The Big Bang (No, I just ended the universe)

[3Phase]

Also, I'll take the alternator cover off and see if anything is loose, bent, etc., or rattling around in there where it shouldn't be.

Kevin, it's really difficult to troubleshoot engine noises without being there but metallic scraping sounds definitely aren't normal and neither are loud metallic bangs unless you are actually at a Metallica concert.

Make sure you disconnect the battery negative wire and disconnect the TCI just to be certain the engine cannot start before you try to turn the crankshaft by hand. Don't forget to plug it back in and reconnect the battery before you try to start the engine.

While you have the alternator cover off, check the crankshaft thrust bearing. Watch out for sharp bits of metal on the rotor and try to move the crankshaft in and out of the engine to see if there's too much end-play. If it moves in and out and clunks the thrust bearing is on its way out.

Still at the alternator side of the engine, turn rotor again and rotate the engine counter-clockwise through several revolutions. Try to listen/feel where the scrape happens if you can. If you hear something scraping or 'clicking' by the lower rear of the clutch cover it could be the transmission end bolt hitting the back of the clutch basket as the clutch turns.

You can sort of check for a worn or binding primary chain by rotating the engine until you feel the slack in the primary chain pull tight. Rotate the engine the other direction until the primary chain tightens up again. If the crankshaft rotates more than 1/8 of a turn or you can feel or hear the chain slapping the chain guide the primary chain is worn. If it's too worn it could bind or jump and skip a tooth but it would have to be really worn for that to happen. It would make noise.


Part of the transmission is always turning with the engine when the clutch is engaged, even when the transmission is in Neutral. When you turn the rear wheel by hand you're 'feeling' for any gears that are binding/hitting each other or the clutch shaft, or if the end bolt on the transmission rotating against the clutch basket.

[Ken Talbot]

Years ago, Syd used to get 'big bang' events that were caused by a broken pick-up coil wire. The wire would go open for a bit, allowing a nice build-up of fuel-air mixture into the header. When the wire re-connected, the two dead cylinders came back to life, and the unburned fuel lit off with spectacular effect.

When this happened along the Port Renfrew road during a group ride at Big Island Deuce, the whole group us us were diving for cover in the ditch!

YMMV...

[3Phase]

Yeah, it may have been a fuel explosion from flooding or bad pickup wires but now it's scraping while it's running. I'm assuming it is running on all four cylinders and not just three but you know how that goes.

The compression check should tell if it's likely to be something easier to get to and fix than a broken valve, cracked piston or a bad bearing.

[DGXSER]

Don't be fooled!!

Either way centerstand or side stand the fuel can and will at times go into the cylinders and the crankcase. BTDT, it aint no fun.

[Larrym]

Prima Non Nocere: CCT

Regardless of whether the Bang event was due to carbs or an ignition problem, a backfire large enough and at the incorrect moment has a predictable result.

Ordinarily all mechanical "pulses" from the individual serve to turn the crankshaft CW. This Drives the valve train by keeping tension on the rear side of the valve chain. (Slack is kept to the forward side of the chain where the CCT can deal with it...)

Under a backfire/misfire situation there is an abrupt change in the crankshaft speed as one or more errant cylinders may try to stop the crankshaft or even try to drive it backwards. This can and does X-fer the tension/loading from the back side of the valve chain to the front side if only for the duration of the "big bang" event.

CCT are historically the "weak link" in the chain and I'd predict that the "event" caused enough force to overcome the CCT "adjustment". It's only a bolt head properly torqued against a sliding rod after all.

Sadly, once the CCT properly adjusted has the "irresistible" force applied and the slider overcomes the resistance of the adjuster.....it may/may not have deformed the slider rod, weakened/broken the adjuster threads, or otherwise made the CCT unreliable IMO.

My Suggestion (along with finding/correcting whatever caused the misfire of course...):

Pull the CCT off the bike and disassemble and examine it thoroughly. If it is less than perfect then replace it. Period. (Your choice of the newer Auto-adjusting CCT or just the same OEM style.)

WARNING: Read and follow ALL precautions regarding R+R'ing the CCT. Some folks have had the cam chain jump teeth during this procedure. Safest/most reliable procedure thus far includes removing valve cover and manually keeping tension on the cam chain during this removal/replacement.

Lot's of material already here on this CCT "feature".

HTH

[3Phase]

DO NOT try to start or run the engine without the alternator cover in place!

Kevin,

I'm an idiot, please forgive me! It's probably way too late by now but DO NOT try to start or run the engine with the alternator cover off!

There is a small metal plug in the main oil galley that's held in place by the bottom of the alternator cover. Turning the engine with the starter motor (sometimes even by hand!) will build enough oil pressure to pop that plug out of the case and shoot oil all over everything. It's really not good if the engine starts and pops the plug: instant loss of oil and pressure.

[Post and PM]

[ManagerMike]

Added thoughts.... if this bang occurred within the exhaust, the timing of the event could have caught one of the pistons in the midst of an exhaust stroke, shocking the crankshaft and therefore the cam chain, and therefore the cam chain tensioner. Same train of thought that larrym is following.
Just maybe, but maybe the noises are from a cam chain flopping about due to a tensioner forced out of adjustment. Not out enough to allow the chain to jump (at idle) but enough to cause 'scraping' noises as it makes contact with other internals.... Just a thought. something else to investigate. If was still running as described, (ildling normally but with a new noise) then it's hard to imagine many scenarios that result in major engine damage, a few, but not many.

[79XS11F]

In my opinion you heard an explosion within the exhaust system. It's pretty much the only thing that could produce the very loud bang you describe and still allow the engine to run. The noise may have been even louder then one might expect if parts within the exhaust system became dislodged and or repositioned by the ignition of the fuel in the exhaust system.
Rob

[XS1100_OEM4ME]

Bang!!

Hey guys, I've been a lurker since joining this very informative site and now I come to you requesting your expertise.

First, before the issue, i'll give a few details about the motorcycle. It is a 1979 Yamaha XS1100 Special. I put on a set of MAC 4-2 Turnouts on it as well as had the carbs cleaned and the bike tuned in the spring of this year. Have had next to zero issues since having that work done. It has around 23,000 miles.

This evening, I was going to go ride for a while...that was until I started my bike. It was a little sluggish to start (which is usual, just have to give it a bit of gas). Immediately upon starting, it produced the aforementioned "big bang" yet still held an idle. From what I could tell in the garage, it was not a backfire. The sound did not exit the exhaust. The RPM's went up like usual at full enrichment so I pushed the choke in to middle position, When the engine was little quieter, I heard what sounded like a metal on metal scraping noise. It wasn't grinding as much as scraping. However, it was still idling normally. I immediately killed the engine. My roommate, who crapped himself inside at the bang, came outside to see what the hell happened. He suggested it could have been a blown or cracked connecting rod on one of the pistons assuming one of the cylinders filled with gas. We removed the timing the cover and manually turned the engine to see if it would produce the same noise and if the cylinders went through full stroke. They appeared to function normally I however do not possess the tools necessary to perform a compression test.

I have no idea where to go from here. Any suggestions, tips, solutions, whatever will be greatly appreciated.

Thanks in advance!

Take off that POS exaust, put on the stock exaust, tune/syc carbs

[3Phase]

I dunno, Rob, there are several mechanical things that could go, "BANG!" and still allow the engine run and seem like nothing was wrong when it's on the centerstand and I've encounted a couple of them with Columbo. The bike wouldn't actually work, at least not for long, but it would start and run until you tried to ride it and then it's, "Don't look, Etheeeellll!" "BANG!" "Bang!" "Bang!" "BANG!" "Too late ...."

If it was a fuel explosion and not a hard mechanical break like the OP thought at first then Larry and Mike probably got it pegged with the cam chain tensioner getting slammed back into its housing; loose cam chain flinging around, hitting the valve cover and ready to jump a tooth or several.

[Larrym]

Bad Dream?

I dunno, Rob, there are several mechanical things that could go, "BANG!" and still allow the engine run and seem like nothing was wrong when it's on the centerstand and I've encounted a couple of them with Columbo. The bike wouldn't actually work, at least not for long, but it would start and run until you tried to ride it and then it's, "Don't look, Etheeeellll!" "BANG!" "Bang!" "Bang!" "BANG!" "Too late ...."

Huh??!!

I'm having a real hard time picturing Columbo EVER doing that. Spitting and popping like Chitty Chitty Bang Bang? No...Can't seem to imagine that your bike has ever run less than perfect.

Maybe cause we did that ride together and I've watched (re-watched) those videos you made. Nothing but flawless performances on the flats, the sweepers, the mountain ups/downs.....

Surely you must be remembering a bad dream you had.

Something like one where after kickstarting your bike it idles like:

"Potato...Potato...Potato...Potato"

[Kevin11]

Gents...all of your input is extremely handy...thank you!

3Phase, no worries, I didn't try to start it with the alternator cover off

OEM4ME, that'd be nice...if I had one. Trust me, the Mac 4-2 system was a huge upgrade from what was on there when the bike was purchased. The system on it when purchased was a 4-2, but paired cyls 1 and 3 on the left side, and cyls 2 and 4 on the right. Made it tough for the PO to realize cyls 3 and 4 weren't firing The baffles were long gone, and it was rusted terribly. But I digress...

[Kevin11]

Update for the moment, haven't been on here in a couple days, so I apologize guys:
-Checked for scraping/weird noises moving the rear wheel with engine off through all gears, no noticable problems
-No loose, visibly bent/deformed bits under timing or alternator covers
-Oil was drained, and was embarassingly low. Didn't smell overtly like fuel, and the color wasn't that bad either.
-Changed oil; went with Rotella, full dino. Noticed that the last person who changed the oil decided that two crush washers were necessary on the drain bolt no wonder I was hemmoraging oil there...
-Adjusted the CCT per instructions in the Clymer manual; it's not all the way in nor out, so there's adjustability either way
-Brushed off the spark plugs & reinstalled (I'm going to get a fresh set when I sych/tune)

Wrapped it all up and tried to start it again. We both had earplugs in, just in case. She fired up, no huge explosion, idled normally. Let it warm up and I took it around the block, no problems. I didn't hear the metallic whirring like the day of the "bang". Jeremy seems to think the valvetrain "chatter" is louder than before, though I don't think so (he always has his helmet on before starting the bike).

So, as of right now, it's running, it drives, and leaks way less oil. It still weeps a couple drops out the tip of the CCT, but it always did that.

Still to do list:
-Compression test
-Take valve cover off: check for damage, check cam chain, check valve clearances, check CCT
-Synch and tune

Again, I really appreciate the help!

[79XS11F]

I dunno, Rob, there are several mechanical things that could go, "BANG!" and still allow the engine run and seem like nothing was wrong when it's on the centerstand and I've encounted a couple of them with Columbo. The bike wouldn't actually work, at least not for long, but it would start and run until you tried to ride it and then it's, "Don't look, Etheeeellll!" "BANG!" "Bang!" "Bang!" "BANG!" "Too late ...."

If it was a fuel explosion and not a hard mechanical break like the OP thought at first then Larry and Mike probably got it pegged with the cam chain tensioner getting slammed back into its housing; loose cam chain flinging around, hitting the valve cover and ready to jump a tooth or several.

Well they are describing this BANG as being very loud. If the bike was on main stand and in neutral everything behind the trany would not be involved. If the issue was with the trany the engine would ether stop dead or cause the rear wheel to rotate. This would be easily seen. So, going forward we have the primary chain, crank, rods and their bearings, pistons and top end. It's highly unlikely the the primary drive chain broke and the same goes for the cam chain. Cam chain breaking would have made a mess of the valves and the bike would not idle "normally". I also don't think the engine would idle "normally" if a rod was broken ether. A bearing might have seized and then spun but that would not likely result in a big bang. Not at idle anyway. It will be interesting to find out just what caused this big bang. My bike in the early stages of tuning has produced some very loud bangs out through the exhaust but even then it was only when the ignition times was off by 180 degrees.
Rob

[3Phase]

"Potato...Potato...Potato...Potato"

NNNNnnnnOOOOOOOOOoooooooo!!1 1111!1 ONE!

3Phase, no worries, I didn't try to start it with the alternator cover off

Whew! Swing and a miss! At least it's running now be careful!

Well they are describing this BANG as being very loud. If the bike was on main stand and in neutral everything behind the trany would not be involved. If the issue was with the trany the engine would ether stop dead or cause the rear wheel to rotate. This would be easily seen.

Rob, that pesky bolt on the end of the transmission drive shaft can come loose and hit the back of the clutch basket. There is a very loud "BANG!" when it backs out far enough to hit the back of the clutch basket and there is a scraping noise until the clutch basket wears down the head of the bolt. The bolt eventually backs out far enough to fall into the clutch cover then down to the oil pan if it doesn't get bounced into the clutch/oil pump gears or clutch/primary gears and force the bevel springs on the primary shaft to go to work to unload the crankshaft.

The clutch always turns the drive side of transmission whenever the engine is running. If any of the gears are too close together on the drive or the driven side of the transmission because of a loose/missing end bolt, wear, a bent shift fork or a even a bad trans repair <hangs head in shame > then the dogs and slots can hit.

They're more likely to hit when the bike is on the sidestand and everything is pulled to the left. When the bike is on the centerstand the gears can still bump into each but they'll usually move away, Either way, they make a soft, almost inaudible, knock that sounds like a bad rod bearing. You can 'feel' the gears or the bolt hitting the back of the clutch basket when you turn the rear tire but there's nothing you can see externally. Eventually the gears will hit and engage far enough to cause a loud "BANG!" before they pop away from each other. If it gets too bad the transmission will try to go into two gears at the same time and make an even louder "BANG!" when the gears pop away from each other and the engine recovers..

This whole time the rear wheel doesn't move. It may jump for a split second when the transmission gears collide but so does the entire bike. No one (okay, no one but me) looks at the back tire when there's a loud noise in the engine.