KD 80 bike knocks What do i do!!!!!!!!!

[robby12]

My kd 80 runs strong and fast but knocks in 5th gear what is the knock rod or piston or what ever elce it could be

i have anther kd80 with a unknown motor that knocked for years till the piston was disintegrated and i don't know what motor it is so i cant put a new piston in it.(posted it 2 min ago help me find out what motor plz)

[RichLockyer]

Could be a dozen things.
How does the plug look? Is it running lean?

[robby12]

yes it is running a little lean but could that do it

[RichLockyer]

Absolutely... a lean condition will result in pinging, high cylinder temperatures, and eventually burning a hole through the piston.

"A little lean" is "a little bad". Very lean can destroy that engine in 30 seconds.

[robby12]

well how do i tune the carb i have never been good a air flue ratios and gas mixing i run it a 40 to 1 on 105 octane gas

[RichLockyer]

First, you're just wasting money on 105 octane gas. These bikes were designed for regular or premium pump gas.

As far as tuning the carb, without spending the time and money on a dyno, it's just trial and error. Check what the jetting is now, go one or two steps, test ride it, and check the plug.

[robby12]

ok i pretty sure right now there is a 85 main jet in it ill get back to you tomarw after i mess with it
i have a second ? what kind of oil should i run in my gear box? What brand and weight would you recommend

[RichLockyer]

Sorry, I'm not familiar with 2-strokes... can't help you on the gear oil.

[zoro]

Like Rich said, Stop running the high octane! Check the crankcase oil for silvery flec. If those bikes have an oil pan, drop it and check the piston rod for any free play. If that engine is knocking you might as well prepare for a rebuild, its not that big of deal with those single cylinder two strokes..........it least it didnt used to be. "Nothing is like it used to be!"

[RichLockyer]

Part of the problem is we can't hear it.
What kind of knock is it? Is it a deep, heavy impact? That would be the bottom end.
Is it a lighter, higher pitched knock? That could be from detonation or preignition from running too lean.

Only in 5th gear... 5th is probably an overdrive, which will increase the tendency to detonate/preignite, but it could also be something in the tranny.

[zoro]

Originally Posted by robby12

ok i pretty sure right now there is a 85 main jet in it ill get back to you tomarw after i mess with it
i have a second ? what kind of oil should i run in my gear box? What brand and weight would you recommend

20w50 Castrol, and the carb issue wont cause a "knock", only "ping" (pre-ignition detonation). Is it knocking or pinging?

[zoro]

What did the used crankcase oil look like? Silvery flec? Does the Knocking speed increase and decrease with the rpm? what area of the engine is the sound coming from, cylider head,crankcase,tranny?

[robby12]

It knocks when you rev is up high and makes a clattering knock noise in 5th gear its not loud its a light knock.
it has a new clutch,piston,bored .20 over new rod,bearings and head all new electrical system points and all it is all most a re built motor

The crankcase oil looks fine it sounds like its coming form the crank to me but all the rod bearings a new
and its defiantly a light knock

[zoro]

Originally Posted by robby12

It knocks when you rev is up high and makes a clattering knock noise in 5th gear its not loud its a light knock.
it has a new clutch,piston,bored .20 over new rod,bearings and head all new electrical system points and all it is all most a re built motor

The crankcase oil looks fine it sounds like its coming form the crank to me but all the rod bearings a new
and its defiantly a light knock

You have two choices only you can make, actually Three. If a shop did the work you described above,take the bike back and have them do it right! If thats not possible, fill the crankcase with 20w50 castrol and ride the bike until you notice the "knock" getting louder and hammering and rebuild the engine. Or tear the engine down now and find the cause. Were the crank bearings good when the rebuild occured? They can be the source of the knock also.

[robby12]

i did the rebulid my self and i re placed any baring that showed sings of play both the rod bearings are new and both crank bearings
iv rebuilt 100s of these motors im 100% sure i did not do any thing wrong

guess ill start taring it down again!!!!!!!! ill find out either way what it is

[RichLockyer]

Originally Posted by zoro

If thats not possible, fill the crankcase with 20w50 castrol and ride the bike until you notice the "knock" getting louder and hammering and rebuild the engine.

KD80 is a 2-smoke... he'd have an interesting afternoon with the crankcase loaded with 20w50

[zoro]

Originally Posted by RichLockyer

KD80 is a 2-smoke... he'd have an interesting afternoon with the crankcase loaded with 20w50

Where did the oil he drained out of it come from? Gear box?

[robby12]

Yes the gear box

[RichLockyer]

Originally Posted by zoro

Where did the oil he drained out of it come from? Gear box?

Ya...
That does create a problem troubleshooting 2-strokes, as there is no oil to drain from the case to analyze. The oil is mixed with the fuel (either by an onboard oil injector or premixed in the fuel tank by the owner), and the fuel is drawn in from the carb into the case on the piston's upstroke where it lubricates the bottom end (normally needle bearings).
As the piston comes down on the power stroke, it pressurizes the case (which closes the reed valves and prevents the fuel/air/oil from being blown back out the carb)... as the piston moves down, it first exposes the exhaust port, and the exhaust gases start to exit. It then uncovers the transfer port, where the pressurized charge in the case is allowed to move into the cylinder, pushing out the remainder of the exhaust gases.
The piston reverses and begins to move up, closes the transfer port (at which point the case begins it's next intake cycle).... and then, if the RPM is right and the exhaust system is tuned properly, the reversion pulse from the pipe will hit the exhaust port just before the piston covers it. This will return a portion of the fresh fuel/air mix that migrated into the headpipe back into the cylinder, increasing both compression and power (called being "on the pipe" or "in the powerband").


Okay... I lied when I said I wasn't familiar with 2-strokes... I know a little about the theory of 2-stroke engines
My only direct hands-on experience has been with RC aircraft engines up to about 3hp (18cc engine). Those things are ball-bearing and can spin as high as 25k.

[zoro]

Originally Posted by robby12

Yes the gear box

No wonder its knocking! Get some oil in that crankcase!

"Just Kidding"