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Discussion Starter #1
'90 GSX-R 750
Carbs are '90 stock Mikuni BST38SS with dynojet stage 3 jet kit. (I have drilled one lift hole larger on the slider and drilled main air jet, as listed on instructions)
Airfilters are K&N RU-2922, as listed on dynojet stage 3 jet kit. They have been washed and oiled.
Exhaust is full vance&hines SS2R.
I have never been able to test these 38mm carbs with stock jetting.

Dynojet kit is E3335.
http://www.dynojet.com/pdf/european/E3335.pdf


Problem:
No power on high gear, first 3 gears goes all the way to redline, but then problem gets worse. Been changing now main jets.

DJ146 (same as mikuni 137)
- Very poor top end power, no power on fourth gear. Max speed 140kmh.

DJ150 (same as mikuni 141)
- Little bit better top end power. No power on fifth gear. Max speed 180kmh.

DJ155 (same as mikuni 146)
- When going over half throttle, starts stuttering on fourth gear. Max speed 160kmh.



Tried so far:
  • Ultrasonic cleaned carbs.
  • New carb gasket kit with with needle valves.
  • Synced carbs.
  • Changed main jets. Instructions says that 150 is the correct one.
  • Changed new spark plugs. Tried with NGK CR9EK and CR10EK (current).
  • Tested spark. It seems good strong blue.
  • Measured coil resistance, both are ok.
  • Fuel lines are replaced.
  • Battery is charged.
  • No visible air leaks.
  • Carbs diaphragms seems ok.
  • Float level adjusted.
  • New fuel tap.
 

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Is your tank venting properly? Correct size float valves? Are the carb breather/overflow tubes in the wind?- this can cause the carbs to pressurize and lower the float level. this was an issue on a recent build of mine and took forever to find the source of the problem.
 

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Discussion Starter #3

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I once ordered a carb rebuild kit for a KZ750 and it came with 2.0 float valves, i didn't look at them and assumed they were correct for the model. On the highway it would loose power when i gave it wide open throttle until i backed off a little and the float bowls refilled. I switched to 2.5 valves (which is stock) and it fixed the problem.

Cruzin image isn't known for high quality parts.
I'm not sure what the stock size is on these carbs (or if they even come in different sizes n these carbs), but they usually are like 2.5-3.0 for a bike of this displacement. I f you still have the stock ones throw them back in there and see if that makes a difference.

is the fuel tap vacuum actuated?

have you done a plug chop?
 
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