Riding a 2007 650 vstar encountered some acceleration issues when opening the throttle about 1/2 way. Engine seems to stumble and miss. Added some stabilizer and some Lucas injector cleaner. Changed oil at 4800 mile have about 5050 on it now. I have read that there are some issues concerning the ethanol in the fuels and that the stabilizer and injector cleaner could help.

Well an oil change is probably not the cause.
What IS it doing? You're telling me what it isn't doing...
Does the motor rev up?

1. add an octane booster to the fuel or try some high octane fuel (93 or above) and see if it stops rattling. Also make sure that the engine is not being lugged (throttled hard at low engine speeds) while in driving. If the rattling goes away or decreases check the fuel quality or, it probably has some engine issues like carbon build-up in the engine, overheating, or the timing may be off, or the engine is not running on all 4 cylinders.

P0121 - Throttle Position Sensor/Switch A Circuit Range/Performance Problem

The Throttle position sensor is a potentiometer that measures the amount of throttle opening. As the throttle is opened, the reading (measured in volts) goes up. The powertrain control module (PCM) supplies a 5 Volt reference signal to the throttle position sensor (TPS) and usually a ground also. A general measurement is: at idle = .5 Volts; full throttle = 4.5 Volts. If the PCM detects that the throttle angle is greater or less than it should be for a specific RPM, it will set this code.

Potential Symptoms: Symptoms of a P0121 trouble code could include:
Malfunction Indicator Lamp (MIL) illumination (Check Engine Light or Service Engine Soon light)
Intermittent stumble on acceleration or deceleration
Blows black smoke on acceleration
No start

Causes: A code P0121 may mean that one or more of the following has happened:
TPS has intermittent open or short internally
Harness is rubbing and causing an open or short in the wiring
Bad connection at the TPS
Bad PCM (less likely)
Water or corrosion in connector or sensor

Possible Solutions:
1. If you have access to a scan tool, see what the idle and WOT (wide open throttle) readings are for the TPS. Check if they're close to the specifications mentioned above. If not, then replace the TPS and re-check.
2. Check for an intermittent open or short in the TPS signal. To do that, you can't use a scan tool. You'll need an oscilliscope. The reason is because scan tools take samplings of many different readings over just one or two data lines and can miss an intermittent drop out. Hook up your oscilliscope and watch the signal. It should sweep up and down smoothly with no drop outs or spikes.
3. If no problems were noticed, perform a wiggle test. Do this by wiggling the connector and harness while watching the pattern. Does it drop out? If so, replace TPS and re-check.
4. If you have no TPS signal, check for 5 Volt reference at the connector. If it's present, check the ground circuit for open or shorts.
5. Make sure the signal circuit isn't 12V. It should never have battery voltage. If it does, trace circuit for short to voltage and repair.
6. Look for any water in the connector and replace TPS as necessary.

Other TPS sensor and circuit related DTCs: P0120, P0122, P0123, P0124

All I am getting from your discription, is that you
have a code & a misfire ,you have yet to diagnose.

Except for replacing the wires, that should last the life
of the car,your doing okey

You now are embarking on throwing parts at it,
not diagnosing.

I can't tell you how to continue,as I don't know
if have any test equip or skills required.

You may have a bad ignition module or other
issues

Again,test,don't install 1 part,unless something
tests bad.

I tried to answer this from an e-mail I got from FixYa, don't know if you got it. So, let's do it again.

Take the air cleaner cover and filter off so that you can see into the carb. With the engine OFF, crank the throttle wide open. As you twist the throttle grip, you should see a stream of fuel squirting into the carb. If you don't see this, the accelerator pump may not be working but this would cause the bike to stumble badly when trying to accelerate.

Now, start the engine and rev it a bit. Don't hold your face to close to the carb. Watch to see if the slide moves up and back down rapidly. If it doesn't, you may have a hole in the diaphragm that controls it.

Both of these scenarios are highly unlikely but possible. Try changing the fuel and see if that helps. What is so unusual is that you say the engine is not missing. When I asked if it had been sitting up I meant for about a year or longer. Sometimes the gasoline in the float bowl evaporates and gums up the jets in the carb but usually the bike spits back out of the carb and generally runs very badly.
Good Luck
Steve
[email protected]

Is the check engine light on? If it is, take the truck to an autoparts store and have them use a scan tool to retrieve the fault code(s) stored in the computer. I would suspect a faulty throttle position sensor or bad EGR valve, but the code(s) should narrow the problem down. Sure hope this helped.

check the throttle position sensor and also check for air intake leaks at the throttle body

check your coils and wires to them had same proplem and found 1 of the grey wires to the coil was disconnected

I had to rebuild my engine at 97000 miles cuz of low engine oil pressure and bad berrings