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ozkat
15-01-2006, 11:54 AM
Bowl pressure
If pressure in venturi of carbs is raising due to boost, so should the pressure in bowl raise. Because if pressure in bowls is lower as it is in venturi, no fuel can be taken into the engine.
In my experience dynamic boost is absolutely nessecary, and it's best to give each connection on the carbs it's own spot on the tube going to the plenum: not in the plenum itself as there is to much pulsing. Make sure they are "angelcut" and in the middle of the airstream. (also known as pitot tubes)

On the GSXR1100 model 91/92 with the 40mm carbs you need to fasten the rubber T’s for instance with steel wire to prevent leakage and pressure drop.

Membrane pressure (CV carbs only)
Pressure above slidemembrane is not needed: it got it’s signal/pressure through the hole in the slide. Don’t enlarge the holes because the slides go up too fast and cause stuttering. (beware of dynojet kit modifications: larger holes and softer springs are a real pain in the butt for the midrange!)

Pressure below the membrane is needed. On the GSXR1100 model 91/92 with the 40mm carbs you have a seperate “venting” system with external hoses. Those are not suitable to pressurize. I removed that system, plugged the holes and drilled holes from the bellmouth towards under the diafram. When you drill you cross a not used hole. You have to plug that up also to prevent leakage and pressure drop. So it is made like the earlier models: the 36 and 38 mm CV carbs do not have this system.

Fuel pressure
The pressure of the fuel going to the bowls should be higher than the pressure in the bowl. If not, not any fuel is flowing into the carbs causing starvation as soon as boost starts to build.
Therefore you need a pump capable of making enough pressure to overcome boost + 2-3 psi at sufficient amount of fuel. The standard membrane pump on some carb bikes is definitely not up to the job: don’t even try it. An automotive EFI pump coming out of a car with the same amount of horsepower you are aiming for should work allright.

If you don’t use a regulator: you will have the maximum pressure on the bowls the pump can handle, and that will be around 90 psi and the end of your carbs.
A regulator is used (I use a malpassi and highly recommend it) to give the carbs a 2-3 psi above boost, so difference in pressure between pump and carbs is always the same independent of boost. Make sure you use a bypass type, not a deadend type as it is less accurate. If you can get away with 5 psi on the carbs without leaking, you can use static pressure and don’t need to modify the malpassi. It you have trouble with it, as I have, you need to shorten the spring in the malpassi a bit to achieve 2-3 psi. I also needed a bigger 8mm return line to be less restrictive. Than you definately need the dynamic pressure to prevent starvation under boost!

Jetting+tuning
Don’t make holes in the slides bigger, or use soft dynojet springs! They make things worse in the midrange area.
If you have your carbs and fuelsupply correctly setup: you don’t need to make big adjustments to needles and jets. Mine is actually on 125 mainjets (stock!) functioning just fine at 11.5:1 A/F
Don’t get an A/F any leaner then 12:1 A/F when you use some serious boost. You will burn up some pistons/valves. 11.5:1 is safe imho.
Use an A/F meter to see what’s going on. Looking at plugs is not saying too much, because if their getting hotter then normal on a turbocharged motorcycle they are white anyway.
Symptoms of lean and rich while driving can sometimes be very similar so in doubt always consult the A/F on a dyno!
Because of the lowered fuellevel it is nessecary to give more fuel for the idle, in my case 8 complete turns out for the mixture screws.

Extra tips:
Place a fuelfilter between pump and regulator. Not before pump because you restrict too much, and not after regulator because you messing with pressure which is critical on a carb/turbo setup. Make sure the fuelfilter can hold the pressure!
The return line of the regulator should be as less restrictive as possible: inside minimum 6mm returning into TOP of tank, not below fuellevel in tank. Otherwise fuel pressure cannot go as low as 2-3 psi. Don’t ever (I mean ever!) bend this line because fuel pressure can reach scary levels damaging your carbs seriously!!!!
Mount a fuelpressure gauge direcly next to the boost gauge so you can easy troubleshoot. Remove later if you want to.
Remove filter in your fuel tap, or better mount a less restrictive fuel tap without filter. Pingel makes these, but I machined a custom one with build in fuelreturn to the top of the tank so you don’t have to weld in a separate fitting in the tank
If it’s a vacuum operated fuel tap make sure you use the prime mode: otherwise on boost the fuel tap will shut! This is a nice one, as you easily overlook it. On some models the fuel tap is really restricted in prime mode so you have to modify that in some way.
If carbs overflow: put in new needles and valves. Make sure fuellevel is not too high: pressurized bowls have a slightly higher fuellevel so you maybe have to adjust it a few milimeters.
On very high horsepowerlevels there can’t flow enough fuel past the needles. Put in thinner/sharper dynojetneedles to solve this. Typical symptom is a sharp leaning out around peak torque, were you need the most fuel every stroke. This makes it harder to get the midrange ok.
At higher boost you can press out the choke plungers! This causes unwanted rich situations and stalling at closing the throttle. Put in stronger springs to solve.

Disclaimer
This info is true for my bike and my application. Some of this info is also true on other bikes/carbs but you have to check yourself. If it doesn’t work or you burn your engine up: dont blame me. If you crash due to an exploding engine: I’m really sorry and I will send flowers to your family but I am not responsible!

Good luck!

Gix11
15-01-2006, 10:30 PM
I've just discovered that if you do a spell check on "OZKAT" it suggests "MOZART" as the correct spelling. Give us a tune OZ. Cheers for the work by the way. Check it on the "how to" to make sure I've put it up right mate.

ozkat
16-01-2006, 06:01 PM
Thanks Gix but its only info ive gathered from other people. Motzart? you have got to be kidding

crazymofo
16-01-2006, 07:40 PM
well written oz *thumbsup*

for what its worth.. im running a ea falcon fuel pump (good for 200kw and only 25bux from a wrecker) i removed the filter bag thats normally on it.

the fuel tap is stock and has been gutted and constantly left on prime. i have a billet/brass filter between the pump and the fuel reg (N power rising rate)

the carbies are the standard 99 model gsxf750 ones (36mm i think!!) ive left them as per standard this time round.. last set had a 140 main and a dynojet stage 3 kit put thru them.. it was a pig to ride) i intend on fitting it all up and just trailoring the bike down to the dyno shop and let them do all tuning (know of any good ones??)

im currently going to run boost reference to the floats thru the carbs original vent line points (popular method in the US.) with 10 psi of boost... 13 psi of fuel pressure is dialed in (so constant 3+ psi).

fuel return is handled via a 8mm return line to a hole in the tank cap (OZKAT... any chance u feel like making another one of them fuel taps?? hehe)

one thing i have added in to my set up is i have fitted a exhaust gas sensor(lifted from a ea falcon again for 25bux). its fitted in the collector of the turbo header, it then runs to a autometer digital A/F meter that is mounted to the bars with the boost and fuel pressure guages.

also fitting a externally venting Blow of valve is highly recomended for blow thru set ups. its saves the delicate carbs a hammering from boost spikes when the throttle closes. mine is fitted directly to the plenum. and speaking of plenums.. general rule of thumb is for 10-15 psi a plenum chamber should be between 1.5lt and 2.0lt on a 750cc/1000cc bike, this gives a good balance of mid range punch and top end breathabilty. im currently making up a bellmouthed 1.8lt plenum from thick wall alloy.


anyways.. enough rambling.

cheers.joe.