metalic rattle

[BA80]

The vacuum advance isn't supposed to go to the boot, it goes to the nipple on the #2 carb.

[jjz28]

Yeah, that makes a huge difference too. I put it where it goes and it runs 100% better, powerful bikes for sure. When i pulled out i accidentally burned out and the rearend slid sideways, i didnt know they had that much power.

[PorterTheCelt]

So this just started happening with my bike. I am at about 43.5k miles. Is a cam chain replacement doable with the engine still in the bike? I want to ride this year if at all possible, and I would like to minimize the things I need to disconnect, take apart, for the time being.

Sad, but grateful that this happened now instead of in the middle of the long trips I was looking at doing this summer.

[motoman]

With all the previous suggestions, my bet is that the rattle from what you stated on your first post is the vacuum advance can under the left side ignition cover. For a quick test of that, remove the vacuum hose from the carb port on second carb from the left.

[motoman]

BTW, if my previous suggestion eliminates the rattle, don't just assume the vacuum advance can is faulty. A lack of carb sync or being way off and not done using a manometer can and will cause a weak vacuum in this case on cyl. #2 causing the vacuum can to be erratic dancing shaft up and down. Also, if having to replace vacuum can, the '78-79' models vacuum cans were set different internally based on cam lobe duration and overlap timing. '80-81' vacuum cans were internally set differently for vacuum draw.....how much and when. 82' and later XJ's vacuum cans were set differently again. Any of the year model vacuum cans will work, just vacuum advance of timing will be a bit different depending on year model used. The vacuum advance cans are pretty much non- repairable. If the shaft is dancing up and down with everything else mentioned set correctly, the internal diaphrams will normally have developed a pin-hole in them. In which case, replace the vacuum advance assembly with a known good one. Is JAT as I've BTDT several yrs. back.

[bikerphil]

Is a cam chain replacement doable with the engine still in the bike?

Yes it is, but with 43K, you have plenty of life left. I have over 100K on my chain with the auto cam tensioner mod and the camshaft timing dots/TDC marks still line up perfectly.

[motoman]

The vacuum advance isn't supposed to go to the boot, it goes to the nipple on the #2 carb.

Also, as an FYI, the #2 carb brass port has a built-in restrictor in order to eliminate erratic vacuum advance cam shaftassembly from pulsating.....and being noisy with the shaft dancing with hose hooked to a fully unrestricted port such as the carb boot port. The carb port where hose attaches is also ported vacuum as it is supposed to be, not manifold vacuum like the carb boots have.

[motoman]

did anyone figure this out cause mine did this when i hook the vacuum advance to the 1st carb boot nipple. When i take off the advance it stops.

Yes.......the carb boot vacuum is unrestricted MANIFOLD vacuum. The #2 carb brass vacuum port has a built in restrictor to eliminate the erratic pulsing of vacuum advance diaphragm and shaft assembly. #2 carb is also the correct PORTED vacuum inlet.

[motoman]

The '81 still has the vacuum, it was the XJ that eliminated it finally...

Thanks Steve! Haven't been around an XJ up close to notice if it was eliminated or not.

[motoman]

BTW, if my previous suggestion eliminates the rattle, don't just assume the vacuum advance can is faulty. A lack of carb sync or being way off and not done using a manometer can and will cause a weak vacuum in this case on cyl. #2 causing the vacuum can to be erratic dancing shaft up and down. Also, if having to replace vacuum can, the '78-79' models vacuum cans were set different internally based on cam lobe duration and overlap timing. '80-81' vacuum cans were internally set differently for vacuum draw.....how much and when. 82' and later XJ's vacuum cans were set differently again. Any of the year model vacuum cans will work, just vacuum advance of timing will be a bit different depending on year model used. The vacuum advance cans are pretty much non- repairable. If the shaft is dancing up and down with everything else mentioned set correctly, the internal diaphrams will normally have developed a pin-hole in them. In which case, replace the vacuum advance assembly with a known good one. Is JAT as I've BTDT several yrs. back.

Correction......in another thread, Crazy Steve stated the vacuum advance assembly was eliminated on the XJ model. Assuming that all was done electronically using the ported vacuum source.