Discussion Anyone else running Keihin FCR's on their ZXR250C?

I recently purchased a fresh set of Keihin FCR's through Webike Japan. They came from JB Power. I had assumed they would be assembled pretty close to perfect and I'd just have to fine tune.
Welp, no sir not that simple. I've spent a couple hours on the dyno today and it's still way too rich on the mains. They came with 125's and I'm down to 108's and it's still off the chart rich. This doesn't even address the other circuits that may be incorrect for the application. I've also learned how sensitive the air jet circuit is and even an 1/8th turn on the thing is a huge difference.

I wanted to reach out and see if anyone else is racing with these carbs on their ZXR and, if so, might have some advice for baseline. Right now even with 108 mains it's so rich on the air/fuel ratio that it doesn't register on the dyno after 12,000RPM. I've ordered smaller sizes to try. This is pretty wild. The dyno work perfectly and the sniffer for the air/fuel has been calibrated. It's definitely a carb tunning issue. Any new information would be great. After I find the main jet size that works I'll be turning my attention back to the midrange and off idle throttle response and could use more info as far as how the main air jet next to the velocity stack affects throttle response.

Thanks in advance. The whole point of this effort is to race the bike here locally alongside the ULWS bike I normally race.

 

I wish I had anything positive to contribute here, but I'm hoping to follow your path when you're done.

Fingers crossed someone has some info here. Good luck.

 

Hi have you checked the plugs and done a plug chop?
I dont trust those probes stuck up the zorst somethings not right thats a pretty big jet change for no see-able gain...
check float level...check spark (one plug being snuffed out will throw off the probe reading) try dropping the needles one notch to see if any difference in the transition....how much smaller/larger are the new carbs compared to stock..are you running then through the airbox ect ect
also try googling the carbs on similar bikes in regards to jet sizes/settings

 

I was going to suggest the "are the slides lifting" - until I remembered that FCR's are not the stock carbs

 

Well that debunks the sales pitch that the carbs come already set for the application - I wish you the best of luck in your quest, that does sound distressing to be tuning from that baseline

 

Have a read here , some good gotchas

http://www.factorypro.com/tech/tech_tuning_procedures/tuning_FCR_Burns,Pat.html

 

Maelstrom is truly a repository of wisdom, unlike our former PM the suppository of wisdom

 

Great minds think alike

 

I exalted you in praise, WHY OH WHY did you go for the suppository option?

 

Jesus. How'd I never find this when I did the FCR41s on the Monster?!

Between the gaskets you make, and this knowledge base, I'd be so screwed if you weren't contributing. Thank you!

 

thank you all for the replies. to answer a few comments quickly here's the facts

New plugs
It's a brand new set of carbs
I'm tuning the bike on a dynomometer not by feel or sound
The valve clearance is in spec
The carbs are synced
I just ran the bike at a local race track a couple weekends ago with the stock carbs and it was fine
it was tuned with the stock carbs and was nearly perfect on the dyno's air/fuel meter. Only issue was I couldn't get it to run with the stock airbox closed up. It ran way too rich and the Uni filter I bought for the airbox was destroyed by one of my dogs before I got a chance to try and tune it with the stock setup
The new carbs should make the bike run better, provide a 1/4 turn throttle for racing, and it should allow for screens to filter sand or dirt if I crash racing.
The tuning links provided in here don't reference the FCRs. I already understand the carb circuits and tuning and I'm making adjustments and running it on a dyno to check results. I'm aware of what each circuit does. I'm hoping to find someone else that's done this mod because I'd like to avoid having to buy a ton of jets and needles to find the range I need to be within. Not having the carbs close from the person that sold them to me means I have a wide range of changes I could potentially make to finally get close to a decent tune. As I'm hoping to not have to buy (4) of everything, a baseline would be great.

 

Shane - I feel for you. I've set up a few sets of FCR's now and frankly, that yours are so far out is surprising.
Couple of questions : What size air bleeds are in them now ? What size float valves - and associated, what fuel pressure are you running ?
Here in NZ I'm a long way from OE jet suppliers. I've made a lot of jets over the years....Keihin numbers are the metric bore size.
Once I've got it sorted with the rough jets, then spend the money on the real ones...