******HELP!****** Theories as to why my bike is running lean?

[Egaeus]

Of course, the tube would be perfectly clear of debris.  And when I took it out to check it, I discovered that it was messed up.  It must have been sitting against the exhaust at some time.  Whee!  More stuff to replace!

[galahs]

So did this fix your problem? 

[Egaeus]

No, it was just brittle, not obstructed.  I tried running without it just to make sure.  Nothing changed.  Don't know about the gas leak.  I don't really feel like losing more gas...if I have any more to lose.

[galahs]

With your float bowls off, have you tried blowing through your breather hose. Does the air flow easily out of the carbs?

[Egaeus]

I haven't done anything since I checked the line.  I won't be looking at it again until probably next week.  Just don't have time....

[Egaeus]

For the still-interested, here's today's progress:

Cliff notes version:

I took the carbs apart and found a little residue on a jet needle and main air jet, but IMO nothing that would be significant.  The fuel leak was caused by a pinched seal at the fuel T.  I will test ride tomorrow.

The full version:

I haven't touched it for 2 weeks, but I had some free time so today I took apart the carburetors with the appropriate pages of the service manual printed out for reference.  Unfortunately, I didn't put them back in and test before it got dark, but here are some observations I made of both carbs:

Diaphragm and piston constant velocity system:  Diaphragms are intact.  Pistons move freely and equally.  I have seen the pistons operate on the bike, and they seem fine.  The o-rings that everyone loses are there and in good condition. I replaced all o-rings about a year ago.

Idle fuel system:  Pilot jet clear.  Pilot air jet clear.  4 pilot bypass ports clear.  1 pilot outlet port clear.  Idle mixture screw and o-ring show no visible signs of wear.  Screws are set at 3 turns out.

Main fuel system: Needle jets (emulsion tubes) clear and all holes in the tubes are open.   Jet needles show no wear.  I cleaned some apparent gasoline residue off of one jet needle.  Main air jets are clear.  One might not have been completely clear, but it definitely is now. 

Choke system: starter jets clear, air passages clear, starter pipes clear, plungers move freely.

Float system: Floats have no gas in them (no leaks), needle valve seats are conical (no wear), system moves freely. 

Air vent and fuel line T's are all clear.  I did find my fuel leak though.  The rubber ring that seals the fuel T was pinched the last time I put it back together.  It didn't seal correctly.  Those rubber pieces are quite poorly made.  I replaced them a year or so ago, and they're already cracking....

Mods: 40 pilot jets, 127.5 main jets, K&N in the airbox, and I have raised the jet needles 1.2 mm with #4 washers.  This seemed to be working okay for quite a while, but I'd be willing to adjust it if necessary. 

Other than the extremely minor issues noted above, I see absolutely nothing wrong inside the carburetors.

Possible adjustments:
- I could adjust the needle, but lowering it would make it run leaner, and raising it more than 2 washers for a nearly-stock setup is (IMO) getting ridiculous.
- Adjust the idle screws out a bit.  I don't know why it would all of a sudden be wrong, but it's definitely worth a try.

After seeing nothing wrong, why am I still obsessed with the carb?  Because the problems are definitely related to throttle position.  Idle and maintaining speed causes the missing and backfiring.  Full throttle is great.  I wonder what the cops would think if I tell them that I can only ride with the throttle wide open....

[Egaeus]

I thought I'd start another reply to address some of the suggestions that were mentioned that I didn't respond to earlier, though I did try most of them.  I figure it's the least I can do.

try backing the fuel mixture screws out a half turn.

I am going to try this tomorrow if it's still acting up.  3 turns is the recommended amount for my setup and it was working before, but who knows what has changed, and I've exhausted many other possibilities. 

If you (or the PO) have had the tank off, double-check the tank petcock. Make sure the screwdriver slot is exactly parallel to the two feed tubes. It's a bit awkward to turn on and off, so fairly often it ends up not perfectly aligned, which can cause fuel starvation at speed- and setting the frame petcock to "Prime" won't help, because not enough flow is getting that far. You want to check with a good flashlight or somesuch. IMHO, some fair percentage of "phantom" GS "diaphragm" problems are just tank petcock mis-alignment. It doesn't take much.

If that's not it, well, it's a long shot, but try cracking the filler cap open, see if that cures the sputtering (this requires a little forethought and a spare key if you want to wait for it  to happen). If for some reason there is NO air vent to the gas tank, the gas will flow fine for a while, until you develop a partial vacuum in the tank, then it will get progressively weaker.

Longer aftermarket fuel lines and/or filter? Make sure they're not contacting the engine head. They're not as far away as you might think, and if they touch, heat can build up in the line and vaporize fuel in it while you're riding.

Just some thoughts.

Definitely not fuel flow.  I bypassed the filter that I had added, and it flows great.  It will empty the tank in no time.
Definitely not the filler cap.  I have an extra key, and I opened it while it was running.  No change in the missing.
It starts almost immediately after I get on the road so the engine's still cool and it's been fairly cool on most days I've ridden.  It couldn't be vapor lock.

I took a look at the videos... seems unlikely that our little carbs could suck fuel faster than that, but I've never done anything similar with mine, so I really have no frame of reference to judge what I'm seeing.  With some scenarios, though, like the lack of venting to the tank, overheating fuel line, or lots of water sloshing around in the tank under the gasoline, it might flow just fine in a test like that and still be a problem when you're under way... there's just so little pressure with the gravity feed and short drop, it doesn't take much.

Definitely not water sloshing.  There's no "blob" on the bottom of the tank, and the tank was filled with ethanol for a bit during the brief ethanol experiment.  It would have absorbed most if not all of the water.

could it be somthing with valve adjustment. where the engine is bringing in to much air for the amount of fuel?  what exhuast are you running? I had a problem with my car one time that was like this. no matter what engine I swapped in it (3 to be exhuast USDM and JDM)  it always ran lean ,,, too lean . needless to the say all three engines bit the dust and I couldnt ever figure it out.  Untill I figured out what they all had in common. the header on the motor, it wasnt properly designed and I was hard on the car My father can explain it to me again, I'm no engineer but it was causing major problems with the engine.

its just an idea. I could be way off but I thought I would share.

I bragged about the valves not ticking, and they started.  I checked the valves again, and two were very slightly off.  I switched the shims, and they're all perfect.  The measured clearance for all of them is between 0.04 and 0.06 mm.  Exhaust system is stock.

[galahs]

You sure its not running rich?

Usually if it requires full throttle to run its because there is too much fuel...

requiring you to have the trottle wide open to let more air in.

[Egaeus]

You sure its not running rich?

Usually if it requires full throttle to run its because there is too much fuel...

requiring you to have the trottle wide open to let more air in.

Hehe, you threw me for a loop there for a second.  No, it's not running rich.  The plugs are white.

[Egaeus]

So, remember a couple of pages back when I took the carbs apart, cleaned the choke, and thought it was fixed.  Well, it wasn't my imagination.  I finally rode it tonight, and miraculously, doing nearly nothing to the carburetor fixed it again.  I rode for about 15 miles, and stopped to get gas, since I didn't know how much I had left after the leak.  By the time I got to the station, it was back.

So what is it about the act of taking apart the carburetors and putting them back together temporarily fixes the problem?  It's not heat related.  The problem shows right back up when it's cold.  It's the taking apart of the carburetors that stops the problem.  Double-u-tee-eff?

[Egaeus]

So how's this for a theory.  What do you do when you take apart the carbs?  Drain the fuel right?  But it is never completely drained.  Then you take off the top caps, and turn it over and take off the float bowls.  I then have been inspecting everything, putting it back together, and it will run okay for a bit. 

My theory after my experience last night that I didn't want to share for fear of polluting anyone else's thought process was that there was sediment in the carburetors that was being washed up into the carb body when I turned it over.  This sediment was then gradually falling back down into the bowls when the bike was running due to the vibrating and splashing, subsequently causing the bike to run like crap as it made it's way into the jets. 

So just now, I take them off, drain the bowls, take them off without turning the carb over, and sure enough, there are significant amounts of tiny rust particles.  The particles are small enough that they move freely and are easily suspended in an agitated liquid, but plenty big enough to clog jets (at least temporarily).  Here's hoping that I have figured it out.  I am going to get a cellulose filter (the current one is just another screen) and see if that helps the rust situation until I get the plastic painted and my new tank on.

[Jarrett]

I have had similar issues with my bike after taking my carbs apart and putting them back together.  I put the blame on the carb boot seal.  If it's not perfect, the bike runs horrible/lean.  Rust doesn't help either.  Mine is rust free.

[Egaeus]

Just so you know, for the record, the fine rust particles *were* the cause of the problem.  I have been moving, so the bike has been sitting forlorn and wanting to be ridden.  My test ride consisted of riding it to the new place.  It ran great.  No missing.  I hate rust. 

In related news, the tank, the cause of all of my problems, has finally sprung a leak around the JB Weld.  It's just enough to keep the epoxy looking wet, but it's a great...opportunity to finally get around to changing the tank.  Now that I have a "real job" I can afford to have the replacement plastics painted to match.  Woohoo!