Broken KX420

BlazinBike · Aug 1, 2005

I let my friend take out my KX420 (2 stroke) :bang: and I chased him 2 miles down Pismo beach to find him standing there with my broken bike.

He told me that the chain fell off and he heard the engine rev so he turned the bike off. Then once he got the chain back on he tried to kick it, but the kick start felt "chunky" eventually the kick start siezed up. I got the bike home, drained the oil and noticed that it was green. I just replaced it with Mobil 1 synthetic before the trip and the plug looks like it's carbon fouled, it was a new plug too. I tried to spin the motor with my hands by grabbing the clutch pack and turning it. The motor only moved a few inches before it would stop spinning. (The plug was out so there was no compression)

So gurus... please diagnos. Thanks -Adam

uts · Aug 1, 2005

Green tranny oil= hole in water pump and no coolant left in motor. Is there any coolant in the radiator. It is possible that all the coolant leaked into the transmission and the engine seized. Hope not for your sake When did KX make a 420 anyway.
UTS

Kevin003 · Aug 1, 2005

uts said:
Green tranny oil= hole in water pump and no coolant left in motor. Is there any coolant in the radiator. It is possible that all the coolant leaked into the transmission and the engine seized. Hope not for your sake When did KX make a 420 anyway.
UTS

That could very well be your problem. Hopefully its not for your sake. A lesson learned, thou shalt not let other friends ride his bike!

John Cena · Aug 1, 2005

I've never heard of a kx 420. I know they made a 81 kdx 420. its not liquid cooled though.

dirt bike dave · Aug 1, 2005

The KX420 was made in '80-'81. Air cooled.

Where do you normally ride? Running in the sand on a cool day at sea level is going to require richer jetting than running the high desert. Then a burst of high revs on a lean, hot motor...even with no load on it, it could be ugly.

The plug is probably not carbon fouled - more likely it is dust from what used to be bearings. As to why the tranny oil is discolored - could be the right side crankshaft bearing and seal got wiped out and premix was leaking in while your budddy was kicking the bike over.

You are going to have to tear it down for examination, but brace youself. I suspect the lower rod (big end) bearing or crankshaft bearings.

Even in the best case scenarior, rebuild costs are likely to exceed the value of the bike. Especially if you need a new crankshaft, cases or cylinder. A new piston, rings, a complete gasket set and a few bearings + your labor and tools are the best case scenario, IMO, and could easily go $300, if you can even find the parts. If cases, cylinder, and/or crank are damaged, you could spend over $1,000 trying to fix it.

Better off putting the money toward a newer bike, IMO.

BlazinBike · Aug 1, 2005

The worst part of all this is that I was planning on selling the bike after this trip. And I normally ride in Glamis, so yeah I really should have had a bigger jet in there.

And I hate to say it but it sounds like you're right Dave. I'll pull out the motor and start examining. And where can I buy bearings/ parts for this bike? Do you know if they'd be the same as a KX250 or another KX bike? Also I've always wondered is the KDX420 the same bike?

On a second thought how hard would it be to swap in a different motor? Possibly a 400-450 4 stroke engine? Does anything bolt up? And if so, would it ever be possible to make this bike street legal?

dirt bike dave · Aug 1, 2005

Your best chances for a direct transplant are from a similar era KX/KDX 400, 420 or 450, MAYBE a first generation air cooled KX500 (though that motor reportedly had lots of problems).

First step is to tear the motor down and see how bad it is.

As for a more modern motor or street legalization, anything is possible with enough time and money. But my advice would be to avoid spending lots of money on the bike. It's hard to put an old war horse down, but sometimes that is the only option that makes sense. If the total parts & repair costs are going to be anything over a few hundred $, sell it as a basket case and put the money toward a newer bike, IMO.

BlazinBike · Aug 1, 2005

Definitely good advice. If I can fix it cheap I'll do so and then most likely sell it, otherwise I sell the parts on E-Bay. It's too much bike for me anyways. I want something a little smaller like a 250, I don't need so much power.

I pulled off the stator cover to find that one of the two bolts holding the stator in shook loose causing the other bolt to bend and the edge of the stator to crack. I pulled the stator off and I could spin the end of the crankshaft. It'll move through most of the stroke easy but it gets difficult to turn when the piston reaches TDC. (Once again the spark plug is out) Is this normal? If not, I guess bearing failure is a safe bet.

BlazinBike · Aug 2, 2005

I found a KDX420 motor that has been in storage for over a decade. It spins, and it shifts, seller wants $80. Should I go for it? -Adam

dirt bike dave · Aug 2, 2005

Sounds like an excellent find. I would do it, especially if it includes the carb, and the motor has spark and compression.

FWIW on the older Kawasakis the stators can go bad, causing weak spark - usually costs about $100 to rewind.

My GUESS is it will bolt right in and the KDX motor will have more mild porting, lower compression and a heavier flywheel plus a lighting coil. Best case scenario is you drop it right in and can get it started with little trouble (might take a while to get it tuned and running right).

I think it would be a lot less work and $ than splitting the cases and doing a rebuild on your old motor. Even if the KDX motor won't run, it will likely provide some good, hard to find parts (like the crank) that might have got trashed on the KX.

ErcDraven88 · Aug 2, 2005

I had this problem with my 2000 Cr 250. The bottom end went out. I finally got the hint after 3 new top ends. Parts and all would have cost me $400 +. I eventually just sold the bike as a basket case and rolling frame.

rm_racer · Aug 2, 2005

If you need parts, you might find them on www.bikebandit.com , my friend just bought a top end for a 1980 RM400. Couldnt find the parts anywhere else, and he was looking for over two months.

BlazinBike · Aug 2, 2005

I bought it. It looks clean too. And you're right guru Dave, it came with a lighting coil and a much larger flywheel. This KDX motor looks like it has 100K less miles than my KX motor.

One more question for ya. The owners manual says to use 10-30 or 10-40W but the guy at Cycle Gear told me to go with Castrol 80W synthetic gear oil. Is this OK?

Thanks again! :)

dirt bike dave · Aug 3, 2005

Either gear oil or motor oil will be fine. I usually go with motor oil, as one time on a cold morning in the mountains with my KDX250 and 80 or 90 w gear oil, I could not change gears until the motor got warm.

The KDX will have an extra wire from the flywheel side for the lights. Usually that is the yellow one, but a manual would be helpful (BTW, the old Kawasaki owner's manuals that came with the bike were very good - almost as good as a shop manual. If you can find one cheap for either bike, get it).

Did the motor come with the electronics, too? If so, good! Use the KDX black box if available, but I'll bet your KX stuff will wire up easy enough.

I think you were pretty lucky to find a donor engine so quick and cheap! Hopefully your good fortune will continue and the job will go easy and you'll have it running soon.

Broken KX420

BlazinBike

uts

Kevin003

John Cena

dirt bike dave

Sponsoring Member

BlazinBike

dirt bike dave

Sponsoring Member

BlazinBike

BlazinBike

dirt bike dave

Sponsoring Member

ErcDraven88

rm_racer

BlazinBike

dirt bike dave

Sponsoring Member

Similar Topics

Broken KX420

Sponsoring Member

Sponsoring Member

Sponsoring Member

Sponsoring Member

Similar Topics

Privacy & Transparency

Privacy & Transparency