just a little too advanced..?

[cheftay]

So i've been messing with the timing on my 71 w/pamco ignition and i've got the pamco backed up as far as it can go and my timing mark is just slightly to the left from the correct marks on the alternator and there no more room on the pamco for me to retard it.

I've adjusted my camchain but that's all that my knowledge is aware of to solve this problem.

Any other factors you guys know of?

Thanks a lot!

Taylor

 

[pamcopete]

cheftay,

Try moving the timing light clamp to the other plug wire first.

Loosen the PAMCO rotor and the slotted disk on the advance side. Turn and hold the PAMCO rotor in the clockwise position as you re tighten the nuts.

There is also a bit of movement available with the advance plate if those two do not provide enough movement.

This could be an indication of a stretched timing chain.

 

[cheftay]

hmm i'll check all those things when i get off work.

the timing chain is brand new from mikesxs so i hope that's not the case!

thanks!

 

[XSLeo]

On my 75 I had the same problem.
When I put the cam chain on I had it off one tooth. I was using points then, I filed the backing plate enough so it would run, but it wasn't right.
I pulled the engine half out of the frame to pull the rocker cover. I put the cam right. Put the engine back in place.
This moved the backing plate back to the center of the original slot.
The width of the slot is about 20 degrees. When the cam is off one tooth it puts the cam 10 degrees off. This puts the ignition 10 degrees off. With a 20 degree slot, the 10 degrees is right at the end of the slot.
An easy way to tell if the cam is off is to put your timing marks at TDC, Look at the pins that hold the ATU and Pamco rotor in place. They should be straight up or down. If it's not then the cam should be checked.

 

[grizld1]

Leo, 1 tooth=10* at the camshaft for 447 motors, but the error is doubled to 20* at the crank due to 2:1 gearing. Taylor, a stretched cam chain results in retarded ignition and valve timing. The camshaft moves CW as the tensioner takes up slack, so that valves and ignition are actuated later in the travel of the crank (CCW). If that were the case with your motor you'd run out of adjustment to the advance side.

There's a bit of play between the ATU alignment pin and its slot, and you can often loosen the stepped nut and reposition the ATU enough to get your timing adjustment back in range. Be sure to smack that nut down tight with a drift; things get seriously ugly if it loosens up.

 

[cheftay]

What's an example of seriously ugly?

Posted via Mobile

 

[cheftay]

Okay got it fixed. Just took the whole advance system/unit apart and greased the rod and re-assembled everything and it adjusted just fine. Thanks for the assist everyone!

 

[grizld1]

Congrats, Taylor! To answer your question, "seriously ugly" means the following pretty common scenario. The stepped nut loosens up, the alignment slot in the ATU slams against its pin and deforms, and the ignition goes way advanced. If that occurs the rider's best hope is that he's sharp enough that day to shut down in a hurry. What usually happens is that the engine starts running badly, the rider tries to limp the bike home, and detonation burns a hole in a piston crown.

 

[cheftay]

welp grizld1... i'm afraid that just happened just not the hole in the piston. I even loctited the nut and it still did it! i feel like a bozo...

The pin in the cam has disappeared and the whole unit exploded into about 20 pieces. I dont think the cam is damaged though.

 

[pamcopete]

grizld1,

If the pin for the ATU loosens up, wouldn't the cam keep moving and leave the ATU behind, so to speak, thus causing a retarded condition?

 

[grizld1]

It isn't the pin that causes the grief, Pete; it's just there for alignment. Anyhow what you've written sounds intuitively right; but in practice, that doesn't appear to be what happens. Every time I've opened up one of these motors with a holed piston--that would be around a dozen of them by now--I've found the stepped nut loose and the ATU deformed by the pin. On the rarer occasions when I've been able to question the riders, they've reported trying to limp the bike home after "It started running real bad." Detonation is clearly the culprit, caused by securement failure between the ATU and the camshaft.

Taylor, don't try to tighten that nut with a claw wrench, etc. and forget about the Loctite. Smack it down with a drift and a BFH.

 

[pamcopete]

ChefTay,

Here's a couple of tips regarding securing the ATU:

1. On some ATU plates, the pin that keeps the locating pin from failing out sometimes protrudes to the other side where it interferes with the spanner nut, preventing the spanner nut from making maximum surface contact. So, file this pin down so it is flush with the plate surface.

2. Also, the spanner nut should be installed with the lip to the outside:

3. When you use the punch to tighten the nut, tap it all the way around.

 

[pamcopete]

grizld1,

No doubt you saw what you saw, but I'm wondering if perhaps the ATU was not installed properly in the first place. I've had people install the ATU without benefit of the locating pin and then report extreme advance or extreme retarded timing. Of course, that's with the PAMCO which is not sensitive to being 180 out on the cam, so the engine will probably run even if you just throw the ATU at it from across the room.

About the only other thing I can think of is if the ATU plate comes loose because the spanner nut comes loose, then perhaps it rotates and just happens to jam again in an advanced position?

 

[grizld1]

Pete, you've had me scratching my head on the "retarded but detonating" scenario. Here's what I think happens. In all examples I've seen the severity of the deformation in the alignment slot indicates impact damage: the ATU was clearly slammed back and forth with considerable force, maybe due to load variations on the camshaft as the lobes clear and engage the rocker arms. What clearly happens, though, that the pin strikes the loose ATU forcefully and repeatedly, bouncing it back and forth, with the bounce to the advance side occurring regularly enough approaching the power stroke to trigger severe detonation.

 

[grizld1]

BTW, I misspoke above; all holed pistons I've seen in XS650's have been associated with ATU condition, but not all with the nut loosening; a few were due to folks who through laziness or ignorance dialed in the timing to the retard position and didn't check the advanced mode timing. If the bobweights are worn a bit that's all it takes, since the advance curve stretches with bobweight wear.

 

[Sundie]

What's an example of seriously ugly?

Posted via Mobile

This is a good example.

 

[Sundie]

BTW, I misspoke above; all holed pistons I've seen in XS650's have been associated with ATU condition, but not all with the nut loosening; a few were due to folks who through laziness or ignorance dialed in the timing to the retard position and didn't check the advanced mode timing. If the bobweights are worn a bit that's all it takes, since the advance curve stretches with bobweight wear.

This is why i don't check idle timing. I check at full advance, then just do a cursory check of idle.

 

[grizld1]

That's what to do! As long as idle retard doesn't go far enough to create a big off-idle lag or start popping carbies out of the boots on startup, it's fine. BTW, I'm a dunce; I assumed steady throttle state. If the ATU is loose momentum will pitch the trailing edge of the slot against the pin under engine deceleration. Axial slop probably does some dirty work as well.

 

[pamcopete]

This is why i don't check idle timing. I check at full advance, then just do a cursory check of idle.


Well, some exaggeration, sundie....

One of the purposes of the idle timing marks is to allow for a variation in the spread of the timing from idle to max advance. If the timing is less than the minimum mark at idle with it timed to the maximum mark at 3000 RPM, then it's time to replace the advance unit, something that for whatever reason, everybody resists doing.

Running the engine at some idle timing less than the minimum will result in sluggish engine performance at lower RPM's.

If the timing is not within the brackets for idle at idle then the only time it will be in correct time is at 3200 RPM and above. And, don't forget the possibility that the timing could be too far advanced at idle when it is correct at 3000 RPM if the springs are too tight, so it would be too far advanced at all speeds below 3000 RPM, which is also not a good thing. You can have some pretty destructive detonation at lower engine speed as well. In fact, detonation can occur just as you are starting out in first gear if it too far advanced at idle.

So, the timing has to be correct at idle as well as 3000 RPM. You shouldn't just take a cursory look at it at idle.

Additional notes:

Regarding the springs being too tight. If you check the timing at 3000 RPM and set it for the maximum indicated mark, you should also rev the engine up higher as well to ensure that the springs and the bob weights are at their maximum travel. That is the other purpose in checking for correct timing at idle and 3000 RPM, to ensure that the springs are not too tight, which would result in a too far advanced timing for all engine speeds below 3000, and the advance would continue to advance beyond 3000 RPM.

When installing new springs, you should check / set the timing first at idle, and then rev up to 3000 RPM to see if the timing advances. If you checked the timing at 3000 RPM first, and the springs were tight (all new springs are tight), then you would have extreme advance at all engine speeds below 3000 RPM.

Shine the timing light on the advance weights when setting max advance to see if the weights are actually at their maximum extension.

 

[pamcopete]

That's what to do! As long as idle retard doesn't go far enough to create a big off-idle lag or start popping carbies out of the boots on startup, it's fine. BTW, I'm a dunce; I assumed steady throttle state. If the ATU is loose momentum will pitch the trailing edge of the slot against the pin under engine deceleration. Axial slop probably does some dirty work as well.

Well, except for the fact that the points rubbing blocks apply a constant "braking" action on the points cam, and have a tendency to keep the ATU from doing as you describe.

Still can't wrap my little brain around the phenomena, but I think it has something to do with the points rubbing blocks as they slid down the slope of the reverse lobe that closes the points they would have a tendency to move everything forward if the ATU spanner nut was loose, just in time for the rising slope of the reverse lobe to open the points early. I like this theory because the PAMCO doesn't have any points and would not be susceptible to the phenomena!