CLACK at 2k RPMs

[treybrad]

Symptoms:
1. Idle has dropped by about 800rpm
2. Seems to be more backfiring on engine braking (could be the colder weather?)
3. If SLOWLY rev'ed to 2k rpms, there is a distict "CLACK", metal on metal sound that comes from the top of the engine.

Aside from that it still seems to run just fine. Accelerates smoothly and strong, my butt dyno can't tell that it's lost any power.

Situation:
Changed the oil today along w/ some other small things like bar ends and all seemed well. Bike has been running GREAT with the cooler weather, no problems at all. So I go meet a co-worker and we're riding 2 up on the way to a meeting. I know i'm getting low on gas but not too worried about it. We pull into the parking lot and the bike acts a little strange and actually backfires pretty hard. I thought I could feel air puff on my leg and she definitely heard and felt it. It was idling low and choppy but we were there. So we parked and I attributed it to being low on gas.

So we get done w/ the meeting and I have to switch to RES to start it, further confirming to myself that was all that was wrong... (being low on gas), but as soon as we start to take off I hear, and feel, a CLACK sound from the top end. I think... crap. We go get gas and I check the oil which was still clean and full. Still does the CLACK noise if you rev'ed it really slowly at around 2k, but otherwise seemed to be running fine. I noticed it seemed like there was a lot more backfiring, but it was still a little cold, it was cold outside, and I don't ride 2 up much, so maybe that was it.

My thoughts:
Valves? Why would it only do it at 2k? Everything is in constant time, seems like if a valve was hitting a piston it would do it at all RPMs...

Can a bikefire like that bend a valve? That could explain the poor idle or backfiring I guess. Exhaust or intake? Both?

You can FEEL the clack sometimes, like the engine has lost a little momentum.. BAD. Needless to say I'm not riding it anymore until I figure out what the heck is going on.

I "checked" my valves when I got the bike... I didn't have a feeler guage small enough to fit, so they were less than .004", but the shims would spin in the buckets, so there was some clearance.. All of them were the same, which makes me think the prev. owner had them done since they all seemed to be the same tightness.

What do you guys think? I guess I need to take off the valve cover and take a look at my valves or something... I just would love to have some suggestions on where to start and what to look for. Bike has 27,000mi, only the last 1,500mi or so have been mine, but she's been running great since I got the carbs taken care of.

So yeah, help PLEASE. Just need some guidance on what to look at/for.

trey

[Diderich]

Mine does the exact same thing at 2k, and I discovered that it was caused by a backfire through the carb.  In my case, it was a vacuum leak plus some other factor, probably lean.  Once i fixed the vacuum leak, it only backfires if I very delibrately keep it at 2k.  I'm sure it could be a million other things though.  You can start there until someone smarter chimes in.

[Kerry]

Yeah, pop the valve cover.  Check the valve clearances, as well as any end-to-end freeplay in the camshafts (which is what I think is causing the noise).

[scratch]

Yeah, I concur with Kerry. The fresh oil may have changed the clearance. (not really, but it doesn't hurt to check)

[GRU]

yeah mine does that too but it's not bugging me since it does it once a day

[treybrad]

Wow, you guys really aren't that concerned with it?? Seems to me like it's a fairly big deal... maybe not. Have you checked your valve clearances and stuff? Anyway, I'll have a chance to look at mine tomorrow and let you know how it turns out.

trey

[90GS500rehab]

Mine was loose tension on the cam chain. I had to manually set the adjuster but the sound is gone now.

[Kerry]

Mine was loose tension on the cam chain. I had to manually set the adjuster but the sound is gone now.

I think 90GS500rehab may be onto something here.  When I had a '96 with (what I suspect was) camshaft endplay I described the sound as a "soft clunk".  I kept thinking it was a loose camchain and wishing I had a better understanding of how to tighten it.

But treybrad is describing a (loud?) metal-on-metal CLACK.  That could be slack in the camchain, causing it to whip upwards and hit the valve cover.  It makes sense that the sound would disappear when you rev the engine and put the camchain under tension.

If it IS a loose chain then you should take care of it right away, before it jumps a tooth on one (or both) of the camshaft sprockets and results in bent valves.    Pop the valve cover and push down on the camchain between the two sprockets.  There should be very little slack.  If the chain deflects a suspicious amount, let's see if we can get 90GS500rehab to post some "How To" steps for tightening the tensioner.  :mrgreen:

While you're in there, go ahead and check for end-to-end play in the camshafts.

[Diderich]

I thought the cam chain tensioner was automatic?  Now if it's gone bad that's another thing...It seems I've seen posts to that affect.

Make sure you post what you find.  Because mine does the exact same thing at 2k.  I checked my valves 1000 miles ago and they were plenty in spec, I wouldn't suspect that was the problem.  Mine also sounds like a clank, and is accompanied by a momentary loss in RPMs.  The reason I said backfire, is because I was using clear tubing to synch the carbs and whenever it clanked, the tube on the vacuum port lit up like a neon flashbulb, and the tube was scorched when I took it off.

[Kerry]

The cam chain tensioner IS automatic, in that it's spring-loaded and supposedly adjusts to take up any change in chain slack.

But like you said, the spring can break or eventually reach its service limit.  Once upon a time someone posted about re-tightening the spring (but I don't remember seeing any details).

Manjul created a FAQ thread with Manual Cam Chain Tensioner Info, if you don't mind shelling out some money for the privilege of tweaking the tension yourself with a wrench.

[treybrad]

Well I just went and fired it up for the first time since it happened and I'll be damned if I can get it to make the noise. I was hoping to snap a short movie to let you guys hear the sound. Sound or no sound I'm going to take off the valve cover and look for anything out of the ordinary tomorrow. I'll take a look for all the stuff you guys have mentioned and keep you posted.

trey

[90GS500rehab]

The spring in mine is apparently shot. What I did was back the tensioner out of the engine block by a couple threads and SLOWLY tighten it back up. You have to be careful not to create too much tension on the chain. Now mine is leaking oil through the tensioner so I will likely be ordering another one.

Mine was loose tension on the cam chain. I had to manually set the adjuster but the sound is gone now.

I think 90GS500rehab may be onto something here.  When I had a '96 with (what I suspect was) camshaft endplay I described the sound as a "soft clunk".  I kept thinking it was a loose camchain and wishing I had a better understanding of how to tighten it.

But treybrad is describing a (loud?) metal-on-metal CLACK.  That could be slack in the camchain, causing it to whip upwards and hit the valve cover.  It makes sense that the sound would disappear when you rev the engine and put the camchain under tension.

If it IS a loose chain then you should take care of it right away, before it jumps a tooth on one (or both) of the camshaft sprockets and results in bent valves.    Pop the valve cover and push down on the camchain between the two sprockets.  There should be very little slack.  If the chain deflects a suspicious amount, let's see if we can get 90GS500rehab to post some "How To" steps for tightening the tensioner.  :mrgreen:

While you're in there, go ahead and check for end-to-end play in the camshafts.

[treybrad]

Well, here's what I found out today:

Cam chain seems tight enough. If I press on it pretty hard, actually, really hard, I can maybe get it to deflect 3/8 of an inch. That's pressing hard, and it's not enough to be jumping teeth, especially under normal circumstances I don't think.

Side to side play on the cams was minimal. Maybe 1mm if that. So no problem there.

Valve clearances were about the same. All were tighter than my .038mm feeler but spun freely in their buckets. Actually, the right intake valve was at the loose end of the spectrum, right at .08mm. But, in spec either way. I'm waiting to win an auction tomorrow for some smaller feelers just to know where the others sit.

Only thing I could find actually WRONG was a missing vacuum cap on the carburetor. The left side one must've popped off, maybe the backfire? That was the idle problem. I could plug the port w/ my finger and it'd idle fine, let off and it'd die. So I put a new cap on and that's taken care of.

Ran it for a while, not completely up to operating temp, but I couldn't get the clank noise to happen. It was doing it cold last time, so I don't know why that'd be different. Oil is full and clean, nothing else under the valve cover seems out of whack, so I'm going to write it off as an anomaly of the evening.

Unless you guys say my cam chain is too loose, even though it seems tight, I don't see any problems. Thanks a bunch for all the suggestions on what to look at, I definitely appreciate the help   . I can't even imagine how much money this board saves me on a weekly basis...

Thanks again guys!  

trey

[Kerry]

Sounds like a good day of wrenching!     Good work on the vacuum cap.

The camchain tension sounds OK ... to me, anyway.  But 1mm of play on the camshafts could easily result in some (harmless) noise.  Remember ... the 4 available camshaft adjustment shim sizes only differ by .1mm, with a total range of 1.0 - 1.3mm.  On my '99 I've never detected ANY play, and on my old '96 I could only detect some on the intake shaft.  I can't remember if I ever tried to measure it, but it wasn't much at all.

Let us know if you hear something again.  But if you do, and it seems like camshaft play would explain it ... relax!