1981 xs 650 Sharp Hollow noise out the left pipe

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Bruce Wayne

XS650 Enthusiast
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My bike has been Backfiring/popping like crazy upon decelleration and at idle. I rebuilt the carbs and cleaned them last week because it sat last winter without stabilizer in the tank. It has straight pipes, pods, and i rejetted both the main jet and pilot jets a couple sizes bigger last year.

Rebuilding the carbs didn't seem to help the situation much at all, It still popped and sounded sputtery, just out the left side.

I set the cam chain adjustment and the valve timing (which was way off) and now the bike isn't popping/backfiring at idle, but the left side still sounds hollow and doesn't have it's usual low thump. It runs much more even now, just doesn't sound good.

I richened the mixture screw and there was no change...

What's next? maybe exhaust leak? Thanks for the help!
 
Have you synched the carbs?

My 79 had a funky idle, backfired out the left cylinder on deceleration and just generally sounded a little off until I synched the carbs.
 
I haven't synched the carbs but I but new plugs in there last week, the old ones looked how they were supposed to though. Do I need to buy a carb synch guage to do synch them?
 
All you need to use is some tubing and a yard stick and some transmission fluid.
Make this:
carbsync.jpg

Warm up your bike, remove vacuum barb covers, attach hose to barbs, start the bike, let idle and use a long standard head screwdriver to, slowly, adjust the sync screw between the carbs.
Simple!
 
Awesome yeah i just found the thread that had that in there. I'll give it a try and go from there.
what else might cause that issue?
 
Most backfires are caused by cold, ambient air meeting the hot exhaust gases. There has been a lot of "back and forth" about the left side running hotter than the right. That being said, some same the left sounds different.
So, your straight pipes. Are they just original ones that have been cut? Original pipes are 2 pieces, an inner and an outer. There could be a hole in one causing an odd sound. You may want to try and figure out if it is an exhaust sound or a vibration sound also. i.e. the hot exhaust is doing it or something is rattling in there. You may also want to check the exhaust pipe gasket on the head.

Check the sync first. This is common when they are out of sync.
 
They are original pipes that have been cut, but they didn't always sound this way. I just went out and messed with the sync screw, I havent built the tool yet but i wanted to see if anything would change if i adjusted it either way. They sound more "in synch" now but still has that loud tinny noise out the left pipe.

It sounds like to me that it's an exhaust sound but ill see if i can get a video of the bike on here in the next couple of hours because that might help diagnose it...
 
Well I'm not sure about the way they are cut. Usually if you chop them it is right next to the foot peg where they begin to go into the muffler.
This is my old bike.


Notice where the pipes are cut. These original pipes, just cut.

I'm really not sure man. It's hard to tell.
 
I'd probably do a compression test just to be sure that the valves were seating ok .Is the noisy exhaust getting hotter than the other pipe? Plugs should tell you whats going on ....worth checking inlet and exhaust for air leaks .
The noise sounds like a badly adjusted valve maybe ?
 
I chopped the pipes just before the muffler, but about 8 or so inches past yours

I'll run a compression test tonight and maybe readjust the valves again just to see if that does anything. Then ill check for air leaks

Thanks for all the help guys i appreciate it...

also i checked the spark plugs, the left side was fouled a little bit. Not wet, just some carbon.
 
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well 100psi is reckoned by some to be near the minimum before considering a rebuild and there is a significant difference between the pots.

How did you do the compression test ?
Both plugs out ?, full open throttle ? 10 revolutions of the crank each pot or until no further pressure build ?

I'd be inclined to try a leak down test to be sure a valve wasn't starting to let by .

hey Bruce hows the weather there right now ? Just realised you're not far from where my partner just returned from a 2x week mustang drive from Medicine Lodge ranch Idaho falls
 
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I just did one plug out, not at full open throttle, and until no more pressure built up.

Its warm today but it's been pretty cold the last couple weeks.
 
needs to be both plugs out and full open throtle for the results to be meaningful or you could get a false reading as Reid says

Your low reading suggest to me a cylinder compression leak somewhere
 
Alright well ill recheck it tomorrow.

I just redid the valves again because i used the wrong clearances the first time. Runs a lot better now but still has that dang ticky noise... I think i'll take it to a local shop tomorrow to see if they can listen to it after i synch the carbs and redo the compression test.

Also, since my bike is an 81 does it not have points? I checked under the caps where the points are supposed to be and they weren't there. since it's newer is the timing all electrical?
 
You have a TCI ignition, no points. Might want to see if there is anyone on here that is close to you before going to a shop. Some shops wouldn't know what to do if you had points.
If it is running ok, then save the money and build a sync tool. Lol, 2 carbs might be 1 too many for some shops.
Oh, and remind the shop that it is a TCI not a CDI. Two different things. That's a good test question for shops too.
 

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