Carb size for these engine specs.

[BoostedRooster712]

Howdy,

I'm wanting to get another set of carbs for the 1216 in the RK. I've decided FCRs are the go for various reasons.

I'm struggling to work out if I should go 39 or 41mm FCRs.

It's an 1127 out of an 1100 K.

Head is mildly ported, stage 2 yoshi cams, wiseco R1216 kit which I've seen quoted between 12:1 and 13:1 compression, kibblewhite heavy duty valve springs and abuso oversize valves and titanium retainers. Adjustable cam sprockets and cams will be dialled in to spec.

Exhaust is a full Skorpion (akrapovic) 4 into 1 system.

Ignitech ignition system.

Bottom end is solid with carrillo rods.

Drag racing isn't on the cards, just spirited road riding and the occasional track day.

I'm leaning towards the 39s but the go big or go home part of my mind has driven me to come here to seek your opinions.

With these specs I have not upgraded the cylinder studs, I guess I proably should get my hands on some APE ones. Thoughts on that as well?

Any assistance would be greatly appreciated. 

Edited July 18, 2023 by BoostedRooster712
spelling got me again!

[Upshotknothole]

I'd say go with the 41s and accept that it's going to be boring around town if you take it out. The 39s will probably give you a little bit more usable real world power, but the 41s will force you to keep it up in the power band more. That just sounds like it'd be more fun to ride, but I like peaky bikes.

[Joseph]

Spirited road riding would probably point towards using the smaller ones ?

It does sound like quite the engine though 

[Gixer1460]

A M8 of mine has got a similar spec in a Blandit (with maybe not so much headwork and CR) and tried my new set of 41's, got dyno'd and mega power but ultimately quite unsatisfying on the road! As said above, at town speeds they were lumpy and a fraction too much twist action was a potentially licence losing scenario! I got them back and gave him some 36mm RS's which although down on power (maybe 10-12hp) are perfect for 'everyday' whilst still having licence loss potential 

ps. the 41's were / are destined for my 1460 engine - should be perfectly suited, if I ever get my arse in gear and do it!

[clivegto]

9 hours ago, Gixer1460 said:

 

ps. the 41's were / are destined for my 1460 engine - should be perfectly suited, if I ever get my arse in gear and do it!

 

Is it done yet 

[dupersunc]

41s. You've built the rest of the engine for power, why try and strangle top end.

 

[Duckndive]

Jonny1bump runs 39s on his mildly tweaked one  

https://oldskoolsuzuki.info/forums/topic/256-final-evolution-of-borderline-road-race/page/28/

[BoostedRooster712]

Thanks for all the great input gents.

A bit of history on this engine, with stage 1 cams, no other valve train changes, the mild porting and a set of 38mm CVs, is has put out 145 hp at the rear wheel.

That seemed more than enough back then but that was a long time ago. I remember it being what I thought at the time, an absolute torque monster.

There's a set of 39s at work which have just recently been rebuilt. If I can finally convince my boss to sell them to me I will give them a try.

If I find them a bit asthmatic I will chase a set of 41s.

[Upshotknothole]

I have RS38s on a stock 1127 engine and it's boring around town, but far from unrideable if you're used to flat slides. Curious what yours puts out if you can get the 39s on it. Are the FCRs still easy to get down there? They're getting harder and harder to find in the states, easier to get RS carbs or order Yosh TMRs from Japan/Europe if you're looking for bling.

[BoostedRooster712]

I have a set of RS38s as well, I plan on running them back to back on the dyno against whatever FCR I buy.

FCRs are getting harder to find down here. I found a set of 41s in New York but when you add freight in and they were stated as doing hardly any work but had been rebuilt (why if they've done hardly any work?) I decided to pull the pin.

There's a set of 39s at work, I saw them being rebuilt last year, I'm just having trouble extracting them from the boss who in his 70s, still likes to hoard nice parts he'll never use.

Looks like you can still get 39s new, just not 41s.

https://www.keihin-fcr.com/index.php?route=product/product&path=68&product_id=72

[Gsxrsam]

My old 1127 with ported head, bore, etc has fcr 39s on for about 12 years now.

The limiting factor was not the carbs, it was having a fuel tap big enough to feed them at high revs.......

 

[BoostedRooster712]

56 minutes ago, Gsxrsam said:

My old 1127 with ported head, bore, etc has fcr 39s on for about 12 years now.

The limiting factor was not the carbs, it was having a fuel tap big enough to feed them at high revs.......

 

I've been a bit concerned about that. I have a RG250 outlet in the tank which let's me feed to a remote tap. The tap was fine with CVs but I'm not sure if it will with flatslides. 

This is how it looks on a diagram. 

I have two pingel taps, the dual outlet high flow one has bugger all chance of fitting in 7/12 as it is massive. The other is a standard pingel. I'll have to see if I can find taps for each outlet to put in the fuel lines. I've bought some metal dry break connectors but plugging and unplugging them all the time will be a PITA. 

What solution did you use? 

[Gsxrsam]

It was a yamaha tap with 8mm feed instead of 6mm..... fz 750 if i remember right..... close to hole centres too.

[baldrick]

i ran 39s on my 1340 gsxr.

no shortage of top end and more useable id say