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Old 09-12-2014, 01:08 PM   #1
GoGoGavin41 OP
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Sputtering at High Speed

2005 DRZ 400 with keihin carb jetted for higher altitude (+4k' is what PO told me). Recently took tank off and disconnected gas to the carb (only fuel related thing I've done to it in the few months I've owned it).

The past few rides, bike starts strong, pulls strong through all gears until I get to upper highway speeds. It'll hold speed (+85-90) for a few min, then will sputter out like I'm running out of fuel until I drop below 80. From there, it will run fine so long as I don't try to go above this threshold speed - it just sputters out again until I slow back down. It does this regardless of whether I have the fuel in On, Reserve, or Prime.

Have run a few gallons of gas with Seafoam in it, doesn't seem to have made any difference.

Should I take the thing off and look inside? Was hoping to sell it soon and I can't really give it to someone while it's doing this.
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Old 09-12-2014, 03:47 PM   #2
Danjal
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Run it up to speed,once it starts sputtering take off/loosen your gas cap. See if that alleviates it.
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Old 09-12-2014, 03:59 PM   #3
D.T.
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Fuel level low in the float bowl? Carb vents plugged?
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Old 09-12-2014, 04:09 PM   #4
Navin
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Agreed above, also test at high RPM in lower gear too, 1st, 2nd. See if air flow at highway speed is funking up vent hoses or if it RPM relative.
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Old 09-12-2014, 04:20 PM   #5
Danjal
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Agreed above, also test at high RPM in lower gear too, 1st, 2nd. See if air flow at highway speed is funking up vent hoses or if it RPM relative.
Yep. Fuel cap I'd the easiest I can think of,so it's a start.

Is it also worse going uphill?
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Old 09-12-2014, 05:30 PM   #6
GoGoGavin41 OP
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Originally Posted by Danjal View Post
Yep. Fuel cap I'd the easiest I can think of,so it's a start.

Is it also worse going uphill?
You know, now that you mention it, I think it may be. Is that float related?
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Old 09-12-2014, 05:45 PM   #7
Danjal
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Not entirely but a plugged vent will show good fuel flow at the start and zero or a vac under high load. Uphill can be a float issue or in my case of asking,load. Free revving or on a stand wont show issues like it will under load. You can bench tune like a mofo,but until you strap on your bucket and ride your settings or get ridden experience back,tuning an unknown carb is a shot in the dark or swagging it at best.

Carb vents being plugged tend to cause fuel overflow and high rpms.

Float height can cause all sorts of fun issues depending on the carb setup and application. One main issue is restricted or out of spec air flow in the bowl causing your main to not run right.

Fcr carb by chance? Any other mods? Did it sit for a while? Any recent maintenance? Possibility of bad gas?

Danjal screwed with this post 09-12-2014 at 05:56 PM
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Old 09-12-2014, 09:21 PM   #8
victor441
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could be a partially plugged filter, screen, etc. in the carb or fuel line restricting fuel flow too.....
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Old 09-13-2014, 09:42 PM   #9
k-moe
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Your float could be set too low. Check the fuel level and adjust as needed.
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Old 09-14-2014, 11:25 AM   #10
Tim McKittrick
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I had a very similar problem on an older BMW airhead and I chased vents and such for a long time before I discovered it was a poor flowing fuel filter. If you have an aftermarket inline filter, change it: mine looked new but were still restricting flow just enough to drop the fuel level under high loads for long enough periods- ie cruising at 80.

If you do not have a filter remove the petcock from the tank and make sure the filter sock over the inlet tubes isn't covered in gook.
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