1978 XS750 high speed problems

[Ken Talbot]

Man!!!!
Started the bike up tonight and I heard the clanking of either the valves or the cam chain.

YIKES! Let's hope for the best...

BTW - don't crank it over again till you remove the cover and check the timing marks. Only turn it by hand, very slowly, carefully...

[IanDMacDonald]

Similar Problems

This is a link to a youtube video of a guy who's bike problem sounds almost exact to mine if it helps any. http://www.youtube.com/watch?v=s1vsv6UTIuQ

[IanDMacDonald]

Okay, my buddy took a look at it this morning and found that all the marks line up. He will do a compression check in a few minutes to confirm that I do not have bent valves. Anyways, what else could be making that racket? Possibly one of the guide arms breaking or coming loose? I looked through the tensioner hole and did not see anything out of place, not that you can see much anyways. Something my buddy pointed out though: When the tensioner is in place and you turn the crankshaft, the tensioner pin does a very quick triple in & out motion, instead of a solid one in & out pass. Does that make sense? I do not remember seeing that before I screwed with the tensioner. Hopefully you guys can lend a hand and diagnose this for me. The youtube link in my above post is almost duplicate to the sound I hear from my engine.

Thanks,

Mac

[IanDMacDonald]

Bad Cam Chain Tensioner?

Fellas-
My buddy did his best to look my bike over as best he could, and did not find anything broken, short of taking the motor completely apart. Is there a possibility that the cam chain tensioner itself could be bad, and if so, how would I know? It is a very simple part, how would I check to see if it does not work properly? Is there any way of knowing if the guides (dampeners) are broken w/o breaking the engine all the way open?

Thanks fellas,

Mac

[DGXSER]

Are you certain this is not just normal valve noise. Not to familiar with the 750/850 line. Only owned one over 20 years ago. But the 11s are a loud valve train engine.

[IanDMacDonald]

No way DGXER! This only came about after I messed with the cam chain tensioner. It sounds as ugly as the youtube video I have posted.

[cywelchjr]

No way DGXER! This only came about after I messed with the cam chain tensioner. It sounds as ugly as the youtube video I have posted.

That sounds like it might be cam chain noise. It's certainly possible that the tensioner is not functioning right and leaving the cam chain too slack.

[rpgoerlich]

On most of the XS750's the Primary chain is the noise culprit... They stretch and start hitting the crankcase on decel and when the carb's aren't synced properly. How many miles on it?

Another would be the starter clutch. The bolts tend to back off and the whole assembly rattles. You can remove the LH crankcase cover to check/fix this. Either lean the bike on it's right or drain the oil. While you have this cover off you can also check the tensioner's as they bolt up to the crankcase right behind the crank starter gear. Check your manual as it shows all this under electric starter gears.

On your limited speed thing, what type of carb's are on it MIC1 or 2's? The MIC1's will have a lift lever for the choke are a pain to sync and the jets are mounted in the bowl. The MIC2's will have the pull out lever(like the 11's). I'd say you found your biggest problem with the diaphragms. When you cleaned them did you pull the emulsion tubes(MIC2)? It is possible but unlikely some one swapped the carb's for 850 Hitachi's.

[IanDMacDonald]

Received the cam chain tensioner today! Wish me luck that it's the problem.

[DGXSER]

Luck!!!! I hope it is the problem!!

[IanDMacDonald]

Yesssirrrr!!!! It's fixed. Now, running the bike with everything fixed, she's lagging top end again. I stuck an airfilter on the end of the crankcase breather hose, will this affect the performance? I will take that off first, and if that does nothing, I will drop the jet sizes down starting at 135's and work my way up. Thank you for all of your help fellas. She is almost ready to be winterized.

Mac

[DGXSER]

Awesome!!! Top end performance is much easier to fix than bad cam chain or guides!!!

[IanDMacDonald]

droppped the jets down to the 135's and the carbs still seem to be chugging, which leads me to believe that the diaphragm in one of the carbs (probably the middle one) has lost vacuum. I know for a fact that it's only running on two cylinders now. Weekend project, here I come. Someone mentioned something about the emulsion tubes. Are those the brass tubes going into the float bowls? Are you supposed to be able to see light through them? How do I clean them out properly?

Thanks,

Mac

[DGXSER]

The emulsion tubes are also sometimes referred to as the needle jet, the needle in the slide goes down into it. To remove them pull the main jet and the slide diaphram assy and it will push/slide out of the diaphram side, push from the float bowl side.

To clean all those small openings, I use a piece of wire out of one of my large wire brushes, and generous amounts of carb cleaner.

[tadtl1]

search for cct catastrophe

Hi,

There is at least one episode of xs 11 catastrophe written up on this site regarding cam chains being off the bottom sprocket. And this happened after the cct was taken off without knowing all the possibilities of pitfall. If I were you, I would read up on that episode to see what the potential problems are. Better to be forewarned than blunder into disaster with bent valves. Just my two cents.

Do what don said.

Secondarily, regarding the emulsion tubes, I had almost the exact symptoms that youre having only my bike wont go over 55, I just cleaned the carbs thouroughly, and found the emulsion tubes to be rather plugged. I dont have the carbs back on the bike yet, but am hoping that was all it was.