Cam chain jumped a tooth after rebuild?


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Manitoba, Canada
Hi guys,
I have been benefiting immensely from all the tech. posts and discussions on this site.

Over the course of the summer I finally got to working on a 1979 XS650 SF that I had purchased in '05 or '06. It is mostly stock with the exception of some paint.
When I purchased it from my brother in law, he stated that the cam chain was rattling and probably in need of replacement. I went ahead and replaced the chain while cleaning and learning along the way. I replaced the front guide with a unit from Hoos racing (thanks to Gary) while i was in there. New head gasket went in along with copper washers for the top end and a replacement cap nut for the chain adjuster as the old one was missing the o-ring. The front brake reservoir had leaked fluid from the big o-ring at the base of the nylon cup, so that got replaced along with repainting the metal housing and brake lever. I rebuilt/repainted the the front brakes as well as fluid had seeped through the piston seal. The carbs got a cleaning and kit, new air to carb gaskets.

That covers most of my adventures in repair/rebuilding so far.

The issues left to deal with now are #1. After the first reassemble I could not get it to fire. After a couple of days trying everything else I finally listened to Bro in laws' first suggestion of switching the spark wires which solved it (started on the second kick). It did have a lot of lag/rough running in the first bit of throttle twist, but would idle like a dream. Moved the clip on the needle from slot 3 to second from bottom and that seemed to help. I hope it's not too rich.
#2. the timing has gone out after my first ride. When I took off the tensioner cap nut I noticed that it had moved in from flush by about 3-4mm. Is the chain able to jump a tooth with just that amount of slack? I think it was running hot too, judging from the color of the spark plugs (not to mention the smoke coming off the engine after getting it back to the garage).

That's enough for now I think.


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Yep, that's a lot for a first post. If pics 13 & 16 were with a timing light it appears retarded. We're looking for spark at the F mark @ idle, then advancing clockwise with some revs. Have you checked the mechanical advance unit for free movement? The cam timing can be double checked while servicing the advance as neeeded. We'll get to carbs and the stumble in a bit after the timing is sorted.

The cam chain would stretch a bit when new - how long was your ride?
Thanks for the reply.

The first ride wasn't very long, just up and down the street a few times with a couple of high acceleration pulls and then around a couple of blocks to feel turning and test signals.

Unfortunately, I don't have a timing light and the engine wasn't running. Pic 13 shows TDC on the flywheel with pic 14 showing cam slot off to the right at same point.
15 shows points just separating with 16 showing flywheel way off to the right (retarded, correct?).
As for your jumping time question... no, 3-4mm is not enough slack for the chain to jump teeth.
I checked the other end of the cam (timing advance?) and it seems to be free.


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To check cam timing, you're looking at the wrong pin. You need to look at the one on the other side that locks the advance unit onto the cam .....


The pin hole in the cam itself will face straight up or straight down, depending on which cylinder is at TDC on it's compression stroke.
The advance was too free...

After review: jpdevol right on the money about checking advance. Thanks 5twins, I see the error I made there.
The advance unit had come completely loose with the locking pin rolling around inside the cover. I must have forgotten to tighten the notched nut after installing. After putting everything back as it should be, the locking pin is exactly vertical at TDC mark on crank.

A little more history on needle settings. The last guy to work on it had the clip on the #4 slot (2nd from bottom). I was attempting to take things back to original and installed the clip in #3 (center slot). With it not running well, I reverted to last "known to run" configuration.
Super - glad it's fixed!

Do you remember what pilot and main jets are in the carbs?
I might have to take a look when I go to winterize.
If it means anything, I used the recommended kit from xs650 direct which I think is the MikesXS of Canada.
The carbs are B38s with no mods to either intake or exhaust which might have inclined me towards a different size.

Jpdevol, is the mix usually richer in moving the clip down on the needle?
Jpdevol, is the mix usually richer in moving the clip down on the needle?
Yes sir.
You can go out to maybe 3 turns on the fuel crews in 1/4 turn increments - see if that helps.

Once you replace pilot jets, then you'll go back to 2.25 turns to start and find its happy spot
All the pilot jets thread in. You may be remembering the needle jet (?)