Classic Fuel Starvation Trouble Shooting

[DoD#i]

Upon going through the carbs last night I returned to an issue I noticed when first breaking them down.  The gaskets on the bottom end of the guides look pretty shoddy on both carbs, and they weren't included in the rebuild kits.  Does this picture look like something that could cause issues?  And if so, does anyone know where to get that part, be it just teh o-ring or the whole guide?

This thread suggests that that would be 1mm thick 10 mm ID 12mm OD (should be square-section but improvised with round section.)

http://gstwins.com/gsboard/index.php?topic=34265.msg381563#msg381563

Mind, some of the measurements there don't seem to coincide with my measurements, but mine are hampered by the state of my o-rings - and I haven't taken this part apart, nor do I intend to at this time.

This is one of the parts that is not supposedly removable, though you have obviously removed it. The way to get a new one, per any of the parts sellers, is to get a whole new carb body - it's "not supposed to be repairable" and parts are not separately available.

[Badot]

My el cheapo compression tester from Harbor Freight had the proper threads for the GS.

Now, to those who know the GS better than I: Would it harm the bike to run it on one cylinder temporarily? If one cylinder runs fine and the other does not, you would at least narrow down your problem area not to mention give you something to compare (ex. if one cyl. is fine and the other isn't, and both those o-rings look the same, it's probably not those)

[gilkeyb]

Well, i let the carbs soak temporarily in thinned mineral spirits today.  Upon taking them out I sprayed them out with carb cleaner and then let them sit for a number of hours.  After leaving work I again sprayed out every passage, this time with Jet spray Gum-Out.  To fix the hole in the "choke jet" (for lack of a better term), I used a very thin walled piece of brass tubing and some jb weld.  The gastket at the bottom appears to possibly be available with the purchase of a new slide guide, at the price of ~70 dollars each.  So, I put a very thin layer of RTV on this and clamped it in place.  RTV is probably not the best option here, as its not entirely fuel proof.  But, I suspect very little, if any, fuel will be in that area.

Lastly, I purchased a compression tester from Advance (o'reillys didn't have one for sale or rent).  I only used the 12 mm adapter and my own compression tester, so heres hoping I can return it.  The long and short of it is that the compression checks out to 120 psi on both cylinders with the carbs off.  From what I have read, this is slightly below recommended (140-190psi), but not bellow minimum (114psi).  So although it is marginal, I suspect that compression is acceptable in both cylinders.

I ordered a set of diaphragms today as well.  If that fixes the problems, it'll be the best $50 that has been spent on this whole project.  If it doesn't, i'll have some new in package diaphragms for anyone interested 

Brian

[gilkeyb]

Well, I got the carbs all back together tonight.  Started it and it started idling perfectly.  I let it warm up and sprayed starter fluid, looking for leaks:  Nothing.  No air leaks.  Took it for a ride, experienced lean surge at 55mph.  Darn it.  I shut it off mid-lean-surge and coasted the last 1/2 mile home.  Checked the plugs and as to be expected, VERY WHITE.  Definitely still lean.

At this point I am:
40 pilots
125 mains
1 shim (3x1mm washer)
3 turns out
No petcock (both main and reserve on tank running straight to carbs)

I'm so beyond lost at this point.  The last remaining thing to check, which will have to wait til tomorrow, is running with the tank open.  I forgot my spare key tonight otherwise I would have done it then.  I talked to a mechanic friend, his advice was "the pilot circuits on those mikunis tend to get clogged up".  I would not be surprised at all if that were the case, but i've been through them 3 times with a copper rod and sprayed them out multiple times with gum out.  I would have hoped to get all the gunk.

Thanks to all that have weighed in.  Hopefully the cause of this can be found eventually...

Brian

[bombsquad83]

Are your exhaust header bolts tight, and do you have exhaust header gaskets in good shape?

[gilkeyb]

Are your exhaust header bolts tight, and do you have exhaust header gaskets in good shape?

I can honestly say I haven't checked that beyond the visual "Well, its not dangling off, so I think its ok" check.  I'll go through tonight and make sure that everything is in order with the exhaust. 

I'm really starting to sell myself on the gas cap not flowing (although I had already checked that in some fashion).  Then again, i've talked myself into believing my fixes would work a lot of times on this project!

Brian

[BockinBboy]

Are your exhaust header bolts tight, and do you have exhaust header gaskets in good shape?

+1 on this. Bomb beat me to it 

- Bboy

[bombsquad83]

If it's not the exhaust, then it could still be a fuel flow issue.  If you run straight up to 55 and keep pushing, does it start sputtering right away, or does it take a while and then start to sputter?  After checking the carb breather tube, and the gas tank vent for proper air flow, all that is left in your situation is the needle valves (since you eliminated the petcock).  It could be possible that they are adjusted too tight, but they are allowing some fuel flow just due to wear.  However, when the bowl empties fast enough, there is not enough flow past the needles to keep up.

Also, I would be very careful running with no petcock...if the needles stick and start allowing gas to flow freely, you will have a flooded mess.

[gilkeyb]

If it's not the exhaust, then it could still be a fuel flow issue.  If you run straight up to 55 and keep pushing, does it start sputtering right away, or does it take a while and then start to sputter?  After checking the carb breather tube, and the gas tank vent for proper air flow, all that is left in your situation is the needle valves (since you eliminated the petcock).  It could be possible that they are adjusted too tight, but they are allowing some fuel flow just due to wear.  However, when the bowl empties fast enough, there is not enough flow past the needles to keep up.

Also, I would be very careful running with no petcock...if the needles stick and start allowing gas to flow freely, you will have a flooded mess.

From a stop it will run up to speed fine and then gradually will surge more and more aggressively, forcing you to slow down.

I considered the needle valve.  But, those were replaced when I rebuilt the carbs ~1 month ago.  So I would hope that they were ok.  But what you describe certainly is possible.  I set the float height according to this recommendation

http://cgi.stanford.edu/~sanjayd/gs500/Maintenance/FloatHeight

I definitely agree with you that it feels like the bowls are running out of fuel, and thats when the bike starves.  Which is what leads me to believe its above the carbs thats causing the issues.  But, everything appears to be clean between the tank and the carbs.  I'll have to be sure to go through them one more time...or two.

Thank you for looking out for my lack of petcock.  I have been sure to clamp off the line running to the carbs when I let the bike sit.  I pull the clamp off only when the bike is running. 

All hope isn't lost yet.  More things to check and double check at least means I don't have to give up on it.  Thanks for your help bombsquad

Brian

[bombsquad83]

I don't trust aftermarket (K&L) needle valves.  I had a set that I was never able to get to work properly.  I finally replaced them with OEM and haven't missed a beat since.  Ronayers.com is a good source for these, or you can try your local dealer.

You could verify the carb bowls are running low.  When the surging gets bad, stop quickly and safely.  Quickly clamp the gas line, then measure the gas in bowls with the U-tube method.  I sure wish they would make carb bowls with a little window in them...

I like vicariously wrenching through others   .

[iclrag]

...I sure wish they would make carb bowls with a little window in them...

I like vicariously wrenching through others   .

I should try to make one, maybe it just wouldn't work because all the fluids moving? but that sounds like a good idea

[gilkeyb]

I don't trust aftermarket (K&L) needle valves.  I had a set that I was never able to get to work properly.  I finally replaced them with OEM and haven't missed a beat since.  Ronayers.com is a good source for these, or you can try your local dealer.

You could verify the carb bowls are running low.  When the surging gets bad, stop quickly and safely.  Quickly clamp the gas line, then measure the gas in bowls with the U-tube method.  I sure wish they would make carb bowls with a little window in them...

I like vicariously wrenching through others   .

I like this idea a lot.  As soon as I can get a chance, and it stops raining, i'm going to give this a try.  It sure feels like the bowls are getting drained dry.  But I never considered checking mid-surge because it just doesn't make sense for me to be running out of gas...

Thank you for your help, bombsquad.  Its nice to at least keep some hope alive on this project.  So keep coming up with ideas for me to check!

Brian

[mab32]

The conveluted fuel delivery system on these bikes is nuts.
I ripped it all out, used the on/off petcock from the tank, soldered two copper pipes up into the petcock joining the copper tubes as in a "Y" configuration, soldered that into a nipple for 5/16" fuel line.
I attached the fuel line to the nipple, ran it down beside the carbs, added a motion pro on/off valve.
From the on/off valve I added a few more inches of fuel line and a motion pro quick disconnect, and from the quick dissconnect enough fuel line to get to the carbs.
I leave the original tank petcock ON, shutting the fuel off with the motion pron on/off valve.
Don't want gas in the oil.....
When I want to take the tank off I uncouple the quick disconnect and am good to go.
Works great, no wanting for fuel at any speed.
No reserve, so learn to watch your fuel. 
The End

[bombsquad83]

That's way too much work...if you really want to get rid of the vacuum petcock see my build thread.  Use a 2007 crf230x petcock.  Only takes minor modification and it retains reserve functionality.

[mab32]

Sure, to each their own, if it works it works.
No real work involved, just a little soldering. The petcock already fit, it's stock. And it cost me nothin
Nice paint job though !

[jestercinti]

I'll add some things.

1.  Are your plugs on tight?  Lately I was having the same issue, and I learned that my plugs were backed out 2 turns.  Oops.
2.  Carb Vent Tube not routed correctly.  If the carb vent tube that runs on top of the airbox is not routed correctly, it causes pressure issues that hurt fuel flowing freely.  Make sure the end is routed somewhere out of the wind like behind the battery box.

The rest of the suggestions were my first thoughts as well.

[gilkeyb]

I'll add some things.

1.  Are your plugs on tight?  Lately I was having the same issue, and I learned that my plugs were backed out 2 turns.  Oops.
2.  Carb Vent Tube not routed correctly.  If the carb vent tube that runs on top of the airbox is not routed correctly, it causes pressure issues that hurt fuel flowing freely.  Make sure the end is routed somewhere out of the wind like behind the battery box.

The rest of the suggestions were my first thoughts as well.

Based on the fact that the plugs have been in/out/changed/switched and every variation in between, I know its not the plugs.  They are in tight.

I do have the vent line from the carb routed to the same location as the vent for the airbox (lower right on trans case).  I'll move that and give it a try.  Unfortunately I have been out of town and won't get a chance to work on the bike at all until friday most likely.  But, thanks to you guys, I have a nice long list of possibles to check out.  I also have a new set of diaphragms waiting at the powersports store.  I'll pick those up sometime this week, as well as some oil for my Fz6

[bombsquad83]

That was probably one of the worst places you could put the carb vent for wind interference.  Hopefully terminating it behind the airbox will help out.

Actually on second thought, the worst place to put the carb vent is connected to the tank overfill drain like another member had from a PO.  Sure it works fine...until it rains and puts water straight into your carb bowls.

[bombsquad83]

Sure, to each their own, if it works it works.
No real work involved, just a little soldering. The petcock already fit, it's stock. And it cost me nothin
Nice paint job though !

Thanks for the compliment.  Didn't mean to slam your work.  Glad it works for you and didn't cost anything.  I hope for your sake that you never run out of gas!

[jestercinti]

The carb vent tube kinks EXTREMELY easily.  I had the bike stall and run like hell for 10 minutes before I figured it out.  Make sure that there is enough slack, and if necessary go to your local FLAPS and get a hose connector to extend the hose.

By the way, FLAPS = Friendly Local Auto Parts Store

If mounted the wrong way or into the wind just right, it will cause your symptoms.  I'm not saying that it's 100% your problem, but it's worth a shot.

More info on "Crosswind Engine Stumble"  http://gstwins.com/gsboard/index.php?topic=60976.msg705248#msg705248