I'm posting this here in hopes of getting more hits/contributions to this project. My appoligies in advance if the mods feel the need to move it...
I'd like to see what the Genesis 120 is capable with bolt ons before jumping inside it, and since I already have a nice header, the next bottle kneck looks like it could be with the intake side of things, which has me considering taking the carburetion on my Rage to the next level... that to me is a set of Keihin 39mm flat slide FCR carbs. We have all heard horror stories about them being stumbling non-starting top end only horrors... but after much reading and with updates done to this carburetor in recent years, I feel they might be tuned to work very well on the 120 engine.
So here's some info I've acquired...
Stock 40mm (act. 39mm) CV carbs flow about 10% less than a slide carb of equivalent diameter. That would place the stock CV flow at roughly a 36mm slide.
FCR carbs are available with choke, TPS sensors, and a large slew of adaption parts.
A non choke 39mm FCR kit exists for mid 90's Triumph triples which could be good for development of a kit for the Yamis. Triumphs are a 900cc 3 cyl dohc 4 valve engine producing roughly 100hp over the same RPM range as the Gen 120 engine. The center to center spacing on it 89mm, the Gen 120 is 88mm. It should get things in or close to the ballpark for jetting and setup. Cost for the kit is $800 - 900.
Rumor has it that Yamaha used Flatslides on their overseas snowcross Vector sleds.
Any input or first hand experiences using FCR's is appreciated.
I'd like to see what the Genesis 120 is capable with bolt ons before jumping inside it, and since I already have a nice header, the next bottle kneck looks like it could be with the intake side of things, which has me considering taking the carburetion on my Rage to the next level... that to me is a set of Keihin 39mm flat slide FCR carbs. We have all heard horror stories about them being stumbling non-starting top end only horrors... but after much reading and with updates done to this carburetor in recent years, I feel they might be tuned to work very well on the 120 engine.
So here's some info I've acquired...
Stock 40mm (act. 39mm) CV carbs flow about 10% less than a slide carb of equivalent diameter. That would place the stock CV flow at roughly a 36mm slide.
FCR carbs are available with choke, TPS sensors, and a large slew of adaption parts.
A non choke 39mm FCR kit exists for mid 90's Triumph triples which could be good for development of a kit for the Yamis. Triumphs are a 900cc 3 cyl dohc 4 valve engine producing roughly 100hp over the same RPM range as the Gen 120 engine. The center to center spacing on it 89mm, the Gen 120 is 88mm. It should get things in or close to the ballpark for jetting and setup. Cost for the kit is $800 - 900.
Rumor has it that Yamaha used Flatslides on their overseas snowcross Vector sleds.
Any input or first hand experiences using FCR's is appreciated.
RJH
TY 4 Stroke Junkie
Brian – you need to do that job – maybe Yamaha racing will see it and put FCR’s back on the race sled. I believe they went to FI after their win. I can’t see yet…how FI can work in motocross style riding. A carb can do some thinking… FI only does what’s it told.
Anyway – I believe the early big 4/ Yamaha had FCR’s (or maybe I'm thinking about the quad) and I think you are right about Europe. I would try and get a set of those or at least the jet specs to start. The engine you are suggesting does not have the same air flow, load or fuel requirements as your engine.
Much upgrading has been done to the new FCR’s to solve the problems you suggested. If you can’t find newer models – then you can probably do a fix with an aftermarket fuel screw.
Let’s get this done – I want to see the results. If I had 4/ race sled support – FCR’s would be my first purchase.
Anyway – I believe the early big 4/ Yamaha had FCR’s (or maybe I'm thinking about the quad) and I think you are right about Europe. I would try and get a set of those or at least the jet specs to start. The engine you are suggesting does not have the same air flow, load or fuel requirements as your engine.
Much upgrading has been done to the new FCR’s to solve the problems you suggested. If you can’t find newer models – then you can probably do a fix with an aftermarket fuel screw.
Let’s get this done – I want to see the results. If I had 4/ race sled support – FCR’s would be my first purchase.
I've been considering EFI also... but I'm not so sure I want to take the time to add all the sensors and such to the engine... not that jetting flatslides will happen quickly... Carbs intrigue me though. I've built quite a few hi-po injected engines, never really built a nice carbed engined though.
Yamaha definetly chokes the top end of this engine. Time to see if it can be opened up a bit w/o killing the great low - mid. I'm guessing these carbs could add 30hp all alone... NO OTHER mods... seriously... maybe 7 - 9hp up top.
Yamaha definetly chokes the top end of this engine. Time to see if it can be opened up a bit w/o killing the great low - mid. I'm guessing these carbs could add 30hp all alone... NO OTHER mods... seriously... maybe 7 - 9hp up top.
Some info from another post does indeed indicate the use of flat slides in Sweden. On a stock displacment sled with head work/big valves and flatslides 146.5hp was produced.... no mention of cams or header/exhaust work, but I'm sure it saw some.
Anoth option I have been entertaining is the downdraft version of the stock CVK40. Jetting those sure would be easier
Why not maximize what's there? Going to a downdraft would get rid of those crappy carb to head turned adapters. In an old post, Supertuner mentions how the RX-1 also uses sidedraft carbs on a downdraft head... definetly some power hiding in the RS sled due to that. Another benefit may be that the main and pilot air jet intakes along with the cresent shaped port for the CV diaphram would be moved out of the velocity stack... that would help flow and velocity too... Hmmmmm. Might require hood mods....
Input???
Anoth option I have been entertaining is the downdraft version of the stock CVK40. Jetting those sure would be easier
Why not maximize what's there? Going to a downdraft would get rid of those crappy carb to head turned adapters. In an old post, Supertuner mentions how the RX-1 also uses sidedraft carbs on a downdraft head... definetly some power hiding in the RS sled due to that. Another benefit may be that the main and pilot air jet intakes along with the cresent shaped port for the CV diaphram would be moved out of the velocity stack... that would help flow and velocity too... Hmmmmm. Might require hood mods....Input???
RJH
TY 4 Stroke Junkie
Good work
Probably what they have done is remove the FI and put some carbs on. You see…FI is told how to work so you have no real input from the engine throat telling them what to do. Sure…they have all the sensors – but I still can’t see how to get that fuel in right in high end snocross racing.
I can’t see them being the best option….at this time. MC road racing is good for F1–you can get the map right to prevent wheel spin and many other neat things etc. Well – you can do that with a carb as well.
From what I have seen in the nozzles – and throttle bodies – there is no way you can get the same atomization a carb can deliver. Atomization = HP and that is probably why they are seeing the bigger #’s.
Also – you need to be at a level of experience to fully get the benefits of the carb – or you may as well leave the injectors on. Someone who just changes MJ would have no chance.
Take a look at the old GYT Yamaha injectors. People still think they are great – but the difference between a bad set (which was weekly for us back then) was the difference between 1st working or 3rd or 4th not working. Depending on who was there? They loose the ability to atomize for some reason. You would get the same feel as using the incorrect high speed air jet in a Mikuni.
Downdraft…go for it – I once saw a wicked fast 2/ (back in the day) with a set of dual throat downdraft Webbers. The thing had a fantastic sound and was so fast …..well , until he seized. Never saw him again.
Probably what they have done is remove the FI and put some carbs on. You see…FI is told how to work so you have no real input from the engine throat telling them what to do. Sure…they have all the sensors – but I still can’t see how to get that fuel in right in high end snocross racing.
I can’t see them being the best option….at this time. MC road racing is good for F1–you can get the map right to prevent wheel spin and many other neat things etc. Well – you can do that with a carb as well.
From what I have seen in the nozzles – and throttle bodies – there is no way you can get the same atomization a carb can deliver. Atomization = HP and that is probably why they are seeing the bigger #’s.
Also – you need to be at a level of experience to fully get the benefits of the carb – or you may as well leave the injectors on. Someone who just changes MJ would have no chance.
Take a look at the old GYT Yamaha injectors. People still think they are great – but the difference between a bad set (which was weekly for us back then) was the difference between 1st working or 3rd or 4th not working. Depending on who was there? They loose the ability to atomize for some reason. You would get the same feel as using the incorrect high speed air jet in a Mikuni.
Downdraft…go for it – I once saw a wicked fast 2/ (back in the day) with a set of dual throat downdraft Webbers. The thing had a fantastic sound and was so fast …..well , until he seized. Never saw him again.
Ok carb guru types... I'll need some assistance on the potential of using the downdraft CVK40 carbs on here. I #*$&@ u me d the stock jets and such would be a an easy swap over from the Yami CVK40, but you know how that goes. The downdraft has a main jet and holder that is set off to an angle from the needle and needle jet. I have some pictures that will explain it and I'll post those up later.
I'm dumping the downdraft CVK40 project, OEM carbs just are not that easy to retro on to another engine.
Back to the real (hopefully) power maker. After some digging through the Sudco parts catalog, airbox adapters exist that should be perfect, they will adapt the stock velocity stacks to the FCRs. The spigot (engine side) adapters will have to be custom machined, but those are pretty easy..... then adapt the TPS and we'll see how the throttle cable works. I'll also look at the both heating systems..
Back to the real (hopefully) power maker. After some digging through the Sudco parts catalog, airbox adapters exist that should be perfect, they will adapt the stock velocity stacks to the FCRs. The spigot (engine side) adapters will have to be custom machined, but those are pretty easy..... then adapt the TPS and we'll see how the throttle cable works. I'll also look at the both heating systems..
Kråkan
TY 4 Stroke Junkie
The sled in sweden u are talking about is Fredrik Granberg or Patrik Abrahamssons old snowcross vector. they took the old 973 and MC-Xpress made alot of head, cam and exhaust mods... They also put other carbs on it... It actually had app 167hp.
There was talk about a kit with cams etc that made about 150hp that u could buy from MC-Xpress. But I havent heard anithing else about that.
All i can say is that the Nytro engine is WAY better than the 120. So the FI cant be that bad..... And according to Yam there is about 180hp in the race engine this year. And it runs FI..
There was talk about a kit with cams etc that made about 150hp that u could buy from MC-Xpress. But I havent heard anithing else about that.
All i can say is that the Nytro engine is WAY better than the 120. So the FI cant be that bad..... And according to Yam there is about 180hp in the race engine this year. And it runs FI..
The carbs yamaha used where supposed to be worth 10hp on the 120 engine.
So since the Triumph carbs are sidedraft, most of this seasons set-up will retain the stock curved carb to head adapters along with the airbox. Sudco part number 021-216 airbox adapter will mount up the stock black rubber velocity stack in the same manner to which it mounts to the CVK carb. I'll probably mount everything up, get it running, and if things work out a trip down to Freddie at ECP for dyno tuning of the carbs.
SO WHAT DID YOU FIND???
I did not run them last season, they are 95% adapted and will be installed in the next few weeks. Then on to jetting. Stay tuned!
Deadman
Expert
I have a 39 FCR on my quad, and yes it is a complicated carb, but damn its got alot of nice adjustments. I love the carb now that I understand it! 
Adapted....
All the entering air now sees, besides the needle. One can see the valve guide in there, not bad for a side draft carb on a down draft head.
All the entering air now sees, besides the needle. One can see the valve guide in there, not bad for a side draft carb on a down draft head.
GotJuice
Expert
very interested in this topic!
This is awesome, I would love to have the ability to run a Lightning pro turbo set up on my Apex mnt. but it has to be converted to a carbed set up before the turbo or something similar to that. Keep us posted, very interesting!
This is awesome, I would love to have the ability to run a Lightning pro turbo set up on my Apex mnt. but it has to be converted to a carbed set up before the turbo or something similar to that. Keep us posted, very interesting!
