1996 GSX 787 Bogs Down After Warm Up

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K Huf

K Huf

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1996 GSX 787. Here is what we have done so far.
New SBT exchange engine
Rebuilt carbs with OSD Mikuni "Back to OEM" kits
Replaced grey fuel line with black automotive fuel hose
New fuel selector valve
New fuel/water separator filter
Replaced oil lines and oil filter
Rebuilt jet pump with new bearings and wear ring
New carbon seal & boot

We began the engine beak in process. Used pre-mix fuel. Did the up to 20% thottle for 30 minutes, let cool down. Up to 50% throttle for 30 minutes, let cool down. Now we are at the up to 90% throttle and are having an issue. Runs great and gets up to 6000 to 6500 RPM with great power but after about 10 minutes of run time it bogs down to about 2000 RPM. If we shut off the engine and restart it it runs great for 10 to 15 seconds then bogs down.

What should we be checking?
 
K Huf said:
1996 GSX 787. Here is what we have done so far.
New SBT exchange engine
Rebuilt carbs with OSD Mikuni "Back to OEM" kits
Replaced grey fuel line with black automotive fuel hose
New fuel selector valve
New fuel/water separator filter
Replaced oil lines and oil filter
Rebuilt jet pump with new bearings and wear ring
New carbon seal & boot

We began the engine beak in process. Used pre-mix fuel. Did the up to 20% thottle for 30 minutes, let cool down. Up to 50% throttle for 30 minutes, let cool down. Now we are at the up to 90% throttle and are having an issue. Runs great and gets up to 6000 to 6500 RPM with great power but after about 10 minutes of run time it bogs down to about 2000 RPM. If we shut off the engine and restart it it runs great for 10 to 15 seconds then bogs down.

What should we be checking?
Click to expand...
SBT exchange engine., So its a new Engine. My problem is similar but my bog is only temporary. I bought the ski and the guy just had a new engine installed. My first thought was to check compression which i plan on doing.
 
Hhmm…maybe an electrical issue,,,a component gets warm/hot…and fails temporarily…like a coil,,,check all grounds,a connections,,,,,load test battery….
 
Run a volt meter to the battery and drive it check to see if voltage is dropping.then run volt meter to ground side and see if loosing that side.dont start replacing parts it will get expensive
 
Also check rave valves they open around that rpm.have had a few machines where valve will open then drop down and bog motor down and not recover.was thr replacement fuel selector the same size?also run the ski on reserve and see if bog goes away when it bogs pull the choke to see if it goes away but pu it slowly.and look at plugs for lean condition
 
I had pulled the rave valves and they seem to be moving freely. The fuel selector valve was the same size. I had run it on reserve and still bogged down.
 
I load tested the battery. Tested O.K.

I swapped out the coil with one from our other running GSX

Took it out today and had problems right from the beginning. After running several minutes at the dock it accelerated right up to 7,000 RPM then dropped to 5,000 RPM. After letting up on the throotle it wouldn't go past 3,000 RPM. Shut off the engine and restarted, same issue. When it dropped to 5K it bounced up and down from 5K to 6K. I tried slowing pulling the choke when it bogged down like poormanracing suggested and it just slowed down more.
 
Im leaning towards an electrical problem . After your machine gets warm electronics can change. Had a cam shaft positioning sensor make a car not run after it was hot. The thing that puzzle me is if you shut it off and restart it runs good for a minute. possibly resetting electronics to fail again. CDI box , Took my machine to a dealer that has a test pool and they ran mine in there for a half hour and the problem never showed up , And it is getting cold here . 50s. Have to tried running it with a fuel pressure gage on it to see if it drops off.
 
I would bypass everything in the fuel system. Run hose from the fuel pickup directly to the carb. No valves, filters or anything. That will let you know for sure that part works. The next test is a pain, but it will tell you a lot. You can pull the electrical box out of the 97 along with the harness that goes to the motor on the 97. Then connect it into your 96. You only need the dess, the power to the electrical box, and the start stop switch. And connect the 97 harness plug to the motor. You do not need the gauges connected. Test and see what it does.

Also looking at the thread, you never said if you checked the voltage regulator. You can just unplug it and test. You just can't ride for a long period of time. Do this before you try the full gray box swap.
 
I did unplug the rectifier red wire and it did not help. It is getting late in the season here and I don’t think I’ll get it back in the water until spring.
 
I say this alot here because I chased it for a summer once. Check the rear waterbox to outlet hose for soft spots all over, including right up next to the outlet.
 
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