1998 KX-80 project... Eric Gorr now, or Eric Gorr later?

reepicheep

Member
Apr 3, 2009
670
2
Well, my 11 year old has pretty much outgrown the KX-60, not so much because of the bike, but because of it's wheels. We ride woods at Big Rock near Maysville KY, and that 14 inch front wheel on the 60 is just killing him on long rocky hill climbs.

I found an advertised 1999 KX-80 (which from the VIN is actually a 1998-KX80) with a bad crank and top end issues (had some new parts with the bike). That gives a nice upgrade over the KX-60, disc front and back brakes, KIPS valve, a carb with an air mixture screw (finally!), upside down front forks, 17" front wheel, looks like a really nice bit of kit.

I got it for $400... cheap, but it needs crank for sure, and likely top end work, and had no title (just bill of sale). So probably a pretty fair deal for both me and the seller at the end of the day. I kinda wanted a project bike to rebuild with the kid anyway... I want him to be the one kid on the planet when his generation grows up that won't go all wobbly kneed at the thought of fixing something.

So now the fun part begins... rebuilding my own broken stuff stinks, but buying other peoples broken stuff cheap and fixing it makes me feel great for some reason. Not rational I know, but as hobbies go, it's a pretty cheap one. :)

First, the crank. I'm seeing some great looking Wiseco crank kits on ebay for $130 or so. Stupid cheap (IMHO), as it comes with a new crank and complete bottom end bearing and seal kit. So that's kind of a no brainer, unless somebody here knows something I don't. Anbody have any experience with these?

Now the trickier question. The bike was probably bought new in 1999, ridden for two years, then the piston (predictably to everyone but the original owner) let go. That first owner then let the bike sit for... just about forever, then traded it to the second owner for a mountain bike.

Since the thing sat for so long, and probably because the second owner didn't split the cases and de-shrapnel everything, the crank was toast. Sticking in spots, excessive up and down play, and excessive side play.

Unfortunately, he dropped $100 (or whatever) on a brand new .020 over wiseco piston, and just slapped it in there. The second owner also said they had the bore replated. They put it together, ran it briefly (it would not stay running), and came to terms with the state of their crank. Then they parked it until I bought it.

So here I sit with the slightly damaged top end. There is scoring on the cylinder that you can just barely feel with a fingernail, but it starts pretty low... maybe 80% of the way down before you get to the ports in the cylinder wall.

The piston has some light damage. The skirt is lightly scuffed (from the bad crank no doubt), there is a little chip on the crown (not bad, maybe a quarter way down to the ring), and one end of the wrist pin hole looks like it was flared out a bit by some sort of impact (maybe the wrist pin came out and got jammed on a port or something).

They dropped the $100 for that Wiseco and $whatever for the plating, so I'm not out anything. But I have that piston and cylinder.

So I have two choices at this point...

1) Do the new bottom end, put the existing "slightly abused" piston in the existing "slightly abused" cylinder, run it for a few hours, then tear it down and see how it looks. Obviously, tearing down the top end on a KX-80 to check it is laughably easy. In fact I might even make the 11 year old do it while I observe silently. So that "extra work" is almost a plus.

2) Take the head and cylinder and ship it to Eric Gorr and let him do the magic he did on my KDX-200 (now 225). So for $400, it gets bored to a 100cc, nicisil plated, and comes back with new piston and piston parts. But try and use the stock 80 carb and exhaust but rejet appropriately to make it decent in the woods.

3) Take the head and cylinder and ship it to Eric Gorr and let him do the magic he did on my KDX-200 (now 225). So for $400, it gets bored to a 100cc, nicisil plated, and comes back with new piston and piston parts. Then buy new carb and exhaust parts to match the new 100cc motor.

Were it just option 2, I'd ship it to Eric tomorrow. A KX-80 chassis with a 100cc big bore kit with a low end or "mo-betta" porting would be a killer woods bike for a talented 11 year old rider. The kids woods bike (KDX) Kawasaki never built. :(

My only concern that makes me think I would end up at #3 is that the increased displacement would start some sort of financial death spiral. If I go to the 100cc cylinder, do I then need the new bigger carb? And do I need the new exhaust? It has a stock pipe on it now, which includes a nice beefy header and nice aluminum silencer, so I wouldn't mind sticking with that. Unless it's stupid loud, in which case I'll need an FMF regardless.

At the end of the day, a $400 bike with a $400 top end and a $125 bottom end and $100 in carb and $200 in pipe isn't a horrible deal for a really well dialed in woods bike (under $1300). And that's worst case.

But even better would be a $400 bike with a $125 crank and everything else using "parts on hand". The 60 had enough power for Jack already, so an 80 with a Kips valve and a carb I can actually dial in (air fuel screw) is plenty for the next 3 years at least.

And maybe the best is a $400 bike with a $400 top end and $125 crank, but still stock carb and exhaust. $925 for a great woods bike with stupid good midrange, even if peak HP isn't quite what would be achievable with the finely tuned 100cc carb and exhaust.

Any thoughts and experience from people that have been down this road?
 

reepicheep

Member
Apr 3, 2009
670
2
Curious. The aftermarket pipes are advertised as one model that fits *both* the KX-80 and the KX-100. So does the same pipe really work well on both? Or are the aftermarket pipes just tuned for the KX-100 and sold (even though not well tuned) for the 80?

So maybe with the pipe at least the stock KX-80 pipe would work with the big bore kit. That being said, while I haven't heard this thing run yet, I may want to replace the exhaust no matter what to make the thing quieter.
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
3
I'd be tempted to go Option 2 plus whatever you need to do to quiet the bike down for the trail.

The smaller carb might help with low end anyways. In my experience with a KDX200 to 240, the jetting that worked best on the 200 was pretty darn close to what worked best on the big bore. Basically, the motor is sucking in harder, so it gets the extra fuel it needs. This is my way of saying the 80 jetting might not be too far off.

If you ride it with the stock KX80 pipe for awhile, you will know what it needs in terms of a replacement. IOTW, if the stock pipe lacks low end, you can search for a low end pipe.
 

reepicheep

Member
Apr 3, 2009
670
2
That was my experience as well. I have a KDX-200, and tuned it, and also tuned a buddies KDX-220 (the bigger motor with the smaller carb). The 220 was easier to tune and pulled every bit as hard top and bottom.

I can see the way this is going, and it's not going to be cheap. The "right" carb according to Eric Gorr is $80 or so on ebay. The Q series silencer is $140 or so. Expansion chamber is $60 to $120 depending on condition.

But dang would this be a fun little bike for a 12 year old at the other end of that tunnel... And I'm going to make him pay for the crank. :)
 

helio lucas

~SPONSOR~
Jun 20, 2007
1,020
0
we don´t have the kx 100 over here, just kx 80 small and big wheel. i believe, motor wise, they share the same components, only the cylinder is bored over. so the pipe is the same.
most aftermarket will work on both bikes :/

a 100 cylinder ported for low end will not cost you a lot in parts, it work well with stock components. although it will run flat at higher rpm. good thing since will prevent jack hurting himself while he is growing on the bike, and will prevent killing the motor as well.

those crank kit are cheap indeed. i will buy one this week for a buddy. cheaper than buy a con rod and rebuild the original crank :coocoo:
 

reepicheep

Member
Apr 3, 2009
670
2
Good point Helio, thank you. I probably *want* a fairly flat top end at this point, at least for the first year or so. So I can work on bottom end and quiet now, then build it out for top end later.
 

2strokerfun

Member
May 19, 2006
1,500
1
Eric Gore Now. Carb later. (exhaust leave the same).
But whatever you decide, avoid option 1). Don't put that cylinder and piston back on top of a new crankshaft. Or you might be buying a second new crank before you know it.
 

reepicheep

Member
Apr 3, 2009
670
2
Got the Wiseco crank... fast shipping, and it looks fantastic. No idea how they can sell the crank, bottom end seal and bearing kit, and gasket kit, for $125 shipped. But I'm glad they do!
 

reepicheep

Member
Apr 3, 2009
670
2
Got them back, look great.

I found another trick that I figured I should post somewhere that Google can find it for the next person... this 1998 KX 80 (like lots of other KX-80s) had a clutch basket nut that was impossible to get off. Absurdly tight.

Even with the impact wrench and heat, I was getting nowhere.

The trick was finally to "pre load" the drive train... so I had a piece of rebar through the rear sprocket to lock it from the wheel, I put the bike in 5th gear, and used a screw driver to push the inner clutch basket to where it took up all transmission and chain slop.

That finally let the impact wrench do the job. Before then, it was just flopping the shaft around, and not delivering the impact to the nut. The energy from the impact wrench was just shaking the drive train, not loosening the nut.

This looks like its going to be a fun little bike...

For what that's worth, it should help somebody.
 

reepicheep

Member
Apr 3, 2009
670
2
Just another update in case anyone doing this same thing stumbles on this thread...

The earlier KX-80/100's used a Keihin PE26 carb. For example, my 1998. The later KX-100's used a PWK-28 carb. Eric Gore also recommends changing to a PWK-28 carb.

The jetting guides for the PE-26 and the PWK-28 carbs are different, so if you look up the jetting and buy a $30 worth of jets from Dennis Kirk so you are sure you have all the sizes you need, make sure you are looking at jetting guides for the right year. DAMHIK. :(

I guess the obvious solution is to go find a PWK-28 on ebay. After all, I already have the jets. :)
 

reepicheep

Member
Apr 3, 2009
670
2
Update again... finally got it to make smoke this weekend, and boy does this thing run nice. Even without the right jets, it starts great and pulls *hard*. Even with my way overweight butt on the seat, and some coughing at high RPM, it would do roll on throttle only wheelies in the first three gears from mid RPM's on up.

It feels like exactly what I wanted it to be, which is a "mini-me" KDX-220.

The whole rebuild went well. The weisco crank kit had all the gaskets, seals, and the main bearings for the bottom end. I didn't replace all the other little bearings (shifter drum, etc) as they were in good shape. I replaced them all when I rebuilt my KDX, and it was just a waste of money to replace perfectly good bearings.

The top end went well also. The replate and porting from Eric Gorr was perfect, and the bike runs and pulls great. I didn't recognize the piston as Wiseco this time, and didn't recognize the brand. That doesn't mean it's a bad piston, I haven't researched it yet. And maybe it doesn't matter so much on a 100, where the piston is less stressed than a big bore bike anyway.

The FMF Q series is unbelievably good. Amazingly quiet (yay!), and pulls hard. Most of the noise is coming from the header / expansion chamber (which means it won't carry).

The FMF Gnarly header I got was for a 100, and apparently the exhaust port diameter is bigger. So while it looks like it would bolt up right, it won't fit in the head. I just used the stock header instead, which seems to be working well, and isn't very heavy on this bike, so I'll probably just stick with that for the time being. The bike for sure is not hurting for power.

The kickstand is coming along well. I had a spare that came in a parts box with a buddies used KDX-220 (that was NOT for a KDX-220, I think its for some suzuki 4 stroke or something). It was on the KX-60, but was really too long for that bike. So I pulled it off, and brought it over. I had to cut off a long top tab on the bracket, and drill a new hole, but it would actually fit in very nicely. The top bolt went through the idler, the bottom bolt went through the old side stand tube. I had to cut down that tube (as it stuck out), and because the back side of that tube would interfere with the idler pulley if I had a nut on there, I tried filling the hole with weld and tapping it out so I can just bolt it straight in. It worked OK, but the stand bracket was angled out (bad, as it was unsupported) and I didn't have the right drill bit size to be able to tap it nicely... it worked, but I need to redo it (now that i have the right bits). I'll build up an angled ramp (probably grind down a nut into a ramp and weld it on after filling the hole again). I was chicken about frying an expensive CDI, so I used oxy actylene instead of MIG.

Last night I started my quest to lower it (my 11 year old is a bit small for it, but his KX-60 just had too small a front wheel for where we ride, it won't climb over rocks). I had sourced a cheap set of rear shock dog bones off ebay. I cut those, and welded in about a 28mm section of bar stock. This REALLY dropped the bike, unladen, it probably dropped 6-8 inches. That's either "too much" or "the maximum you would ever want to try". I'll have to watch the kid bounce it over a few curbs and see what rubs to tell. I'd like to make it taller (maybe 4 inch drop), so I'll probably pull them back out and take 4mm off their length. For what it's worth, you will have a hard time welding them with a 110v MIG unit... I started there and it wasn't going well, but when I switched back to Oxy/Act it went fantastic. I'd say instead of 28mm that 20mm to 23mm would be a better starting point. I dropped the front triple clamps on the forks as far as they would go as well... to the bottom of the raised section the top clamp engages. No idea if I have created stability or handling problems yet, the bike hasn't moved. Its got a really low long look to it though, so it'll be a mean little hill climber. Ground clearance obviously suffered... but its still as good or better than an XR-80.

The fact that i used spare dog bones cheap off ebay means that I can put it back to stock in about 20 minutes, and keep cutting and welding the old bones every few months as he grows and gets more comfortable with a taller bike.

I'll post up some detail pictures when I get it all sorted, showing the dog bones and side stand fabrications. I'll post up final jetting as well once I get that sorted.
 
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