Topic: Throttle position

[bwaller]

My son has a set of FCR 32mm on his FZR4. Last season he struggled through set up with these carbs. During the season's last race we decided the final issue he experienced was probably in the pilot circuit since the bog was getting on the throttle out of low speed corners. The engine would be at around 10K rpm but probably back on the gas from 1/8 throttle position.

He has worked with different needle tapers and mainjets and those circuits seem good.

(When on the dyno Rick Stetson had some adjustable low speed airjets and installed those. Just as a ballpark setting those were set at 2.75 turns out.)

It appeared to be lean at that spot and different IMS settings didn't really gain much. By setting the adjustable low speed airjet down to 1.25 -1.5 turns from seated seems to help eliminate the bog but negatively affects idle. The mixture screw doesn't help enough with idle so he has ordered 1 larger & 1 smaller pilot jet to try.

Does anyone have experience with FCR carbs enough to recommend baseline settings for airjets and mixture screws in this pilot circuit. (example between 1 & 2.5 turns before changing pilot jets??)

[Captain]

The very area of throttle position you are struggling with is mostly controlled by the slide cutaway. Also after reading your discriptions of the changes attempted I think you are attempting everything around this as a "crutch" and it's not solving your problem.  Change the cutaway for one with a stronger signal and you will find that all your setting and adjustments will come back into range.

 Captain

[MRieck]

The very area of throttle position you are struggling with is mostly controlled by the slide cutaway. Also after reading your discriptions of the changes attempted I think you are attempting everything around this as a "crutch" and it's not solving your problem.  Change the cutaway for one with a stronger signal and you will find that all your setting and adjustments will come back into range.

 Captain

Different cutaways are not available for the FCR's. The accelerator pump also comes into play. Unfortunately the FCR pump isn't as adjustable as the Mikuni RS or TMR. It can be tweaked for starting point and duration

[turboguzzi]

32mm on a 400? gosh brent, this sounds way overcarbed to me.... am not surprised you guys struggle to dial them in. these would be a great size for a 750 that's sucking almost twice the amount of air....

[bwaller]

Brent I say 1/8 throttle and it's in that ballpark, but Mike is right, different slide cutaways are not available that we have found. On the dyno these carbs really performed well, but "in the air" there were several tuning issues and this one circuit is last to be addressed.

Part of the problem is in a four race series, tuning time is short. Tyler always explained what it was doing and we were both fooled by the rpm not considering throttle position until the last round last season.

On the shop floor if the throttle is opened to that position and held there it will bobble for a few seconds then lose rpm and even stall, unless more throttle added. He claimed on the track it eventually pulled through that spot after a second or so but that lost time killed drive.

Just closing the air jet to 1 turn off the seat and the issue disappears but naturally there are 3 adjustments involved in the pilot circuit and more time is needed. I feel sure we can solve this with a little more time but mostly wondered about adjustment range for that airjet & the IMS.

Mike, the pump timing is good and although I originally thought that might be the problem I don't anymore....at least for now. 

TG, these are the recommended FCR carbs for this bike, actually some use 33mm. Really large in comparison to what some other brands would use I agree. Tyler really likes the power increase they bring. 

[MRieck]

Those carbs produce a really good signal with that floating valve. Flat slide design CV carbs reached 38mm with 600's if I remember correctly.