Help!! loud bang under gas tank?!?!?!

[88kat-rider]

OK well i posted a question about the direction the fuel petcock is suppose to be pointed for prime,reserve, and on....

I found out that on my 88 i have it set with the arrow pointed backwards(towards the tail) and that this is prime...

I pointed the arrow down and then went to start the bike.

I pulled the choke out and turned the key on and the kill switch on..

It cranked over once and then went bam! like something litteraly hit the tank or underside im not sure.... Then if i tried to start it...it would sound like the starter motor was jamed or not enough juice in the battery to turn it over..
I put the bike in gear and pushed it a little bit then back into neutral. I tried to start it again and the bike cranked half rotation then jammed starter noise....

I thought forsure I broke something good after the loud bang but I put the petcock arrow pointed backwards again instead of down and it fired up no problem....

anyone have an idea what happened here?

[scottynoface]

The bang was a backfire through the carbs.

[arsenic]

Hey, didn't you say you had to set the A/F screw at like 3.25 turns out? You still sure that's right?

[88kat-rider]

any ideas as to why? I never ever ever heard of a bike actually backfiring before, so rare... why on the normal run position would it do that? and then jam the starter??

its running right now outside but on the prime setting...should i try turning it again? any explanation for whats going on here?

it might be 3 turns but in and around that area why??? it was awhile since i last set them and i dont remember exactly what i set them to..

[arsenic]

It backfired because you have the A/F screws at 3.25 turns out. That's REALLY rich, like way richer than it needs to be. That is why it backfired, cause it's too rich. A quater of a turn makes a big difference, don't believe me, I think you're bike just made that fairly clear.

[scottynoface]

any ideas as to why? I never ever ever heard of a bike actually backfiring before, so rare... why on the normal run position would it do that? and then jam the starter??

its running right now outside but on the prime setting...should i try turning it again? any explanation for whats going on here?

it might be 3 turns but in and around that area why??? it was awhile since i last set them and i dont remember exactly what i set them to..

Why are you inside if the bike is running outside? You know these things have no cooling system when stationary right?

[88kat-rider]

it was suggested to me to do that while i am running the bike without the jet kit in because of the full exhaust i have on it.

I can set it back to 2.5 out but don't understand why its running on prime and backfire on normal....i don't want to look or sound like an idiot but can you explain this to me>?

Why are you inside if the bike is running outside? You know these things have no cooling system when stationary right?

laptop in the garage

not running anymore, it was on for a minute then turned off

[scottynoface]

not running anymore, it was on for a minute then turned off

thank god, there's a lot of stupid that runs around this place...

[88kat-rider]

i thought 2.5 out for stock setup....I'll clean and soak my carbs as was suggested earlier, ill drop in the jetkit, set the A/F screws back to 2.5 out and then test?

Any conclusions as to the cause for the starter not cranking and jamming? I understand the reason for the bang.....what about the starter jam?

[None2Slow]

Is it possible that the petcock vacuum lines are on wrong? It could have cranked, then jammed due to hydrolock from dumping to much fuel into the cylinders. Had this happen to me when I left it in the prime position instead of run. Filled the crankcase full of fuel as well.

[88kat-rider]

that would have made sense but i verified the engine did not hydrolock and does not flood the engine on prime( my needle seats are still good).

Only thing i can think of is that when the bike backfired the drive for the starter was still engaged and jammed. When i moved the bike back and forth in gear it came free....

I ran the bike after the backfire on the prime setting, and it was fine...it ran like it did before but i am still unsure if its all good to re-try on the "on" setting and why it backfired on the "on" setting and not on prime....this has me really scratching my head (assuming the A/F screw adjustment was the cause of the backfire..)

I adjusted the A/F screws back to 2.5 turns out (previously set to 3-3.25 out)
and this should fix the backfire.

anything i am missing here?

[88kat-rider]

UPDATE....

I triple checked everything...compared it all to the haynes manual and made sure everything was right. vacuum lines are all correct (Thanks KREY for the link)

I re-adjusted the A/F screws to 2.5 out, turned the petcock to "on" not prime...

crank...crank...BANG!

starter motor jammed making that electrical surge sound(kinda like my welder sounds like when laying a thick bead down..) doesn't spin....engine is not hydro locked!

turn the petcock back to prime.....roll the bike forward and backwards a little bit and the starter un jams...try to start on prime...it fires up(needs choke) after a couple of seconds with choke out RPMS climb and then i push choke back in and it idles smooth at 1200 rpm.

I took it for a drive...runs nice...still have some tuning to do to the carbs to get the top end right.

I need to pick your brains about this backfiring with the petcock turned to the "on" position..It has me stumped...

Any harm in just running the bike on prime? I understand if my needles dont seat it will flood and hydrolock but any other concerns about running on prime?

[arsenic]

I honestly don't need what's causing that to happen. I do know how to fix it though. Rebuild the petcock.

[Mojoe]

in all honesty, about the only thing that makes any sense to me is that you are confusing your on/prime settings.

you know, I have heard a dozen "debates" on here about backfire being one thing, and some other kind of "(fill in blank)fire" that does something else. I won't get into that because I am ol' skool where that skool taught just one term...backfire, be it out the tailpipe or up through the cabs(s).

with that aside, whenever you have any "misfire" that is intense enough, your engine will actually spin backwards...which is what the term "backfire" means to me personally,,,the engine fired backwards, reversing it's rotation for a brief moment. one a car...no big deal...but a motorcycle has a starter clutch, and if you rotate the engine backwards, you will flip over a dawg or two. I guess some people call them pawls? anyway, it makes a hell of a snap when it happens. this sound will come from beneath your tanks, of course. mix that with the sound of the backfire and you have quite a loud thing going on. btw, it takes a hell of a good backfire to not only stop an engine's rotation, but to also reverse it enough to flip this dogs...or pawls....but it can happen. when they are flipped over, no way in hell your starter will turn it over. it just isn't strong enough. you need to push the bike in gear, or turn it over with a strong arm on the front sprocket.

how do I know all this? cuz when I was putting my 1127 engine together, I inadvertently rotated the engine the wrong way with a strong arm on the front sprocket, while the engine was on my work bench. when I did it, something in the engine made a really loud "snap" that scared the crap outta me and I thought for sure I smashed something. before doing anything else I got on the horn with my buddy whose family owns the local suzuki shop and asked him what was up with that. he told me that is what happened, and to just turn the engine back the other way, and listen for it to snap back. it did and all was fine after that.

so...that might explain your starter problem.

[88kat-rider]

normally i would agree Arsenic....but i am pretty sure its not the cause.

I did a "test" if you will on it.....I removed the tank. With the tank off and full of gas i turned the petcock to on and reserve...nothing comes out...with it on prime it starts flowing like mad!.

When I apply vacuum to the vacuum port on the petcock with my handy hand held vac pump from the fuel shop(used on diesel injection pump testing) i get flow when its on the on and res position and it stops when vacuum is cut off.

I had already on order a petcock rebuild kit for doing over the winter and I'll slap it in now just for the heck of it(filter screen looks rough)....but How does the relation between the petcock and backfire come together?? The petcock gets the fuel to the carbs and does not affect how much fuel goes through them into the combustion chamber... my carbs dont know if i have it on prime or on so why does it backfire? or do they?

in all honesty, about the only thing that makes any sense to me is that you are confusing your on/prime settings.

you know, I have heard a dozen "debates" on here about backfire being one thing, and some other kind of "(fill in blank)fire" that does something else. I won't get into that because I am ol' skool where that skool taught just one term...backfire, be it out the tailpipe or up through the cabs(s).

with that aside, whenever you have any "misfire" that is intense enough, your engine will actually spin backwards...which is what the term "backfire" means to me personally,,,the engine fired backwards, reversing it's rotation for a brief moment. one a car...no big deal...but a motorcycle has a starter clutch, and if you rotate the engine backwards, you will flip over a dawg or two. I guess some people call them pawls? anyway, it makes a hell of a snap when it happens. this sound will come from beneath your tanks, of course. mix that with the sound of the backfire and you have quite a loud thing going on. btw, it takes a hell of a good backfire to not only stop an engine's rotation, but to also reverse it enough to flip this dogs...or pawls....but it can happen. when they are flipped over, no way in hell your starter will turn it over. it just isn't strong enough. you need to push the bike in gear, or turn it over with a strong arm on the front sprocket.

how do I know all this? cuz when I was putting my 1127 engine together, I inadvertently rotated the engine the wrong way with a strong arm on the front sprocket, while the engine was on my work bench. when I did it, something in the engine made a really loud "snap" that scared the crap outta me and I thought for sure I smashed something. before doing anything else I got on the horn with my buddy whose family owns the local suzuki shop and asked him what was up with that. he told me that is what happened, and to just turn the engine back the other way, and listen for it to snap back. it did and all was fine after that.

so...that might explain your starter problem.

K++

This sounds exactly like what happened....especially the starter part..I had to roll the bike in gear forward to get the starter un jammed.

I think i will have to tear my carbs down and do them up properly %100...it's the only thing that i can think of, aside from doing the petcock which im in the process of doing.

If i get a whole lot of fail from that...I'll wait for Arsenic to have a 90-96 set of carbs rebuilt forsale and swap them in with a new air box and left bar control for the choke...

or just run it on prime