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

[jkoeringMN]

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!

[DGXSER]

If it turns over normally by hand several rotations with no noises, then check the cam chain tensioner to see that is is seated correctly. If that checks out, then pull all of the plugs and look at them to see what they look like, white, black, wet or dry. With the plugs out, and the engine cool, put your thumb down hard over the plug holes one by one while cranking the engine and see if it does not blow your thumb off the hole with some force. (A makeshift/poormans compression test.) They should all feel about the same and push your thumb off the hole.

[dbeardslee]

Backfiring is sometimes caused by out of sync carbs, and if they're far enough out of sync they can make the bike sound like it's going to fall apart. It can also cause them to fart out the carbs, but that's not usually very loud. How do you know it didn't come out of the exhaust pipe? Could you tell where the sound did come from?

[CatatonicBug]

I'd check for 2 things - carbs needing "proper" tuning, and a loose exhaust. It's amazing what kind of noise can be created when the pipes are not held tight. As mentioned, check your plugs and see what color they are. That will tell you if you're running rich/lean, and could be causing a backfire.

[Kevin11]

Hi all-
I'm the aforementioned roommate...and yes, I did crap my pants.

The exhaust is torqued correctly with the appropriate gaskets, so I highly doubt a leak at the headers. This was a massive bang, almost as if a shotgun were fired.

The bike has been leaking oil progressively worse in the last week or two, though the oil level was OK prior to last evening's bang. Most of the leaking is coming from two spots: the oil drain plug and the cam chain tensioner.

As Jeremy mentioned, we pulled the plugs after the bang. All four are sooty, but consistent across all four.

Because of the metallic whirring noise, I fear starting the bike again before we run a few more checks. Thanks for your help guys!

[ManagerMike]

Was it on the centerstand? If on the centerstand, and the carbs leak, they will leak back into the airbox. On the sidestand they can leak into the crank case. Either way, leaked fuel could explain the bang on startup. Check the air filter for fuel and check the crankcase for fuel odor and or being overfilled.

[jkoeringMN]

Backfiring is sometimes caused by out of sync carbs, and if they're far enough out of sync they can make the bike sound like it's going to fall apart. It can also cause them to fart out the carbs, but that's not usually very loud. How do you know it didn't come out of the exhaust pipe? Could you tell where the sound did come from?

I'm not 100% positive that it didn't come out of the exhaust, but the way the bike was positioned in the garage and where I was in relation to the bike i.e right next to it - made it appear to come from somewhere up front. If it was a backfire then thats not a huge deal, it does that sometimes although not that insanely loud. Regardless, there is still the deal with the metal scraping noise.

We'll definetly look into the suggestions thus far! Thanks guys

Was it on the centerstand? If on the centerstand, and the carbs leak, they will leak back into the airbox. On the sidestand they can leak into the crank case. Either way, leaked fuel could explain the bang on startup. Check the air filter for fuel and check the crankcase for fuel odor and or being overfilled.

It was on the centerstand. When im not riding, I always store it on the centerstand.

[ManagerMike]

Check the air filter then, if my hunch is right you'll likely find a mutilated filter element and the 'scraping' you heard was the normal sound of the air intake no longer muffled by an air filter. Then all you have to do is solve the carb leaking issue...
Good luck, and welcome to the club!

[natemoen]

I am fighting some issues with my bike and it does backfire both through the carbs and out the exhaust. It even feels like something is slapping me in the leg when I am riding it and it backfires through the carbs. Also I know it comes through the carbs because there is some melted material on the air filter. Like was mentioned check the filter and see what it looks like!

[Kevin11]

Was it on the centerstand? If on the centerstand, and the carbs leak, they will leak back into the airbox. On the sidestand they can leak into the crank case. Either way, leaked fuel could explain the bang on startup. Check the air filter for fuel and check the crankcase for fuel odor and or being overfilled.

Thanks Mike I can picture what you're saying...so leaky petcocks plus leaking float valves = gas overfilling the float bowls, and draining back into the airbox. We push the ignition, and somehow the spark makes its way to the airbox, igniting any fuel/vapor trapped there, hence the explosion. Jeremy, if that's the case, you're lucky the airbox didn't explode and send plastic shrapnel into your junk

We'll pull the airbox and try to get some pics!

[jkoeringMN]

Thanks Mike I can picture what you're saying...so leaky petcocks plus leaking float valves = gas overfilling the float bowls, and draining back into the airbox. We push the ignition, and somehow the spark makes its way to the airbox, igniting any fuel/vapor trapped there, hence the explosion. Jeremy, if that's the case, you're lucky the airbox didn't explode and send plastic shrapnel into your junk

We'll pull the airbox and try to get some pics!

You're telling me...imagine the clean-up you'd have to take care of.

[prometheus578]

Startin' to agree with Manager Mike about the leakin' carbs.

All plugs were black and fouled means a fuel problem... too much fuel.

Ok, let's assume you have a petcock/fuel octopus issue...
Carbs are flooding when sitting (and also when riding, but sitting is the issue right now). This extra fuel, due to the bike on the center stand, is pooling in the cylinders.
You crank and start the bike... One of the cylinders, being on the exhaust stoke, then pumps a bit of liquid fuel out into the exhaust system. On it's next cycle, it ignites the remaining fuel in the cylinder, and as there's still too much fuel in there that doesn't completely burn... on the exhaust stoke it pumps this remaining, still burning fuel into the header which then ignites the rest of the fuel trapped in there and... boom.

Black, fouled plugs means fuel problem. A fouled plug that fires intermittently... means a cylinder that is pumping raw fuel into the exhaust. This plug could eventually spark... igniting a very dense fuel mixture which then gets expelled (still burning) into the fuel laden exhaust system....BOOM.

Unplug the fuel lines at the carb and see if fuel flows constantly... bad petcock/octopus.

Also, recheck your oil level at the little window with the bike on the center stand. The window shouldn't be filled... you should see a line of separation... Oil at bottom, empty space at top.
If you see no separation in the window, chances are that there's too much oil in there... you're looking through an oil/fuel mixture.
This mixture is then a very thin, fuel diluted oil... which will be thin enough to leak from a lot of places that it normally wouldn't.
Hell... maybe thin enough to provided no cushioning lubrication at all... hence the scraping noises you hear.

(Just a quick, early morning, pre-coffee assessment)

[Kevin11]

All plugs were black and fouled means a fuel problem... too much fuel.

You crank and start the bike... One of the cylinders, being on the exhaust stoke, then pumps a bit of liquid fuel out into the exhaust system. On it's next cycle, it ignites the remaining fuel in the cylinder, and as there's still too much fuel in there that doesn't completely burn... on the exhaust stoke it pumps this remaining, still burning fuel into the header which then ignites the rest of the fuel trapped in there and... boom.

Black, fouled plugs means fuel problem. A fouled plug that fires intermittently... means a cylinder that is pumping raw fuel into the exhaust. This plug could eventually spark... igniting a very dense fuel mixture which then gets expelled (still burning) into the fuel laden exhaust system....BOOM.

Hell... maybe thin enough to provided no cushioning lubrication at all... hence the scraping noises you hear.

Here's my tenative checklist of stuff to do...
1) Compression test, dry and wet
2) Change oil and examine (fuel contamination, etc.)
3) Check cam chain tensioner for adjustment/damage
4) Check petcock function (fyi, when parked, the petcocks are always in the "off" position)
5) Synch carbs & adjust idle mixture down a tad (they're all too rich)

Before the next attempt to fire this thing up again, what can I do to make sure it doesn't blow up...again?

Thanks again, guys!

[3Phase]

Kevin, be careful. I'm wondering if something jammed the primary shaft and the relief springs kicked and/or the oil pump drive gear came loose.

It could also be the bolt behind the clutch basket on the transmission drive shaft came loose and hit or is hitting the clutch basket. The bang/shock could have warped the alternator rotor enough to make it hit the stator or the alternator cover on the right side of the engine.

There are a few quick checks you can make that might help to narrow down the noise to the engine or transmission.

With the engine not running! put the bike on the centerstand and slowly turn the back wheel by hand while listening for metallic noises or any unusual feel in the transmission. If that's good, put the transmission in gear, pull in and hold the clutch then try it again. Do that for each gear.

If the transmission seems okay, check the engine. Disconnect the TCI, remove the spark plugs and make sure the transmission is in Neutral. Press the Start button and listen for the noise. Put your thumb over each spark plug hole while cranking the engine for a quick test just to see if the pistons are all present and accounted for and the crankshaft isn't broken.

[Kevin11]

Thanks Scott, I will definately get to these tonight!

One thing I tried yesterday, prior to reading your post, was cranking the engine w/no spark plugs in. All pistons are moving and air's getting pushed out the spark plug holes on all four pistons.

I'll try the process you suggested, checking each gear and turing the rear wheel. However, the bike wasn't put into gear/ridden at any time after the bang.

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.