What's the ideal findings, a nice shinny piston?

As for reving, it rev'ed better after closing the air screw 1/4 turn. I haven't tried to close it more, but will tonight. I'll also pull the pipe off when I get home. Hopefully you can guide me more on that issue.

Thanks again my friend! ?

[Wayward Son]

it will be obvious if the piston is damaged.

[sceason]

Didn't see anything significant. ? Guess it's ok then?

[sceason]

Didn't see anything significant. ? Guess it's ok then? We have a TT race tomorrow so we'll see how she goes! I'll check back in by Sunday!

[Wayward Son]

sounds good.