GT250 EFI Throttle Body Synchronization
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Hello everyone!
I recently purchased a 2012 GT250 EFI. The bike was having stalling/idling problems after warm up so I started digging around to diagnose the issue. After going back and forth on this forum I came to learn that someone had bypassed the ISC by plugging the two vacuum hoses, one from each throttle body into blocked ports on the air box, leaving the ISC ports open. I proceeded to plug those vacuum hoses back into the ISC and started the bike.
The bike started up right away, but revved up quite high considering it was getting air from the ISC and the open throttle plates. This is where I loosened both the throttle bodies and zeroed them out (forbidden in the service manual) but cannot get it to run right anymore.
It idles around 1600 rpm with closed throttle bodies, but one cylinder cuts in and out, making the idle jump 1600-1800 rpm intermittently. If I try to ride it, it bogs, sputters and cuts out under certain conditions. There could be more to this but I’m pretty sure my throttle bodies are out of sync now. I am making my own manometer, where am I going to plug in the hoses? Is there any other way to approach this?
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Just like the carbs , you can look at the throttle body ports , and use a “sync tool” that measures the vacuum that is being pulled by each cylinder, you want them to be almost identical to each other
As you don’t want 1 cylinder to pull more air than the other one when you open throttle , it can disregulate all the other sensors that detect air in the bike from air box down to the lambda sensor , usually there is 1 port per cylinder with a pipe going to the ISCV
So you can connect the Idle Solenoid Valve (ISCV) as normal but in the middle of each pipe , get a “t-junction” piece that will allow you to connect a third hose each cylinder , then those extended extra hoses go to your carb sync tool to see how well your throttle is in sync or not.
Also, it sounds very stupid but sometimes I used to have this issue dealing with customer bikes , after many struggles , i replaced the stator and it was working okay but the stator did not give me symptons …but sometimes it will act very strange causing the tachometer to jump around 1000 to 2000 rpm , so the ECU is not sure what to do as the flywheel timings are erratic.
Hope this gives you ideas ?
// Meditation doesn't mean you have to sit still....
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Thank you for the instructions Marcel.
Currently, my bike idles nearly smooth at the following settings:
Front TB: slightly open
Rear TB: closed
If I move either of these a little, the misfires start taking place.
Connected a manometer with the given instructions and the results were not satisfying. The rear cylinder was pulling significantly more than the front and upon adjusting the TB settings on the running bike to bring them closer, the running situation got worse. Still couldn’t get the manometer to match on both sides and the rear cylinder ended up sucking some of the hydraulic fluid and burned it off. I gave up at that point and just wanted to ride back home from the shop. Tomorrow, I am going to try again by eliminating the ISCV, taking vacuum readings straight off the cylinders, because the bike idles just fine with the two vacuum lines blocked off and ISCV bypassed. I will post an update after.
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Update:
I connected the manometer directly to the throttle bodies and started it up. Rear TB was pulling more than the front, which meant that it had a stronger vacuum from the motor and needed more air, so I slowly opened up that TB butterfly until both cylinders matched. It idled at 2100 rpm. So I turned each throttle body down one by one until the idle sat at 1400rpm and it idled very well with both cylinders pulling same vacuum. Here, the idle stopper is set!
Moving onto on-throttle settings, the front TB was pulling more vacuum under throttle (needed more air), so I had to increase the tension on this cable until both TBs responded equally to throttle being pulled. Engine pulled equal vacuum (or close to equal) at idle and on-throttle now, so the cable tension is set!
Now I re-route the vacuum lines back into the ISC and re-introduce it into the air system. Bike starts up right away, idle settles down at 1500-1600, revs freely and seems happy.
Note: If your TBs are set from the factory, do not mess with them! If its already too late and the damage has been done, this method should get you 99% close to it. Use a good quality manometer and a cold pint of your favourite beer, have fun with it. Good luck!
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To build a manometer at home, I used the following:
- 20″ clear vinyl tubing
- flat thick cardboard/wood plank for the structure
- couple zap straps/zip ties
- Some high viscosity fluid (I used 10W-40 engine oil)
Take the flat piece of cardboard/wood and make equidistant horizontal lines across, with your favourite sharpie.
Drill holes on sides, so that you can secure the clear vinyl tubing to this board using zip ties.
Center the vinyl tubing on the board, making a U-shape (loop) at the bottom, and strap it to the board.
Use a syringe or funnel to pour your choice of oil into the tubing, until it sits at the desired level.
I’m having trouble attaching images to this thread, however there are lot of examples on Google, check out some images for inspiration, and good luck!
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2012 GT250 EFI
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