snap,crack and pop

[southerndime333]

pop , purrrr, pop,, BANG!!, crackle *dead*

thats what happend when i put it in gear to pull off,

so i left it for the night because i was pissed off with it. right i took off the points/advance cover.. AND BEHOLD! Bent advance governer boss , the metal thingy behind the locking nut, were the pin goes it bent the lip. the pin fell off before it did anymore damage.. my question is what could of caused it??

 

[grizld1]

That's a no-brainer. The pin is there only for alignment. When the nut loosens, the ATU rotor slaps back and forth against the pin, and the slot deforms.The result is usually a holed piston; I hope your motor died before that happened.

 

[Sundie]

why a holed piston? usually, the timing isn't that bad for that long when this happens. and until the nut entirely falls off, is only a few degrees out. and while it's doing it, the bike's acting weird, and you don't push it.

When the nut falls off, exciting things happen for an instant, then the engine dies.

 

[southerndime333]

it sounds like whats happend..
no holey pistons... whitch is good..
thanks for the fast replys too.

i dont really want to fork out lots of money for a new one for it to happen again..

the pin looks like it forced its way round , if it was loose it was my fault stupid error. and ive lernt from it.

 

[jay760]

Use thread lock on it when you put it back together, I had one go once and snapped the end of the cam off!

 

[grizld1]

Sundie, I'm not going to speculate on theory; but every salvage XS650 motor with a holed piston that I've bought and broken down or helped to rebuild, and that's been a few, has had a trashed ATU, deformed and sometimes even with a chunk broken out at the alignment slot. Every one of them. No exceptions.

True--if the rider has sense enough not to try to "ride through" the misfire and shuts down, or if things just happen to occur fast enough to kill the engine completely, a holed piston isn't inevitable. The nut doesn't "fall off"--it can't unless the whole ATU rotor has been shoved off the camshaft, and there's no room in the cover for that. The nut just has to be a little loose for impact between slot and pin to start, and once the impact starts the slot will start to wallow, and the more it wallows, the more momentum increases between impacts, and the damage accelerates. You soon get the timing advancing much more than "a few degrees" (remember, that camshaft is turning at half crank speed, any error up top is doubled down below). All this happens pretty fast, it's not a gradual progression where you notice performance deteriorating slowly.

Suggestion: smack that nut down hard with a flat punch and a 4-pound hammer and you won't need thread lock.

 

[Sundie]

Sundie, I'm not going to speculate on theory; but every salvage XS650 motor with a holed piston that I've bought and broken down or helped to rebuild, and that's been a few, has had a trashed ATU, deformed and sometimes even with a chunk broken out at the alignment slot. Every one of them. No exceptions.

Well, the only thing i would point out is that the inverse is not true. Every xs650 motor that had the timing advance nut fall off did not hole a piston. An example in this thread, and my own tend to disprove the absolute "this happens if this happens".

True--if the rider has sense enough not to try to "ride through" the misfire and shuts down, or if things just happen to occur fast enough to kill the engine completely, a holed piston isn't inevitable. The nut doesn't "fall off"--it can't unless the whole ATU rotor has been shoved off the camshaft, and there's no room in the cover for that.

My own example would tend to disprove that - since that's exactly what happened. Nut came off, things imploded. Broke the ATU, punched a hole out through the backing plate. Interestingly, i found the pin still inside.

The nut just has to be a little loose for impact between slot and pin to start, and once the impact starts the slot will start to wallow, and the more it wallows, the more momentum increases between impacts, and the damage accelerates.

You know, at least in my case, this damage might have been entirely ameliorated by the fact i was running a pamco - which doesn't put any back/forth pressure on the atu, like the points lobe does.

You soon get the timing advancing much more than "a few degrees" (remember, that camshaft is turning at half crank speed, any error up top is doubled down below). All this happens pretty fast, it's not a gradual progression where you notice performance deteriorating slowly.

Just checking - doesn't the cam shaft go *half* the speed of the crank? Since the crank spins 720 for every 360 of the cam... Thus, the max 5 degrees that pin can slam back and forth (before total loss) only equates to 2.5 degrees of advance/retard.

Don't disagree that it happens pretty fast though.

Suggestion: smack that nut down hard with a flat punch and a 4-pound hammer and you won't need thread lock.

A bit of blue thread lock is * cheap* insurance.

 

[southerndime333]

righty ho then.. thred lock and a big hammer it is... grizld1 that is what happend then, the nut became loose and started getting tempremental.

i took the head off. the piston did have a fresh mark on it. like the valve hit the piston.. as everyone here seems to be smart, do you think it would bend the valve at all??

sundie cheers for the tip

 

[Sundie]

valve bending: no. This would not (normally) cause the valve to hit the piston. The only thing that can make that happen is your cam slipping a tooth, or cam chain breaking. Unlikely.

take pics of the mark and post.

ps - *blue* thread lock, not red. You'll have to cut the thing off if you use red.

 

[grizld1]

Sundie, you got yourself all dyslexicated there (my excuse for it is old age, you'll have to find your own). The crank turns twice as fast as the cam; 360* of crank=180* of cam, that's how it has to be for a 4-stroke cycle. 1* error at cam=2*error at crank. The only thing that could have allowed the nut to come fully off would be the ATU rotor actually breaking up. Also, load of points springs on the ignition shaft isn't what's responsible for slapping the ATU to death. Rapidly shifting loads on the cam lobes and acceleration/deceleration working against the inertia of the ATU assembly do the dirty deed.