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General Category >> Rubber Side Down! >> Wide open throttle fuel starve symptom
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Message started by ckramer on 11/25/24 at 16:00:10

Title: Wide open throttle fuel starve symptom
Post by ckramer on 11/25/24 at 16:00:10

My 2013 S40 runs flawlessly at low to mid RPM in any throttle position, but acts like it's starving for fuel as it revs under sustained wide open throttle (like when winding out 3rd under hard acceleration, or 4th at 60 in wide open throttle).
From low throttle load to sudden downshift and high revs, no symptoms for 5 secs, then surging and hesitating, like when it hits reserve at 55mph.

Diaphragm in carb is brand new (replaced because engine was pinging badly.. pinging is completely resolved now.)
No changes to any carb settings or needles or exhaust or manifold, spark plug is new, air filter is new, valves are on spec. Bike is mechanically 100% stock.

Carb was completely clean when I disassembled it. Jets are all clean, fuel filters in petcock and at needle valve were perfectly clean.
58K miles of flawless riding (running ethanol free premium) on the bike until recently.

Rectifier was replaced 200 miles ago (was charging 14.8v & cooking bulbs.)

What makes for bucking & sputtering, only at WOT high rpm?

Title: Re: Wide open throttle fuel starve symptom
Post by Dave on 11/25/24 at 16:41:20

Turn your petcock to the PRI position and see if it runs better.

If so - then your vacuum petcock is in need of replacement with a manual one.

Title: Re: Wide open throttle fuel starve symptom
Post by ckramer on 11/25/24 at 17:53:37

I tried running on PRI. Same symptoms there.

Title: Re: Wide open throttle fuel starve symptom
Post by JOG on 11/25/24 at 20:24:25

Get in a safe place, get it to Bukkin and Spittin, hit the kill,get it stopped and open the drain on the bowl. Catch what comes out.
If it's A teaspoon, you're burning gas faster than the bowl is being filled.
Floats set too low.
The screen is dirty.
The screen on the petcock is dirty.
You have an inline filter.
Your fuel line is too skinny.
Pull the clutch lever in. Don't let the engine put vacuum on the petcock.

Title: Re: Wide open throttle fuel starve symptom
Post by Dave on 11/26/24 at 02:16:55

Try loosening the fuel cap....maybe the vent in the cap is clogged.

Title: Re: Wide open throttle fuel starve symptom
Post by JOG on 11/26/24 at 04:33:14

And That just might be it.. Totally forgot about that one.

Title: Re: Wide open throttle fuel starve symptom
Post by TheBabyDerp on 11/26/24 at 12:05:47

5B606D7A6B677C7A6169647B080 wrote:

Try loosening the fuel cap....maybe the vent in the cap is clogged.

Just curious how one would do that with the locking fuel cap? Mine only goes in one way. Not a twisting cap.

Title: Re: Wide open throttle fuel starve symptom
Post by Dave on 11/26/24 at 13:17:56

7D616C6B686B706D6C7B79090 wrote:

[quote author=5B606D7A6B677C7A6169647B080 link=1732579210/0#4 date=1732616215]Try loosening the fuel cap....maybe the vent in the cap is clogged.

Just curious how one would do that with the locking fuel cap? Mine only goes in one way. Not a twisting cap. [/quote]

Well I didn't say it would be easy.......maybe just set it in place without pushing it down far enough to lock - and use some tape to keep it from bouncing down the highway.

Title: Re: Wide open throttle fuel starve symptom
Post by JOG on 11/26/24 at 17:17:31

If you're ripping and it starts bucking, if you back off the throttle, does it start running right?
If so, then it would be good to get a second key and get it messing up, and open the NOT EVEN Almost full gas tank.
Actually, if you're down almost to Reserve, the tank is mostly air.
And you open it up and in seconds, it's bucking.
You fill up,open it up and, same amount of Time passes,and it's bucking
It's not the cap.
It takes a while to use enough gas to make the tank a low pressure zone when it is low on gas, a,lot longer than when it's full.