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Fuel pressure and carb tuning on turbo GSXR

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I have a couple of years experience with building and tuning N/A oilburners and now I made my first FI bike.

Bike: 1100 GSXR 1991
Fuel pump: PIERBURG 7.21440.78.0
Filter: WIX WF8033
Turbo: TD05-16G
Carbs: Stock 40mm
- Drilled 5mm holes from carb intake to under diaphragm
- Blocked stock breather hoses with glue and aluminium pieces
- Vent bowl breathers connected into pitot tube located on charge pipe 
- Jetting: stock 125 main jets, stock (14.6mm) fuel level, needle on third (middle) slot, mixture screw 2 turns out.
Fuel regulator: Malpassi
Gauges: fuel pressure, boost pressure, (PLX AFR will come also, delivery problems...)

Ignition system stock for now. Atm plan is to run with mild under 0.7bar boost.


After very short ride it will hit 0.5bar boost without problems.

Question 1: Problem is that revs will hang after hitting boost and pull clutch. Also revving on idle will cause rpm fluctuation and slowly dropping. Just try to play with mixture screws or is there anything else?
- Renewed intake boots and o-rings. All hose clamps are tight. Tried to spray carb cleaner into rubbers, etc, no change.

Question 2: What should be fuel pressure on atmosphere/idle? Currently it is 0.1bar.

Is there anything else to consider?


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There is several things you can have a look at to try to eliminate the problem.

As stated the fuel pressure has to be set to 1 - 2 psi at idle otherwise the carbs will flood.

The return line back to the tank needs to be at least as big if not bigger than the supply . 

Make sure you haven't got dyno jet springs in the carbs as there weaker than stock .

Have you got a dump valve fitted ? If not I would fit one .

As regard to jet sizes , the size and design of the pitot tube to the area of the boost pipe will have an affect on the  size of main jet you run . Too small a pitot tube you end up with massive main jets. There was a really good article on line somewhere about this.

I would get some afr readings as possibly it's going really rich once you snap the throttle shut.


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Thanks for your replies!

I raised fuel pressure to 0.15bar and that seemed to fix this fluctuation problem. I just received my plx afr gauge and will install it next.

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