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  #1  
Old 12-15-2015, 01:58 PM
tqs335 tqs335 is offline
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knocking noise at RPM above 3000.

Hi all.
My 81 Special has had a fairly loud knocking noise above 3K RPM for some time. It also seems to be affected by engine load. If I'm cruising at say 60 mph in 4th it is loudest but if I accelerate or climb a hill it seems to go away. If I actually knew anything about this engine, I would say it sounded like either a connecting rod big or small end or maybe a crank bearing. the nature of the noise doesn't really sound like the cams or rockers but I suppose that could be it. I did have one mechanic say he thought it was the vacuum advance mechanism. The thing runs great and has no unusual vibration or oil usage or leaks. Any suggestions are welcomed.
Thanks.
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  #2  
Old 12-15-2015, 03:31 PM
motoman motoman is offline
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Vacuum advance hose plugs onto top frontside of #2 carb.
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  #3  
Old 12-15-2015, 05:16 PM
mack mack is offline
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Advance

Not enough info to really diagnos yet. When did it start? Suddenly or has it developed and worsened over time. With what we know to date. The advance seems a likely culprit and motoman has given you the most common cause. Ensure the small vac hose is not connected to the intake boot but rather the #2 carb.
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  #4  
Old 12-15-2015, 06:20 PM
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Read this TSB on the Vacuum advance.......

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Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.

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80 SG Ol' Okie;79 engine & carbs w/pods, 45 pilots, 140 mains, Custom Mac 4 into 2 exhaust, ACCT,XS850 final drive,110/90/19 front tire,TKat fork brace, XS750 140 MPH speedometer, Vetter IV fairing, aftermarket hard bags and trunk, LG high back seat, XJ rear shocks.

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  #5  
Old 12-15-2015, 06:53 PM
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Has the bike always had this noise? Have the carbs been off recently and vacuum advance hose is on the wrong place like Motoman said a few posts ago?
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  #6  
Old 12-15-2015, 09:36 PM
Ranger_xs1100 Ranger_xs1100 is offline
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Knock Knock

Hi tqs335, and welcome to the forum.

I have 2 xs1100LG's (MNS)
One has the same knocking around 3 to 4k rpm.
The other one doesn't!

Disconnecting the vac advance seems to minimize the knocking but also
seems to have a negative affect on overall performance.

This problem has caused me to start knocking my head against the wall!
and that didn't help either!

Good luck tqs335..
Please let me know if you find a resolution...
I'll be watching this thread closely!
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  #7  
Old 12-15-2015, 09:46 PM
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Quote:
Originally Posted by Ranger_xs1100 View Post
Hi tqs335, and welcome to the forum.

I have 2 xs1100LG's (MNS)
One has the same knocking around 3 to 4k rpm.
The other one doesn't!

Disconnecting the vac advance seems to minimize the knocking but also
seems to have a negative affect on overall performance.

This problem has caused me to start knocking my head against the wall!
and that didn't help either!

Good luck tqs335..
Please let me know if you find a resolution...
I'll be watching this thread closely!
If you read the TSB I posted the fix back in the day was a modified vacuum pod. A part that is no longer available. Some were fixed and some weren't that explains why you have one that does, and one that doesn't. Try swapping the vacuum pods and see if the problem moves from one bike to the other.

Also, in the TSB it says that the detonation is NOT damaging to the engine.
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Greg

Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.

― Albert Einstein

80 SG Ol' Okie;79 engine & carbs w/pods, 45 pilots, 140 mains, Custom Mac 4 into 2 exhaust, ACCT,XS850 final drive,110/90/19 front tire,TKat fork brace, XS750 140 MPH speedometer, Vetter IV fairing, aftermarket hard bags and trunk, LG high back seat, XJ rear shocks.

The list changes.

Visit XS11.org too......
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  #8  
Old 12-16-2015, 05:54 AM
Ranger_xs1100 Ranger_xs1100 is offline
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Knock Knock

I replaced that vacuum pod/canister/plunger unit (whatever it is called) with one from a 79 standard, which I believe used the same part number that the tech service bulletin recommends. There seemed to be some improvement but not an elimination of the knocking. I haven't swapped the one between my 2 LG's which will be my next step but it will have to wait til warmer springtime weather now.
Thanks BA80.
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LG, Military theme.
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The link to my past and current projects:

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https://rucksackgrunt.wordpress.com/

...
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  #9  
Old 12-16-2015, 06:41 PM
tqs335 tqs335 is offline
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Thanks gentlemen!

Thank all of you who generously responded to my post.
Im going to dive into the problem based on your advise and information. I really hope to make some progress on this. I love riding my bike but the noise gets a little annoying sometimes.
Thanks again!
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  #10  
Old 12-18-2015, 01:00 AM
mack mack is offline
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Ba80

You have any other service bullets? Are they available on this site?
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79 XS 1100 SF Special
HERMES
original owner
http://i946.photobucket.com/albums/a...ps6932d5df.jpg

81 XS 1100 LH MNS
SPICA
http://i946.photobucket.com/albums/ad305/mack-055/2.jpg

78 XS 11E
IOTA
https://youtu.be/wB5Jfbp6SUc
https://youtu.be/RaI3WYHSuWA



Have recovery trailer and shop if you breakdown in my area.
Frankford, Ont, Canada
613-398-6186
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  #11  
Old 12-18-2015, 07:37 AM
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BA80 BA80 is offline
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No, sorry Mack. I have that one saved from several years ago when I still had the original 3J6 engine. I had the same knock at highway speed.

I believe that one came from TC.
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Greg

Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.

― Albert Einstein

80 SG Ol' Okie;79 engine & carbs w/pods, 45 pilots, 140 mains, Custom Mac 4 into 2 exhaust, ACCT,XS850 final drive,110/90/19 front tire,TKat fork brace, XS750 140 MPH speedometer, Vetter IV fairing, aftermarket hard bags and trunk, LG high back seat, XJ rear shocks.

The list changes.

Visit XS11.org too......
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  #12  
Old 12-18-2015, 08:15 AM
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MPittma100 MPittma100 is offline
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Fuel

Have you tried different octane fuels? JAT
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  #13  
Old 12-21-2015, 02:30 PM
motoman motoman is offline
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Quote:
Originally Posted by BA80 View Post
No, sorry Mack. I have that one saved from several years ago when I still had the original 3J6 engine. I had the same knock at highway speed.

I believe that one came from TC.
Vacuum can has to match year motor and initial static timing assembly. They go hand in hand.
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81H Venturer1100 "The Bentley" (on steroids) 97 Yamaha YZ250(age reducer) 92 Honda ST1100 "Twisty"(touring rocket) Age is relative to the number of seconds counted 'airing' out an 85ft. table-top.
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  #14  
Old 12-21-2015, 02:40 PM
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Bonz Bonz is offline
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tqs335, has it been established you have the correct vacuum cannister line connection to the #2 carb boot?
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2019 Rally Link:
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  #15  
Old 12-22-2015, 10:08 PM
motoman motoman is offline
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Re-read all above, and I suspect the vacuum hose is misplaced on carb boot and not on #2 carb where it's supposed to be. If that is hooked correctly, I highly suspect either seriously out of sync carbs..........or rubber internal diaphram of vacuum advance has a hole in it and is trash.
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