While perusing the Durbahn site looking at all the shiney things that you couldn't buy if you sold half your organs, I saw their Keihin FCR carbs for the RC30. And well, that got me thinking. Carbs aren't that complicated. Surely less complicated to make a rack for carbs than convert to FI (as their NC30). There's a lot of information to go over, so who wants to help?
For the point of the discussion, let's assume that there are no bottlenecks in the system, and ignore jet and pilot settings. I just want to see what it would take to get a set of flat slide carbs on. Not that I'm planning on starting another project any time soon.
Durbahn advertise a 7hp gain just from switching the carbs from the OEM to the Keihin FCR39s for the RC30. What is giving the extra power? Jetting can always be adjusted and tuned, so it has to be something more than that.
What are the sizes on the intakes for the 3rd and 4th gen. I think they are different. 3g=39mm. 4g-41mm? I can always check my 96 carbs for the size, but not the 3rd gen. What are they on the RC30?
Does Keihin make an individual carb unit so that 4 units could be assembled and mounted in the orientation needed for the VFR?
Just spit ballin here, but I think flat sides are supposed to flow faster with the same size. I vaguely recall something about the different venturis created by the shape of the slide. This also would effect the power curve. And this would also assume your cam timing is setup to deal with the air, but I think we are ignoring this part based on your stipulations.
I don't know of a single Keihin that would work on VFR, maybe something for a HD? You'd have to make a custom manifold, but that is not terribly hard. I don't know what if any the advantages would be of the single carb set up. It would make jetting for elevation change easier, although less precise.
When I said single carb, I didn't mean one carb for the engine. I meant one carb unit of the correct size and orientation. Put four of them together to make the carb rack.
Flat slide carbs are awful in the lower RPM ranges.
The limited edition Kawasaki zx-7 RR (with the adjustable swingarm pivot, steering head, and close ratio gearbox) had flat slide carbs from the factory.
You really had to nurse the throttle open at lower revs or the motor would bog as the slides were opened too much. I rode one brand new back in the 1990s.
Constant velocity (vacuum slide) carbs don't do that, as the slides rise in response to engine vacuum.
But if we were building a bike targeted for the track, I wouldn't worry too much about the low RPMs. Good to know though.
CV carbs do not work well with pod filters. I do not remember the reason at the moment. Think it had something to do with the pressure/vacuum created. Are flat slide carbs the same? I think an airbox would still be very beneficial to the function.
I know my FZR with CV carbs did not respond well to pod filters, but everyone said it had more to do with the length of runners into the carbs being to short especially in the lower RPMs. It felt like there was a hole around 6k. My experience did not seem to be unique.
The only bike I had with a flatslide (and a Mikuni at that) was an NSR 50. It wore a Uni filter. I didn't notice any issues at lower rpms, but I basically only rode that bike at WOT. I hit 62 and change on that bike naked (the bike, not me thank goodness).
VFROEM put RC30 carbs on his 3rd gen. I think they are a direct fit. 3rd gen carbs are supposed to be an upgrade for the 4th gen since they are bigger, so one could assume the RC carbs will also fit straight on the 4th gen.
That's kind of where I want to take this. Can't really buy RC30 carbs, and the special order Durbahn ones are $2500+.
There's not really a whole lot to a carb assembly. Fuel lines, a spacer, the throttle pulley, top plate, and a handful of lines. Maybe it could be done for under a grand if you start with four individual carbs.
Four FCRs isn't gonna be a cheap thing to do, plus the addition of the linkages and other conversion parts. I'm looking into what a fuel injection conversion would cost at the moment, I think it can be done for under a $1000.
Did you see what Durbahn did on the NC30 fuel injection conversion? There's not much information on it other than it's working and how he used GSXR 750 bodies on it. Someone else here was also working on a FI conversion. Think it was flyguyeddy?
Quick glance at prices for the Motec units. $4k(ish), so something less fancy would have to be used.
Yeah, Megasquirt would have to be utilized to make it any kind of economical.. Yes, its known to have some bugs and hiccups, theres a reason why a lot of the high end tuners refer to it as "Mickeysquirt". But it can be made to work well, maybe not as well as a motec unit, but for the price, it could come damn close.
One of my good friends over here in WNC is one of the top EFI tuners in the country, people happily fly him around the country to have him come and tune their setup. He's listed as an approved tuner on every standalone website out there. So I'll be taking advantage of his know-how, and get him to help me setting up the ignition trigger aspect of things, which is the only part I'm not really clear on. I even considered bringing him a VFR PGM-Fi box, as thats the type of Fi used to tune most of the honda vehicles he does. There may be a way to tune those, if there was, this is the guy who would figure it out.
Seeing that a 5th gen fuel injection/ throttle body assembly sells complete for about $50 used is what got me started on this. Would be nice if we could figure a way to get a complete conversion for around $1000, would certainly make my future turbo addition easier to dial in too. I'll keep you guys updated if when I make some more progress, certainly don't have the cash to be buying project parts at the moment.
I believe the 3g are 36mm, and sizes are generally taken from the venturi. From what I've read, the 4g transitioned to a smaller flat slide carb because the incoming air flows faster at a lower rpm, helping midrange response.
You may want to look at http://www.sudco.com...alogJPG/014.jpg . It stipulates a 35-39mm carb for a 750. They also list some sample kits they have; instead of getting 4 singles, it may be easier to buy one fourset from a ZX-7R (or similar) and break the rack. Another idea maybe to call them and see if theyhave something from a parallel twin 400cc. I figure any headstart could be useful.
The spacing between carbs would be key and tricky to get. It would make sense to get a rack and break it just for the extra springs and bits if they were cheaper as a rack than individual.
I've been doing a bit of research, and think I can sum it up. Flat slide carbs maybe easier to do, but will work terrible except at the tippy top. I found the thread of the guy who had his NC30 converted to FI by Durbahn, and he seemed to not like the flat slides very much and said they didn't work well until 9k rpm.
FI is probably harder, with the biggest challenge being the camshaft sensors. Would want an exhaust with a place for a wideband O2 sensor as well.
Yeah, Megasquirt would have to be utilized to make it any kind of economical.. Yes, its known to have some bugs and hiccups, theres a reason why a lot of the high end tuners refer to it as "Mickeysquirt". But it can be made to work well, maybe not as well as a motec unit, but for the price, it could come damn close.
only haters, fools, and people not willing to learn it would call megasquirt this (or people with a vested interest in you buying THEIR shit). you cant TOUCH any other fi system with the featureset that ms3/ms3x now has for the price.
i had my impreza running better than stock on ms2. and i was still in need of tuning for idle and such (i just got lazy and stopped caring)
microsquirt is the way i plan to go when i ever get around to it. i am severely stalled on my vfr project due to the impreza engine swap and my rf900 needing a transmission.
BUT from what i have already learned from the vfr stuff. you need 01 gsxr600 throttle bodies. they fit right into the carb boots with a little trimming of the throttle body. linkages are another matter entirely.
4th GENs are 34mm, 3rd GENs are 36mm. I have no idea what the result would be to put the 3rd gen carbs on a 4th gen but I always thought it would be a fun "what if" project if I ever had the time and was awash in money. Just dialing it in would be a project in and of itself.
End of thread · 23 posts recoveredWayback Machine snapshot 13 Apr 2016
