Carb ports and their functions

RocketRider · February 12, 2014, 08:01:37 PM

Working on my CBR 900 turbo project. Everything together so I need to piece together what the right combination of venting routing makes the carbs operate how they're supposed to (IE, as if in a higher pressure atmosphere). If anyone is experienced in the particular functions of carb vents shoot me a message please! Bonus points if you know the CBR 900 carbs (Keihins)

288RR · February 12, 2014, 08:23:10 PM

Well If I remember right. On car engines when you turbo an older carbed engine you have to out the whole carb in a box so when you build the boost it pressurized the entire carb in order to get everything to work right. And fuel pressure will have to raise with boost pressure, and you already have that part covered.
The reason for the box is because the carbs use vacuum to draw the fuel from the bowl, and with the pressure it will just push air into the bowl.

Super charged engines don't need the inclosed box due to the carbs be mounted on top of the blower. It just suck the extra air in fuel.

GreenMachine · February 12, 2014, 08:30:36 PM

On the Keihins from my bike, the float-bowl air pressure is equalized with that which the pressurized airbox receives. There are many adaptations that'll work besides the OEM way, but I'd still recommend starting with the OEM configuration...which I'm unfamiliar with for your bike. The symptoms of not getting enough pressure inside the float bowls is that the bike will starve for fuel and literally stall out completely the faster you ride. Generally above 45 MPH is where the bike will become intolerant and refuse to go any faster. I don't recall if your CBR had an elaborate RAM air system or not, so it may not be a big issue for you. I know Kawasaki's depend on a pressurized airbox and thus the float bowl plumbing to compensate.

The OEM on my bike uses a snorkel that sticks out in the mouth of the air intake at the front of the bike and leads all the way back to a union that feeds the carb floats. Some refer to it as a Y-snorkel because it resembles that shape, but whatever, you'll want the float bowls to get as much air pressure from the front of the bike as what is being packed into the clean side of the carbs from the airbox. The workaround, is to plumb the vacuum union into the airbox on the clean air side of the air filter.

You'll probably run into another issue with turbocharging a high-compression ratio combustion piston: heat. My understanding is that turbos generally have very low compression ratios bound to packing in more air in than would otherwise be available at sea-level. Watch out for detonation and boost your octane rating accordingly. I'm guessing you're going to need to run 95 - 100 octane just to keep cylinder temperatures under control.

GreenMachine · February 12, 2014, 08:37:01 PM

Quote from: 288cbr on February 12, 2014, 08:23:10 PM
Well If I remember right. On car engines when you turbo an older carbed engine you have to out the whole carb in a box so when you build the boost it pressurized the entire carb in order to get everything to work right. And fuel pressure will have to raise with boost pressure, and you already have that part covered.
The reason for the box is because the carbs use vacuum to draw the fuel from the bowl, and with the pressure it will just push air into the bowl.

Super charged engines don't need the inclosed box due to the carbs be mounted on top of the blower. It just suck the extra air in fuel.

GreenMachine · February 12, 2014, 08:38:07 PM

Oh, and don't make the mistake other motorcyclists do in running AVGAS. It's leaded. You'll fry your motor.

RocketRider · February 12, 2014, 09:02:50 PM

Quote from: 288cbr on February 12, 2014, 08:23:10 PM
Well If I remember right. On car engines when you turbo an older carbed engine you have to out the whole carb in a box so when you build the boost it pressurized the entire carb in order to get everything to work right. And fuel pressure will have to raise with boost pressure, and you already have that part covered.
The reason for the box is because the carbs use vacuum to draw the fuel from the bowl, and with the pressure it will just push air into the bowl.

Super charged engines don't need the inclosed box due to the carbs be mounted on top of the blower. It just suck the extra air in fuel.

Entire carbs need to be boxed because they're not designed to hold pressure. Ours arent really either but they DO. At least the small amount I am asking of them. I've got the rising rate regulator is there like you said.

I have 3 lines I need to figure out, all of which are just currently hooked to my plenum. One is the carb float vent, one I THINK vents the slide top hats but I'm not sure which one it is or if that's its only function. The third is a mystery to me :\.

Quote from: GreenMachine on February 12, 2014, 08:30:36 PM
On the Keihins from my bike, the float-bowl air pressure is equalized with that which the pressurized airbox receives.

This is a point i've seen in forums...supposedly I need to use a pitot tube in my charge pipe and route it to my float vents to keep the bowls nice n positive. I don't know that it's a requirement though? At the moment I can't get above 4k rpm so I'm not sure that's the issue (yet)

I assume my issues thus far are purely fuel, having a hard time finding a balance particularly at idle. Gonna do a test tomorrow to try to dial in the fuel regulator. All information is appreciated thanks guys

dub · February 13, 2014, 09:16:33 AM

carburetors...no thanks.

RocketRider · February 13, 2014, 07:46:54 PM

Plumbed the carb vent to the pitot tube and left the other vents to the plenum...it worked. Holy...shit...it worked...

GreenMachine · February 13, 2014, 08:20:04 PM

Awesome. Now prove it! We wanna see the beast in action.

RocketRider · February 13, 2014, 08:50:49 PM

Wanna go for a ride this weekend? Find me a nice clear runway. I wanna do a nice long run on it!

GreenMachine · February 13, 2014, 09:40:52 PM

Is that such a good idea after having so many things changed?

Personally, I'm not a fan of 6th gear attempts on streets.

RocketRider · February 13, 2014, 10:09:43 PM

Top of third would be plenty. The downside to our beasts is that top of FIRST is freeway speed. Lol.
I do need to do some general maintenance/tidying up from trouble shooting but she's gonna be reliable by end of saturday...practically stock at typical rpms

GreenMachine · February 17, 2014, 03:30:02 PM

I didn't ride this w/e. Did you?

RocketRider · February 19, 2014, 08:08:24 PM

Got a short ride in. Discovered I was barfing oil due to poor return line routing. Fixed. I think I can sufficiently say good enough for the whole project! The last things I wanna do are enlarge my oil restrictor (just for safety, just a quick drilling operation), reinstall the fairings (when I find out WTF I did with the bolts...or buy new ones), and add a fuel pump kill switch for cold starts. The pump tends to overwhelm the carbs when the idle is low due to cold. Never a prob once the engine warms up and the idle picks up.

A short video from the weekend:
http://www.youtube.com/watch?v=EasuzhCGdPk

Keep it within the SSA plz, I will be deleting it sooner rather than later and posting a similar vid in a more secluded area.

288RR · February 19, 2014, 10:22:33 PM

Sounds good, and looks like its a blast to ride.