I believe that I have solved the power issues I was having with my Suzuki Bandit 1250. The really long story short was that the bike was not running right at certain RPM’s. After replacing the throttle body, replacing every vacuum line, and setting the valve clearances, the bike runs a lot better. There’s still a little work to do, but the bike runs much better now.
Some background information on the issue: When I acquired the bike, the previous owner had installed a gear position indicator, which would tell you which gear you were in. However, according to him, when the gear position sensor (an OEM sensor on the bike that’s part of the fuel injection system), the bike would run really rough at about 4000-5000 RPM’s. The bike was borderline almost unridable when the gear position sensor was plugged in, so he just rode with it unplugged. This would leave the neutral switch light off, even when the bike was in neutral.
Over the last few years of owning the bike, I did minor tests to see if anything would have an effect. I removed the aftermarket gear position indicator, which had no effect. The previous owner also installed an aftermarket speedometer component. Removing that also did nothing. After spending a good few months reading service manuals and reading online, I narrowed the issue down to some issue with the throttle body. At first I believed the issue to be related to the air screws that are used to balance the throttle bodies. I replaced the o-rings inside the air screws. However, that did not have any effect on it. I decided to take the throttle body off, and clean it, thinking there was carbon build up within the throttle body or possibly the fuel rail.
While cleaning the throttle body, I noticed that many of the hoses were beginning to crack due to being hard and dry with age. I replaced the hoses, and cleaned the throttle body. Putting the throttle body back on the bike, and firing it up, not only did cleaning and replacing hoses not fix the issue, it made it worse. The issue likely was not with the throttle body, but either with a sensor on the throttle body, or the valve clearances. I opted to check the valve clearances. The valves were out of spec, and getting them back in spec was a necessary repair I needed to do. However, setting the valve clearances also did not solve the issue.
I had finally eliminated it down to a sensor. In my research, there were 4 sensors that could be the culprit:
- The Throttle Position Sensor (TPS)
- The Secondary Throttle Position Sensor (STPS) and Secondary Throttle Valve Actuator (STVA)
- The Intake Solenoid Controller (ISC), also known as an Intake Air Controller (IAC)
- Intake Air Pressure (IAP) sensor
I replaced the throttle body with a used throttle body that I purchased on eBay. The bike rode better, but still ran a little rough. I balanced the throttle bodies, which required disconnecting the IAP sensor. The bike ran fine with the IAP sensor disconnected. I concluded that there was likely at least one sensor, likely the TPS, that had failed, and the IAP sensor is beginning to fail. Balancing the throttle bodies after replacing the throttle body did help, but the bike still runs a tiny bit rough, and the only component of the fuel injection system that I have not replaced was the IAP sensor (the replacement throttle body came with a fuel rail with fuel injectors on it).
Mechanical work requires thinking logically, and trying to test one solution at a time. I kind of broke that rule with replacing the throttle body, as I do not know for certain which sensor on the throttle body was actually bad, or if it was always the IAP sensor that was bad. I believe that it was a variety of small issues that had built up into one major issue with the bike not running well with the Gear Position Sensor plugged in. The previous owner had thought it was the Gear Position Sensor causing the issues. It turned out that unplugging it did not actually solve the issue, and just hid it, making it harder to tell what was going on. I spent well over $500 in tools and parts trying to fix this issue. However, I know a lot about the Suzuki Bandit, and fuel injection in general, and have an idea of what to look at in the future.