Makita 6421 Build Problems

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CobraChevelle

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I was able to score a deal (at the time I thought it was) on a "new 6421" that was broken in the box. It looked like someone dropped the box and broke the chain brake ear. So, I had to replace the clutch side case half and decided to throw on an oem 7900 top end assembly.

Well, I have a couple of problems:
1.) I can get the saw to run but It vibrates bad and the throttle is surging like crazy. It almost feels like the vibrating it causing the throttle surging. All the screws are tight so idk what else

2.) My clutch keeps coming loose. I don't know why, I modified a spark plug socket into 3 prongs and put a rope in the cylinder so i can reinstall it. Is there a torque spec? should I use thread locker? Is this causing my severe vibration?

I 'm at a lost.

Thanks
 
Did the crank get bent when the saw was dropped?
I couldn't see how. The clutch cover has a small scratch and thats about it. It looks like it was dropped between the handle and the clutch cover. How do I check? If its bent? It doesnt feel like it when turning it over.
 
I couldn't see how. The clutch cover has a small scratch and thats about it. It looks like it was dropped between the handle and the clutch cover. How do I check? If its bent? It doesnt feel like it when turning it over.
Might be possible to turn it over without the spark plug and clutch in place and see it wobble.

The parts are all the same minus the cylinder and piston
 
Might be possible to turn it over without the spark plug and clutch in place and see it wobble.

The parts are all the same minus the cylinder and piston
I took the clutch off and I dont see anything when rotating the crank with the pull cord.
 
How did you remove and install crankcase halves. What method?
Did you press them together, or use heat?

I bought a case splitter...I didn't make my own. And for putting the halves back together, I coated the crank and bearing with oil and pulled the two halves together with the screws. It was pretty easy. Well, I thought it was at the time...
 
I've learned the hard way that there needs to be a metal spacer, or "shim", centered between the counterweights of the crank while under any removal or install stress.
This is done on "screw pulled together" crank halves in order to keep the crank weights from bending closer together, and thus causing a wobble vibration. At 11,000 rpm, it doesn't take much to vibrate alot. Next thing you know, a seal will pop out, and the bearing will be hammered loose in the press fit bearing pocket.
The shim or (I use a tapered bolt), needs to be only tight enough to keep it from falling out.
If you tapped the crank nut with a hammer to remove it, there's a good chance it'll be bent too.
Some models are more sensitive than others.
Husqvarna makes a nice tool that pulls the crank into the bearing from each side.
 
I've learned the hard way that there needs to be a metal spacer, or "shim", centered between the counterweights of the crank while under any removal or install stress.
This is done on "screw pulled together" crank halves in order to keep the crank weights from bending closer together, and thus causing a wobble vibration. At 11,000 rpm, it doesn't take much to vibrate alot. Next thing you know, a seal will pop out, and the bearing will be hammered loose in the press fit bearing pocket.
The shim or (I use a tapered bolt), needs to be only tight enough to keep it from falling out.
If you tapped the crank nut with a hammer to remove it, there's a good chance it'll be bent too.
Some models are more sensitive than others.
Husqvarna makes a nice tool that pulls the crank into the bearing from each side.

It does makes sense, but this thing came apart and went together so easy. I did watch a lot of videos online and I don't recall people using a shim when pulling the case apart, but it makes sense what you said. I did find some videos of home made tools using the threads on the shaft and pressing the halves together after they heat up the bearing.

Question is, do I want to gamble $140-200 on a crank?

Could the clutch be bad? Possibly causing the the wobble?
 
I've learned the hard way that there needs to be a metal spacer, or "shim", centered between the counterweights of the crank while under any removal or install stress.
This is done on "screw pulled together" crank halves in order to keep the crank weights from bending closer together, and thus causing a wobble vibration. At 11,000 rpm, it doesn't take much to vibrate alot. Next thing you know, a seal will pop out, and the bearing will be hammered loose in the press fit bearing pocket.
The shim or (I use a tapered bolt), needs to be only tight enough to keep it from falling out.
If you tapped the crank nut with a hammer to remove it, there's a good chance it'll be bent too.
Some models are more sensitive than others.
Husqvarna makes a nice tool that pulls the crank into the bearing from each side.

I've never had any these problems or had to do any extra precautions.

OP have you checked for the circlips? Forgot one maybe?
 
I'll update on here also.

Well, after using an actually piston stop (instead of rope) my clutch doesn't come loose and my crazy hand numbing vibration went away. So maybe its safe to say my crank is good...

My next problem is I can't get this thing to run right... I can get it to idle ok when I lean it out,but obviously I can't get throttle, but when I richen the L screw and hit the throttle the saw will still race after I let off on the throttle...

From what I was told and read in other forums it might be an air leak. First i'll check the throttle cable and then try to find a way to find this potential air leak.
 
Definitely sounds like a leak. sandwich rubber like from a bike tube between the carb and boot, and muffler and cylinder. Use the impulse line to add pressure/vacuum. Be the easiest route if you don't have the proper chainsaw specific tools.
 
Well, I found my problem... Its the crankshaft seal on the clutch side.... I'm very surprised cause this is a brand new case half with bearing.... What are the chances of this bearing and seal bad being bad brand new? Any pointers on removing and reinstalling the bearing.

Thanks
 
Heat the case up and knock it out. Should come out pretty easy if the case gets hot enough.
 
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