Pulled the bike out today for a carb balance.Started it up and let it warm.Went for the idle screw and BANG.Backfire!Blew the vacuum caps off the left carb along with that side of my K&N lunch box filter.Pulled the carbs to find the diaphram on that side blown to hell.The top of the slide hit the cap so hard that it left indentations in it.Any clue why.I have checked the valves.They're in spec.New plugs.Carbs were gone thru over the winter.The ignition system checks out fine using the Clymer manual technique.Tank is clean with new lines,fresh petcocks(both) and a brand new filter.It was running fine until this.All I want to do was some fine tuning. 

A really BIG nasty gremlin?   

Quote from: Teek on July 02, 2008, 12:21:37 AM
A really BIG nasty gremlin?   

I checked the Mikuni site.Appears the only thing that can cause this is an air leak or clogged main jet. My carbs are better than new internally and my intake boots are in like new condition.No other places for air leaks.  I will not attempt to start it again until I solve the problem.I can't afford new diaphrams every time I start the bike. 

Chain tension for cams OK???
Maybe the valves jumped timing.

If it happend with the throttle closed the problem is related to the pilot system. It wont be the main jet if it happened on tickover. You have ruled out timing and air-leaks, that only leaves an incorrect (probably lean) mixture as the cause. So its down to a blockage in the fuel/airways of the pilot circuit, wrong pilot screw setting.......or, and this is my best guess......its still incorrectly pilot-jetted for the K&N.
Did you pull the plug on that cylinder and have a look at it?

Quote from: birdman on July 02, 2008, 08:36:04 AM
Chain tension for cams OK???
Maybe the valves jumped timing.

This was checked during the valve check.

Quote from: sledge on July 02, 2008, 09:22:03 AM
If it happend with the throttle closed the problem is related to the pilot system. It wont be the main jet if it happened on tickover. You have ruled out timing and air-leaks, that only leaves an incorrect (probably lean) mixture as the cause. So its down to a blockage in the fuel/airways of the pilot circuit, wrong pilot screw setting.......or, and this is my best guess......its still incorrectly pilot-jetted for the K&N.
Did you pull the plug on that cylinder and have a look at it?

This happened while the bike was idling with the choke off.Engine was warm and outside temps were in the low 90*F.My jetting is spot on for my set up.40/142.5/1 washer 2.5 turns.Everything has been clean and double checked.I replaced all fuel lines,fuel filter and installed a freshly mod no vacuum petcock.Rechecked the fuel levels in the float bowls.That's spot on.Pulled the plug and it appears to be running rich.They were black and sooty.This could be from using the choke for cold start up.I never really rev'd it to burn any carbon off the plugs.I had to replace the left carb body,but the replacement was in good shape after a good cleaning.All my fuel lines are correct.I have another set of carbs.I may clean them and rejet them.Give it another go.Could I possibly have broken loose some carbon, got it stuck in the intake valve, hanging it open ever so slightly.I may pull the intake boots and have a peep down the intake ports.

mine used to backfire wometime when i was starting it...

the little flange/ring thing that holds the header on was loose...

both sides

fresh air into the pipes + fire (exhaust) = BOOM in the pipe

Quote from: starshooter10 on July 02, 2008, 04:02:44 PM
mine used to backfire wometime when i was starting it...

the little flange/ring thing that holds the header on was loose...

both sides

fresh air into the pipes + fire (exhaust) = BOOM in the pipe

That's a backfire thru the exhaust.No problem there.My bike backfire thru the left carb and blew it apart inside.I've done a lot of carb work but never experienced this before.I am very particular about my bike.Everything as to be spot on an rechecked 3x before I will even connect the battery.Especially when dealing with the fuel system.I blew up,literally exploded,an 82 full size Bronco because my boss was rushing me.I didn't recheck everything.The metal fuel line wasn't seated correctly and leaked fuel onto a bad spark plug wire.By the time I went 50 feet to grab an extinguiser,I was laid out on a concrete floor from the explosion.Luckily,we start vehicles outside that have had fuel system work.It burnt to the ground before the fire department could drive around the block.So I now check everything 3x or more before I start the bike.Thing that worries me is, the bike was running fine, just vibrating a lot.I knew the carbs were slightly out of balance from where I had rejet.I didn't take time to rejet until I got the exhaust completely finished.So,I am at a loss.The teacher becomes the learner,on this issue. 

Yea my 89 did that a couple times. Back in 98.
Yea lost a diaphragm, and the vacuum spigot cap. Then till I replaced it it was terrible. heck it barely ran.
Post back here what happens.
Cool.
Buddha.

I have the unmolested all original 89 carbs.Mix screws still have there brass caps.They run ok with the stock exhaust.Sooooooooo,I'm going to break them down, clean them, and move my jets over from my rejetted 90 carbs.I'm also going to try checking the intake ports for carbon and junk.May be a valve hanging open.Hope to report back soon.

Mmm.. I have the same problem (from the left carb), some time now. I first noticed it when I rejeted and installed the lunchbox. Later I went stock again and it still persisted. Then I realized that I had this problem even before the rejeting (Same idle characteristics like sudden drop of rpm, though you don't realize it's a backfire when you have the airbox on)

Today I checked the valve clearances, the left cylinder Intake AND exhaust valves where less than 0.03mm, so I guessed this was the cause of the problem.. But now this makes me wonder...

Quote from: ben2go on July 01, 2008, 08:47:24 PM
I have checked the valves.They're in spec.

I still haven't messed with mine.I got the carbs done just haven't put them on and tested them yet.My girl friend had 2 strokes in a month and ended up in the hospital 3 times.Then her mother had eye surgery and we spent 6 days away from home.When I get my shipments out and everything here back in order, I will try another test and tune session.

Sorry to hear about your girlfriend.  I hope that turns out for the best.

Carb problems seem like the smallest of your worries, but I hope you get that figured out as well. 

Keep us posted.

WOW!I forgot all about this thread and I am back to somewhat of the same issue,again.

Problem 1-Occasional backfire through left carb,when idling,at a stop.
Problem 2-Occasionally requires a lot of rpm 3000-3500 to pull off without stalling.
Problem 3-Power level is off until 4000-4500 rpm.Then she screams.
Problem 4-Slow to return to idle,but has a stable idle.

So far I have completely dismantled and cleaned the carbs.I replaced all soft parts except float needles.I set the float height where the fuel level is at the split between the float bowl and carb body.Three turns out on the enrichment screws.My carbs only have a pilot jet and main.I haven't sync'd my carbs yet.Working on putting together a yard stick manometer.I did a valve adjustment while the carbs were soaking,and installed new plugs.

I believe I have narrowed it down to these issues.
1-Float level to low.
2-Enrichment screws need another half to one turn out.
3-Carb slide needles need another washer to enrich the mixture.Sorry can't remember the specific name for those needles.
4-Carbs out of sync.

Any other things that could be causing my issues?

Bike Specs
Exhaust-Right side D&D muffler from a Hayabusa.
Air filter-K&N lunch box
Carbs-40/142.5/1 washer/3 turns out

Reminder
Fresh valve adjustment and new plugs.

Quote from: laserred97gt on July 26, 2008, 11:10:55 AM
Sorry to hear about your girlfriend.  I hope that turns out for the best.

Carb problems seem like the smallest of your worries, but I hope you get that figured out as well.  

Keep us posted.

Thanks.She has gotten better since I orginally posted this.

Spark Plug Pics.

LEFT-The side that back fires through the carb.


RIGHT-The side that seems to run normally.

Have you tried swapping the left and right coils?

Quote from: Big Rich on May 22, 2011, 04:53:12 PM
Have you tried swapping the left and right coils?

Good idea.
I have not, but they both check out when tested with my electrical meter.I know,it could be an intermitten problem after the coils warm up.I will give the swap a try after I double check for a vacuum leak at the carb boots.

Quote from: ben2go on May 22, 2011, 12:11:41 PM
Thanks.She has gotten better since I orginally posted this.

Thats great!

On the carby thingy, another thing could be ignition advance.
If I am not mistaken pre 04 models have two pick up coils.. could it be that the one of them is misplaced slightly??

The left plug seems to be running a bit richer than the right one.

Quote from: lamoun on May 23, 2011, 03:13:09 AM

Quote from: ben2go on May 22, 2011, 12:11:41 PM
Thanks.She has gotten better since I orginally posted this.

Thats great!

On the carby thingy, another thing could be ignition advance.
If I am not mistaken pre 04 models have two pick up coils.. could it be that the one of them is misplaced slightly??

The left plug seems to be running a bit richer than the right one.

I checked the ignition pick ups.They are tight and there is no adjustment on mine.I do have a 5 degree advanced ignition rotor,but that wouldn't cause only one side to back fire.I agree that it appears the right side is running just a little lean.Over all the plugs look pretty good.