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Discussion Starter · #1 ·
Hi again everyone,

My bike (750k2) idles but the engine stalls when revving it.
There is a lot of hesitation when opening the throttle.

Spark plug tips are brown, I have seafoam in the tank, removed/rechecked airbox, secondary throttle butterflies are off, there are no FI codes.

Bike was running fine. I removed the airbox to do some cable tucking and now all of a sudden the bike runs very rough. Don't know what to check next.
 

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Discussion Starter · #2 ·
Damn. Thought I fixed it bcoz I had a short on a melted connector from the R/R to 3 cables going into the clutch (?) cover on the left side.

Still no luck.

Btw, when I wiggle the electric plug on the injectors, there is gas coming out and drips a little on the electric connector. Is that normal?
 

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Elbow Grease is Cheap
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Damn. Thought I fixed it bcoz I had a short on a melted connector from the R/R to 3 cables going into the clutch (?) cover on the left side.

Still no luck.

Btw, when I wiggle the electric plug on the injectors, there is gas coming out and drips a little on the electric connector. Is that normal?
I'm gonna say that isn't normal and could result in your bike turning into a fireball.


On your original question: accidentally unplugged a hose going to the throttle body, possibly resulting in a vacuum leak?
 

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The 3 wires going into the case is the 3 phase AC from your stator, going to the rectifier. This is the electiricty the bike generates when it's running. The connector between the stator and rectifier is a common point of problems but I don't think is your problem.

A motor that idles but dies when you rev it is probably running very lean. There's just enough gas for a no-load idle, but crack the throttle (which only directly regulates airflow), and it goes too lean and stalls. You could have low fuel pressure, clogged injectors, or a vacuum leak.

Wiggling the fuel injector with the system pressurized may result in a little fuel leaking. Injectors are just a press-fit in the rail. It's not going to explode. You can drop a match into a tank of gas and nothing will happen. Injectors run on very low current and the connectors are specifically designed to be intrinsically safe.

I'd agree that you disconnected a hose somewhere and now have a vacuum leak while removing and replacing the airbox.
 

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Vtwin
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Discussion Starter · #6 ·
Checking hoses, removed, reinstalled air box again..... still nothing.

When I slam the throttle open the engine dips and stalls, but when I do it a bit slower it will rev higher. Does that certainly mean that I'm running lean?

I don't know exactly how and where to check vacuum leaks.
 

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Vtwin
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Ok, when I tune the quads and dirtbikes with single carb and they dog out when the throttle is slammed I just have to adjust the air/fuel screw usually back it out a little


Sent from Motorcycle.com App
 

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Discussion Starter · #8 ·
Ok, when I tune the quads and dirtbikes with single carb and they dog out when the throttle is slammed I just have to adjust the air/fuel screw usually back it out a little


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Hm.. don't you mean, tighten it in more?

Do you maybe know where the a/f mix screw of a 750k2 gsxr is?
 

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Discussion Starter · #9 ·
Ok, so I read up on this forum:
http://forum.motorcycle-usa.com/default.aspx?f=18&m=13605

I don't hear or see any rpm changes when spraying wd40... Sprayed in the throttle bodies, in the PAIR, nothing....

Anyone knows where the a/f mix screw is? Although the same morning that day when the problem started, the bike ran fine.
Btw, revving it up to about 4000-5000 rpm and then slamming open results in a deep hesitation dip.

I'm inspecting the vacuum hoses again; all the t-pieces seem to be fine....
 

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I don't believe you have a A/F screw because your bike is fuel injected.

Later, Doug
 

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Vtwin
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FI!!? Ooops.....well dump some Seafoam in it, that fixes everything


Sent from Motorcycle.com App
 

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Discussion Starter · #14 ·
Ok, the situation is getting more crazy.

I removed and cleaned the injectors, changed the spark plugs, changed fuel filter and now.... the bike is running on 3 cylinders... The header of the 4th cylinder is not getting heated up when the engine is running ...

Pulled the injectors again, yes, they spit when engine is turning over. Yes, the plug sparks, when pulled out of cylinder head when cranking.

Bike symptoms are about the same if I recall correctly: takes a couple of tries before it starts, bike is idling rough, hesitation when opening throttle (the more throttle, the deeper the hesitation until it stalls).
 

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Discussion Starter · #15 ·
Ok, I got a bad spark plug. So the 3 cylinder problem is solved. The leaking of the injection is solved by replacing the rubber seals.

Bike seems to rev better now. I was playing (connecting, disconnecting, blocking off) with the IAP sensor vacuum hose and the bike seems to rev better with it unplugged/blocked off, but engine stalls after coming down from a high rpm.
 

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Discussion Starter · #16 ·
Now this is confusing.. when the vacuum hose is pulled off the IAP sensor the idle is better and the lower revs are much better. Bike responds like it should. But at higher revs when slamming the throttle the bike just bogs...

When the vacuum hose is connected the idle is noticeably more rough.

.......
 

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If you've ruled out vacuum leaks, then your manifold pressure sensor could be bad. I've seen EFI motors that get all sorts of wacky running issues when one dies. The service manual should have a way to test it with an ohmmeter, though usually if it's out of spec it will trip the FI light.
 

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Discussion Starter · #18 ·
If you've ruled out vacuum leaks, then your manifold pressure sensor could be bad. I've seen EFI motors that get all sorts of wacky running issues when one dies. The service manual should have a way to test it with an ohmmeter, though usually if it's out of spec it will trip the FI light.
No FI lights come up.

Manifold pressure sensor?
 

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Discussion Starter · #19 ·
Swapped the air pressure sensor with the atmospheric one, resulting in the same thing. So I guess my air pressure sensor is good.

Went for a test ride:
- vacuum hose disconnected from the air pressure sensor: bike runs fine at lower rpms but bogs a lot to go higher.
- vacuum hose closed with screw: takes some revving to be able to take off
- vacuum host plugged into air pressure sensor: runs like a pos, even the idling is rough

At this point I think that there is no vacuum leak.. ?
 

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More specifically, a manifold vacuum sensor. It's an important input to the ECU to calculate fueling needs.

If the hose is disconnected (vent to atmosphere), you will have a major vacuum leak and the bike will run very lean, which will give you a lot of bogging at higher throttle positions. With this hose disconnected from sensor but plugged, the ECU "thinks" there is atmospheric pressure in the manifold i.e much more air than there actually is and you are running very rich resulting in the hesitation until the throttle is opened up.

With it plugged it, it runs poorly. I think the sensor is bad. The atmospheric pressure sensor (usually in the airbox) and the manifold sensor (downstream of throttles) are usually not interchangeable even if they look the same.
 
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