WTF just happened?! Loud pop under gas tank, followed by smoke, on ignition ON

[Protonus]

Ok, I am rather mechanically and electrically inclined, and quite savvy in general, however, most of my knowledge comes from fuel injected cars.  I've taken a fair share of this bike apart in restoring/repairing/upgrading/modding it, so I'm starting to be familiar with it. But carb'd bikes are brand new to me and this is my first.  So this one has me pretty baffled as to what just happened:

I have a 1982 RJ, late model with the airbox mod and full '83 RK style fairings.  ~16k miles. YICS ports are currently plugged.  I got out of work after riding my bike into work and while on lunch, and it was running great/perfect as it had been, no issues.  I stood next to the bike and turned the key to the ON position.  I did NOT hit START.  All I did was turn the key to ON.  As soon as I did (turn it to ON), I heard a relatively loud BANG or POP from under the gas tank area.  It sounded kind of muffled, and it sounded like it could have been gas exploding (like the sound of a small potato cannon etc), or it may have been an electrical POP.  It sounded more like fuel though.  It caught me off guard of course so I wasn't listening for it.  A second or two later, a whiff of smoke or vapor rose up from the front of the tank near the triple tree, which lasted a second or two.  It didn't much smell like exhaust and looked whiteish to clearish so my first thought was the BANG was an electrical pop from something arcing and frying and the smoke was the component letting the "magic smoke out".  I looked for fires and further smoke and none came.  I tested the horn, low/high beam, turn signals, gauge lights, sidestand relay, etc etc - I could find nothing electrical that didn't work as normal.  So I decided to start it, and it started right up, and sounded fine/normal.  Revved it, re-checked all the lights/gauges etc, found nothing wrong.  Let it idle up and warm up and it continued to be fine, no smoke/fire or anything out of the usual.  So I decided I'd chance it and ride it home.

It ran fine, EXCEPT - at constant throttle, at anything over say, 1/8 throttle, between say, 3k and 8k on the tach, the bike is now surging/bucking a bit.  It feels a lot like someone is blip'ing the throttle - a quick gain and loss of power, followed by a slight wobble of the tach needle.  It's not very bad, didn't think much of it at first even but it did it the whole way home even once it warmed all the way up, and it's been otherwise running great or dare I say perfect.  The whole way home, other than that bucking/surging, it ran perfect, idled perfect, still seemed to have good power, and never found anything electrically that didn't work.

So I'm pretty stumped as to WTF just happened!  Could the rotating assembly have been in just the right position that a coil fired, when I turned the key on, and maybe burnt some gas that was there already in the cylinder? I know the accelerator pump can spray gas even with the bike off, maybe I blipped the throttle and there was enough to make combustion?  That would seem harmless if that's the case, although alarming... But it wouldn't explain the bucking/surging it seemed to have after?   My other thought is I cooked an ignition coil (since they're in that area under the tank) and the pop was arcing thru it?  But the bike seemed to run and idle fine which I wouldn't expect with a bad coil?!

I'm hoping these symptoms/story will ring a bell with someone right away, so I wont' have to pull the bike apart looking for "something" when I don't even know what I'm looking for.  Thanks in advance guys. 

[Night Vision]

they all do that  ;)

just usually not after sitting for a period time... its called wasted spark and if you have a bunch of vapors in the combustion chamber and/or pipes... you'll get quite a bang

this usually happens after one is trying to start the bike... loads it up / floods it and then when she does get a good spark... she clears her throat...

since it's "running fine except for some bucking" I'd suspect a bad plug / wire / cap / coil..... something that is making her not combust fully 

[QBS]

NV+1.  You're on the right track with your suspicions regarding the position of the "rotating assembly".  My experience has been major bangs out the exhaust system.  I mean major, as in large calibre gun shot. Really.  My next move would be to remove the tank and air cleaner assembly, investigate all high tension electrical connections, the status/appearance of all fuel and vacume lines, and air cleaner placement.

Search "wasted spark".  The V spark plugs always fire simultaneously, one on a fueled cylinder and one on an exhausted one (hence the term "wasted spark").  This simplifies ignition system design and the spark plugs don't seem to mind.  I think Hardlies also use a similar system.  Also, not real sure, but I think that the plugs fire whenever the key is turn on from off even in the absence of crankshaft rotation.  If a fueled cylinder has open valves at that time, bang happens.

Hopefully, others that are knowledgeable on these matters will clarify/correct as needed.

[JohnAMcG]

Rings a bell for me!
I had this happen a couple times, till I finally tracked it down.  It may not be the same issue, but the bike does spark when the key is turned on.  Be sure you are not holding a plug wire when you turn the key. 
I don't know if you are having the same issue as I did, but for me, when this happened, it blew out the valve cover gaskets.  That's where the smoke came from.  The first time it happened I didn't even notice the gasket had blown and there was oil leaking down the side of the cylinder when I ran it.  Also, I would have those little surging issues, and when I did I would see a puff of smoke in my mirrors.  I realized it was coming from that gasket, and I pushed the half moon thing back in and it ran fine for a while, till it happened again.  It always seemed to do it after it had been running, and sat a few minutes then turn the key on. 

The next time it happened was at my girl's house, and it blew the half moon across the yard.  I topped off the oil and limped it home.  (incidentally, this is when the rear brake bar bolts broke, and tried to kill me, so change those if you havent.  See the sticky)  It sprayed oil all over my leg on the side with the blow out.  There has been some debate as to what caused this, as the common cause of the gasket blowing is a crimped crankcase ventilation hose (the hoses that go from the cylinders to the air box).  This will cause excessive pressure in the valve covers, but what I had was an explosion.  I am fairly certain I had a bad petcock causing this.  Gas was leaking into the vacuum line that is supposed to control the petcock.  I figured it out once, when I took the tank off after running it, and found gas dripping from the vacuum line to the petcock.  Once I repaired the petcock, I have never had the problem again. 

Not sure if you are having the same issue, but I hope this helps.  Good luck -JM
 

[QBS]

Proto, based on JohnAMcGs' report of fuel entering his vacume line, if you have a similar issue that may explain your newly aquired drivability problems.

[Protonus]

I am fairly certain I had a bad petcock causing this.  Gas was leaking into the vacuum line that is supposed to control the petcock. 

John: Thanks for the suggestion!  I think that is the exact same problem I have, sorry I just got to updating this again today but I worked on it last week.  I recently did a "ghetto rebuild" of my petcock because it was leaking externally AND into the vacuum line.  I took two petcocks and used the gaskets and metal etc that was the best between them.  I used silicone grease on the rubber that needed to move, and fuel safe gasket sealant on the ones that didn't, and reassembled it all.  It worked good, no external leaks.  But it may have started leaking into the vacuum line again since. I pulled my line after running the bike for a bit, and it seemed like it may have leaked, it smelled like gas at least.  To stave off the problem for now if that was the cause, I capped off both ends of the vac line going to the petcock.  When I want to ride, I turn it to prime, and then back to reserve when the bike is off - basically making it a manual petcock for now.  I ordered the K&L petcock rebuild kit for $22 shipped off eBay, and just got it in today, will install it in the next few days.  Also, I think the bigger problem in terms of drive-ability is, I found the vac cap for my YICS port on the front head had popped off, and was laying in the valley between cylinders.  I swear I checked this when I first had the problem, but I'm thinking the backfire/fuel pop, blew that cap off or loosened.  Since capping off the pet cock line and recapping the YICS port, it's back to normal, no more drive-ability issues and it's running great.  I looked at all 8 of my "half moons" and while none were missing or popped out, the two on the right side of the rear cylinder, had small 1/4" long cracks or tears in them.  No visible leaking of fluid from them, and they may have been like this since I got the bike, not sure.  But I don't' think they've been always like this so maybe they blew thru when the fuel pop happened like what happened to you, instead of blowing out.  For now, I put a few layers of liquid electrical tape on them.  I'm going to change them out, when I do the petcock rebuild in the next few days.

Either way, since capping off the lines etc, I haven't had another one of these fuel pops since. 

[js9_20]

Rings a bell for me!
I had this happen a couple times, till I finally tracked it down.  It may not be the same issue, but the bike does spark when the key is turned on.  Be sure you are not holding a plug wire when you turn the key. 
I don't know if you are having the same issue as I did, but for me, when this happened, it blew out the valve cover gaskets.  That's where the smoke came from.  The first time it happened I didn't even notice the gasket had blown and there was oil leaking down the side of the cylinder when I ran it.  Also, I would have those little surging issues, and when I did I would see a puff of smoke in my mirrors.  I realized it was coming from that gasket, and I pushed the half moon thing back in and it ran fine for a while, till it happened again.  It always seemed to do it after it had been running, and sat a few minutes then turn the key on. 

The next time it happened was at my girl's house, and it blew the half moon across the yard.  I topped off the oil and limped it home.  (incidentally, this is when the rear brake bar bolts broke, and tried to kill me, so change those if you havent.  See the sticky)  It sprayed oil all over my leg on the side with the blow out.  There has been some debate as to what caused this, as the common cause of the gasket blowing is a crimped crankcase ventilation hose (the hoses that go from the cylinders to the air box).  This will cause excessive pressure in the valve covers, but what I had was an explosion.  I am fairly certain I had a bad petcock causing this.  Gas was leaking into the vacuum line that is supposed to control the petcock.  I figured it out once, when I took the tank off after running it, and found gas dripping from the vacuum line to the petcock.  Once I repaired the petcock, I have never had the problem again. 

Not sure if you are having the same issue, but I hope this helps.  Good luck -JM
 

So, how did you cure the gas from leaking into the vacuum line?  ... I ask because, I've just rebuilt 2 petcocks and each of them are leaking fuel into the vacuum line.   Fortunately I discovered the issue before running the motor...
I've now used 2 rebuild kits and I'm out of spare petcocks.  Any suggestions?

I did notice when I pulled each of them apart that there was these small round plasticky discs...  (2 from the first petcock and, only one from the 2nd.)  Since I could not find any reference to these at the enlarged view of petcock parts (Yamaha parts list...)  I dont even have a clue where they belong...  I dont recall seeing these the last time I rebuilt one of these.  Anyway, since i still havent figured out how to attach a photo here, I've attached a link to my flickr page, where Ive posted a photo of this thing next to a 6mm wrench.
... Anyone have some enlightening knowledge about this?
Thanks...
http://www.flickr.com/photos/67369011@N02/6899197569/in/set-72157629376885639

[Re-Vision]

On the petcock directly below the plate labeled "ON" is a Phillip's screw. This is the drain screw and it has a gasket made of plastic that is likely what you have. There should be only one of these.     BDC

[Rikugun]

I don't see a through hole in that black thing in the pic and if that's a 6mm wrench then the black thing is too small to be the sealing washer as the screw is 6mm.

Did the part(s) come from the small spring side of the diaphragm or the fuel side of the diaphragm? Or did it come from under the plate Re-Vision described? Either way I don't think it belongs in there.

If gas is entering the vacuum line I'd think it was due to a hole or tear in the diaphragm.

[Re-Vision]

I think you are right Rikugan, sometimes the holes in the Fuel Cock Gasket don't get punched out cleanly and they don't break loose until after the petcock is in use. They don't come out until the petcock is dis-assembled. Loose debris could cause problems. The wrench is likely a 12 mm.        BDC

[js9_20]

Hey guys...  Thanks for your response.  Well, the wrench is 6mm.  The round thing is tiny..  Now that you mention it, maybe about the diameter size of the 4 holes in the round black plastic piece on the thumb turn (lever) side of the petcock, or slightly smaller.. tomorrow I will measure the size with a caliper.  Its not the plastic bushing/gasket from the drain screw... much smaller and there is no hole in it. My photo doesn't show as good of an image as I'd hoped, since it's actually not black but dark brown.. its a fuel resistant material for sure, fibered hard resinous material (not flexible/not rubber.)  Anyway, my thought was it didn't belong there but, as I mentioned previously.. I found two from the first petcock and one from a separate petcock...  weird?  Unfortunately I don't know which side of the petcock or diaphragm they came from because, I actually didn't see them at all until after soaking the parts in Pinesol for a day.  When I removed all the parts from the container, they showed up... unexpected and mysterious like.  By the way, I did soak all the parts from both valves separately from each other.
Anyway, I'm assuming its some kind of freakish thing that both petcocks had this mysterious part mixed in with the rest of the parts that actually belong there.
More importantly though... is the fact that both of these freshly rebuilt petcocks are leaking fuel through the vacuum port...  The diaphragms are both brand spanking new (out of the sealed packages)with no tears, as well as the rest of the parts (o-rings.)
I have noticed though, that both (new) diaphragms have the small pinholes slightly misaligned.  They're overlapping the chambers/holes in the body of the petcock but not centered, even after screws are inserted through the corresponding holes.
I hate to buy a whole new petcock valve but, I've spent a considerable amount of time with these things with disappointing results.  I'm about to give up.   ... The last time I rebuilt one of these (from a Vision,) it went together and worked quite well.  I don't recall any misalignment with the small pinholes though.
It's really quite a simple task. Not really much to them, except neither of them are working properly!  Hmmm,  .. I must be doing the same thing incorrectly, on both of them.  .. Wouldn't be the first time for that ever to happen.  haha...

Well,... thanks for listening (reading..) I'd appreciate any more suggestions/advice on why there's fuel in the vacuum.

[Lucky]

IF there is a tiny pinhole in the center of that disk, it is a choke used to cut down the amount of vacuume the diaphram recieves. it goes on the inside of the plate with the vaccume nipple, covering the hole.

if your leaking fuel thru the vacuume hose, then either the diaphram has a hole, is not installed right or it's cut incorrectly

[Rikugun]

Has the filter screen been used on both petcocks? If not, maybe the mystery parts came from inside the tank? They look too small to be from the 4 holed gasket - maybe 2-3mm? The gasket holes appear to be 5+. Also, I'm not convinced the holes are punched but rather that 4 holed gasket is cast in it's final shape. Anywho, getting back to your leak...

The misalignment of the locating pin holes sounds like it's causing some assembly woes. I wonder if they are assembled incorrectly from the manufacturer? BTW, when assembled, does fuel weep from the area where the diaphragms are sandwiched? I'd try to align them prior to assembly. The conical seal half of the aluminum bit and the disk part on the vacuum side will rotate independent of each other. Carefully grasp the two parts and try rotating the 2 diaphragms into alignment. You may have to gather up a bit of the diaphragm while rotating the fuel shut off plunger to get it to spin with the plunger. It's tedious to do and describe as well! Incidentally, this may be where the fuel is getting through. Not necessarily in a hole in the field, but rather at the center where the diaphragms are captured by the aluminum disk/hub/plunger.

This may not cure the fuel leak into the vacuum line but it doesn't cost anything to try. Are both new sets of diaphragms of the same brand? They may be from a production run that is bad? If so, buying more of the same will result in the same outcome until you purge all the defective units from circulation. :o  :( Good luck with it.

[Lucky]

"The conical seal half of the aluminum bit and the disk part on the vacuum side will rotate independent of each other."

You may have had some that you have been able to turn, but they absolutley shouldn't. that would constitute a vacuume leak & would draw fuel into the line. that assembly should be crimped tight. when i make an 83 dual diaphram, one of the things I make sure of is that the diaphram is sealed well in those spots...

[Rikugun]

I respect your opinion and suggest that the crimp can be tight enough to seal and still be moved with some effort. Note that I also caution this is where the leak may be.

[Re-Vision]

Remember seeing a gasket that still had flashing on it that needed to be removed, I calculated the wrench size if the foreign object was indeed the size of the gasket holes. A 6 mm wrench would make the object approximately 1.5mm.    BDC

[js9_20]

Hey guys.. Thanks for all the input.  So, the leak problem appears to be a simple fix.  I will know for sure when I put it all back together.
Lucky, .. I think you were spot on in your analysis of the fuel through the vacuum problem.  The problems appears to be a human error.. I have my fingers crossed that all will be well when I put "all" the parts back.  ha-ha
I kinda figured "I must be doing the same thing wrong" with both cases, since both are new kits....
I knew the diaphragm holes should align better and, I also had noticed that there were no alignment pins present when I reassembled the thing...  My cloudy memory was telling me that there were alignment pins present the last time I put one of these together...
Anyway, to get to the point... In both cases, I put the rubber diaphragm in the petcock body without slipping it into the plastic spreader thing, which also possesses the alignment pins, ... Duhhh!
I'm thinking, this is a pretty good explanation of what would be causing my troubles.
I'll let you all know what the outcome is but,... I think you'd be correct with one guess...
So, back to the disc things..  no hole in them.  I do find it odd that both petcocks had them present.  I'm also thinking that its a weird coincidence, since no one seems to be familiar with them.  I'm also gonna forget all about them once I have these things working correctly, especially since I managed to lose the first two that came out of the one petcock...  I only have one left, so I'm hoping they are as unnecessary as they are mysterious.
Thanks to all.

[Lucky]

I respect your opinion and suggest that the crimp can be tight enough to seal and still be moved with some effort. Note that I also caution this is where the leak may be.

I guess it might be semantics at this point, but all i'm saying is that, imo, if it was a quality part, the three pieces (diaphram, button & disk) should not move/rotate, but should act as one...  If the disk moves relative to the sealing button, I can see the potential for leakage..


btw, being a fiber type material, i really think that mystery piece goes at the vacuume port...  you really have to look for the hole, it's tiny, especially if it's clogged by dirt/crud/sealer, etc.

[Rick G]

I always take a pair of small needle nose pliars and increase the wave in the wave washer, that puts pressure on the petcock handle , to insure a positive seal.

[js9_20]

Hi all,
Well, as suspected... once I got the spreader in between the two sides of the diaphragm, she's working just fine.  No more fuel through the vaccum line, woohoo! Additionally, no leaks either... once I finally got the diaphragm plate to seal up tight.  Also, it seems to be operating just fine, without that mystery disc...

Odd to me that no one here appears to be familiar with this thing so, I'm still assuming it's not so important... but, I'm not gonna throw it away either.

I did pop off a few more photos of it with much better descriptive/quality imaging than the previous.  I've downloaded them here, if anyone is still interested...
http://www.flickr.com/photos/67369011@N02/sets/72157629376885639/

Lucky, I did look much closer for a hole in this thing but still haven't found one.
The photo's give the impression of a filter(ish) material but, I suspect its not so.  The material it's made of is much like that you will find as a backer board for electronic components (the brown ones.. not the green.)  ... Hard plastic over a fiberous material.  I would suspect maybe a backer for punching holes through the diaphragm rubber...? Or, something along that line... except, these disc's came out of the "petcock" along with the old parts, rather than the new parts package.  I did check the diameter of the disc and found it to be 4.5 mm.  Looks like the screw holes on the diaphragm are 4.5 - 5mm...
Perhaps I'll never know what this is for but, as long as
my machine is running fine without it, ... "No worries!"