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Discussion Starter #1
Bike runs mint around town, including pulls all the way out to 10k, but after a couple minutes of sustained highway speed it starts cutting out and eventually dies. Rolling with the clutch in for 15-30 seconds and then re-starting the engine brings it back to life.

I’ve removed the orange tank cap vent valves, and the tank vent line is clear. I have not performed the FOG mod (yet). Previous owner had the petcock and carbs rebuilt last year at a reputable shop.

Any ideas? I’m leaning towards running too rich and consuming fuel faster than the bowls can replenish, but wanted to hear if anyone else has encountered this.
 

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Welcome to the forum!

hmm, my first suggestion would be the vent but it sounds like you already though about that. is this a new issue or has it been happening all along.
 

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Discussion Starter #3
Welcome to the forum!

hmm, my first suggestion would be the vent but it sounds like you already though about that. is this a new issue or has it been happening all along.
Thanks! I bought the bike 2 weeks ago, but yesterday was my first time getting it on the highway. So I guess it’s been an all along issue.
 

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Previous owner had the petcock and carbs rebuilt last year at a reputable shop.
Any ideas? I’m leaning towards running too rich and consuming fuel faster than the bowls can replenish, but wanted to hear if anyone else has encountered this.
Thats what they (sellers) all say. I'd have a 2nd look at both, for sure, for sake of process of elimination. Never know, you may get "Gomered" as in...surprise surprise surprise, looky what I found! Loose part, missing part, defective part, assembly or adjustment error....all on the table till eliminated via careful inspection.
Furthermore...I'd suggest having new float bowl and pilot screw orings in hand before cracking into carbs. Just in case......
 

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Discussion Starter #5
Thats what they (sellers) all say. I'd have a 2nd look at both, for sure.
Oddly enough, that’s the only service record I got with the bike. I noticed when I wack the throttle at lower RPM (about 2k) it hesitates before pulling. Leaning towards a shitty carb job. Checking now.
 

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also remember these CV carbs are not equipped with accelerator pumps. At 2K you're not even close to being in the useful powerband.

I got Gomered just the other day within a EX carb refurb....look very closely at the belled, deformed ends of these emulsion tubes, evidence of gross overtightening.

aex7.jpg
 

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Re shop rebuild....part of the FI generation...majority of the shop help these days were in diapers (or perhaps not even born) when carbs were "it".
 

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....

Any ideas? I’m leaning towards running too rich and consuming fuel faster than the bowls can replenish, but wanted to hear if anyone else has encountered this.
I would imagine that being this rich would produce plumes of black smoke when you hit the gas. And your spark plugs would be covered in black soot. In fact, pull your plugs and inspect the color on the tips.

You definitely sound as though you have fuel starvation, so I would inspect the routing of your fuel line from the tank to the carbs. I would also inspect the petcock internals to make sure they are clean and not partially clogged.
 

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Discussion Starter #11
out of curiosity, is there any fuel filter installed?
No fuel filter, just the petcock screens.

I pulled the carbs, and with floats removed cleared out the fuel rail using carb clean. Installed new (clear) fuel line and vacuum lines, which helped verify the petcock is working correctly. Made sure the line was angled down or at the most level. Hopped on the highway, and got a good bit farther than before but it still felt like it was cutting out. I’ll give it another shot here in a bit.

For the sake of being thorough, is there any way it could be an ignition problem rather than a fuel problem?
 

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Discussion Starter #12
Updating this thread, because I just made a good trip on the highway without issue. I'll test further tomorrow when it's warm out, and update with the results.

Here's a complete list of what I did, none of which seemed to fix the issue:
  • Fuel tank cap vent delete (orange discs).
  • FOG mod, which consisted of a 1" hole in the rear of the left airbox cover as well as pilot screw tuning (2 1/4 turns if memory serves ended up being best).
  • Dry float height verification.
  • Cleaned out fuel rail.
  • Replaced vacuum and fuel lines, making sure the line was running downward or at the least level.
  • Took apart petcock to verify screens were clear and petcock was functioning correctly.
Here's the last thing I did, which seems to have fixed the issue (pending verification in warmer weather):
  • Carb vent tube delete.
 

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what way was the vent tube pointing? Someone else had an issue where it would die if there was a wind from the right hand side, turns out that the vent tube was pointed that way (so wind was blowing up the vent tube)
 

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Discussion Starter #14
what way was the vent tube pointing? Someone else had an issue where it would die if there was a wind from the right hand side, turns out that the vent tube was pointed that way (so wind was blowing up the vent tube)
It ended about 8” past the battery, on the center-left side of the bike. It didn’t go terribly far down. I’m thinking the rear fender was catching air and creating a high pressure spot, or something to that effect.
If a half hour zip on the highway goes fine, I’ll put the tube back and see if the issue comes back.
 

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I had a similar problem on my old Bandit 1200. On a faster road like a freeway the bike would slightly stutter under certain wind conditions. I had a vent tube incorrectly routed, and the problem went away when I routed the vent tube correctly.
 

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Discussion Starter #16
Grrrr... still doing it.

Handled the 29 mile mostly highway trip just fine, but coming home it died out twice.

Is there something CDI related that could be overheating? I was coasting in gear (in other words, keeping the petcock open) and it wasn't coming back.
This is leading me to rule out fuel starvation as the cause.
 

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Grrrr... still doing it.

Handled the 29 mile mostly highway trip just fine, but coming home it died out twice.

Is there something CDI related that could be overheating? I was coasting in gear (in other words, keeping the petcock open) and it wasn't coming back.
This is leading me to rule out fuel starvation as the cause.
The CDI would likely misbehave on any 29 mile ride. For the bike to actually die it sure sounds like fuel starvation; the speed is demanding more fuel than the carbs are getting. Can you take the same journey at a slower pace and see if you removes the issue. Or ride on a quieter road in a lower gear to create higher revs to simulate highway revs/fuel demand.
 

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Discussion Starter #18
Will update this thread in 1-2 weeks. Tires are on order, so while it's down for those I'm going to go HAM on the carbs and do a correct WET fuel level setting. Last time I neglected to do a proper deep cleaning.
 

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Discussion Starter #19
Update: Solved! (For now?)

Despite working (that is, allowing fuel to flow when the bike is cranking over), I frequently observed a large air bubble in the fuel line with the OEM petcock.

Replaced the vacuum petcock with a standard non-vacuum unit I picked up for $11 on ebay, and went on an 80 mile highway trip, maintaining a speed of ~80mph. Not once did it hesitate or act like it was going to cut out.

Better yet, I looked at the fuel line when I got home and it was bubble free and full of fuel.

I’ll update this thread later if the issue comes back, but it feels pretty well solved. It’s finally deserving of a good wash :)
 
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