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

[Lucky]

I still think its a choke, i've seen these in petcocks before..

[Rikugun]

I'm glad you got it working right - and with all the parts this time DOH!  :D 

[js9_20]

Haha... Yeah, its amazing what can be accomplished just by putting all the correct parts in place.   

...  Possibly a choke as you say Lucky.  I don't know but the bike is running as good as ever without it.
I'm just in from a 3 hour ride.. cruising at various speeds up to about 85 mph, with no apparent issues.

Anyway, I'm just happy to have the fuel/vacuum issue resolved.  On to other things...

So, I'm curious what action did you take when you've seen these "chokes" in the past... did you reinstall them?  .. I'm wondering what signs would indicate or be apparent if these things were required, since I've left mine off.   ... Also, I'm not really sure I would want to choke the vaccum at the petcock.  I would think that a more abundant supply of fuel would be desired to feed the fuel pump, and the carbs are metered through the pilot jets.. right?
I'm no engineering genius by any means but, it just seems like, the more fuel "available".. the better.  ????

[Lucky]

that side of the petcock isn't shown in the parts fiches, so my term "choke' is a guess.

if there is no hole, & it goes where i think it goes, it could act as more of a shim, to keep the diaphram from being pulled too far, or possably a seal, to make sure there still vacuume on a long decelleration.

I seem to remember it going in with a tiny spring as well.  next time i pull one apart, i'll take a closer look, i just don't remember ....

I wouldn't worry about not having it in at this point though,  sometimes things are 'overengineered' for possabilities that never come to fruition.

ride & enjoy!

--Lucky

[Rikugun]

On decel there is more vacuum available, not less. Mine did not have one and has worked fine. js, your's is apparently working well so I wouldn't worry about it. There is a spring though - did you reuse it? It goes on the vacuum side to help push the fuel plunger closed when the engine is off.

[Lucky]

yeah, brain fade..  :D

[Rick G]

By the way , I haven't seen anyone mention it , but the "half moon thing" is part of the valve cover gasket.Just pushing it back in is not the answer. you need to remove the valve covers and replace the whole gasket.

[js9_20]

Ride and enjoy... I like that!   

Yeah, I have the spring in at the diaphragm...

Looking at it again, I think it might belong in the hole that the brass nipple threads into.  The one that the vaccum line actually attaches to.  Looks like it would fit in there quite well actually, and probably have a big effect on the amount of vacuum as well.  I never found any small spring, though.

Anyway, my attentions are moving elswhere.   :)

[Rikugun]

I think it might belong in the hole that the brass nipple threads into.

That brass nipple is a restrictor of sorts and can get carboned up. It's worth checking if anyone is having problems with a seemingly good petcock not flowing properly.

[jasonm.]

just jumping back to the beginning. The service manual states...the ignition does a "check" when the ignition is turned "ON". Meaning it fires the spark plugs even if the engine is NOT rotating. This usually happens within 3 seconds of the ignition "ON". You'll hear a mild pop if there is enough fuel in the cylinder. Some call it wasted spark. I call it "the ign. check".  If your bike is now running odd since this POP. Then check the vacuum hoses, especially the petcock hose. It can pop off if the intake valve was not closed when this happened.

[Protonus]

My issue (I'm the OP) was definitely a petcock that was leaking fuel back into the vacuum line.  Though I've gotten very tiny pops out of the carb's or exhaust on IGNITION ON before, they have been nothing like the ones I got out of the carbs on the day in question.  Since rebuilding the petcock (fully, with new gaskets etc) I haven't had this problem since and the drive ability issue went away.  Also I had to cap off my YIC ports again as I lost a vac cap at some point, and I found multiple vacuum lines were not routed to the "correct" ports that they were  OEM (having a spare parts bike that is virtually unmodified/original is REALLY handy!).  All this happened with in the same few months of eachother, but I think my petcock was the root cause for sure.

[mechaman]

I had something similar happen today, it sounded like a shotgun going off! :o It scared the crap out of me! It didn't back fire out of the exhaust, it came from between the head and block at the head gasket, not valve cover, and started streaming coolant from the head gasket.  I believe I flooded it without having any spark, so I took every connector apart and cleaned terminals with electrical connector cleaner, inspected and put back together.  This is when it happened, I turned the key on and BLAM! :o After that it started and ran fine after warm up. ;D  I'm concerned about the head but it stopped leaking coolant and I haven't seen any traces of it in the oil and no more unusual smoking. I'm hoping not to have to replace the head gasket, but I wouldn't be surprised if I end up doing it down the road. :-\

[Rikugun]

That is messed up right there! Sorry to hear that. I doubt a blown head gasket will resolve itself.   :( You might want to start looking for gaskets now  :o

[mechaman]

That is messed up right there! Sorry to hear that. I doubt a blown head gasket will resolve itself.   :( You might want to start looking for gaskets now  :o

Yeah, I'm looking for that now among other things (gas tank, carb kit, etc) I gave next to nothing for the bike and just cant get myself to give more for a good tank than what I gave for the whole bike.  I hope to get it road worthy before long, I'm itching to take it out more than just up and down the roads close to the house with a makeshift tank on it. 

[Lucky]

Unless you find a NOS tank (german Ebay sometimes) you won't find one that doesn't have holes, unless the p.o. fixed it (properly)

If you haven't noticed the corners of the tank sit lower than the petcock & rust out.  i just stripped down my wife & sons tank & they look like a spagetti strainer.  they'll get fiberglass repairs, the inside coated with POR-15 (.com) and painted.  don't use any coating if it's vinal.  it MUST be an epoxy.  i hear good things about Caswell.

[mechaman]

I've attempted to weld mine and the whole right underside of the tank is gone as in not there anymore :-[ ( I knew it was a bad idea to try,but i did it anyway) not sure if it can be fiberglassed or not. There is one on ebay for $49 thats in about the same shape as mine was before I destroyed it. http://www.ebay.com/itm/1982-YAMAHA-XZ550RJ-XZ550-XZ-550-RJ-550RJ-VISION-GAS-FUEL-TANK-11H-/280807832276?pt=Motorcycles_Parts_Accessories&hash=item4161737ad4&vxp=mtr#ht_8208wt_1084
What do you guys think? Should I go for it?

[Lucky]

First. you'll have to fix that one too, but i think you can do better for $50.. that tanks pretty ratty....

i've gotten good at using my butaine torch for soldering.  more heat than an iron, less than a propaine torch (welder is way too much heat, these tanks are thin...) and i'm thinking of taking an old tank i have & trying my hand at using silver bearing solder & silverflux. possably with some metal screen backing if i have any.  & seeing what i can do. (this tank is looong dry)

EDIT: found some fine mesh brass screen.  this might work...  or i might blow up! lol

[Lucky]

Update: Soldering is working extreemly well.  only used the screen once. it's hard to get flush...

bare clean metal, silver solder paste flux & silver bearing solder.  work the flux into the holes & use only as much heat as needed. don't overheat it...

[fret not]

"don't over heat it" . . . . .that's always the trick.   It takes some skill to get it right.

I was seriously thinking of making fiber reinforced resin molded tank until I found a tank from Germany. There is a fuel resistant resin for laying up fiber glass called "ISO" that I heard of from the Bonneville guys.  It is also much more heat resistant than the standard polyester resins commonly used for fiberglass work. 

[Lucky]

my problem is i hate working with materials like resin, fiberglass & bondo.  hate the dust.  but, i'm better at it than i used to be. i tend to get more on the tank now that wearing it :)

I've got both tanks in a layer of primer now (one is fiberglassed & resined, the other is soldered & resined) i have to clean the insides out some more & POR coat them, then i'll continue with the bodywork.

I know some guys can do entire tank start to finish in 2 days. am i doing a better job since it takes me a month?