Vacuum Advance and detonation

[GtrMn]

Hey T.C.! This time I wasn't talking about the XJ...haha

[scott0757]

I have an '80 XS SG and experience this knock often. It's most noticable when at steady low-load crusing about 3-4K RPMs. I don't like the noise and shift gears up or down so get out of that RPM range at the speed I am stuck at (unless, of course, I can crank it well past those RPMs!).

Nice to know it's nothing to worry about, unless of course it really is a rod knock (gulp).

[Axel]

Huh

How do you change the timing ?

[webbcraft2150]

Hot dang!!! Whadda know. When I first bought my 80g,(with full fairing) it had that noise, suspected i had been ripped. Took it to two different Yahama dealers who said not to worry about about it, so I haven't. Noise is still there at 63k, right where they said it would be. I either ride out of it or turn the radio up.

[GtrMn]

Anyone have any ideas on why it only effected the '80 and '81 models?

[scott0757]

Given my luck, it's because that's one of the years I have.....
Murphy knows where I live and how to find me...

[TopCatGr58]

Originally posted by GtrMn
Anyone have any ideas on why it only effected the '80 and '81 models?

Well, Yamaha was having to do some tweaking for EPA standards, and those years are when they went to the lesser compression piston/head/valves combo!

In 80 they used those funky break off bolts to lock the timing plate in place. In 81 they changed the TCI, removed the mechanical centrifugal advance parts, so these may have something to do with it?
T.C.

[GtrMn]

In 80 they used those funky break off bolts to lock the timing plate in place.

Figured it was EPA related...seemed like they changed quite a bit of things...including jet sizes etc.

How the heck do I get those break off bolts off if I want to retard the timing a couple degrees? Thanks in advance!

[Ken Talbot]

Originally posted by GtrMn
How the heck do I get those break off bolts off if I want to retard the timing a couple degrees? Thanks in advance!

Break them off, drill the broken stub, and use an e-z-out. It was meant to be difficult to discourage people from doing it.

[mro]

Break them off, drill the broken stub, and use an e-z-out. It was meant to be difficult to discourage people from doing it .

Don't remember it being that hard.
Don't think I needed an eazy-out


mro

[Ken Talbot]

Do they break off and leave enough of a stub that you can just grab with pliers? That would certainly be easier...

[GtrMn]

That would certainly be easier...

Sure would...wondering also if just drilling and using an easy-out would work too. A bit worried about breaking the head off and the possibility of bending something.

[TopCatGr58]

Technique from Clymer's Addendum!

1. Remove the center allen bolt and remove the timing plate.
2. Rotate the engine until the projection on the Cent. Adv. mechanism aligns with the slot on the PU coil plate. This is necessary for removal of the backing plate.
3. Flatten the remaining portion of the sheared off bolt with a drift or flat punch.
4. Use a center punch and hammer and make a deep enough impression in each bolt for a drill guide.
5. Drill a hole appx. 0.39in/10mm deep, using a 3mm drill bit, in each bolt.
6. Tap a screw extrator into the hole with a hammer and unscrew the bolt, repeat for other bolt.

The rest is about removing the cent. adv. unit and such! Then it says to reinstall using the same type of bolts!!!! Bwaaa Haa Haa!
T.C.

[tarzan]

I have this exact problem.My bike runs great but that detonation sounds bad between 3k-4k under light load conditions.I actually thought it was a rod at first.Now it makes sense.The SG has lower compression than the earlier years.
Anyway did you guys with this problem adjust your timing and eraticate the knock?Because if this really fixes it I,ll drill them damn screws out tomorrow and adjust mine.

[trbig]

If you have some decent carbon deposits on your pistons and combustuion chamber, these could be causing the knocks too.... hot spots of carbon exploding your air/fuel mix before it's time.


Tod