YZ80 Shifter Cart

[pacman]

I just bought an old shifter cart with what I believe is a Yamaha YZ80 2-stroke motor. I'm not very experienced with 2 strokes, so I though I'd post a couple questions and see if you guys can help me out.

1. Yamaha is stamped on the side, along with the displacement in CC, but I'm not 100% sure what this motor is. Is there are way to look it up by the serial number stamped on the case? I need to know what motor it is, and it'd be nice to know the year.

2. The compression is really low. About 70psi. The previous owner said he had the top end "redone". I assume this means that he replaced the piston and rings. He also said he had it bored out and a larger piston put in. When he got it back from whoever did the work, the motor had really low compression. He got tired of messing with it and let it sit for who knows how long, then decided to sell it. We pulled the head and noticed that there are 2 O-rings (rather than a head gasket) between the head and cylinder. My first step is to replace those 2 o-rings. Assuming the piston, rings, and head "gasket" (o-ring) are good, what else could cause low compression on a 2-stroke?

That's it for now, but I'm sure there will be more to follow. Thanks for any help or advice you can offer.

BTW, pics to follow soon. This thing is gonna be wicked fun when it's running. :trust:

[bluedogok]

Try the eKartingNews.com - Forums (http://www.ekartingnews.com/forum.php?).

I have been looking at Rotax karts on/off, a friend of mine in Denver runs them and has been trying to get me into them.

[Jack Giesecke]

It uses O rings instead of a gasket and that's a good thing. The TZs, GP roadracing engines, are all O ringed. Compression is way low. Did you open the throttle or pull off the carb when you tested it for compression? If not, open the throttle and try it again. You've got to have air in it to compress.

If it's 70 psi, it's not stuck? NO scaring on the cylinder wall? Check squish clearance. To do this, insert a chunk of solder through the spark plug hole such that it reaches to the cylinder wall in the compression/squish area and roll the motor over several times through TDC, check the thickness of the solder. If it's much higher than .025", you have way too much squish clearance and the head will need to be decked to reduce clearance. Could be worn rings. Take the rings off the piston, insert them loose one at a time into the top of the cylinder, turn a piston over to push them down a quarter to a half inch or some (above the exhaust port) and use a feeler gauge to measure ring end gap. If it exceeds spec (will be in the manual), replace. Worn rings will reduce compression. Sounds like something more serious than rings, but they could be totally clapped out. Long as you have the piston off, insert it right side up into the bore and use feeler gauges and measure clearance between the piston and cylinder, rotate 90 degrees and check again (to make sure it's a round bore). A worn bore will be more worn front to back than side to side and will reduce compression.

BTW, if they did bore it, I hope they had it replated. They didn't put a liner in it or something stupid, did they? It should be a nickacil lined cylinder. I bought a TZ 250 motor once that a genius had installed liners in, nice job, of course it didn't work. :rolleyes: Seized that thing a couple of times and put me on my head before I figured out steel expands at a different rate than a nickacil lined aluminum bore (thermal expansion for all you engineers out there). The piston is designed for aluminum, not steel. :doh:

Sounds like you got a hack job of a motor to me. No tellin' what's the deal without some hands on inspection and measurements. Be aware, too, that the crank in those little race engines don't last forever, especially if raced on big tracks. You might wanna check crank clearance and play as per the manual against the run out limits long as you have the piston off. Yamaha's manuals are usually good about stating run out limits for things, unlike Honda. But, it might not be in a Clymer manual.

[Jack Giesecke]

Another thing that can kill compression I just thought of is if the guy went crazy with a porting tool on the exhaust port height. He'd have to go stupid on it to cut it down to 70 psi, though. :rofl: But, I've seen some stupid stuff done, did 'em myself before I knew any better and thought just because some cycle magazine said so.....well, we all go through the moron stage before we start reading and learning, put it that way.

Moral of the story is, either get a mentor or read a LOT (better both) or leave the tuning to the pros.

[pacman]

Thanks for all the good advice, Jack. We actually did get the motor running - briefly. We fired it up in the garage, let it warm up, and it actually sounded pretty good. Then we got it out on the street, put a little load on the motor in 1st gear and it promptly blew the reed cage, intake manifold and carb right off. Turns out three of the four bolts holding it all together were stripped.

I really doubt the cylinder was plated after the bore. It's got a 1st (.25mm) overbore and no sleeve. The piston and rings seem well clearanced, although I haven't spec'd them yet. Probably won't bother, as I'm pretty sure the compression problem is directly related to the stripped out bolts on the intake side. It's pretty much impossible to get a tight seal around the reed cage

You're right, the motor is a hack job. Almost every fastener is stripped. If we keep this motor, there's a lot of thread chasing and helicoil in our future. If we buy a new (used) motor, there's a potentially large dent in our wallets in our future. ;)

That's probably the route we'll take. A late model yz smoker will pretty much bolt up to the frame with very little, if any, mods. A Honda or Kwakker will work with a few tweaks, so I'm pretty open to anything right now. Heck, I'm even up for a rebuilder, as long as there're no stripped out bolt holes. :giveup:

[Jack Giesecke]

If it was leaking around the cage, then you're blowing gas back out of the crank case and not fully charging the cylinder. Yes, that would explain things. Like I said about the throttle, you have to have gas to compress.

Now, look at the cylinder, does it look shiny, polished or like raw aluminum? It will last maybe 5 minutes if they didn't re-plate it. :rofl: It HAS to have that nickacil coating. I had a place in Wisconsin plate the KX last time I seized and tore the lining. It was about $150, but they did a great job. That nickacil will outlast any steel liner by far so long as you don't stick the motor and tear it. Most of the modern sportbikes have the nickacil bores anymore. It has advantages, transfers heat much better than a steel liner in an aluminum block, the primary thing, but it's also harder, tougher, wears better, has less coefficient of friction than steel liners.

Yamaha's been chrome plating or nickacil plating the TZs since their inception in 1975. The YZ line wasn't far behind.

[pacman]

I'll have to pull the head again. The kart is over in my buddy's garage (we're splitting the cost of this project and he has more room than me), so the nxt time I get over there I'll have a look.

To be honest, I'm about finished with this motor. The more we tinker with it, the more we find wrong. Very frustrating that someone would jack something up this bad.:argh:

[pacman]

Finally got some pictures.

http://i69.photobucket.com/albums/i77/wstecker/kart.jpg

http://i69.photobucket.com/albums/i77/wstecker/kart1.jpg

http://i69.photobucket.com/albums/i77/wstecker/kart2.jpg

http://i69.photobucket.com/albums/i77/wstecker/kart3.jpg

As you can see, it's a lay-down, endurance style kart. It's a bear to get into the seat, but once you're situated, it's remarkably comfortable.

I can't wait to run it wide open on a closed course. Lots of work to do before that, though.

[igo-wfo]

I can't wait to run it wide open on a closed course.

AKA around the neighborhood...:trust:

Hooligans UNITE!! :twisted:

On a more serious note,
Have you looked into picking up another motor for it?
There's the place on Renfro right by the car wash( EGX powersports ) that does a lot of dirtbike type stuff, they might know where you could get a new lump for it.

[pacman]

AKA around the neighborhood...:trust:

Hooligans UNITE!! :twisted:

On a more serious note,
Have you looked into picking up another motor for it?
There's the place on Renfro right by the car wash( EGX powersports ) that does a lot of dirtbike type stuff, they might know where you could get a new lump for it.

Yup, EGX is right across the street from me (I'm in Oak Valley). I'm still looking for a smokin deal on a smoker. :trust:

[Jack Giesecke]

I'm going to be looking for a KX65 motor soon as I get some money ahead. I want to shovel it into my KX80 chassis for superbike legal in TMGP. They were very unlimited class friendly last year and this year I can only run two friggin' sprints. :rolleyes: To top off the cutback, they made 80s illegal in my favorite class, backwards. So, I'm going to try to find a late model KX65 motor for something I can afford. The late model ones are as fast as my old 88 model KX anyway. I won't miss much in horsepower and what with the stock ports, it ought to be easier to ride, to boot.

My motor is built, though, and pulls well, never dynoed. But, I'd guestimate 20 or so at the rear wheel. If you find a good 65 motor and ar intent on an 80 and want to swap (I'm picky, want a late model KX) we could work a deal. I wouldn't mind keeping it if TMGP does another about face on the class structure, but I think I could live with the 65 in either case because it's superbike legal and is legal for endurance if I wanted to run it.

With a 65, my old KX would be a KILLER endurance ride. I run 17s on it and it's very comfy for long stints, unlike squattin' down on a bike with 12" tires. NSRs KILL my old knees anymore. It also handles rather well with Bridgestone ultra soft slicks on it. I could get a ringer rider, maybe bring Bill Cotton back out of retirement, again, and go kick some butt. :evil:

[pacman]

I'm going to be looking for a KX65 motor soon as I get some money ahead. I want to shovel it into my KX80 chassis for superbike legal in TMGP. They were very unlimited class friendly last year and this year I can only run two friggin' sprints. :rolleyes: To top off the cutback, they made 80s illegal in my favorite class, backwards. So, I'm going to try to find a late model KX65 motor for something I can afford. The late model ones are as fast as my old 88 model KX anyway. I won't miss much in horsepower and what with the stock ports, it ought to be easier to ride, to boot.

My motor is built, though, and pulls well, never dynoed. But, I'd guestimate 20 or so at the rear wheel. If you find a good 65 motor and ar intent on an 80 and want to swap (I'm picky, want a late model KX) we could work a deal. I wouldn't mind keeping it if TMGP does another about face on the class structure, but I think I could live with the 65 in either case because it's superbike legal and is legal for endurance if I wanted to run it.

With a 65, my old KX would be a KILLER endurance ride. I run 17s on it and it's very comfy for long stints, unlike squattin' down on a bike with 12" tires. NSRs KILL my old knees anymore. It also handles rather well with Bridgestone ultra soft slicks on it. I could get a ringer rider, maybe bring Bill Cotton back out of retirement, again, and go kick some butt. :evil:

Thanks for the offer Jack!:thumb:

I haven't found a good anything for less than about 700, and those are about 10 years old. Anything less than that and it needs a top end, bottom end, or both. I'm resigned to the thought of at least doing a top end job on whatever I find. Really late model (5 years old or less) are running 1500 and up.

I'll keep the offer in mind and if I run across a nice 65 I'll let you know.

[focus frenzy]

I got a Kawasaki ZN1300 engine here, all it needs is new clutch plates!! fuel injected!! :eek:

[pacman]

I got a Kawasaki ZN1300 engine here, all it needs is new clutch plates!! fuel injected!! :eek:


Yikes! :eek2: